NBS MONOGRAPH 88

Heat Treatment and Properties

of Iron and Steel

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin. Director

Heat Treatment and Properties of

Iron and Steel

Thomas G. Digges, Samuel J. Rosenberg, and Glenn W. Geil

National Bureau of Standards Monograph 88

Issued November 1, 1966

Supersedes Circular 495 and Monograph 18

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 35 cents
Contents

Page

1. Introduction 1

2. Properties of iron 1

2.1. Transformation temperatures 1

2.2. Mechanical properties 2

3. Alloys of iron and carbon 2

3.1. Iron-carbon phase diagram 2

3.2 Correlation of mechanical properties

with microstructures of slowly cooled carbon steels 6

4. Decomposition of austenite 6

4.1. Isothermal transformation 7

a. To pearlite 7

b. To bainite 7

c. To martensite 7

4.2. Continuous cooling 8

5. Heat treatment of steels 9

5.1. Annealing 10

a. Full annealing 10

b. Process annealing 10

c. Spheroidizing 10

5.2. Normalizing 10

5.3. Hardening 11

a. Effect of mass 11

5.4. Tempering 11

5.5. Case hardening 14

a. Carburizing 14

b. Cyaniding 16

c. Carbonitriding 16

d. Nitriding 16

5.6. Surface hardening 17

a. Induction hardening 17

b. Flame hardening 17

5.7. Special treatments 17

a. Austempering 17

b. Martempering 18

c. Cold treatment 18

d. Ausforming 18

6. Hardenability 18

7. Heat treatment of cast irons 20

7.1. Relieving residual stresses (aging) 20

7.2. Annealing 20

a. Malleabilizing 22

7.3. Normalizing, quenching, and tempering- 22

7.4. Special heat treatments 22

Page

8. Practical considerations 22

8.1. Furnaces and salt baths 23

a. Protective atmospheres 23

b. Temperature measurement and

control 23

8.2. Quenching media and accessories 24

8.3. Relation of design to heat treatment 25

9. Nomenclature and chemical compositions of

steels 26

9.1. Structural steels 26

9.2. Tool steels 31

9.3. Stainless and heat resisting steels 32

10. Recommended heat treatments 34

10.1. Structural steels 34

10.2. Tool steels 35

10.3. Stainless and heat resisting steels 35

a. Group I—Hardenable chromium

steels (martensitic and magnetic) 38

b. Group II—Nonhardenable chro- mium steels (ferritic and magnetic) 38

c. Group III—Nonhardenable chro- mium-nickel and chromiumnickel-manganese steels (austenitic and nonmagnetic) 38

11. Properties and uses of steels 38

11.1. Structural steels 38

a. Plain carbon structural steels 38

b. Alloy structural steels 39

11.2. Tool steels 39

11.3. Stainless and heat resisting steels 40

a. Group I—Hardenable chromium

steels (martensitic and magnetic) 41

b. Group II—Nonhardenable chro- mium steels (ferritic and magnetic) 41

c. Group III—Nonhardenable chro- mium-nickel and chromiumnickel-manganese steels (ausstenitic and nonmagnetic) 41

d. Precipitation-hardenable stainless

steels 42

11.4. Nickel maraging steels 44

12. Selected references 45

Library of Congress Catalog Card No. 66—61523

ii
Heat Treatment and Properties of Iron and Steel

Thomas G. Digges/ Samuel J. Rosenberg/ and Glenn W. Geil

This Monograph is a revision of the previous NBS Monograph 18. Its purpose is to provide an understanding of the heat treatment of iron and steels, principally to those unacquainted with this subject. The basic principles involved in the heat treat- ment of these materials are presented in simplified form. General heat treatment

procedures are given for annealing, normalizing, hardening, tempering, case hardening,

surface hardening, and special treatments such as austempering, ausforming, martempering and cold treatment. Chemical compositions, heat treatments, and some

properties and uses are presented for structural steels, tool steels, stainless and heatresisting steels, precipitation-hardenable stainless steels and nickel-maraging steels.

1. Introduction

The National Bureau of Standards receives

many requests for general information con- cerning the heat treatment of iron and steel and for directions and explanations of such

processes. This Monograph has been prepared

to answer such inquiries and to give in simplified form a working knowledge of the basic

theoretical and practical principles involved in

the heat treatment of iron and steel. The

effects of various treatments on the structures

and mechanical properties of these materials

are described. Many theoretical aspects are

discussed only briefly or omitted entirely, and

in some instances, technical details have been

neglected for simplicity. The present Monograph supersedes Circular 495, which was published in 1950, and Monograph 18 (1960).

Heat treatment may be defined as an operation or combination of operations that involves

the heating and cooling of a solid metal or alloy

for the purpose of obtaining certain desirable

conditions or properties. It is usually desired

to preserve, as nearly as possible, the form,

dimensions, and surface of the piece being

treated.

Steels and cast irons are essentially alloys of

iron and carbon, modified by the presence of

other elements. Steel may be defined as an

alloy of iron and carbon (with or without other

alloying elements) containing less than about

2.0 percent of carbon, usefully malleable or

forgeable as initially cast. Cast iron may be

defined as an alloy of iron and carbon (with or without other alloying elements) containing

more than 2.0 percent of carbon, not usually

malleable or forgeable as initially cast. For

reasons that will be apparent later, the dividing line between steels and cast irons is taken

at 2.0 percent of carbon, even though certain

special steels contain carbon in excess of this

amount. In addition to carbon, four other

elements are normally present in steels and in cast irons. These are manganese, silicon,

phosphorus, and sulfur.

Steels may be broadly classified into two

types, (1) carbon and (2) alloy. Carbon steels

owe their properties chiefly to the carbon.

They are frequently called straight or plain

carbon steels. Alloy steels are those to which

one or more alloying elements are added in sufficient amounts to modify certain properties.

The properties of cast iron also may be modified by the presence of alloying elements—such

irons are called alloy cast irons.

2. Properties of Iron

Since iron is the basic element of steel, a knowledge of some of its properties is a prerequisite to an understanding of the fundamental principles underlying the heat treatment

of steels.

2.1. Transformation Temperatures

If a molten sample of pure iron were allowed

to cool slowly and the temperature of the iron were measured at regular intervals, an ideal-

^ Retired.

ized (equilibrium) time-temperature plot of the

data would appear as shown in figure 1. The

discontinuities (temperature arrests) in this curve are caused by physical changes in the

iron.

The first arrest at 2,800 °F marks the temperature at which the iron freezes. The other

arrests (known as transformation temperatures or critical points) mark temperatures at which certain internal changes take place in

the solid iron. Some of these temperatures

are very important in the heat treatment of

steel.

1
TIME Figure 1. Idealized cooling curve for pure iron.

The atoms in all solid metals are arranged in some definite geometric (or crystallographic)

pattern. The atoms in iron, immediately after

freezing, are arranged in what is termed the

body-centered cubic system. In this crystal

structure the unit cell consists of a cube with an iron atom at each of the eight corners and

another in the center (fig. 2, a). Each of the

many individual grains (crystals) of which

the solid metal is composed is built up of a very large number of these unit cells, all oriented alike in the same grain. This high-temperature form of iron is known as delta (S)

iron.

At 2,550 °F (the A4 point), iron undergoes an allotropic transformation (fig. 1); that is, the arrangement of the atoms in the crystal

changes. The new crystal structure is facecentered cubic, and the unit cell again consists

of a cube with an iron atom at each of the

eight corners, but with an iron atom in the center of each of the six faces instead of one in the

center of the cube (fig. 2, b). This form is known as gamma (y) iron. At 1,670 °F (the

A3 point) iron undergoes another allotropic

0 b Figure 2. Crystal structure of iron.

(a) Body-centered cubic (alpha and delta iron); (b) Face-centered cubic (gamma iron).

transformation and reverts to the bodycentered cubic system (fig. 2, a). This structure, which is crystallographically the same as

delta iron, is stable at all temperatures below

the A3 point and is known as alpha (a) iron

(fig. 5, A). The arrest at 1,420 °F (known

as the A, point) is not caused by an allotropic

change, that is, a change in crystal structure.

It marks the temperature at which iron be- comes ferromagnetic and is therefore termed

the magnetic transition. Above this temperature iron is nonmagnetic.

These various temperature arrests on cooling are caused by evolutions of heat. On heating,

the arrests occur in reverse order and are caused by absorption of heat. The critical

points may be detected also by sudden changes

in other physical properties, for instance, ex- pansivity or electrical conductivity.

2.2. Mechanical Properties

Iron is relatively soft, weak*, and ductile

and cannot be appreciably hardened by heat

treatment. Its tensile strength at room temperature is about 40,000 lb/in.-, its yield strength

is about 20,000 lb/in. % and its Brinell hardness

is about 80. The modulus of elasticity is about

29,000,000 lb/in.-. The strength and hardness can be increased, with corresponding decrease

in ductility, by cold working.

3. Alloys of Iron and Carbon

The properties of iron are affected very

markedly by additions of carbon. It should

be realized that in discussing iron-carbon al- loys, we actually are dealing with plain carbon

steels and cast irons.

3.1. Iron-Carbon Phase Diagram

The complete iron-carbon phase (or constitutional) diagram represents the relationship

between temperatures, compositions, and struc- tures of all phases that may be formed by iron

and carbon under conditions of equilibrium

(very slow cooling) . A portion of this diagram for alloys ranging up to 6.7 percent of

carbon is reproduced in figure 3; the upper

limit of carbon in cast iron is usually not in excess of 5 percent. The lefthand boundary of

the diagram represents pure iron, and the

right-hand boundary represents the compound

iron carbide, FcsC, commonly called cementite.

* Iron grown as single-crystal whiskers may exhibit astoundingly

high strengths.

2
3600 —

6.7 % C STEELS CAST IRONS

CARBON, % BY WEIGHT

Figure 3. Iron-carbon phase diagram.

The beginning of freezing of the various

iron-carbon alloys is given by the curve ABCD,

termed the liquidus curve. The ending of

freezing is given by the curve AHJECF,

termed the solidus curve. The freezing point

of iron is lowered by the addition of carbon

(up to 4.3%) and the resultant alloys freeze over a range in temperature instead of at a constant temperature as does the pure metal

iron. The alloy containing 4.3 percent of car- bon, called the eutectic alloy of iron and cementite, freezes at a constant temperature as indicated by the point C. This temperature is 2,065 °F, considerably below the freezing point

of pure iron.

Carbon has an important effect upon the

transformation temperatures (critical points)

of iron. It raises the At temperature and lowers the A3. This effect on the A3 temperature

is very important in the heat treatment of

carbon and alloy structural steels, while that on

the A4 is important in the heat treatment of

certain high alloy steels, particularly of the

stainless types.

It is possible for solid iron to absorb or dissolve carbon, the amount being dependent upon

the crystal structure of the iron and the temperature. The body-centered (alpha or delta)

iron can dissolve but little carbon, whereas the

face-centered (gamma) iron can dissolve a considerable amount, the maximum being about

2.0 percent at 2,065 °F (fig. 3). This solid

solution of carbon in gamma iron is termed

austenite. The solid solution of carbon in delta

iron is termed delta ferrite, and the solid solution of carbon in alpha iron is termed alpha

ferrite, or, more simply, ferrite.

The mechanism of solidification of ironcarbon alloys, especially those containing less than about 0.6 percent of carbon, is rather complicated and is of no importance in the heat

treatment of carbon steels and cast irons. It is sufficient to know that all iron-carbon alloys

containing less than 2.0 percent of carbon (that

is, steel) will, immediately or soon after solidification is complete, consist of the single phase

austenite. Cast irons will consist of two phases

immediately after solidification—austenite and

cementite (FegC). Under some conditions this

cementite formed on cooling through the temperature horizontal ECF will decompose partly or completely into austenite and graphite

(carbon)

.
UJ

q:

Z)

I-< 1 (TU

Q.

2000

1800

1400

1200 -

1000

800

600 -

A3 1670° F

03% C

Ausfenite

+ Cemenlile

Pearlite with Excess Ferrite

Ferrite + Cemenlile

Peorlite with Excess Cementite

1

I Z

0.2 08 10 CARBON.

Figure 4. Phase diagram for carbon steels.

The part of the iron-carbon diagram that is concerned with the heat treatment of steel is reproduced on an expanded scale in figure 4. Regardless of the carbon content, steel exists as austenite above the line GOSE. Steel of

composition S (0.80% of carbon) is designated

as "eutectoid" steel, and those with lower or higher carbon as "hypoeutectoid" and "hypereutectoid," respectively.

A eutectoid steel, when cooled at very slow

rates from temperatures within the austenitic

field, undergoes no change until the temperature horizontal PSK is reached. At this temperature (known as the Ai temperature), the

austenite transforms completely to an aggregate of ferrite and cementite having a typical

lamellar structure (fig. 5, D and E). This

aggregate is known as pearlite and the Ai

temperature is, therefore, frequently referred

to as the pearlite point. Since the Aj transformation involves the transformation of austenite to pearlite (which contains cementite

Fe.sC), pure iron does not possess an Ai transformation (fig. 4). Theoretically, iron must

be alloyed with a minimum of 0.03 percent of

carbon before the first minute traces of pearlite can be formed on cooling (point P, fig. 4). If

the steel is held at a temperature just below Ai,

(either during cooling or heating), the carbide

in the pearlite tends to coalesce into globules or spheroids. This phenomenon, known as

spheroidization, will be discussed subsequently.

Hypoeutectoid steels (less than 0.80% of car- bon), when slowly cooled from temperatures

above the A.^, begin to precipitate ferrite when

the A3 line (GOS—fig. 4) is reached. As the

temperature drops from the A., to Ai, the precipitation of ferrite increases progressively and

the amount of the remaining austenite de- creases progressively, its carbon content being

increased. At the A, temperature the remaining austenite reaches eutectoid composition

(0.80% of carbon—point S, fig. 4) and, upon

further cooling, transforms completely into

pearlite. The microstructures of slowly cooled

hypoeutectoid steels thus consist of mixtures of

ferrite and pearlite (fig. 5, B and C). The

lower the carbon content, the higher is the temperature at which ferrite begins to precipitate

and the greater is the amount in the final structure. Hypereutectoid steels (more than 0.80% of

carbon) when slowly cooled from temperatures

above the Ae,,,, begin to precipitate cementite

when the A^m line {SE—fig. 4) is reached. As

the temperature drops from the Acm to Ai, the

precipitation of cementite increases progressively and the amount of the remaining austenite decreases progressively, its carbon content being depleted. At the Ai temperature

the remaining austenite reaches eutectoid composition (0.80% of carbon) and, upon further

cooling, transforms completely into pearlite.

The microstructures of slowly cooled hypereutectoid steels thus consist of mixtures of

cementite and pearlite (fig. 5, F). The higher

the carbon content, the higher is the temperature at which cementite begins to precipitate

4
i iGL RE 5. Microstructural constituents of slowly cooled carbon steels. A, Ferrite (a iron). All grains are of the same composition. XIOO.

B, 0.25% carbon. Light areas are (errite grains. Dark areas are pearlite. XIOO.

C, 0.5% carbon. Same as B but higher carbon content results in more pearUte and less ferrite. XIOO. D, 0.8% carbon. All pearlite. XIOO.

E, Same as D. At higher magnification the lamellar structure of pearlite is readily observed. X500.

F, 1.3% carbon. Pearlite plus excess cementite as network. XIOO.

All etched with either picral or nital. and the greater is the amount in the final structure. The temperature range between the Ai and

Ao points is called the critical or transformation range. Theoretically, the critical points

in any steel should occur at about the same

temperatures on either heating or cooling very

slowly. Practically, however, they do not since

the A3 and Ai points, affected but slightly by

the rate of heating, are affected tremendously

by the rate of cooling. Rapid rates of heating

raise these points only slightly, but rapid rates

of cooling lower the temperatures of transformation considerably. To differentiate between

the critical points on heating and cooling, the

small letters "c" (for "chauffage" from the

French, meaning heating) and "r" (for

"refroidissement" from the French, meaning

cooling) are added. The terminology of the

critical points thus becomes Acg, Arg, Aci, Ari,

etc. The letter "e" is used to designate the occurrence of the points under conditions of

extremely slow cooling on the assumption that

this represents equilibrium conditions ("e" for

equilibrium) ; for instance, the Aes, Aei, and

5
3.2. Correlation of Mechanical Properties With

Microstructures of Slowly Cooled Carbon Steels Some mechanical properties of pearlite

formed during slow cooling of a eutectoid

(0.80% of carbon) steel are approximately as

follows:

Tensile strength—115,000 Ib/in^

Yield strength—60,000 Ib/in^.

Brindell hardness number—200.

The amount of pearlite present in a slowly

cooled hypoeutectoid steel is a linear function

of the carbon content, varying from no pearlite,

when no carbon is present (the very slight

amount of carbon soluble in alpha iron may be

neglected), to all pearlite at 0.80 percent of

carbon. The balance of the structure of hypoeutectoid steels is composed of ferrite, the mechanical properties of which were given in a preceding section. Since the mechanical properties of aggregates of ferrite and pearlite are

functions of the relative amounts of these two

constituents, the mechanical properties of

slowly cooled hypoeutectoid steels are also lin- ear functions of the carbon content, varying

between those of iron at no carbon to those of

pearlite at 0.80 percent of carbon (fig. 6).

4. Decomposition of Austenite

In alloys of iron and carbon, austenite is stable only at temperatures above the Aei

(1,330 °F). Below this temperature it decomposes into mixtures of ferrite and cementite

(iron carbide) . The end product or final struc- ture is greatly influenced by the temperature at which the transformation occurs, and this, in

turn, is influenced by the rate of cooling.

Since the mechanical properties may be varied over a wide range, depending on the decomposition products of the parent austenite, a knowledge of how austenite decomposes and the factors influencing it is necessary for a clear

understanding of the heat treatment of steel. The progressive transformation of austenite

under conditions of equilibrium (extremely

slow cooling) has been described. Practically,

however, steel is not cooled under equilibrium

conditions, and consequently the critical points on cooling always occur at lower temperatures

than indicated in figure 4. If samples of steel, say of eutectoid carbon

content for the sake of simplicity, are cooled

from above the Aei at gradually increasing

rates, the corresponding Ar transformation

occurs at lower and lower temperatures (fig.

7) . This transformation is distinguished from

that occurring under extremely slow rates of

cooling (ArO by the designation Ar'. As the

rate of cooling of this steel is increased, an

additional transformation (termed the Ar" or

120

100

._ 80 o

o

O 60

40

20

y

200

to

160 3Na

HAR BER

120 _i

_jUJ

z.

trm

80

100

li ^

< 1,1

FERR TE

PEARL.ITE

UJ z

t- UJ

q: "

1,1 UI

0.2 0.4 0.6 CARBON, PERCENT

0.8100

Figure 6. Relation of wcchanical properties and structure

to carbon content of slowly cooled carbon steels.

1330

1050

AUSTENITE

PEARLITE

+ MARTENSITE MARTENSITE

INCREASED RATE OF COOLING- FiGURE 7. Schematic illustration showing the effect of rate of cooling on the transformation temperatures and de- composition products of austenite of eutectoid carbon

steel. 6
Ms) appears at relatively low temperatures

(about 430 °F) . If the rate of cooling is still further increased, the Ar' transformation is suppressed entirely and only the Ar" transformation is evident. It should be noted that

the temperature of the Ar" is not affected by

the rate of cooling, whereas the temperature of

the Ar' may be depressed to as low as about

1,050 °F in this particular steel. The product of the Ai'' transformation is fine

pearlite. As the temperature of the Ar' is gradually lowered, the lamellar structure of

the resulting pearlite becomes correspondingly

finer and the steel becomes harder and stronger.

The product of the Ai-" transformation is martensite (fig. 9, A, and 15, A) , which is the

hardest and most brittle of the transformation

products of austenite and is characterized by a

typical acicular structure. The phenomenon of the occurrence of both

the Ar' and Ai-" transformations is known as the split transformation. The resultant microstructures of steels cooled at such rates as to undergo a split transformation consist of varying amounts of fine pearlite and martensite

(fig. 9, C). The actual amounts of these two

constitutents are functions of the rates of cooling, the slower rates resulting in more pearlite

and less martensite, and the faster rates resulting in more martensite and less pearlite.

4.1. Isothermal Transformation

The course of transformation of austenite

when the steel is quenched to and held at vari- ous constant elevated temperature levels (isothermal transformation) is conveniently shown

by a diagram known as the S-curve (also

termed the TTT diagram—for time, temperature, and transformation) . Such a diagram

for eutectoid carbon steel is shown in figure 8

and the discussion of this figure will be confined to steel of this particular composition.

a. To Pearlite Austenite containing 0.80 percent of carbon,

cooled quickly to and held at 1,300 °F, does

not begin to decompose (transform) until after

about 15 min, and does not completely decompose until after about 5 hr (fig. 8). Thus, at temperatures just below the Aei, austenite is stable for a considerable length of time. The

product of the decomposition of austenite at

this temperature is coarse pearlite of relatively

low hardness. If the austenite is quickly

cooled to and held at a somewhat lower temperature, say 1,200 °F, decomposition begins

in about 5 sec and is completed after about 30

sec, the resultant pearlite being finer and

harder than that formed at 1,300 °F. At a temperature of about 1,050 °F, the austenite

decomposes extremely rapidly, only about 1 sec elapsing before the transformation starts and 5

1200

1000

800

600

200

_-i ! Ae,—

|AII<^TFNITF(A1

FERRITE(F) +

CARBIDE(C)

AUSTENITE Ms .

1 1 mill

1 MIN

1 1 iiiin 1 1 mill

1 HR

1 I'llllll

1 DAY

1 1 II 1 111

I 5 334042

42

4247

62

0.5 1 10' 10" TIME, SECONDS Figure 8. Isothermal transformation diagram (S-curve) for eutectoid carbon steel. (Metals Handbook, 1948 edition, page 608.) The hardness of the structures formed at the various temperatures is given by the scale on the right.

sec before it is completed. The resultant pearlite is extremely fine and its hardness is relatively high. This region of the S-curve where

decomposition of austenite to fine pearlite

proceeds so rapidly is termed the "nose" of the

curve.

b. To Bainite If the austenite is cooled unchanged to temperatures below the nose of the S-curve

(1,050 °F), the time for its decomposition

begins to increase (fig. 8) . The final product

of decomposition now is not pearlite, but a new

acicular constituent called bainite (fig. 15, E)

possessing unusual toughness with hardness

even greater than that of very fine pearlite.

Depending on the temperature, then, a cer- tain finite interval of time is necessary before

austenite starts to transform into either pearlite or bainite. Additional time is necessary

before the transformations are completed,

c. To Martensite If the austenite is cooled unchanged to a rel- atively low temperature (below about 430 °F

for the eutectoid carbon steel under consideration), partial transformation takes place in- stantaneously; the product of transformation is martensite. Austenite transforms into martensite over a temperature range and the

amount that transforms is a function of the

temperature. Only minute amounts will so transform at about 430 °F; practically all of

the austenite will be transformed at about

175 °F. The beginning of this transformation range is termed the Ms (martensite

start) and the end of the range is termed the

Mf (martensite—finish). As long as the temperature is held constant within the Ms—Mf

range, that portion of the austenite that does

not transform instantaneously to martensite

7
remains untransformed for a considerable

length of time, eventually transforming to

bainite.

In ordinary heat treatment of the plain car- bon steels, austenite does not transform into

bainite. Transformation of the austenite takes

place either above or at the nose of the S-curve,

forming pearlite, or in passing through the

Ms—Mf range, forming martensite or both. It

is evident that in order for austenite to be

transformed entirely into martensite, it must

be cooled sufficiently rapidly so that the temperature of the steel is lowered past the nose of

the S-curve in less time than is necessary for

transformation to start at this temperature.

If this is not accomplished, part of the steel transforms into pearlite at the high temperature (Ar'), and the remainder transforms

into martensite at the low temperature (Ar"

or Ms—Ft temperature range) . This explains

the phenomenon of the split transformation

described previously.

4.2. Continuous Cooling

Figure 9 represents a theoretical S-curve on which are superimposed five theoretical cooling

curves. Curves A to E represent successively

slower rates of cooling, as would be obtained,

for instance, by cooling in iced brine, water, oil, air, and in the furnace, respectively.

The steel coded according to curve E begins

to transform at temperature ti and completes

transformation at to', the final product is coarse

pearlite with relatively low hardness. When

cooled according to curve D, transformation

begins at and is completed at ti', the final

product is fine pearlite and its hardness is greater than the steel cooled according to curve

E. When cooled according to curve C, transformation begins at and is only partially

complete when temperature ta is reached; the

product of this partial transformation is very

fine pearlite. The remainder of the austenite

does not decompose until the Ms temperature

is reached, when it begins to transform to martensite, completing this transformation at the Mt temperature. The final structure is then a mixture of fine pearlite and martensite

(typical of incompletely hardened steel—frequently termed "slack quenched" steel) with a higher hardness than was obtained with the

steel cooled according to curve D. The rate of

cooling represented by curve B is just suffi- cient to intersect the nose of the S-curve, con- sequently only a minute amount of the austenite

decomposes into fine pearlite at temperature t^; the remainder of the austenite is unchanged

until the martensite transformation range is reached. If the steel is cooled at a slightly

faster rate, so that no transformation takes

place at the nose of the S-curve, the steel is completely hardened. This particular rate is

INCREASING TIME -

Figure 9. Schematic diagram illustrating the relation be- tween the S-curve, continuous cooling curves, and result- ing microstructures of eutectoid carbon steel. Microstructures: A, martensite; B, martensite with a trace of very fine pearlite (dark); C, martensite and very fine pearlite; D, fine pearlite; E,

coarse pearlite. All etched with nital. X500.

termed the critical cooling rate and is defined as the slowest rate at which the steel can be

cooled and yet be completely hardened. Since

this rate cannot be directly determined, the

rate indicated by curve B, producing only a

trace of fine pearlite (fig. 9, B), is frequently

8
used as the critical cooling rate. The hardness

of the resultant martensite is equivalent to the

maximum that can be obtained. Samples

cooled at a faster rate, such as that indicated

l)y curve A, are also completely martensitic but no harder than the sample cooled according to the critical cooling rate.

It will be noted that the rate at which a steel cools through the temperature range in the

vicinity of the nose of the S-curve is of critical

importance. Somewhat slower rates of cooling

above and below this temperature range can be

tolerated and yet obtain a completely hardened

steel, provided that the cooling through the

temperature interval at the nose of the S-curve

is sufficiently fast. In practice, however, steels are usually cooled rapidly from the quenching

temperature to relatively low temperatures

(about 500 °F) and then allowed to cool in air. Although the above discussions of the decomposition of austenite have been limited to a

steel of eutectoid composition, other steels behave in a similar manner, the temperatures

and times of reactions being different. In

hypoeutectoid steels, free ferrite plus pearlite are formed if transformation begins above the

temperature range of the nose of the S-curve;

the amount of free ferrite decreases as the temperature of transformation approaches the nose of the curve. In hypereutectoid steels,

free cementite plus pearlite are formed if transformation occurs above the nose. The

time for the start of the transformation at the nose increases as the carbon increases up to the

eutectoid composition, and then decreases with

further increase in carbon. That is, the nose

is shifted to the right (with respect to the

time axis, fig. 8) as the carbon is increased to

0.8 percent and to the left with further in- crease in carbon content. The temperature of formation of bainite is not appreciably affected by carbon content, but

the time for its formation increases with the

carbon.

Both the Ms and Mf temperatures are markedly lowered by increasing carbon content, as is shown for in figure 10. The Mf

temperatures of the plain carbon steels have

not been adequately determined; available information indicates that the Mf of high carbon

steels is actually below room temperature.

Slight amounts of austenite are frequently

retained in quenched steels, especially in the

higher carbon grades, even when cooled to room temperature.

5. Heat Treatment of Steels

All heat-treating operations consist of subjecting a metal to a definite time-temperature

1000

200

0 1 1 \ \ 1 1 \ 1 0 0.20 0.40 0.60 0.80 1.00 1.20 1.40 CARBON ,7o Figure 10. Influence of carbon on the start of the martensite {Ms) transformation of high-purity iron-carbon

alloys.

(Digges, Trans. Am. Soc. Metals 28, 597 (1940).)

cycle, which may be divided into three parts:

(1) Heating, (2) holding at temperature

(soaking), and (3) cooling. Individual cases vary, but certain fundamental objectives may

be stated.

The rate of heating is not particularly important unless a steel is in a highly stressed

condition, such as is imparted by severe cold

working or prior hardening. In such instances

the rate of heating should be slow. Frequently this is impracticable, since furnaces

may be at operating temperatures, but placing

the cold steel in the hot furnace may cause distortion or even cracking. This danger can be

minimized by the use of a preheating furnace

maintained at a temperature below the Ai.

The steel, preheated for a sufficient period,

then can be transferred to the furnace at operating temperature. This procedure is also

advantageous when treating steels having considerable variations in section thickness or very

low thermal conductivity.

The object of holding a steel at heat-treating

temperature is to assure uniformity of temperature throughout its entire volume. Obviously,

thin sections need not be soaked as long as thick sections, but if different thicknesses exist

in the same piece, the period required to heat

the thickest section uniformly governs the time

at temperature. A rule frequently used is to soak 1/2 hr/in. of thickness.

The structure and properties of a steel depend upon its rate of cooling and this, in turn,

is governed by such factors as mass, quenching

media, etc. It must be realized that the thicker

the section, the slower will be the rate of cooling regardless of the method of cooling used

except in such operations as induction hardening to be discussed later.

9
5.1. Annealing

Annealing is a process involving heating and

cooling, usually applied to produce softening.

The term also refers to treatments intended to

alter mechanical or physical properties, produce a definite microstructure, or remove gases.

The temperature of the operation and the rate

of cooling depend upon the material being an- nealed and the purpose of the treatment.

a. Full Annealing

Full annealing is a softening process in

which a steel is heated to a temperature above

the transformation range (Aca) and, after

being held for a sufficient time at this temperature, is cooled slowly to a temperature below

the transformation range (Ari). The steel is ordinarily allowed to cool slowly in the furnace,

although it may be removed and cooled in some medium such as mica, lime, or ashes, that in- sures a slow rate of cooling.

Since the transformation temperatures are

affected by the carbon content, it is apparent

that the higher carbon steels can be fully an- nealed at lower temperatures than the lower

carbon steels. In terms of the diagram shown

in figure 4, steels must be heated to above the

line GOSK. The temperature range normally

used for full annealing is 25 to 50 deg F above

this line (the upper critical), as shown in

figure 11. The microstructures of the hypoeutectoid

steels that result after full annealing are quite

similar to those shown for the equilibrium conditions (fig. 5, B and C). Eutectoid and

hypereutectoid steels frequently spheroidize

partially or completely on full annealing (see

later section and fig. 12, D).

b. Process Annealing

Process annealing, frequently termed stressrelief annealing, is usually applied to coldworked low carbon steels (up to about 0.25%

of carbon) to soften the steel sufficiently to allow further cold-working. The steel is usually heated close to, but below, the Aci temperature (fig. 11). If the steel is not to be

further cold-worked, but relief of internal

stresses is desired, a lower range of temperature will suflSce (about 1,000 °F). Rate of

cooling is immaterial.

This type of anneal will cause recrystallization and softening of the cold-worked ferrite

grains, but usually will not affect the relatively

small amounts of cold-worked pearlite. Typical structures of cold-worked, process-annealed,

and fully annealed low-carbon steel are shown

in figure 12, A, B, and C, respectively.

1800

1700

1600

^ 1500

ir 1400

a:

UJ

1^ 1300

1200

1100

1000

Figure 11. Recommended temperature ranges for heat

treating plain carbon steels.

c. Spheroidizing

Spheroidizing is a process of heating and

cooling steel that produces a rounded or globular form of carbide in a matrix of ferrite. It

is usually accomplished by prolonged heating

at temperatures just below the Aci (fig. 11),

but may be facilitated by alternately heating to temperatures just above the Aci and cooling

to just below the Ari. The final step, however,

should consist of holding at a temperature just

below the critical (Ar^. The rate of cooling

is immaterial after slowly cooling to about

1,000 °F.

The rate of spheroidization is affected by the

initial structure. The finer the pearlite, the

more readily spheroidization is accomplished.

A martensitic structure is very amenable to spheroidization.

This treatment is usually applied to the high

carbon steels (0.60% of carbon and higher).

The purpose of the treatment is to improve

machinability and it is also used to condition

high-carbon steel for cold-drawing into wire.

A typical microstructure of spheroidized high

carbon steel is shown in figure 12, D.

5.2. Normalizing

Normalizing is a process in which a steel is heated to a temperature above the Acg or the

Acm (fig. 11) and then cooled in still air. The

purpose of the treatment is to obliterate the

effects of any previous heat treatment (including the coarse-grained structure sometimes

10
Figure 12. Representative microstructures of carbon steels. A, Low carbon steel (0.1% C) as cold-worked. Both ferrite (light) and pearlite (dark) are severely deformed. XIOO. B, Same as (A) after process aimealing at 1,200° F. The ferrite is recrystallized (grains are equl-axed), but pearlite is not affected by this treatment. XIOO.

C, Same as (A) after full annealing at 1,650° F. All traces of cold working are ehminated, and the ferrite grains are larger than in (B). XIOO. D, High carbon steel (1.1% C) as spheroidized. All the carbon is present in the form of spheroids or shghtly elongated particles of cementite in a matrix of

ferrite. X500. E, Hypoeutectold steel (0.5% C) as normalized at 1,600° F in H-in. round (center area). Because of the rapid rate of air cooling such a small section, the

pearlite is quite fine and relatively little free ferrite is formed. XIOO.

F, Same steel as (E) but normalized in 2H-in. round (center area). The slower rate of cooling due to the larger section results in coarser pearlite and more

free ferrite. XIOO.

All etched in picral.

resulting from high forging temperatures) or cold-working and to insure a homogeneous

austenite on reheating for hardening or full annealing. The resultant structures are pearlite or pearlite with excess ferrite or cementite,

depending upon the composition of the steel. They are different from the structures resulting after annealing in that, for steels of the

same carbon content in the hypo- or hypereutectoid ranges, there is less excess ferrite or

cementite and the pearlite is finer. These are

the results of the more rapid cooling.

Since the type of structure, and, therefore,

the mechanical properties, are affected by the

rate of cooling, considerable variations may

occur in normalized steels because of differ- ences in section thickness of the shapes being

normalized. The effect of section thickness on

the structure of a normalized 0.5-percentcarbon steel is shown in figure 12, E and F.

11
5.3. Hardening

Steels can be hardened by the simple expedient of heating to above the Ac., transformation,

holding long enough to insure the attainment

of uniform temperature and solution of carbon

in the austenite, and then cooling rapidly

(quenching) . Complete hardening depends on

cooling so rapidly (fig. 9, A and B) that the

austenite, which otherwise would decompose on

cooling through the Ari, is maintained to relatively low temperatures. When this is accomplished, the austenite transforms to martensite on cooling through the Ms—Mf range. Rapid

cooling is necessary only to the extent of lowering the temperature of the steel to well below

the nose of the S-curve. Once this has been

accomplished, slow cooling from then on, either

in oil or in air, is beneficial in avoiding distortion and cracking. Special treatments, such as time quenching and martempering, are designed to bring about these conditions. As

martensite is quite brittle, steel is rarely used

in the as-quenched condition, that is, without

tempering.

The maximum hardness that can be obtained

in completely hardened low-alloy and plain car- bon structural steels depends primarily on the

carbon content. The relationship of maximum

hardness to carbon content is shown in

figure 13.

sample increase and the possibility of exceeding the critical cooling rate (fig. 9, B) becomes

less. To overcome this, the steel may be

quenched in a medium having a very high rate of heat abstraction, such as iced brine, but, even so, many steels have a physical restriction on the maximum size amenable to complete

hardening regardless of the quenching medium.

The marked effect that mass has upon the

hardness of quenched steel may be illustrated

by measuring the hardness distribution of different size rounds of the same steel quenched

in the same medium. Curves showing the distribution of hardness in a series of round bars

of different sizes of 0.5-percent-carbon steel are shown in figure 14. The quenching medium

used was water; the quenching temperature

was 1,530 °F. The rate of cooling decreased

as the diameters of the bars increased. Only

the i^-in. round hardened completely through

the cross section, whereas with the 4-in. round

the critical cooling rate was not attained even

at the surface.

5.4. Tempering

Tempering (sometimes called drawing) is the process of reheating hardened (martensitic) or normalized steels to some temperature below

the lower critical (Aci). The rate of cooling

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 CARBON ,%

Figure 13. Relation of maximum attainable hardness of quenched steels to carbon content.

(Burns. Moore, and Archer, Trans. Am. Soc. Metals 2G, 14 (1938)).

a. Effect of Mass Previous discussion of the formation of martensite has neglected the influence of mass.

It must be realized that even with a sample of

relatively sm.all dimensions, the rate of abstraction of heat is not uniform. Heat is always

abstracted from the surface layers at a faster

rate than from the interior. In a given cooling medium the cooling rate of both the surface

and interior decreases as the dimensions of a

2 10

RADIUS, INCHES 12

Figure 14. Variation in hardness in different size rounds

of 0.5--percent-carbon steels as quenched from 1,530° F

in water.

(Jomlny, Hardenability of alloy steels, Am. Soc. Metals, d. 75, 1939.)

12
; .rRE 15. Microstructures and corresponding hardness of heat-treated high carbon steel. A, As quenched in brine; martensite, Rockwell C hardness=67. XoOO. B, Same as A after tempering at 400° F; tempered martensite, Rockwell C hardness=61. X500.

C, Same as (A) after tempering at 800° F; tempered martensite, Rockwell C hardness=45. X500. D, Same as (A) after tempering at 1,200° F; tempered martensite, Rockwell C hardness=25. X500. E, Quenched in lead at 650° F; bainite, Rockwell C hardness=49. X500.

F, Heat-treated steel showing decarburized surface layer (light area) that did not respond to hardening. XIOO.

All etched in 1% nital.

is immaterial except for some steels that are

susceptible to temper brittleness.^

As the tempering temperature is increased,

the martensite (fig. 15, A) of hardened steel passes through stages of tempered martensite

(fig. 15, B and C) and is gi-adually changed

2 Temper brittleness is manifested as a loss of toughness (ob- served only by impact tests of notched bars) after slow cooling from tempering temperatures of 1,100 °F or higher, or after temF^")!?^-"? temperature range between approximately 850 ° and

1,100 "F. It is most pronounced in alloy steels that contain manganese or chromium and usually can be prevented by rapid quenching from the tempering temperature. The presence of molyb- denum is beneficial in counteracting the tendency toward temper

bnttleness.

into a structure consisting of spheroids of

cementite in a matrix of ferrite, formerly

termed sorbite (fig. 15, D). These changes

are accompanied by a decreasing hardness and

increasing toughness. The tempering temperature depends upon the desired properties and

the purpose for which the steel is to be used.

If considerable hardness is necessary, the tempering temperature should be low; if considerable toughness is required, the tempering

temperature should be high. The effect of

tempering on the hardness of fully hardened

carbon steels is shown in figure 16.

13
70

60

50

40

30

20

10

0.8(kRBON N

S\ 1 2 % CARBO

0.35 ?o CARBO

200 400 600 800 1000 TEMPERING TEMPERATURE °F

1200 1400

Figure 16. Effect of tempering temperature on the hardness of carbon steels of different carbon content.

Specimens were tempered for 1 hr. (Bain, Functions of the alloy- ing elements in stel. Am. Soc. Metals, p. 38, 1939.)

Proper tempering of a hardened steel re- quires a certain amount of time. At any- selected tempering temperature, the hardness

drops rapidly at first, gradually decreasing

more slowly as the time is prolonged. The

effect of time at different tempering temperatures upon the resultant hardness of a eutectoid carbon steel is shown in figure 17. Short

tempering periods are generally undesirable

and should be avoided. Good practice requires

at least 1/2 hr (or, preferably, 1 to 2 hr) at tempering temperature for any hardened steel. The necessity for tempering a steel promptly

after hardening cannot be overemphasized. If

fully hardened steel is allowed to cool to room

temperature during hardening there is dd,nger

that the steel may crack. Carbon steels and

most of the low alloy steels should be tempered

as soon as they are cool enough to feel comfortable (100 to 140 °F) to the bare hands.

Steels should not be tempered before they cool to this temperature range because in some

steels the Mf temperature is quite low and un- transformed austenite may be present. Part

or all of this residual austenite will transform to martensite on cooling from the tempering temperature so that the final structure

will consist of both tempered and untempered

martensite. The brittle untempered martensite, together with the internal stresses caused

by its formation, can easily cause failure of the

heat-treated part. When it is possible that

such a condition exists, a second tempering

treatment (double tempering) should be given

to temper the fresh martensite formed on cooling after the initial tempering treatment.

70

60

50

2 40

30

400° F

600°F \

\ \

--.^.800° F

^^00°F

10 SECOND

1/2 MINUTES

TIME AT TEMPERING TEMPERATURE

2 5 HOURS25

Figure 17. Effect of time at tempering temperature on

the hardness of 0.8-percent-carbon steel. {Logarithmic

time scale.)

(Bain, Functions of the alloying elements in steel. Am, Soc Metals, p. 233,

1939.)

If structural steels are to be used in the nor- malized condition, the normalizing operation is frequently followed by heating to a temperature of about 1,200 to 1,300 °F. The purpose

of this treatment, which is also designated as tempering, or stress-relief annealing, is to relieve internal stresses resulting on cooling

from the normalizing temperature and to improve the ductility.

5.5. Case Hardening

Case hardening is a process of hardening a

ferrous alloy so that the surface layer or case is made substantially harder than the interior or

core. The chemical composition of the surface

layer is altered during the treatment by the addition of carbon, nitrogen, or both. The most

frequently used case-hardening processes are

carburizing, cyaniding, carbonitriding, and

nitriding.

a. Carburiziag

Carburizing is a process that introduces car- bon into a solid ferrous alloy by heating the

metal in contact with a carbonaceous material

to a temperature above the Acs of the steel and

holding at that temperature. The depth of

penetration of carbon is dependent on temperature, time at temperature, and the composition of the carburizing agent. As a rough

indication, a carburized depth of about 0.030

to 0.050 in. can be obtained in about 4 hr at 1,700 °F, depending upon the type of carburizing agent, which may be a solid, liquid, or gas. Since the primary object of carburizing is to secure a hard case and a relatively soft, tough

core, only low-carbon steels (up to a maximum

of about 0.25% of carbon) , either with or without alloying elements (nickel, chromium, man14
HYPOEUTECTOID ZOiJi j EUTECTOID ZONE HYPEREUTECTOID ZONE A > -^SURFACE .J Figure 18. Structure of a carburized steel. The specimen was cooled slowly from the carburizing temperature. Etched with nital. X7o.

ganese, molybdenum), are normally used.

After carburizing, the steel will have a highcarbon case graduating into the low-carbon

core (fig. 18).

A variety of heat treatments may be used

subsequent to carburizing, but all of them involve quenching the steel to harden the carburized surface layer. The most simple treatment

consists of quenching the steel directly from

the carburizing temperature; this treatment

hardens both the case and core (insofar as the

core is capable of being hardened). Another

simple treatment, and perhaps the one most

frequently used, consists of slowly cooling from

the carburizing temperature, reheating to above the Aca of the case (about 1,425 °F),

and quenching; this treatment hardens the case

only. A more complex treatment is to double

quench—first from above the Acs of the core (about 1,650 °F for low-carbon steel) and

then from above the Aca of the case (about

1,425 °F) ; this treatment refines the core and

hardens the case. The plain carbon steels are almost always quenched in water or brine; the

alloy steels are usually quenched in oil. Although tempering following hardening of car- burized steel is sometimes omitted, a low-temperature tempering treatment at about 300 °F

is good practice.

It is sometimes desirable to carburize only

certain parts of the surface. This can be

accomplished by covering the surface to be

protected against carburizing with some material that prevents the passage of the car- burizing agent. The most widely used method

is copper plating of the surfaces to be

protected. Several proprietary solutions or

pastes, which are quite effective in preventing

carburization, are also available.

The commercial compounds commonly used

for pack (solid) carburizing contain mixtures

of carbonate (usually barium carbonate), coke

(diluent) , and hardwood charcoal, with oil, tar,

or molasses as a binder. Mixtures of charred

leather, bone, and charcoal are also used.

The carburizing action of these compounds

is diminished during use and it is necessary to add new material before the compound is reused. Addition of one part of unused to three to five parts of used compound is com- mon practice. The parts to be carburized are packed in boxes (or other suitable containers)

made of heat-resistant alloys, although rolled or cast steel may be used where long life of

the box is not important. The lid of the box

should be sealed with fire clay or some other

refractory to help prevent escape of the car- burizing gas generated at the carburizing temperature. The depth and uniformity of case is affected by the method of packing and design

of the container.

Liquid carburizing consists of case hardening steel or iron in molten salt baths that

contain mixtures principally of cyanides (poisonous), chlorides, and carbonates. The case produced by this method contains both carbon

and nitrogen, but principally the former. The

temperatures used range from about 1,550 to 1,650 °F or higher, depending upon the compositions of the bath and the desired depth of

case. At 1,650 °F a case depth of about 0.010

to 0.015 in. can be obtained in 1 hr; about

0.020 to 0.030 in. can be obtained in 4 hr.

Considerably deeper cases can be obtained at higher temperatures with longer periods of

time. After carburizing, the parts must be

quenched just as in solid carburizing, but it is usual to do this directly from the molten bath.

In all present-day commercial gas carburizing, two or more hydrocarbons are used in

combination for supplying the carbon to the

steel. The hydrocarbons used are methane,

ethane, propane, and oil vapors. The steel parts are placed in sealed containers through

which the carburizing gases are circulated; the

temperatures used are in the neighborhood of

15
5 0.080 I

(-

Ol

UJ^ 0.060 u

<o

0.040

0.020

0 t—^ 1 1 1 1 1 ^ 1 1 0 4 6 12 16 20 24 28 CARBURIZIN6TIME , HOURS Figure 19. Relation of time and temperature to carbon

penetration in gas carburizPng.

(Metals Handbook, 1939 edition, p. 1041.)

1,700 °F. Average expectation for depth of case in gas-carburized steel is illustrated in fig- ure 19. After carburizing, the parts must be

quench hardened.

b. Cyaniding

A hard, superficial case can be obtained

rapidly on low-carbon steels by cyaniding.

This process involves the introduction of both

carbon and nitrogen into the surface layers of

the steel. Steels to be cyanided normally are heated in a molten bath of cyanide-carbonatechloride salts (usually containing 30 to 95%

of sodium cyanide) and then quenched in brine,

water, or mineral oil; the temperature of operation is generally within the range of 1,550

to 1,600 °F. The depth of case is a function

of time, temperature, and composition of the

cyanide bath; the time of immersion is quite

short as compared with carburizing, usually

varying from about 15 min to 2 hr. The max- imum case depth is rarely more than about

0.020 in. and the average depth is considerably

less.Steels can be cyanided also by heating to the proper temperature and dipping in a powdered cyanide mixture or sprinkling the powder on the steel, followed by quenching. The

case thus formed is extremely thin.

Cyaniding salts are violent poisons if allowed

to come in contact with scratches or wounds;

they are fatally poisonous if taken internally.

Fatally poisonous fumes are evolved when

cyanides are brought into contact with acids.

Cyaniding baths should be equipped with a hood for venting the gases evolved during heating and the work room should be well ventilated. Molten cyanide should never be permitted to come in contact with sodium or

or potassium nitrates commonly used for baths

for tempering as the mixtures are explosive.

Furthermore, care is necessary in preparing

a salt bath and the work pieces should be completely dry before placing in the molten bath.

The advice of salt manufacturers should be

obtained and followed in the operation and

maintenance of salt baths.

c. Carbonitriding

Carbonitriding, also termed gas cyaniding,

dry cyaniding, and nitrocarburizing, is a process for case hardening a steel part in a gascarburizing atmosphere that contains ammonia

in controlled percentages. Carbonitriding is used mainly as a low-cost substitute for cyaniding and, as in cyaniding, both carbon and nitrogen are added to the steel. The process is carried on above the Aci temperature of the

steel, and is practical up to 1,700 °F. Quenching in oil is sufiiciently fast to attain maximum

surface hardness; this moderate rate of cooling

tends to minimize distortion. The depth to which carbon and nitrogen penetrate varies

with temperature and time. The penetration

of carbon is approximately the same as that

obtained in gas carburizing (fig. 19).

d. Nitriding

The nitriding process consists in subjecting

machined and heat-treated steel, free from sur- face decarburization, to the action of a

nitrogenous medium, usually ammonia gas, at a temperature of about 950 to 1,050 °F,

whereby a very hard surface is obtained. The

surface-hardening effect is due to the absorption of nitrogen and subsequent heat treatment

of the steel is unnecessary. The time required

is relatively long, normally being 1 or 2 days.

The case, even after 2 days of nitriding, is generally less than 0.020 in. and the highest hardness exists in the surface layers to a depth of

only a few thousandths of an inch.

Liquid nitriding (nitriding in a molten salt bath, generally composed of a mixture of

sodium and potassium salts) employs the same

temperature range as gas nitriding.

Special low-alloy steels have been developed

for nitriding. These steels contain elements

that readily combine with nitrogen to form

nitrides, the most favorable being aluminum,

chromium, and vanadium. Molybdenum and

nickel are used in these steels to add strength

and toughness. The carbon content usually is about 0.20 to 0.50 percent, although in some

steels, where high core hardness is essential, it may be as high as 1.3 percent. Stainless steels also can be nitrided.

Because nitriding is carried out at a relatively low temperature, it is advantageous to

16
use quenched and tempered steel as the base

material. This gives a strong, tough core with an intensely hard wear-resisting case—much

harder, indeed, than can be obtained by quench

hardening either carburized or cyanided steel. Although warpage is not a problem during

nitriding, steels increase slightly in size during

this treatment. Allowance can be made for

this growth in the finished article. Protection

against nitriding can be effected by tin, copper,

or bronze plating, or by the application of cer- tain paints.

5.6. Surface Hardening

It is frequently desirable to harden only the

surface of steels without altering the chemical

composition of the surface layers. If a steel contains sufficient carbon to respond to hardening, it is possible to harden the surface layers

only by very rapid heating for a short period,

thus conditioning the surface for hardening

by quenching.

a. Induction Hardening

In induction hardening, a high-frequency

current is passed through a coil surrounding

the steel, the surface layers of which are heated

by electro-magnetic induction. The depth to which the heated zone extends depends on the

frequency of the current (the lower frequencies

giving the greater depths) and on the duration

of the heating cycle. The time required to heat the surface layers to above the Acs is surprisingly brief, frequently being a matter of

only a few seconds. Selective heating (and

therefore hardening) is accomplished by suitable design of the coils or inductor blocks. At

the end of the heating cycle, the steel is usually

quenched by water jets passing through the

inductor coils. Precise methods for controlling the operation, that is, rate of energy input,

duration of heating, and rate of cooling, are

necessary. These features are incorporated in

induction hardening equipment, which is usually entirely automatic in operation. Figure

20 shows the hardened layer in induction hardened steel.

b. Flame Hardening

name hardening is a process of heating the

surface layers of steel above the transformation temperature by means of a high-temperature flame and then quenching. In this process

the gas flames impinge directly on the steel surface to be hardened. The rate of heating

is very rapid, although not so fast as with induction heating. Plain carbon steels are

usually quenched by a water spray, whereas the

rate of cooling of alloy steels may be varied

from a rapid water quench to a slow air cool

depending on the composition.

Figure 20. Macrograph of induction hardened gear teeth.

Original size, 4-in. diameter (Osborn).

Any type of hardenable steel can be flame

hardened. For best results, the carbon content should be at least 0.35 percent, the usual

range being 0.40 to 0.50 percent.

5.7. Special Treatments

a. Austempering

Austempering is a trade name for a patented

heat-treating process. Essentially, it consists

of heating steel to above the Ac^ transformation temperature and then quenching into a hot

bath held at a temperature below that at which

fine pearlite would form (the nose of the

S-curve, fig. 8), but above the Ms temperature

(fig. 10). The product of isothermal decomposition of austenite in this temperature region

is bainite. This constituent combines relatively high toughness and hardness. A typical

microstructure of austempered steel is shown

in figure 15, E.

The austempering process has certain limitations that make it impracticable for use with

many steels. In order to assure a uniform

structure (and hence uniform properties), it

is essential that the entire cross section of the

steel be cooled rapidly enough so that even the

center escapes transformation at the nose of

the S-curve. In carbon steels the time required

to start transformation at the nose of the

S-curve is extremely short, so that only relatively small sections (about %-in. maximum

thickness) can be successfully hot quenched

in austempering baths. The time required for

transformation of the austenite of alloy steels

17
to fine pearlite is usually longer, and hence

larger sections can be successfully austempered

(about 1 in. maximum). However, the time

required for transformation to bainite frequently becomes inordinately long with many

alloy steels and the process of austempering,

therefore, is generally impracticable for these

steels.

b. Martempering

Martempering consists of heating a steel to above its Acg transformation and then quenching into a bath held at a temperature approximately equal to that of its M,. The steel is maintained in the hot bath until its temperature is essentially uniform and then is cooled

in air. Severe internal stresses develop in steel during

hardening. Steel contracts during cooling but

undergoes a marked expansion when the

austenite transforms to martensite. Since a quenched steel must cool from the surface

inward, various portions transform at different times. The steel is thus subjected to a

variety of differential expansions and contractions, resulting in considerable internal stress. By equalizing the temperature throughout the

section before transformation takes place, and

then cooling slowly through the martensite

(Ms—Mt) range, the internal stresses are con- siderably reduced. This is the prime objective of martempering.

Modified martempering differs from standard martempering only in that the bath

temperature is lower, ranging from just below

the Ms point to about 200 °F. With the faster

cooling at these lower bath temperatures, steels

of lower hardenability can be hardened to suffi- cient depth. However, greater distortion of

sensitive parts is likely with the modified

process.

c. Cold Treatment The Mt temperature of many alloy steels is so low that complete transformation of austenite

to martensite does not occur on quenching to room temperature or on cooling after tempering. This retained austenite may be partially or completely transformed by cooling below

atmospheric temperatures and such treatment

is called "cold treatment." The beneficial

effects of cold treatment have not been fully

explored. It is known that the retained

austenite of highly alloyed steels is frequently

difficult to transform. Cooling these steels to low temperatures (to the temperature of solid

CO2 or even lower) immediately after the

quench is sometimes effective in transforming

the retained austenite, but with the concomitant danger of cracking. When the cold treat- ment is applied after tempering, the retained

austenite is considerably more resistant to

transformation. If cold treatment is used, the

steel should always be tempered afterwards.

Repeated alternate heating to a temperature

slightly below that used in tempering and cooling to a subzero temperature in a refrigerated

iced brine, carbon dioxide, liquid air, or liquid

nitrogen is commonly used for transforming

the retained austenite (dimensional stabilization) of steel gages, especially those of the ball- bearing type composition ( AISI 52100)

d. Ausforming

In this process, medium-carbon alloy steels are first austenitized and then cooled rapidly

to the temperature range above Ms in the "bay"

of the S-curve, between the pearlite and bainite

transformation bands. While held at a temperature within this bay the steels are plastically deformed and subsequently transformed at lower temperatures to martensite or bainite.

The steels are then tempered. This "ausforming" or "hot-working" technique has been

employed to provide higher yield and tensile

strength values than those obtainable for these

steels by the normal quench and temper

treatments.

6. Hardenability

Hardenability is the property that determines the depth and distribution of hardness

induced in steel by quenching. It is increased

by increasing carbon and by the addition of all the common alloying elements (except cobalt)

,

provided that these elements are completely

dissolved in the austenite at quenching temperatures. The elements most frequently used

for this purpose are manganese, chromium, and

molybdenum. Hardenability is also enhanced

by increased grain size ^ and homogeneity of

the austenite. However, a coarse-grained

austenite increases the tendency of a steel to

' The grain size that influences hardenability is that grain size of the austenite that exists at the quenching temperature. It is usually measured under the microscope In terms of the number of grains per square inch at a magnification of XIOO. The common range

of grain size numbers is as follows (note that the larger the number, the finer is the grain size; i.e., the more grains there are per square inch) :

American Society for Testing and Materials Grain size no. Grains/in'

at XIOO

0 0.50

0.5 0.707

1.0

1.5 2.0

2.5

3.0 3.5

4.0

4.5

5.0

5.5

6.0

6.5 7.0

1.0 1.41

2.0 2.83

4.0 5.66

8.0 11.3

16.0 22.6

32.0

45.3

64.0

Grain size

7.5 —

8.0

8.5 9.0

9.5 10.0

10.5

Grains/in^

at XIOO

90.5

- 128.

- 181.

- 256.

- 362.

- 512.

- 724.

11.0 1020.

11.5 1450.

12.0 2050.

12.5 2900.

13.0 4100.

13.5 5800.

14.0 8200.

18
distort and crack during heat treatment.

Coarse-grained steels also are less tough than

fine-grained steels. A clear distinction must be drawn between

the maximum hardness obtainable in a steel and its hardenability. In straight carbon and

low-alloy steels, the maximum hardness is a

function of carbon content only (fig. 13),

whereas hardenability is concerned primarily

with the depth of hardening.

Numerous methods have been proposed and

used for determining the hardenability of steel. The selection of a method depends largely upon

the information desired and the range in hardenability of the steels. For a fundamental study, hardenability is often measured in terms of the critical cooling

rate (fig. 9, B), but the standard end-quench

(Jominy) test is widely used commercially.

In the end-quench test a bar of steel 1 in, in

diameter by 4 in. long, previously normalized

and machined to remove the decarburized surface, is heated to the hardening temperature

for 30 min. It is then quickly transferred to a

fixture that holds the bar in a vertical position

and a jet of water under controlled conditions

immediately is directed against the bottom end

only. The end of the bar is thus cooled very

quickly while cross sections remote from the

end are cooled more slowly, the rate of cooling

being dependent upon the distance from the

quenched end. After cooling is completed, two

diametrically opposite flats about 14 in. wide

are ground the length of the bar and Rockwell

hardness measurements are made at intervals

of 1/16 in. on one or both of the flat surfaces so prepared. The relationship of hardness to distance from the quenched end is an indication of

the hardenability (fig. 21).

The conditions of the end-quench test have

been standardized and the rates of cooling at various distances from the quenched end have

been determined and are shown at the top of

figure 21. It is thus possible to correlate hardness with cooling rate, which is a function of

the distance from the water-cooled end. The

cooling rates at various positions in the cross- sections of different size rounds, when quenched

in still oil and in still water also have been

determined. Correlations can be made, therefore, between the hardness obtained at various

positions on the end-quench bar and positions

of equivalent hardness of round bars quenched

in still oil or water (assuming that points of

equal hardness have the same cooling rate).

Figure 22 shows this relationship between the

distances on the end-quenched bar and various

locations for equivalent hardnesses in round

bars quenched in still oil; figure 23 shows cor- responding relations for bars quenched in still water.

If the hardenability (end-quench) curve of a steel is known, it is possible to ascertain

COOLING RATE AT I300°F , °F/SEC

310 125 32 16 10.2 7.0 4.8 3.9 3.4 32 3.10

g 40 O

4901195 55 22 12.4 8.4 5.6 4.3 3.6 3.3 3.15

1 1 1 1 ( 1 1 1

C

1 1 1 1

\

8

A

1 1 1 1 1 1

1 1 1 1 1

1 0 4 8 12 16 20 24 28 32 36 40

DISTANCE FROM QUENCHED END , 1/16 IN. Figure 21. Hardenability curves for different steels with

the same carbon content.

A, Shallow hardening; B, intermediate hardening; C, deep hardening. The cooling rates shown in this figure are according to the results of Boegehold and Weinman, N.D.R.C. Report OSRD No. 3743, p. 42 (June 1, 1944).

- 2.5

a.)^ 1.5

SURFACE / 3/4 / / RADIUS/ /center

//

(y ^1/2 RADIUS /

y

0 2 4 6 8 10 12 14 16 IB 20 22 24

DISTANCE FROM WATER QUENCHED END, 1/16 IN.

Figure 22. Relation between distances on the end-quench

bar and various locations for equivalent hardness or

cooling rates in round bars quenched in still oil. (Metals Handboolc. 1948 edition, p. 492.)

from this curve and the curves given in figures

22 and 23 whether or not a selected size round

made from the steel will have a desired hardness at its surface, % radius, 1/2 radius, or

center, when quenched either in still oil or

water. Conversely, the hardness values as determined at various positions in rounds after

quenching in oil or water can be used to advantage in approximating the degree of hardenability of a steel suitable for replacement of

similar parts.

The standard end-quench test is most useful

for steels of moderate hardenability, that is, the low-alloy steels. For shallow hardening

steels, such as plain carbon, a modification of

this test (the L-type bar) is used.

19
a:

SURFACE / 3/4

ADIUS ^1/2RADIUS 7

JT /center

0 2 4 6 8 10 12 14 16 18 20

DISTANCE FROM WATER QUENCHED END .1/16 IN. FinuRE 23. Relation between distances on the end-quench

bar and various locations for equivalent hardnesses or cooling rates in round bars quenched in still water.

(Metals Handtook, 1948 edition, p. 492.)

7. Heat Treatment of Cast Irons

Because of variations in the rate of solidification and cooling, castings may be white at the

surface layers, mottled immediately below, and

gray in the interior.

In nodular cast iron, also known as "ductile

iron," "nodular graphite iron," "spherulitic

iron," and "spheroidal graphite iron," the graphite is present in the form of nodules or spheroids (fig. 24, E and F) bestowing considerable ductility to the cast iron. These nodules form during solidification due to the treat- ment of the molten iron, just prior to casting,

with a few hundredths percent usually of either

magnesium or cerium; the sulfur content must

be below about 0.015 percent for the treatment

to be effective with these elements. Nodular

iron and gray iron have the same total carbon

content. The mechanical properties of nodular

iron can be varied by alloying.

Cast irons may be given a variety of heat

treatments, depending on the compositions and

desired properties. The principles of the heat

treatments applied to cast iron are similar to those already discussed for steels.

Cast irons are of three general types, gray,

white, and nodular. The terms gray and white

are descriptive of the characteristic appearances of the fractures and nodular is descriptive of the shape of the graphite particles.

In gray iron a large proportion of the carbon

is present as graphitic or uncombined carbon

in the form of flakes. The small proportion of

carbon that is present as combined carbon or cementite is in the form of pearlite just as in a

steel. Depending on the amount of combined

carbon, the matrix of cast iron may be similar

to a hypoeutectoid steel with varying amounts

of free ferrite and graphite flakes dispersed

throughout (fig. 24, A, B, and C). If the

amount of combined carbon is about 0.8 percent, the matrix will be entirely pearlitic; if the

amount is greater, the matrix will be similar to a hypereutectoid steel. In white iron most, if not all, of the carbon

is present as cementite (fig. 24, D). The

dividing line between white and gray cast iron

is not clearly defined and iron possessing characteristics of both are called mottled cast iron.

Whether a cast iron will be gray or white

depends on two factors—composition and rate

of cooling. Certain elements tend to promote

the formation of graphitic carbon; the most

important of these are silicon, nickel, and

sulfur. Others tend to promote the formation

of combined carbon; the most important of

these are chromium, molybdenum, vanadium,

and manganese. Rapid solidification and cooling, such as result from thin sections cast in

sand or larger sections cast against chills, promote the foi^mation of white iron. Iron so produced is frequently termed chilled iron.

7.1. Relieving Residual Stresses (Aging)

The heat treatment that relieves residual

stresses is commonly called "aging," "normalizing," or "mild annealing;" the term "stress re- lieving" is more accurately descriptive. This

treatment can be accomplished by heating the

cast iron to between 800 and 1,100 °F., holding

at temperature from 30 min. to 5 hr, the time

depending on the size and temperature, and

cooling slowly in the furnace. Such treatment

will cause only a slight decrease in hardness,

very little decomposition of cementite, and only

a slight change in the strength of the metal.

7.2. Annealing

Sometimes, it is desirable to soften castings

in order to facilitate machining. The temperature range most commonly used for this purpose is 1,400 ° to 1,500 °F, although temperatures as low as 1,200 ° to 1,250 °F have been

used satisfactorily. Highly alloyed irons are sometimes annealed at temperatures as high as

1,800 °F. In all annealing, care should be

taken to prevent oxidation of the casting and

cooling should be slow.

In general, softening for machinability is accompanied by a decrease in strength and in

the amount of combined carbon, and by an in- crease in graphite content. Irons that contain

carbide-forming elements (such as chromium,

molybdenum, vanadium, and manganese) are more resistant to softening than the ordinary

gray iron and, therefore, must be annealed at considerably higher temperatures. Completely

annealed ordinary gray cast iron has a Brinell

20
r

* T > ^^j^

FifiURE 24. Microstructure of cast irons.

A, Gray iron. The dark flakes are graphite. XJnetclied. XIOO.

B, Same as (A). Etched with picral. XIOO.

C, Same as (B). The matrix consists of pearlite with a small amount of free ferrite (white) and is similar to a hypoeutectoid steel. The appearance of the

graphite flakes is not affected by the etchant. X500. D, White iron. The white needles are cementite; the dark areas are pearlite; the dark spheroidal areas are the eutectic (Ledeburite) . Etched with picral. X500.E, Nodular iron. The dark nodules are graphite; the white matrix is ferrite. Etched with picral. XIOO.

F, Pearlitic nodular iron. Nodular graphite (dark) in a pearlitic matrix. Etched with picral. XIOO. G, Malleable iron. The dark nodules are graphite; the white matrix is ferrite. Etched with picral. XIOO. H, Pearlitic malleable iron. Nodular graphite (dark) in a pearlitic matrix. Etched with picral. XIOO.

21
hardness of 120 to 130, and completely an- nealed alloy irons may have a Brinell hardness

of 130 to 180, depending on the composition.

The matrix of nodular iron can be made

completely ferritic by heating at 1,300 °F for 1 to 5 hr, depending on composition, size of casting, and amount of pearlite in the as-cast structure; cooling from the annealing temperature

can be at any convenient rate. If massive or primary carbides are present in the as-cast structure, they may be converted to spheroidal

graphite by heating at 1,600 to 1,700 °F for

1 to 5 hr. Slowly cooling from this temperature range to about 1,275 °F and holding from

3 to 5 hr will convert the matrix to ferrite.

Nodular iron with a ferritic matrix has the

desirable combination of maximum machinability, toughness, and ductility. The hardness of ferritic nodular iron varies with the

alloy content from a low of about 130 Brinell

to a high of about 210 Brinell.

a. Malleabilizing

Malleabilizing (also termed graphitizing) is a process of annealing white cast iron in such

a way that the combined carbon is wholly or

partly transformed to graphite in the nodular

form (fig. 24, G and H) instead of the flaky

form that exists in gray cast iron. The nodular form of carbon is called temper or free

carbon. In some instances, part of the carbon

is lost in the malleabilizing process.

White iron that is to be malleabilized should

contain all its carbon in the combined form.

Any carbon present as graphitic carbon (and

hence in the form of flakes) will not be affected

by the graphitizing treatment and is undesirable in the final structure. The temperatures used in malleabilizing

usually range from about 1,500 to 1,650 °F.

Time at temperature is quite long, frequently

being as much as two to three days. Holding

at this temperature causes the decomposition

of the cementite into austenite plus rather

rounded forms of graphite. If the iron is then

cooled very slowly through the critical range

(less than 10 °F/hr), the resultant structure

will consist of ferrite plus temper carbon.

Cooling less slowly through the transformation

range, or using an iron containing an alloying

element that retards graphitization, results in a final product containing various amounts of

pearlite plus ferrite with, of course, temper

carbon. Irons having a pearlitic structure

plus temper carbon are known as pearlitic

malleable irons (fig. 24, H). As might be ex- pected, pearlitic malleable irons are susceptible

to heat treatment in the same manner as steels.

7.3. Normalizing, Quenching, and Tempering

When heated to above the critical temperature (line SK, fig. 3), cast iron consists of austenite plus excess cementite or graphite and the

austenite can be transformed in the same manner as in steel. Although irons that are almost

completely graphitic (the common soft gray

irons) can be hardened, this is not customary.

Such irons must be held at temperatures well

above the critical for long periods of time in

order to assure the solution of carbon in the

austenite.

For optimum results, the graphite flakes

should be small and uniformly distributed and

the matrix should be pearlitic with not too much free ferrite (combined carbon from about

0.50 to 0.80%). Castings should be heated uniformly to the quenching temperature, which

usually ranges from 1,500 to 1,600 °F. The

quenching medium commonly used is oil; water

is used to a limited extent and some alloy

castings may be quenched in air. The iron, as quenched, generally has a lower strength than

as cast, but the strength is increased by tempering in the range of 350 to 1,200 °F. Softening proceeds uniformly as the tempering temperature is increased above about 350 to 1,200 °F.

A pearlitic matrix can be produced in nodular iron as cast or by normalizing. The

normalizing treatment consists of heating to 1,600 to 1,650 °F, holding at temperature for

solution of carbon, followed by cooling in air. The mechanical properties of the normalized

irons vary with the composition, fineness, and

amounts of pearlite; heavy sections are lower

in hardness and higher in ductility than light

sections. The Brinell hardness numbers range

from about 200 to 275.

Martensitic or bainitic structures are produced in nodular iron by quenching from 1,600

to 1,700 °F in oil or water. The irons as quenched have high strengths but low ductility.

However, the ductility of the as-quenched iron can be materially increased (strength and

hardness decreased) by tempering at about

1,000 to 1,300 °F.

7.4. Special Heat Treatments

Subject to the same limitations as steels, cast irons can be flame-hardened, induction-hardened, austempered, martempered, or nitrided.

8. Practical Considerations

Satisfactory heat treatment of steel (and

iron) requires furnaces that have uniform controlled temperatures, means for accurate temperature measurement, and for protecting the

surface of the material from scaling or decarburizing. Proper quenching equipment is needed also.

22
8.1. Furnaces and Salt Baths

There are many different types and sizes of

furnaces used in heat treatment. As a general rule, furnaces are designed to operate in

certain specific temperature ranges and at- tempted use in other ranges frequently results

in work of inferior quality. In addition, using

a furnace beyond its rated maximum temperature shortens its life and may necessitate

costly and time-consuming repairs.

Fuel-fired furnaces (gas or oil) require air

for proper combustion and an air compressor

or blower is therefore a necessary adjunct.

These furnaces are usually of the muffle type,

that is, the combustion of the fuel takes place

outside of and around the chamber in which

the work is placed. If an open muffle is used,

the furnace should be designed so as to prevent

the direct impingement of flame on the work.

In furnaces heated by electricity the heating

elements are generally in the form of wire or

ribbon. Good design requires incorporation of

additional heating elements at locations where

maximum heat loss may be expected. Such

furnaces commonly operate up to a maximum

temperature of about 2,000 °F. Furnaces

operating at temperatures up to about 2,500 °F

usually employ resistor bars of sintered

carbides.

Furnaces intended primarily for tempering

may be heated by gas or electricity and are

frequently equipped with a fan for circulating

the hot air. Salt baths are available for operating at either tempering or hardening temperatures.

Depending on the composition of the salt bath,

heating can be conducted at temperatures as low as 325 °F to as high as 2,450 °F. Lead

baths can be used in the temperature range of

650 to 1,700 °F. The rate of heating in lead or

salt baths is much faster than in furnaces.

a. Protective Atmospheres

It is often necessary or desirable to protect

steel or cast iron from surface oxidation (scaling) and loss of carbon from the surface layers

(decarburization, fig. 15, F). Commercial fur- naces, therefore, are generally equipped with some means of atmosphere control. This usually is in the form of a burner for burning

controlled amounts of gas and air and directing

the products of combustion into the furnace

muffle. Water vapor, a product of this combustion, is detrimental and many furnaces are equipped with a means for eliminating it. For

furnaces not equipped with atmosphere control, a variety of external atmosphere generators

are available. The gas so generated is piped

into the furnace and one generator may supply

several furnaces. If no method of atmosphere

control is available, some degree of protection

may be secured by covering the work with cast iron borings or chips.

Since the work in salt or lead baths is sur- rounded by the liquid heating medium, the

problem of preventing scaling or decarburization is simplified.

Vacuum furnaces also are used for annealing

steels, especially when a bright non-oxidized

surface is a prime consideration.

b. Temperature Measurement and Control Accurate temperature measurement is essential to good heat treating. The usual method

is by means of thermocouples; the most com- mon base-metal couples are copper-Constantan

(up to about 700 °F), iron-Constantan (up to about 1,400 °F), and Chromel-Alumel (up to about 2,200 °F). The most common noblemetal couples (which can be used up to about

2,800 °F) are platinum coupled with either the

alloy 87 percent platinum—13 percent rhodium

or the alloy 90 percent platinum—10 percent

rhodium. The temperatures quoted are for

continuous operation.

The life of thermocouples is affected by the

maximum temperature (which may frequently

exceed those given above) and by the furnace

atmosphere. Iron-Constantan is more suited

for use in reducing and Chromel-Alumel in

oxidizing atmospheres. Thermocouples are

usually encased in metallic or ceramic tubes

closed at the hot end to protect them from the

furnace gases. A necessary adjunct is an

instrument, such as a millivoltmeter or potentiometer, for measuring the electromotive

force generated by the thermocouple. In the

interest of accurate control, the hot junction of

the thermocouple should be placed as close to the work as possible. The use of an automatic

controller is valuable in controlling the temperature at the desired value.

If temperature-measuring equipment is not

available, it becomes necessary to estimate

temperatures by some other means. An inexpensive, yet fairly accurate method involves the use of commercial crayons, pellets, or paints

that melt at various temperatures within the

range 125 to 1,600 °F. The least accurate

method of temperature estimation is by observation of the color of the hot hearth of the fur- nace or of the work. The heat colors observed

are affected by many factors, such as the conditions of artificial or natural light, the character of the scale on the work, etc. Steel begins to appear dull red at about

1,000 °F, and as the temperature increases the

color changes gradually through various shades

of red to orange, to yellow, and finally to white.

A rough approximation of the correspondence

between color and temperature is indicated in

figure 25.

23
2700

1500 — 1400

— 1200

I 100

1300

Color of

Hot Body

White

1000 Yellow

900

800

700

600

Orange

Light Cherry

Full Cherry

Dork Cherry

Dork Red 500

- 400

700 Temper

Colors

- 300 Blue

Purple Brown 200 Sfrow

100

0

Figure 25. Temperature chart indicating conversion of Centrigrade to Fahrenheit or vice versa, color temperature

scale for hardening-temperature range, and temperingtemperature range.

It is also possible to secure some idea of the

temperature of the piece of a carbon or lowalloy steel in the low temperature range used

for tempering from the color of the thin oxide

film that forms on the cleaned surface of the

steel when heated in this range. The approximate temperature-color relationship for a time

at temperature of about one-half hour is indicated on the lower portion of the scale in

figure 25.

8.2. Quenching Media and Accessories

Quenching solutions act only through their

ability to cool the steeL They have no beneficial chemical action on the quenched steel and

in themselves impart no unusual properties.

Most requirements for quenching media are met satisfactorily by water or aqueous solutions of inorganic salts such as table salt or

caustic soda, or by some tjrpe of oil. The rate

of cooling is relatively rapid during quenching

in brine, somewhat less rapid in water, and

slow in oil. Brine usually is made of a 5- to 10-percent

solution of salt (sodium chloride) in water.

In addition to its greater cooling speed, brine

has the ability to "throw" the scale from steel during quenching. The cooling ability of both

water and brine, particularly water, is con- siderably affected by their temperature. Both

should be kept cold—well below 60 °F. If the

volume of steel being quenched tends to raise

the temperature of the bath appreciably, the

quenching bath should be cooled by adding ice or by some means of refrigeration.

There are many specially prepared quenching oils on the market; their cooling rates do

not vary widely. A straight mineral oil with

a Saybolt viscosity of about 100 at 100 °F is generally used. Unlike brine and water, the

oils have the greatest cooling velocity at a

slightly elevated temperature—about 100 to 140 °F—because of their decreased viscosity

at these temperatures.

When steel is quenched, the liquid in immediate contact with the hot surface vaporizes; this

vapor reduces the rate of heat abstraction

markedly. Vigorous agitation of the steel or the use of a pressure spray quench is necessary

to dislodge these vapor films and thus permit

the desired rate of cooling.

Shallow hardening steels, such as plain car- bon and certain varieties of alloy steels, have

such a high critical cooling rate that they must

be quenched in brine or water to effect hardening. In general, intricately shaped sections

should not be made of shallow hardening steels

because of the tendency of these steels to warp

and crack during hardening. Such items

should be made of deeper hardening steels ca- pable of being hardened by quenching in oil or

air.

24
A variety of different shapes and sizes of

tongs for handling hot steels is necessary. It should be remembered that cooling of the area

contacted by the tongs is retarded and that

such areas may not harden, particularly if the

steel being treated is very shallow hardening.

Small parts may be wired together or quenched

in baskets made of wire mesh.

Special quenching jigs and fixtures are frequently used to hold steels during quenching

in a manner to restrain distortion.

'When selective hardening is desired, portions

of the steel may be protected by covering with

alundum cement or some other insulating material. Selective hardening may be accomplished also by the use of water or oil jets

designed to direct the quenching medium on the areas to be hardened. This also is accomplished by the induction and flame-hardening

procedures previously described, particularly on large production jobs.

8.3. Relation of Design to Heat Treatment

Internal strains arise from many causes, but

the most serious are those developed during

quenching by reason of differential cooling and

from the increase in volume that accompanies

the martensitic transformation. These stresses are frequently sufficient to distort or crack the

hardened steel. Since temperature gradients

are largely a function of the size and shape of

the piece being quenched, the basic principle of

good design is to plan shapes that will keep

the temperature gradient throughout a piece at a minimum during quenching.

Because of the abruptness in the change of

section, some shapes are impractical to harden,

without cracking or distortion, by quenching in

water, but a certain latitude in design is permissible when using an oil-hardening or airhardening steel. Other things being equal,

temperature gradients are much lower in

shapes quenched in oil than in water, and are

still less in air. Thus a certain design may

be perfectly safe for one type of steel, or one

type of coolant, and unsafe for another.

Errors in design reach farther than merely

affecting the internal strains during hardening.

A sharp angle or notch serves to greatly con- centrate the stresses applied during service,

and the design of the part may be entirely re- sponsible for concentrating the service stresses

at a point already weakened by internal strains

produced during hardening. Concentration of

service stresses frequently parallels concentration of heat-treating strains and is frequently

caused and cured by the same combination of

circumstances.

A part is properly designed, from a standpoint of heat treatment, if the entire piece can

be heated and cooled at approximately the same rate during the heat-treating operations.

O

O

Figure 26. Examples of good and bad designs from stand- point of hardening by heat treatment {Palmer and Luerssen,

Tool steel simplified, p. 392, 1948).

A, End view of an undercutting form tool incorrectly designed. B, The same tool better designed from the viewpoint of heat treatment. Heavy sections have been lightened by drilling holes, thus insuring more

uniform cooling. The fillet at (a) minimizes danger of cracking at the sharp

re-entrant angle. Where a fillet is not allowable, treatment as shown at

(b) is helpful. C, Cracking will tend to occur at the sharp roots of the keyways.

D, Fillets at the roots of the keyways will reduce the tendency toward

cracking. The incorporation of the two additional keyways, even though

unnecessary in actual service, helps balance the section and avoid warping.

E, A blanking die with the center rib heavier than the surrounding areas. This may cause warping on quenching.

F, The same die with holes drilled in the center rib to equalize the amount

of metal throughout the die, thus eliminating warpage difficulties. G, A stem pinion with a keyway about one-half the diameter of the stem. The base of the keyway is extremely sharp, and the piece is further weakened by a hole drilled through the center of the stem near the keyway. The base

of the keyway should be filleted and hole re-located. H, A dangerous design consisting of a thin collar adjoining a thick section. When hardening such pieces, the thin section often warps or cracks at the junction with the hub. Extremely generous fillets and drilhng holes through

the hub to lighten its mass will be helpful.

J, When hardening, the concentration of strains at the junction of the two

holes in the center are apt to cause failure. Such holes should be plugged

before hardening. K, A blanking die poorly designed. Crack will occur from point of fork prong to setscrew hole. The position of the setscrew hole should be changed

to eliminate cracking.

25
Perfection in this regard is unattainable be- cause, even in a sphere, the surface cools more

rapidly than the interior. The designer should,

however, attempt to shape parts so that they

will heat and cool as uniformly as possible.

The greater the temperature difference between any two points in a given part during

quenching, and the closer these two points are

together, the greater will be the internal strain

and, therefore, the poorer the design.

When large and small sections are unavoidable in the same piece, the thick part frequently can be lightened by drilling holes through it. Where changes in section are encountered,

angles should be filletted generously. Some

examples of poor and good design are shown

in figure 26.

9. Nomenclature and Chemical

Compositions of Steels

9.1. Structural Steels

In order to facilitate the discussion of steels, some familiarity with their nomenclature is desirable.

A numerical index, sponsored by the Society

of Automotive Engineers (SAE) and the

American Iron and Steel Institute (AISI), is used to identify the chemical compositions of

the structural steels. In this system, a fournumeral series is used to designate the plain

carbon and alloy steels; five numerals are used

to designate certain types of alloy steels. The

first two digits indicate the type of steel, the

second digit also generally (but not always)

gives the approximate amount of the major

alloying element and the last two (or three)

digits are intended to indicate the approximate

middle of the carbon range. However, a deviation from the rule of indicating the carbon

range is sometimes necessary.

The series designation and types are summarized as follows:

Series designation Types lOxx Nonsulphurized carbon steels llxx Resulphurized carbon steels (free machining)

12xx Rephosphorized and resulphurized carbon

steels (free machining)

13xx Manganese 1.75%

*23xx Nickel 3.50%

*25xx Nickel 5.00%

31xx Nickel 1.25%, chromium 0.65%

33xx Nickel 3.50%, chromium 1.55%

40xx Molybdenum 0.20 or 0.25%

41xx Chromium 0.50 or 0.95%, molybdenum

0.12 or 0.20%

43xx Nickel 1.80%, chromium 0.50 or 0.80%,

molybdenum 0.25%

44xx Molybdenum 0.40%

45xx Molybdenum 0.52%

46xx Nickel 1.80%, molybdenum 0.25%

47xx Nickel 1.05%, chromium 0.45%, molybde- num 0.20 or 0.35%

48xx Nickel 3.50%, molybdenum 0.25%

50xx Chromium 0.25, 0.40 or 0.50%

50xxx Carbon 1.00%, chromium 0.50%

51xx Chromium 0.80, 0.90, 0.95, or 1.00%

51xxx Carbon 1.00%, chromium 1.05%

52xxx Carbon 1.00%, chromium 1.45%

61xx Chromium 0.60, 0.80 or 0.95%, vanadium

0.12%, 0.10% min., or 0.15% min.

81xx Nickel 0.30%, chromium 0.40%, molybdenum 0.12%

86xx Nickel 0.55%, chromium 0.50%, molybdenum 0.20%

87xx Nickel 0.55%, chromium 0.05%, molybdenum 0.25%

88xx Nickel 0.55%, chromium 0.50%, molybdenum 0.35%

92xx Manganese 0.85%, silicon 2.00%, chro- mium 0 or 0.35%

93xx Nickel 3.25%, chromium 1.20%, molybdenum 0.12%

94xx Nickel 0.45%, chromium 0.40%, molybdenum 0.12%

98xx Nickel 1.00%, chromium 0.80%, molybdenum 0.25%

* Not included in the current list of standard steels.

Listings of the AISI type numbers and chemical composition limits of the standard struc- tural steels (complete as of January 1966) are given in table 1. The SAE type numbers (not

shown in the table) are the same as the AISI

numbers, except that the latter may be given a

letter prefix to indicate the method of manufacture. These prefixes and their meanings

are as follows : B denotes acid bessemer carbon

steel, C denotes basic open hearth or basic

electric furnace carbon steels, E denotes elec- tric furnace alloy steels. A few of the AISI

steels are not included in SAE listings.

Small quantities of certain elements are

present in alloy steels that are not specified as

required. These elements are considered as

incidental and may be present to the maximum

amounts as follows: copper, 0.35 percent;

nickel, 0.25 percent; chromium, 0.20 percent;

molybdenum, 0.06 percent.

The list of standard steels is altered from

time to time to accommodate steels of proven

merit and to provide for changes in the metallurgical and engineering requirements of

industry.

Many of the alloy structural steels are manufactured to meet certain specified limits in

hardenability as determined by the standard

end-quench test. Such steels, table 2, are designated by the letter "H" following the AISI

number. The chemical composition limits of

these steels have been modified somewhat from

the ranges or limits applicable to the same

grades when specified by chemical composition

only. The hardenability of an "H" steel is guaranteed by the manufacturer to fall within a hardenability band having maximum and

minimum limits as shown by two limiting hardenability curves for that particular steel.

26
Table 1. Composition limits of standard steels NONRESULPHURIZED CARBON STEELS «

Designation Designation number Chemical composition limits, percent number Chemical composition limits, percent

AISI C Mn P(max) S(max) AISI C Mn

•C 1005 0.06 max 0.35 max 0.040 0.050 C 1042 0.40/0 47 0.60/0 90 0.040 0.050 •C 1006 .08 max 0 .25/0 .40 .040 .050 c 1043 .40/0 47 .70/1 00 .040 .050 C 1008 . 10 max [25/0 ^50 .040 .050 c 1044 .43/0 50 .30/0 60 .040 . 050 C 1010 0 08/0 13 .30/0 60 .040 .050 C 1045 .43/0 50 .60/0 90 .040 .050 *C 1011 . 08/0 .13 . 60/O 90 .040 .050 C 1046 .43/0 50 .70/1 00 .040 . 050 C 1012 . 10/O 15 .30/0 60 .040 .050 C 1048 .44/0 52 1 .10/1 40 .040 .050 *C 1013 .11/0 16 .50/0 80 .040 .050 c 1049 .46/0 53 .60/0 90 .040 .050 C 1015 .13/0 18 30/0 60 .040 .050 c 1050 .48/0 55 .60/0 90 .040 .050 C 1016 . 13/0 18 ^60/0 90 .040 .050 c 1051 .45/0 56 .85/1 15 .040 050 C 1017 .15/0 20 .30/0 60 .040 .050 c 1052 .47/0 55 1 .20/1 50 . 040 .050 C 1018 .15/0 20 .60/0 90 .040 .050 c 1053 .48/0 55 .70/1 00 . 040 .050 C 1019 . 15/0 20 .70/1 00 .040 .050 c 1055 .50/0 60 .60/0 90 .040 .050 C 1020 .18/0 23 .30/0 60 .040 .050 *c 1059 .55/0 65 .50/0 80 .040 .050 C 1021 .18/0 23 .60/0 90 .040 .050 c 1060 .55/0 65 .60/0 90 .040 .050 C 1022 .18/0 23 !70/l 00 .040 .050 •c 1061 .55/0 65 .75/1 05 .040 .050 C 1023 .20/0 25 .30/0 60 .040 .050 •c 1064 .60/0 70 50/0 80 .040 . 050 C 1024 .19/0 25 1.35/1 65 .040 .050 •c 1065 .60/0 70 .60/0 90 .040 .050 C 1025 .22/0 28 0.30/0 60 .040 .050 'C 1066 .60/0 70 .85/1 15 .040 .050 C 1026 .22/0 28 .60/0 90 .040 .050 •c 1069 .65/0 75 .40/0 70 .040 .050 C 1027 .22/0 29 1.20/1 50 .040 .050 c 1070 .65/0 75 .60/0 90 .040 .050 C 1029 .25/0 31 0.60/0 90 .040 .050 c 1072 .65/0 75 1.00/1 30 .040 .050 C 1030 .28/0 34 .60/0 90 .040 .050 •c 1074 .70/0 80 .50/0 80 .040 .050 •C 1034 .32/0 38 .50/0 80 .040 .050 •c 1075 .70/0 80 .40/0 70 .040 .050 C 1035 .32/0 38 .60/0 90 .040 .050 c 1078 .72/0 85 .30/0 60 .040 .050 C 1036 .30/0 37 1.20/1 50 .040 .050 c 1080 .75/0 88 .60/0 90 .040 .050 C 1037 .32/0 38 0.70/1 00 .040 .050 c 1084 .80/0 93 .60/0 90 .040 .050 C 1038 .35/0 42 .60/0 90 .040 .050 •c 1086 .80/0 93 .30/0 50 .040 .050 C 1039 .37/0 44 .70/1 00 .040 .050 c 1090 .85/0 98 .60/0 90 .040 .050 C 1040 .37/0 44 .60/0 90 .040 .050 c 1095 .90/1 03 .30/0 50 .040 .050 C 1041 .36/0 44 1.35/1 65 .040 .050

RESULPHURIZED CARBON STEELS •>

Designation Designation number Chemical composition limits, percent number Chemical composition limits, percent

AISI C Mn P(max) S AISI C Mn P(max) S

C 1108 0.08/0.13 0.50/0.80 0.040 0.08/0.13 C 1137 0.32/0.39 1.35/1.65 0.040 0.08/0.13 C 1109 .08/0.13 .60/0.90 .040 .08/0.13 C 1139 .35/0.43 1.35/1.65 .040 .12/0.20 C 1110 .08/0.13 .30/0.60 .040 .08/0.13 C 1140 .37/0.44 .70/1.00 .040 .08/0.13 C 1116 .14/0.20 1.10/1.40 .040 .16/0.23 C 1141 .37/0.45 1.35/1.65 .040 .08/0.13 C 1117 .14/0.20 1.00/1.30 .040 .08/0.13 C 1144 .40/0.48 1.35/1.65 .040 .24/0.33 C 1118 .14/0.20 1.30/1.60 .040 .08/0.13 C 1145 .42/0.49 .70/1.00 .040 .04/0.07 C 1119 .14/0.20 1.00/1.30 .040 .24/0.33 C 1146 .42/0.49 .70/1.00 .040 .08/0.13 C 1132 .27/0.34 1.35/1.65 .040 .08/0.13 C 1151 .48/0.55 .70/1.00 .040 .08/0.13

ACID BESSEMER RESULPHURIZED CARBON STEELS « REPHOSPHORIZED AND RESULPHURIZED CARBON STEELS d

Designation Designation number Chemical composition limits, percent number Chemical composition limits, percent

AISI C Mn P S AISI C Mn P S

B 1111 0.13 max 0.60/0.90 0.07/0.12 0.08/0.15 C 1211 0 . 13 max 0.60/0.90 0.07/0.12 0.08/0.15 B 1112 .13 max .70/1.00 .07/0.12 .16/0.23 C 1212 .13 max .70/1.00 .07/0.12 .16/0.23 B 1113 .13 max .70/1.00 .07/0.12 .24/0.33 C 1213 .13 max .70/1.00 .07/0.12 .24/0.33 C 1215 .09 max .75/1.05 .04/0.09 .26/0.35 tC 12L14 .15 max .80/1.20 .04/0.09 .25/0.35

t Lead = 0.15/0.35 percent. See footnotes at end of table.

27
Table 1. Composition limits of standard steels—Continued

OPEN HEARTH AND ELECTRIC FURNACE ALLOY STEELS "

Chemical composition limits, percent

AISI C Mn P(max) S(max)

1330 0.28/0 33 1.60/1 90 0.035 0.040

1335 .33/0 38 1 . 60/1 90 .035 .040 1340 .38/0 43 1.60/1 90 .035 .040 1345 .43/0 48 1 . 60/1 90 .035 .040

•3140 .38/0 43 0.70/0 90 .035 .040 E 3310 .08/0 13 .45/0 60 .025 .025

4012 .09/0 14 .75/1 00 .035 .040 4023 .20/0 25 .70/0 90 .035 .040 4024 .20/0 25 .70/0 90 .035 0.035/0.050

4027 .25/0 30 . 70/0 90 .035 .040 4028 .25/0 30 .70/0 90 .035 .035/0.050 4037 .35/0 40 . 70/0 90 .035 .040 •4042 .40/0 45 .70/0 90 .035 .040 4047 .45/0 50 .70/0 90 .035 .040 •4063 .60/0 67 .75/1 00 . uoo 040

4118 .18/0 23 .70/0 90 .035 .040 4130 .28/0 33 .40/0 60 .035 .040 •4135 .33/0 38 .70/0 90 .035 .040 4137 .35/0 40 .70/0 90 .035 .040 4140 .38/0 43 .75/1 00 . VoO 040 4142 .40/0 45 . 75/1 00 .035 ;o4o 4145 .43/0 48 .75/1 00 .035 .040 4147 .45/0 50 .75/1 00 .035 .040 4150 .48/0 53 .75/1 00 . Uoo 040

4161 .56/0 64 .75/1 00 .035 !040

4320 . 17/0 22 .45/0 65 .035 .040 •4337 .35/0 40 .60/0 80 .035 .040 E 4337 .35/0 40 .65/0 85 .025 .025 4340 .38/0 43 . 60/0 80 . UoO 040 E 4340 .38/0 43 .65/0 85 .025 !025

4419 .18/0 23 .45/0 65 . UoO 040

•4422 .20/0 25 . 70/0 90 .035 ]040 •4427 .24/0 29 .70/0 90 .035 .040 4615 . 13/0 18 .45/0 65 035 040

•4617 .15/0 20 .45/0 65 !035 '.OiO 4620 .17/0 22 .45/0 65 .035 .040 4621 .18/0 23 .70/0 90 . 035 fiACi 4626 .24/0 29 .45/0 65 .035 .040

4718 .16/0 21 .70/0 90 .035 .040 4720 .17/0 22 .50/0 70 .035 .040 4815 .13/0 18 .40/0 60 .035 .040 4817 .15/0 20 .40/0 60 .035 .040 4820 .18/0 23 .50/0 70 .035 .040

5015 .12/0 17 .30/0 50 .035 .040 •5046 .43/0 50 .75/1 00 .035 .040

•5115 . 13/0 18 .70/0 90 .035 .040 5120 .17/0 22 .70/0 90 .035 .040 5130 .28/0 33 .70/0 90 .035 .040 5132 .30/0 35 .60/0 80 . 035 . 040 5135 .33/0 38 .60/0 80 .035 .040 5140 .38/0 43 .70/0 90 .035 .040 5145 .43/0 48 .70/0 90 .035 .040 5147 .45/0 52 .70/0 90 . 035 . 040 5150 .48/0 53 .70/0 90 .035 .040 5155 .50/0 60 .70/0 90 . 035 . 040 5160 .55/0 65 .75/1 00 .035 .040 •E 50100 0.95/1 10 0.25/0 45 0.025 0.040 E 51100 .95/1 10 .25/0 45 .025 .025 E 52100 .95/1 10 .25/0 45 .025 .025 6118 .16/0 21 .50/0 70 .035 .040 •6120 .17/0 22 .70/0 90 .035 .040 6150 .48/0 53 .70/0 90 .035 .040

•8115 .13/0 18 .70/0 90 .035 .040

8615 .13/0 18 .70/0 90 .035 .040 8617 .15/0 20 .70/0 90 .035 .040 8620 .18/0 23 .70/0 90 .035 .040 8622 .20/0 25 .70/0 90 .035 .040 8625 .23/0 28 .70/0 90 .035 .040 8627 .25/0 30 .70/0 90 .035 .040 8630 .28/0 33 .70/0 90 .035 .040 8637 .35/0 40 .75/1 00 .035 .040 8640 .38/0 43 .75/1 00 .035 .040 8642 .40/0 45 .75/1 00 .035 .040 8645 .43/0 48 .75/1 00 .035 .040 8650 .48/0 53 .75/1 00 .035 .040 8655 .50/0 60 .75/1 00 .035 .040 •8660 .55/0 65 .75/1 00 .035 .040

Ni Cr Mo

0.20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.25

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.30

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.25 .20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35 0.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35 .20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35

1.10/1.40 3.25/3.75

1.65/2.00 1.65/2.00 1.65/2.00

1.65/2.00

1.65/2.00

1 . 65/2 00

1 . 65/2 00 1.65/2 00

1 . 65/2 00

.70/1 00

.90/1 20

.90/1 20 3.25/3 75 3.25/3 75 3.25/2 75

0.20/0 40

.40/0 70

.40/0 70

.40/0 70 .40/0 70

.40/0 70

.40/0 70

.40/0 70 .40/0 70

.40/0 70 .40/0 70

.40/0 70

.40/0 70 .40/0 70 .40/0 70

0.55/0.75

1.40/1.75

0.40/0.60

.80/1.10

.80/1.10 .80/1.10

.80/1.10

.80/1.10

.80/1.10

.80/1.10 .80/1.10

.70/0.90

.40/0.60

.70/0.90 .70/0.90

.70/0.90

.70/0.90

.35/0.55

.35/0.55

.30/0.40

.20/0.35 .70/0.90

.70/0.90

.80/1.10

.75/1.00 .80/1.05

.70/0.90 .70/0.90

.85/1.15

.70/0.90

.70/0.90

.70/0.90

). 40/0. 60 .90/1.15

L. 30/1. 60

.50/0.70 .70/0.90

.80/1.10

.30/0.50

.40/0.60

.40/0.60

.40/0.60

.40/0.60 .40/0.60

.40/0.60

.40/0.60 .40/0.60

.40/0.60

.40/0.60

.40/0.60

.40/0.60

.40/0.60

.40/0.60

0.15/0.25

.20/0.30

.20/0.30

.20/0.30

.20/0.30

.20/0.30

.20/0.30

.20/0.30

.20/0.30

.08/0.15

.15/0.25

.15/0.25

.15/0.25

.15/0.25 .15/0.25

.15/0.25

.15/0.25

.15/0.25

.25/0.35

.20/0.30

.20/0.30 .20/0.30

.20/0.30 .20/0.30

.45/0.60 .35/0.45

.35/0.45

.20/0.30

.20/0.30

.20/0.30 .20/0.30

.15/0.25

.30/0.40

.15/0.25

.20/0.30 .20/0.30

.20/0.30

0.08/0.15

.15/0.25

.15/0.25

.15/0.25 .15/0.25

.15/0.25

.15/0.25

.15/0.25

.15/0.25

.15/0.25

.15/0.25

.15/0.25 .15/0.25

.15/0.25

.15/0.25

28
Table 1. Composition limits of standard steels—Continued

OPEN HEARTH AND ELECTRIC FURNACE ALLOY STEELS '—(continued)

Designation number Chemical composition limits, percent

AISI C Mn P(max) S(max) Si Ni Cr Mo V

8720 .18/0 23 .70/0 90 .035 .040 .20/0 35 .40/0.70 .40/0.60 .20/0.30 *8735 .33/0 38 .75/1 00 .035 .040 .20/0 35 .40/0.70 .40/0.60 .20/0.30 8740 .38/0 43 .75/1 00 .035 .040 .20/0 35 .40/0.70 .40/0.60 .20/0.30 *8742 .40/0 45 .75/1 00 .035 . 040 .20/0 35 .40/0.70 .40/0.60 .20/0.30 8822 .20/0 25 .75/1 00 . 035 .040 .20/0 35 .40/0.70 . '±\)/\j . bU . oU/U . 4U 9255 .50/0 60 .70/0 95 . 035 .040 1.80/2 20 9260 .55/0 65 .70/1 00 !035 .040 1.80/2 20

•9262 .55/0 65 .75/1 00 .035 .040 1.80/2 20 .25/0.40 E 9310 .08/0 13 .45/0 65 .025 .025 .20/0 35 3.00/3.50 1.00/1.40 .08/0.15 9840 .38/0 43 .70/0 90 .035 .040 .20/0 35 .85/1.15 .70/0.90 .20/0.30

*9850 .48/0 53 .70/0 90 .035 .040 .20/0 35 .85/1.15 .70/0.90 .20/0.30

BORON STEELS

These steels can be expected to have 0.0005 percent minimum boron content.

Designation number Chemical composition limits, percent

AISI C Mn P(max) S(max) Si Ni Cr Mo V

50B44

50B46

50B50

50B60

51B60

81B45 94B17

94B30

0.43/0.48

.43/0.50

.48/0.53 .55/0.65

. 55/0 . 65 .43/0.48

.15/0.20

.28/0.33

0.75/1.00

.75/1.00

.75/1.00

.75/1.00

.75/1.00

.75/1.00

.75/1.00

.75/1.00

0.035

.035

.035 .035

.035

.035

.035 .035

0.040

.040

.040

.040

.040

.040

.040

.040

0.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

.20/0.35

0.40/0.60

.20/0.35

.40/0.60

.40/0.60

.70/0.90

.35/0.55

.30/0.50

.30/0.50

0.20/0.40

.30/0.60

.30/0.60

0.08/0.15

.08/0.15

.08/0.15

NITRIDING STEEL •

Designation number Chemical composition limits, percent

AISI C Mn P(max) S(max) Si Al Cr Mo V

0.38/0.43 0.50/0.70 0.035 0.040 0.20/0.40 0.95/1.30 1.40/1.80 0.30/0.40

• Standard steels for wire rods only.

' Silicon: When silicon is required, the following ranges and limits are commonly used: Standard steel designations Silicon ranges or limits Up to C 1015 excl 0.10% max. C 1015 to 0 1025 incl 0.10% max, 0.10/0.20%, or 0.15/0.30% Over C 1025 0.10/0.20% or 0.15/0.30% Copper: When required, copper is specified as an added element to a standard steel. Lead: When required, lead is specified as an added element to a standard steel. Silicon: When silicon is required, the following ranges and limits are commonly used: Standard steel designations Silicon ranges or limits Up to C 1110 incl 0.10% max.

C1116 and over 0.10% max, 0.10/0.20%, or 0.15/0.30%. Lead: When required, lead is specified as an added element to a standard steel. Silicon: Because of the technological nature of the process, acid bessemer steels are not produced with specified silicon content. Lead: When required, lead is specified as an added element to a standard steel. Silicon: It is not common practice to produce these steels to specified limits for silicon. Lead: When required, lead is specified as an added element to a standard steel.

" Grade shown in the above list with prefix letter E generally are manufactured by the basic electric furnace process. All others are normally manufactured by the basic open hearth process but may be manufactured by the basic electric furnace process with adjustments in phosphorus and sulphur. The phosphorus and sulphur limitations for each steelmaking process are as follows: Process Percent maximum

P S

Basic electric furnace 0.025 0.025 Basic open hearth 0.035 0.040 Acid electric furnace 0.050 0.050 Acid open hearth 0.050 0.050 Minimum silicon limit for acid open hearth or acid electric furnace alloy steel is 0.15 percent. Small quantities of certain elements are present in alloy steels which are not specified or required. These elements are considered as

incidental and may, be present to the following maximum amounts: Copper, 0.35 percent; nickel 0.25 percent; chromium, 0.20 percent, and molybdenum, 0.06 percent. Where minimum and maximum sulphur content is shown it is indicative of resulphurized steels.

29
Table 2. Standard H-steels OPEN HEARTH AND ELECTRIC FURNACE STEELS

Chemical composition limits, percent

AISI C Mn Si

1330 H 0.27/0 33 1.45/2 05 0.20/0.35

1335 H .32/0 38 1.45/2 05 .20/0.35 1340 H .37/0 44 1.45/2 05 .20/0.35

3140 H .37/0 44 0.60/1 00 .20/0.35

3310 .07/O 13 .30/0 70 .20/0.35

4027 H .24/0 30 .60/1 00 .20/0.35

•4028 H .24/0 30 .60/1 00 .20/0.36 4037 H .34/0 41 .60/1 00 .20/0.35 4047 jj .44/0 51 .60/1 00 .20/0.35

4118 jj . 17/O 23 .60/1 00 .20/0.35 4130 H ]27/0 33 '.30/0 70 .20/0.35 4137 H .34/0 41 .60/1 00 .20/0.35 4140 H .37/0 44 .65/1 10 .20/0.35 4142 39/O 46 .65/1 10 .20/0.35 4145 H !42/0 49 !g5/1 10 .20/0.35 4147 H .44/0 51 .65/1 10 .20/0.35 4150 jj .47/0 54 . 65/1 10 .20/0.35 4161 H ]55/0 65 165/1 10 .20/0.35

4320 H .17/0 23 .40/0 70 .20/0.35 4337 34/0 41 55/0 90 .20/0.35 4340 H !37/0 44 .55/0 90 .20/0.36 E4340 H .37/0 44 .60/0 95 .20/0.35

4419 H .17/0 23 .35/0 75 .20/0.35 4620 H .17/0 23 .35/0 75 .20/0.35

4621 H .17/0 23 .60/1 00 .20/0.35 4626 H .23/0 29 .40/0 70 .20/0.35

4718 H .16/0 21 .60/0 95 .20/0.35 4720 .17/0 23 .45/0 75 .20/0.35 4815 H .12/0 18 .30/0 70 .20/0.35 4817 14/0 20 .30/0 70 .20/0.35 4820 H !l7/0 23 !40/0 80 .20/0.35

5120 H .17/0 23 .60/1 00 .20/0.35

5130 H .27/0 33 .60/1 00 .20/0.35

5132 H .29/0 35 .50/0 90 .20/0.35

6135 H .32/0 38 .50/0 90 .20/0.35 5140 H .37/0 44 .60/1 00 .20/0.35 6145 H .42/0 49 .60/1 00 .20/0.35 6147 H .45/0 52 .60/1 05 .20/0.35

5150 H .47/0 54 .60/1 00 .20/0.35 6155 H .50/0 60 .60/1 00 .20/0.35 5160 55/0 65 .65/1 10 .20/0.35

6118 H .15/0 21 .40/0 80 .20/0.35 6150 jj .47/0 54 .60/1 00 .20/0.35 8617 fj 14/0 20 .60/0 95 .20/0.35 8620 H !l7/0 23 ] 60/0 95 .20/0.35 8622 H .19/0 25 .60/0 95 .20/0.35 8625 H .22/0 28 . 60/0 95 .20/0.35 8627 H .24/0 30 .60/0 95 .20/0.35 8630 H .27/0 33 .60/0 95 .20/0.35 8637 H .34/0 41 .70/1 05 8640 H .37/0 44 .70/1 05 .20/0.35 8642 H .39/0 46 .70/1 05 .20/0.35 8645 H .42/0 49 .70/1 05 .20/0.35 8655 H .50/0 60 .70/1 05 .20/0.35 8720 H .17/0 23 .60/0 95 .20/0.35 8740 H .37/0 44 .70/1 05 .20/0.35

8822 H .19/0 25 .70/1 05 .20/0.36 9260 H .55/0 65 .65/1 10 1.70/2.20

Ni Cr Mo

1.00/1.45 3.20/3.80

1.55/2.00

1.65/2.00

1.56/2.00

1 . 55/2 . 00

1.66/2.00

1.55/2.00

.65/1.05 0.86/1.25

0.85/1.25

3.20/3.80

3.20/3.80

3.20/3.80

0.35/0.75

.35/0.75

.35/0.75 .35/0.75

.35/0.65

.35/0.75

.35/0.75

.35/0.75

.35/0.75

.36/0.75

.35/0.75 .35/0.75

.35/0.75

.35/0.75

0.45/0.85

1.30/1.80

0.30/0.70

.75/1.20

.75/1.20

.75/1.20

.75/1.20

.75/1.20

.75/1.20

.75/1.20

.65/0.96 .35/0.65

.66/0.95

.65/0.95

.65/0.96

.30/0.60

.30/0.60

.60/1.00

.76/1.20

.66/1.10 .70/1.15

.60/1.00

.60/1.00

.80/1.25

.60/1.00

.60/1.00

.60/1.00

.40/0.80

.75/1.20

.35/0.65

.35/0.65 .35/0.65

.36/0.65 .35/0.65

.35/0.66

.35/0.65

.35/0.65

.35/0.65

.35/0.65

.35/0.65

.35/0.65

.35/0.65

.35/0.65

0.20/0.30

.20/0.30

.20/0.30 .20/0.30

.08/0.15 .15/0.26

.16/0.25

.15/0.26 .16/0.25

.16/0.25

.15/0.25

.15/0.25

.25/0.35

.20/0.30

.20/0.30

.20/0.30

.20/0.30

.45/0.60 .20/0.30

.20/0.30

.15/0.25

.30/0.40

.15/0.25

.20/0.30

.20/0.30

.20/0.30

.15/0.25

.15/0.25

.16/0.25

.15/0.25

.15/0.25

.15/0.25

.15/0.25

.16/0.25

.15/0.26

.15/0.25

.16/0.25

.20/0.30

.20/0.30

.30/0.40

30
Table 2. Standard H-steels—Continued

BORON H-STEELS

These steels can be expected to have 0.0005 percent minimum boron content

Designation number Chemical composition limits, percent

AISI C Mn Si Ni Cr Mo V

50B40 H

50B44 H

50B50 H

50B60 H

51B60 H

81B45 H

94B17 H

94B30 H

0.37/0.44

.42/0.49

.43/0.50

.47/0.54 .55/0.65

.55/0.65

.42/0.49

.14/0.20 .27/0.33

0.65/1.10

.65/1.10

.65/1.10

.65/1.10

.65/1.10

.65/1.10

.70/1.05

.70/1.05

.70/1.05

0.20/0.35

.20/0.35

.20/0.35

.20/0.35 .20/0.35 .20/0.35

.20/0.35

.20/0.35 .20/0.35

0.30/0.70

.30/0.70 .13/0.43

.30/0.70

.30/0.70

.60/1.00

.30/0.60

.25/0.55

.25/0.55

0.15/0.45

.25/0.65

.25/0.65

0.08/0.15

.08/0.15

.08/0.15

* Sulphur content 0.035/0.050 percent. The phosphorus and sulphur limitations for each steelmaking process are as follows: Maximum percent P S

Basic electric furnace 0.025 0.025 Basic open hearth 0.035 0.040 Acid electric furnace 0.050 0.050 Acid open hearth 0.050 0.050 Minimum silicon limit for acid open hearth or acid electric fiirnace alloy steel is 0.15 percent. Small quantities of certain elements are present in alloy steels which are not specified or required. These elements are considered as

incidental and may be present to the following maximum amounts: Copper, 0.35 percent; nickel, 0.25 percent; chromium, 0.20 percent; and

molybdenum, 0.06 percent.

9.2. Tool Steels

Technically, any steel used as a tool may be

termed a tool steel. Practically, however, the

term is restricted to steels of special composition that are usually melted in electric furnaces

and manufactured expressly for certain types

of services.

The current commonly used tool steels have

been classified by the American Iron and Steel

Institute into seven major groups and each

commonly accepted group or subgroup has been

assigned an alphabetical letter. Methods of

quenching, applications, special characteristics,

and steels for particular industries were con- sidered in this type classification of tool steels

which is as follows:

Table 3. Identification and type classification of tool steels

Group

Water hardening

Shock resisting

Symbol and type

Cold work

Hot work .

W

S

0—Oil hardening

A—Medium alloy

D—High carbon—high chromium

H—(H 1 to H 19 incl. chromium

base, H 20 to H 39 incl. tungsten base, H 40 to H 59

incl. molybdenum base)

T—Tungsten base

M—Molybdenum base

L—Low alloy

F—Carbon tungsten

P

The AISI identification and type classification of tool steels is given in table 3. Each

major group, identified by a letter symbol, may

contain a number of individual types that are

identified by a suffix number following the letHigh speed

Special purpose Mold steels

Identifying elements, percent

nation AISI C Mn Si Cr Ni V W Mo Co WATER HARDENING

WlW2

W4

W5

"0.60/1.40

• .60/1.40

« .60/1.40

1.10

0.25

0.25

.50 SHOCK RESISTING

SIS2

S4

S5

S6

S7

0.50

.50

.55

.55

.45 .50

1.50 2.50

1.00 2.00

2.00

1.25

0.55

0.80

.80 1.40.40

.40 1.401.50 3.25

COLD WORK; OIL HARDENING

010206

07

0.90

.90 1.45 1.20

1.00 1.60 0.50 0.50

1.00 0.25

.75 1.75

COLD WORK; MEDIUM ALLOY AIR HARDENING

A2

A3

A4A5A6

A7A8

A9

AlO

1.00

1.25 1.00

1.00 0.70

2.25

.55

.50 1.35

5.00

5.00

1.00 1.00

1.00 5.25

5.00

5.00

1.00

1.00 1.00

1.00 1.00

1.00

1.25

1.40

1.50

2.00 3.00

2.00

4.75 b 1.00

1.25 1.50

1.80

1.00 1.80 1.25

See footnotes at end of table.

ter symbol. The percentages of the elements

shown in the table for each type are not to be

considered as the mean of the composition

31
Table 3. Identification and type classification of tool steels—Continued

Table 3. Identification and type classification of tool steels—Continued

Designation AISI

Identifying elements, percent DesieIdentifying elements, percent

C Mn Si Cr Ni V W Mo

nation Co AISI C Mn Si Cr Ni V W Mo Co

COLD V,'ORK; HIGH (:!ARBO^l-HIGH CHI10MIU>i HIGH SPEED; MOLYBDENUM BASE

DlD2

D3

D4D5

D7

1.00

1.50 2.25

2.25

1.50 2.35

12.00

12.00 12.00

12.00 12.00

12.00 4.00

1.00

1.00

1.00 1.00

1.00 3.00

HOT WORK; CHROMIUM BASE

HIO

Hll H12

H13

H14 H16

H19

0.40

.35

.35

.35 .40

.55

.40

3.25

5.00

5.00

5.00

5.00

7.00

4.25

0.40

.40

.40 1.00

2.50

1.50 1.50

1.50

1.50

5.00

7.00 2.00 4.25 4.25

HOT WORK; TUNGSTEN BASE

H20

H21 H22

H23

H24 H25

H26

0.35

.35

.35 .30

.45

.25 .50

2.00

3.50 2.00

12.00 3.00

4.00

4.00

9.00

9.00

11.00

12.00 15.00

15.00 1.00 18.00

HOT WORK; MOLYBDENUM BASE

H41 H42

H43

0.65

.60

.55

4.00

4.00

4.00

1.00 2.00

2.00

1.50 6.008.00

5.00

8.00

HIGH SPEED; TUNGSTEN BASE

TlT2

T4T5T6

T7T8

T9

T15

0.70

.80

.75 .80

.80

.75

.75 1.20

1.50

4.00

4.00 4.00

4.00

4.50

4.00

4.00

4.00

4.00

1.00

2. 00

1.00 2.00

1.50 2.00

2.00

4.00 5.00

18.00

18.00

18.00

18.00 20.00

14.00

14.00 18.00

12.00

5.00

8.00

12.00

5.00

5.00

ranges of the elenients. Steels of the same

type, manufactured by various producers, may

differ in analysis from the values listed and

may contain elements not listed in the type

identification.

9.3. Stainless and Heat Resisting Steels The stainless and heat resisting steels possess relatively high resistance to oxidation and

to attack by corrosive media at room and ele- vated temperatures. They are produced to cover wide ranges in mechanical and physical

properties. They are melted exclusively by the

electric furnace process.

A three-numeral system is used to identify

stainless and heat resisting steels by type and

according to four general classes. The first digit indicates the class and last two digits

indicate type. Modification of types are indicated by suffix letters. The meaning of this

AISI system is as follows

MlM2

M3«

M3<i M4M6

M7

MlO

M15 M30

M33

M34 M35

M36

M41 M42

M43

M44

0.80

.80 1.05 1.20

1.30 0.80

1.00

.85 1.50 0.80

.90

.90

.80

.80 1.10

1.10

1.25

1.15

4.00

4.00

4.00 4.00

4.00

4.00

4.00 4.00

4.00

4.00

3.75 4.00

4.00 4.00

4.25

3.75

3.75 4.25

1.00 2.00

2.40

3.00

4.00

1.50 2.00

2.00

5.00

1.25

1.15 2.00

2.00

2.00

2.00

1.15 2.00

2.25

1.50 6.00

6.00

6.00

5.50

4.00

1.75

6.50 2.00

1.50 2.00

6.00

6.00

6.75

1.50

1.75 5.25

8.00 5.00

5.00

5.00

4.50 5.00

8.75

8.00

3.50

8.00

9.50

8.00

5.00

5.00 3.75

9.50

8.75 6.25

SPECIAL PURPOSE; LOW ALLOY

SPECIAL PURPOSE; CARBON TUNGSTEN

SPECIAL PURPOSE; MOLD STEELS—OTHER TYPES

12.00

5.00 5.00

8.00

8.00 5.00

8.00 5.00

8.00

8.25

12.00

LIL2

L3L6

L7

1.00 •0.50/1.10

1.00 0.70

1.00

1.25

1.00

1.50 0.75

1.40

0.20

.20 1.50 ''.25 0.35 .40

FlF2

F3

1.00 1.25

1.25

1.25 3.50

0.75 3.50

MOLD STEELS

PIP2

P3

P4

P5P6

P20 P21«

0.10

.07

.10

.07 .10

.10

.35 .20

2.00

0.60

5.00

2.25

1.50

1.25

0.50

1.25 0.20

3.50 0.40

4.00

20 0.30 0.75 0.25

" Varying carbon contents may be available. At producer's option.

<^ Class 1.

<i Class 2. "Al, 1.20%

Series Designation Classes 2xx Chromium-nickel-manganese steels; nonhardenable, austenitic and nonmagnetic

3xx Chromium-nickel steels; nonhardenable,

austenitic and nonmagnetic

4xx Chromium steels; hardenable, martensitic

and magnetic

4xx Chromium steels; nonhardenable, ferritic and magnetic

5xx Chromium steels; low chromium heat resisting.

The chemical composition ranges and limits

of the standard stainless and heat resisting

steels are given in table 4.

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10. Recommended Heat Treatments

10.1. Structural Steels A listing of recommended heat treatments

for many of the steels listed in tables 1 and 2 are given in tables 5 and 6. Treatments for

the standard steels that are omitted can be esti- mated by interpolation (also see fig, 11).

Where different quenching media are suggested, the smaller sections may harden satisfactorily in the slower quenching medium.

The upper end of the heat treating ranges

should always be used for large size sections.

The inclusion of normalizing and annealing

temperatures does not imply that these treat- ments are necessarily a prelude to hardening.

For hardening, the general rule is to place

the cold steel in a furnace containing a slightly

oxidizing atmosphere or neutral salt bath at

the hardening temperature and let it heat

naturallly, hold 1/2 hr/in of thickness, and then

quench; it is sometimes advisable to warm intricate sections or complex shapes before putting them into the high temperature unit and

to remove from the quenching medium while

still warm. Usually, the quenched piece should

be allowed to cool until it feels comfortable to the bare hand and then immediately tempered.

To minimize cracking, it is sometimes desirable

to temper the quenched piece before it reaches

room temperature; a double temper (i.e., repeating the treatment) is recommended for

this condition.

For carburizing, most of the steels may be

given either (1) a single, (2) double, or (3)

triple treatment as is indicated in table 5. The

time at the carburizing temperature will vary

depending upon the temperature used and the

desired thickness of the case. The relation

between time, temperature and case depth is illustrated in figure 19,

Table 5. Recommended heat treatments for carburizing grade steels

AISI or SAE number

Carburizing temperature Cooling method Reheat Cooling medium 2d reheat Cooling mediumTempering

temperature

1008 to 1024; 1108 to 1119; 1211

to 1215

3310.

4012 to 4024; 4118.

4419,4422 and 4427; 8822.

4520.

4320; 4615 to 4626; 4718 and

4720 ___

4815 to 4820.

5015; 5115 and 5120.

6118 and 6120.

8115; 8615 to 8622; 8720; 8822; 9310 and 94B17.

1,650/1,700

1 , 650/1 , 700

1,650/1,700

1,650/1,700

1.650/1,700

1,650/1,700

1,650/1,700

1,650/1.700

1,650/1,700

1,650/1,700

Brine or water

Water or oil Cool slowly do

(Oil

I do

Cool slowly.

( do

[Oil

I do

1 Cool slowly. do

'Oil do

Cool slowly. do

[Oil do

Cool slowly

. do

Oil do

1 Cool slowly _ do

[Oil

I do

I Cool slowly. do

[Oil

I do

Cool slowly. do

,400/1,450

,400/1,450

,650/1,700

Brine or water.

do. Water or oil 1,400/1,450 Brine or water

375/1,425 375/1,425

525/1,575

Oil do

do... 1,375/1,425 Oil-

,425/1.475

,425/1,475

,600/1,650

Oil..do.

.do. 1,425/1,475 Oil450/1.500

450/1,500

625/1,675

Oil-.do.

..do. 1,450/1,500 Oil.

,475/1,525

,475/1,525

,650/1,700

Oil do.

do. 1,475/1,525 Oil.

,425/1,475

,375/1,425

,525/1,575

Oil--do

-do 1,375/1,425 Oil350/1,400

350/1 ,400 550/1,600

Oil..do. .do. 1,350/1,400 Oil.

[Oil

I do

1 Cool slowly do

425/1,475

425/1,475

600/1 , 650

Oil dodo. 1,425/1,475 Oil.

[Oil

I do

Cool slowly do.

,450/1,500

,450/1,500

,600/1,650

Oil or water. do

Oil 1,450/1,500 Oil or water.

450/1,500

450/1,500

550/1,600

Oil..do-

.do- 1,450/1,500 Oil250/300

250/300

250/300

250/300

250/300

250/300

250/300

250/300

300/400

250/300

34
Table 6. Recommended heat treatments for heat treating grade steels (No tempering treatments are given, as these temperatures depend upon the desired hardness)

AISI or Aloi A TQT r\T or SAE Normalizing Annealing Hardening Quenching SAE Normalizing Annealing Hardening Quenching number temperature temperature temperature medium number temperature temperature temperature medium

°F r

° F

0 po rp 1030 1 , 625/1 , 725 1 , 0*iD/l , 0 i 0 Water or urine. 5046 1 . DUU/ J. . I UU J. , 4 ( O/ i. , O^ 0 1 , OUU/ 1 , OOU 1040 1,600/1,700 1,475/1.525 1.500/1.550 Do. 5130 1.600/1,700 1,500/1.550 1 , 550/1 , 600 Oil or water. 1050 1,550/1.650 1.450/1.500 1.475/1.525 Do. 5145 1,600/1.700 1.475/1.525 1,525/1,575 Oil. 1060 1 . 500/1 . 600 1 ,4i5/l .475 1 , 45U/1 , OUU UO, 5160 i. , DUU/ 1 , 1 UU 1 . 40U/ 1 . OUU X , OUU/ X , OOU lJO. 1070 1 . 500/1 . 600 1 , 4io/l , 4 i

1 ,< Kn /I t^nn 1 , 40U/ i , OUU Do 50100 i , DUU/ J. , 1 UU 11 J.00/1 A^^n , tUU/ 1 , ^ou 1 4.fio/i t^oo X , 40U/ X , OUU Do

1080 1 . 475/1 . 575 1 ,0(5/1, 4Zo 1 A (\(\ /I A t^fl Do 51100 J. , DUU/ i. , 1 UU 1A J.00/1 A^C\ , *UU/ X , ^OU 1 47^/1 ^^9^; X , ^ ( O/ X , O^u Do

* 1090 1.475/1.575 1.375/1,425 1 A nn /I /I Pin 1 , 4UU/ i , 45U J-'O. 52100 1J. ,OUU/ fiOO/1J. , 700 1 UU 1 4O0 /I y1 t^O X , 'tUU/ X . 40U X , OUU/ X , OOU UO.

1132 1 .625/1 .725 1 . 5Z5/1 , 575 1 , OOU/ 1

dUU Tin 6150 1A fiOO/1 700 , OUU/ J. , 1 UU X . OUU/ X . OOU X , OOU/ X , DUU UO.

1140 1 . 600/1 , 700 1 , 4 / 5/1 , 0Z5 i, . OUU/ 1 , OOU Do 8630 1J. , fiOO/1 \j\J\J / A , 700 1 \J\J 1 f^OO/l f^f^O A , 0\J\} / A , tJOU 1 , O^tOf X , O 1 o Do

1151 1 . 550/1 . 650 1 ,45U/1 ,oUU 1 . 4 i O/ 1 , o^o Do 8645 1A , fifiO/1 \J\J\} / X , 700 f \J\/ A , t 1 !.>/ A , O^O i . 0\J\} / X . OOU Do

1330 1 . oOU/l . tw 1 KrtA /I t;eA

1 , OUU/ 1 , OOU 1 , O^O/ X , O (

or water. 8660 1 600/1 700 1 4f>0/l f^OO A f'xIiJ/A.O^u Do

1340 1.550/1.650 1.475/1.525 1.500/1.550 Do. 8735 li 600/1 [700 1 . 500/1 . 550 1,525/1,575 Do!

3140 1.550/1.650 1.475/1.525 1.475/1.525 Oil. 8742 1.600/1,700 1 . 500/1 . 550 1,525/1,575 Do.

4028 1.600/1,700 1.525/1,575 1.550/1.600 Oil or water. 9260 1,600/1.700 1.500/1.550 1,550/1,600 Do.

4042 1,550/1,650 1.475/1.525 1.500/1.550 Oil. 9840 1,600/1,700 1.475/1.525 1 , 500/1 , 550 Do.

9850 1.600/1,700 1,475/1,525 1 , 500/1 , 550 Do.

4063 1,550/1,650 1.450/1.500 1.475/1,525 Do,

4130 1,600/1,700 1.525/1.575 1 .550/1 ,600 Oil or water. 4140 1,600/1,700 1.500/1,550 1.525/1.575 Oil. 4150 1,600/1 ,700 1.475/1.525 1,500/1.550 Do.

4340 1,600/1,700 1.500/1,550 1.500/1.550 Do.

10.2. Tool Steels Recommended heat treatments for the different types of tool steels are summarized in

table 7. The time the steel is at the normalizing

temperature, after being heated uniformly

through, varies from about 15 min for a small

section to about 1 hr for large sizes; cooling

from the normalizing temperature is in still air. For annealing, the upper range in temperature is recommended for large and the

lower limit for small sections. The time the

steel is at the annealing temperature, after

being heated uniformly through, ranges from

about 1 hr for thin sections of carbon or low

alloy to about 4 hr for heavy sections of high

alloy steel. Cooling from the annealing temperature through the transformation range

should not exceed about 75 °F/hr for the car- bon and low alloy and 50 °F/hr for the high

alloy steels. For hardening, the protective atmosphere,

rate of heating, hardening temperature, time

at temperature, and rate of cooling vary with

the composition of the steels. A general rule

is to heat the plain carbon and low alloy steels

rapidly in a furnace having a slightly oxidizing

atmosphere or in a neutral salt bath at the

hardening temperature, hold 1/2 hr/in of thickness, and then quench. The high alloy steels,

such as the T and M types, are usually preheated to minimize distortion and cracking and

then heated rapidly in a furnace containing a

highly carburizing atmosphere or in a molten

bath at the hardening temperature, held at temperature for time as indicated in the table,

and then quenched. Tools should be tempered

immediately after quenching; a double temper

is recommended for the high alloy steels. The carburizing steels (type P) may be (1)

quenched directly from the carburizing temperature; (2) quenched directly from the carburizing temperature followed by reheating to and

quenching from the recommended hardening

temperature; (3) slowly cooled from the car- burizing temperature followed by reheating to and quenching from the recommended hardening temperature or (4) slowly cooled from the

carburizing temperature, reheated to the car- burizing temperature and quenched, followed

by reheating to and quenching from the recom- mended hardening temperature. For most

purposes, method (3) is generally suitable.

Tempering should follow in all cases.

10.3. Stainless and Heat Resisting Steels The response of stainless and heat resisting

steels to heat treatment depends upon their

composition. The general methods employed

are similar to those used with other steels and

no special equipment is needed. Heat treat- ment may be performed in any conventional

electric, gas fired, oil fired, salt bath, or induction furnace in which the required temperatures can be obtained. Precautions should be

taken to avoid exposure of the steel to the di- rect flame of the burners. Conditions that will

result in carburization of the surface of the

steel should be avoided. In general, oxidizing

furnace atmospheres are preferred. Bright,

scale-free heat treatment is possible by the

use of specially prepared and controlled atmospheres.

35
Table 7. Recommended heat treatments for tool steels

Normalizing

temperature

Annealing

Temperature

Brinell hardness number

Preheating temperature

Hardening

Quenching

temperature Quenching medium

Tempering

Temperature

1,450/1,700 1.450/1,700

1,450/1,700

1,450/1,700

(«)

(«) W

(•)

(•)

(') 1,500/1,600

1,500/1,550

1,600/1,650

1,600/1,650

Ml

M2

M3 Class 1 M3 Class 2 M4

450

1,600/1,650

1,600/1,650

1,600/1,650

1 , 600/1 , 650 1,600/1,650

1,650

1,650

1,650

op (BHN)

1,375/1 450 159/202

1,400/1 450 159/202

1,375/1 450 159/202 C

1,400/1 450 163/202 C-)

1 ,450/1 500 183/229 C)

1,400/1 450 192/217 1 100/1 200

1 ,400/1 450 192/229 1 100/1 200

1,400/1 450 192/229 1,300/1 400

1,475/1 525 192/229 1 400

1 ,500/1 550 187/223 200/I 300

1 ,400/1 450 183/212 100/I 200 1,375/1 425 183/212 100/I 200

1 ,400/1 450 183/217 100/I 200

1 ,450/1 500 192/217 1 100/1 200

1,550/1 600 202/229 350/1 450

1 ,550/1 600 207/229 1 450

1 ,350/1 400 200/241 150/1 250

1 ,350/1 400 228/255 100/I 200

1 ,350/1 375 217/248 1 100/1 200

1 ,600/1 650 235/262 1 400/1 500

1 ,550/1 600 192/223 1,450

1 ,550/1 600 212/248 1,450

1 ,410/1 460 235/269 1,200

1 ,600/1 650 207/248 1 400/1 500

1 ,600/1 650 217/255 1 400/1 500

1 ,600/1 650 217/255 . 1 400/1 500

1 ,600/1 650 217/255 1 400/1 500

1 ,600/1 650 223/255 1 400/1 500

1,600/1 650 235/262 1 400/1 500

1 ,550/1 650 192/229 1,500 1,550/1 650 192/229 1 400/1 500

1 ,550/1 650 192/229 1 400/1 500

1 ,550/1 650 192/229 1 400/1 500 1,600/1 650 207/235 1 400/1 500 1,600/1 650 212/241 1 500/1 600

1 ,600/1 650 207/241 1,500 1,600/1 650 207/235 1 500/1 600

1 ,600/1 650 207/235 1 500/1 600

1 ,600/1 650 207/235 1 500/1 600

1 ,600/1 650 213/255 1 500/1 600 1,600/1 650 217/241 1 500/1 600 1,600/1 650 207/235 1 500/1 600 1,600/1 650 217/241 1 500/1 600

1 ,500/1 600 207/235 1,350/1 550

1 ,550/1 650 207/235 1 350/1 550

1 ,500/1 600 207/235 1,350/1 550

1 ,600/1 650 217/255 1,500/1 600

1 ,600/1 650 223/255 1,500/1 600 1,600/1 650 229/269 1,500/1 600

1 ,600/1 650 235/275 1,500/1 600

1 ,600/1 650 248/293 1,500/1 600

1 ,600/1 650 217/255 1,500/1 600

1 ,600/1 650 229/255 1,500/1 600 1,600/1 650 235/277 1,500/1 600 1,600/1 650 241/277 1,500/1 600

1,500/1 600 207/235 1,350/1 550 1,600/1 650 212/241 1,350/1 550 1,600/1 650 223/255 1,350/1 500

1 ,600/1 650 223/255 1,350/1 500 1,600/1 650 223/255 1,350/1 550 1,500/1 600 248/277 1,400/1 500 1,500/1 600 217/255 1,350/1 550 1,500/1 600 207/235 1,350/1 550

DOU 241/277 1,500/1 600

1 ,600/1 650 235/269 1,350/1 550 1,600/1 650 235/269 1,350/1 550

1 ,600/1 650 235/269 1,350/1 550 1,600/1 650 235/269 1,350/1 550 lieoo/i 650 235/269 1,350/1 550 1,600/1 650 235/269 1,350/1 550 1,600/1 650 235/269 1,350/1 550 1,600/1 650 248/269 1,350/1 550 1,600/1 650 248/293 1,35C/1 550

1,425/1 475 179/207 C)

1,400/1 450 163/196 C) 1,450/1 500 174/201 C-) 1,400/1 450 183/212 C-) 1,450/1 500 183/212

1,400/1 475 183/207 1,200 1,450/1 500 207/235 1,200 1,450/1 500 212/248 1,200

"F

1,400/1

1,400/1

1,400/1

1,400/1

1,650/1

1 1,550/1

/

<" 1,600/1

I

<i 1,650/1

<! 1,600/1

1,675/1

1,700/1 1,450/1

d 1,400/1

"i 1,450/1

/ 1,450/1

1 1,550/1

1,700/1 1,750/1 1,500/1

1,450/1

1,525/1 1,750/1

1,800/1

1,800/1

1,450/1

1,775/1

1,800/1 1,700/1

1,775/1 1,800/1

1,850/1

1,850/1

1,825/1

1,825/1

1,825/1

1,850/1

J 2,050/2

2,000/2

« 2,000/2

' 2,000/2

• 2,000/2

' 2,200/2

• 2,000/2

' 2,100/2

• 2,150/2

''2,000/2 2,050/2

''2,000/2

• 2,300/2

' 2,300/2

• 2,300/2

' 2,325/2

• 2,325/2

• 2,300/2

• 2,300/2

• 2,275/2

' 2,200/2

' 2,150/2

• 2,175/2

2,200/2

2,200/2

• 2,200/2

• 2,150/2

' 2,150/2 ^2,150/2

' 2,175/2

' 2,200/2

' 2,200/2

• 2,200/2

' 2,225/2

« 2,225/2

2,175/2

2,175/2

2,175/2

2,190/2

1,450/1

1,450/1

1,550/1 1,425/1

1,500/1

1.450/1 1,500/1

1,450/1

1,450/1 1,450/1

500

500

500

500

800

650

700

750 700

750

750

500 475

500

525

625 800

850

600

550 600

800 850

875

500 850

875

800

850 875 950 900

875

875 900

950

150 200 200

200

200 325

250

300

300

175 225

175

375

375

375

400

400

350

375 325

300

225

250

250 250

250

200 240 225

250

250

250

250

275

275

220

210

220

240

550 550

700

500 600

550

600

600

600

600

Brine or water dodo

do

Oil Brine or water do

Oil do

do

Air or oil

Oil do

do Water

Oil

Air do

do

do

do

do

do

dodo

Air do

Oil Air do

do

Air do

do

do

do

Air or oil do

do

do

do

do

dodo

Air, oil, or salt do

do

do

Oil, air, or salt do

do

do

dodo

do

do

do

Oil. air, or salt do

do

do

do

do

do

do

do

do

do

do

do

do

do

dodo

do

Oil or water Water

Oil Water

Oil do

do Water or brine do

Water, brine, or oil

"F

350/650

350/650

350/650

350/650 400/1,200

300/800

350/800

350/800

400/600

400/1,150 300/500

300/500 300/600

325/550

350/1,000

350/1,000

300/800 300/800 300/800

300/1,000

350/1,100

950/1,100

350/800

400/1

400/1

400/1

400/1

400/1

300/1

1,000/1

1,000/1 1,000/1

1,000/1

1,100/1

1,050/1

1,000/1

1,100/1

1,100/1

1,100/1

1,200/1 1,050/1 1,050/1

1,060/1

1,050/1

1,050/1 1,050/1

1,000/1

1,000/1

1,000/1 1,000/1

1,000/1

1,000/1 1,000/1

1,000/1

1,000/1 1,000/1

1,000/1 1,000/1

1,000/1

1,000/1

1,000/1

1,000/1

1,000/1

1,000/1 1,000/1

1,000/1

1,000/1

1,000/1

1,000/1

1,000/1

1,000/1

1,000/1

1,000/1

000

000

000 000

000

000 200

200

200

200

200

250

300 250

250

250

500 200 250

250

200 200

200

100

100

100

100

100

100

100 100 200

100

100

100

100

100 100

100

100 200

100

100 100

100

100

100

100

100

100 300/600 300/1,000 300/600

300/1,000 300/600

350/500

350/500

350/500

36
Table 7. Recommended heat treatments for tool steels—Continued

CARBURIZING GRADES

AISI

type Normalizingtemperature

Annealing

Carburizing temperature

Hardening Tempering

Temperature Hardness Temperature Quenching medium Temperature Rc of case

"F °F (BHN) "F "F °F

rl 1,350/1.650 81/100 1X , fic;n/i DOU/ X , 700 1 1,450/1,475 Water or brine. 350/500 64/58 P2 («) 1,350/1,500 103/123 1,650/1.700 1 .525/1 ,550 Oil 350/500 64/58 P3 (') 1,350/1,500 109/137 1,650/1,700 1 ,475/1 .525 do 350/500 64/58 P4 (=) 1,600/1,650 116/128 1 .775/1.825 1.775/1.825 Air 350/500 64/58 P5 (.') 1,550/1,600 105/110 1,650/1,700 1.550/1,600 Oil or water 350/500 64/58 P6 («) 1,550/1,600 190/210 1,650/1,700 1,450/1,500 Oil 350/450 61/58 P20 1.650 1,400/1,450 150/180 1,600/1,650 1 ,500/1 ,600 do___ 350/500 64/58 P21 1,650 '1,300/1,500 Air or oil. . ' 950/1,025 40/30

» Normalizing:

0.60 to 0.75% C, 1500 °F:

.75 to .90% C, 1450 ^F;

.90 to 1.10% C. 1600 °F:

1.10 to 1.40% C, 1600 ° to 1700 Annealing:

0.60 to 0.90% C. 1360 ° to 1400

.90 to 1.40% C, 1400 " to 1450

Preheating not necessary. Do not normalize.

* Do not soak.

* Time at hardening temperature. 2 to 5 min.

' Double tempering recommended.

8 Normalizing not required.

I" Do not anneal.

' Solution treatment. Aging treatment—precipitation hardening steel.

Table 8. Recommended heat treatments for stainless and heat resisting steels GROUP I. HARDENABLE CHROMIUM STEELS (MARTENSITIC AND MAGNETIC)

Annealing ' Hardening ' Stress relieving Tempering

Temperature Hardness Temperature Hardness Temperature Hardness Temperature

1 . 550/1 , 650

1 . 500/1 . 650

^ 1.200/1,300

1,550/1,650 1,550/1,650

1,550/1,650

1,150/1,225

1,625/1,675

1,625/1,675

1 , 625/1 , 675

1 , 525/1 , 600

1 , 525/1 , 600

75/83 Rb

75/85 Rb

98 Rb/24 Rc

78/83 Rb

80/90 Rb

81/89 Rb

97 Rb/24 Rc

91/95 Rb

94/98 Rb

95/99 Rb

80/90 Rb

72/80 Rb

1,700/1 ,850 1,700/1,850

1,800/1,900

1,700/1,850

1,700/1,850

1,800/1,900

1,800/1,950

1,850/1,950

1,850/1,950

1,850/1,950

1 , 600/1 , 700 m

39/43 Rc

39/43 Rc

42/47 Rc

40/44 Rc

39/43 Rc

53/56 Rc 42/46 Rc

55/58 Rc

57/59 Rc

60/62 Rc

35/45 Ro

"F

450/700 450/700

450/700

450/700

450/700 300/700

450/700

300/700 300/700

300/700

40/37 Rc 40/37 Rc 44/40 Rc

40/37 Rc

40/37 R,

53/48 Rc

42/36 Rc

56/51 Rc

58/53 Rc

60/55 Rc

1,000/1,100

1,000/1,400

1 ,100/1 ,300 1,000/1,400

1,000/1,400

1,000/1,200

400/1 .200

GROUP II. NONHARDENABLE CHROMIUM STEELS

(FERRITIC AND MAGNETIC)

Annealing « AISI

type Hardness, Temperature Rb

op

405 1.350/1.500 70/80 430 1,400/1.500 80/90 430 F 1.350/1,500 80/90

430 F Se 1,350/1,500 80/90

442 1,300 80/90

446 1,450/1,600 80/90

GROUP HI. NONHARDENABLE CHROMIUM-NICKEL AND

CHROMIUM-NICKEL-MANGANESE STEELS

(AUSTENITIC AND NONMAGNETIC)

AISI Annealing AISI Annealing

type temperature ' type temperature '

op op

201 1,850/2,050 309 1,900/2,050

202 1,850/2,050 309 S 1,900/2,050

301 1,850/2,050 310 1,900/2,100

302 1,850/2,050 310 S 1,900/2,100

302 B 1,850/2,050 314 2,100

303 1,850/2,050 316 1,850/2,050

303 Se 1,850/2,050 316 L 1,850/2,050

304 1,850/2,050 317 1,850/2,050

304 L 1,850/2,050 321 1,750/2,050

305 1,850/2,050 347 1 ,850/2,050 308 1,850/2,050 348 1,850/2,050

» Cool slowly to 1,100 °F. Although these steels harden appreciably on air cooling, quenching in oil is preferable.

<^ Full annealing impractical; may be air cooled from indicated temperature.

Generally used in annealed condition only.

^ Air cool.

' Cooling may be in air or by quenching in water. The resultant hardness of all of these steels will be in the range of approximately

Rockwell B 80 to 90.

37
The stainless steels may be divided into three

general groups as follows

a. Group I—Hardenable Chromium Steels (Martensitic

and Magnetic)

These steels respond to heat treatment in a manner similar to most lower alloy steels and

will, by suitable thermal treatment, develop a wide range of mechanical properties. The

tempering range of 700 ° to 1000 °F. should be

avoided because it is conducive to temper

brittleness.

b. Group II—Nonhardenable Chromium Steels (Ferritic and Magnetic)

These steels are essentially nonhardenable by

heat treatment. They develop their maximum

softness, ductility and corrosion resistance in

the annealed condition.

c. Group III—Nonhardenable Chromium-Nickel and

Chromium-Nickel-Manganese Steels (Austenitic and

Nonmagnetic)

These steels are essentially nonmagnetic and

cannot be hardened by heat treatment. Cold

working develops a wide range of mechanical

properties and some of these steels, in the

severely cold worked condition, may become

slightly magnetic. They are annealed by

rapidly cooling (air or water) from high temperatures to develop maximum softness and

ductility as well as corrosion resistance.

Recommended heat treatments for the steels

listed in table 4 are given in table 8.

11. Properties and Uses of Steels

11.1. Structural Steels The strength properties of heat treated

(quenched and tempered) structural carbon

and alloy steels are closely related to their

hardness and are surprisingly similar for a se- lected hardness provided that the steels originally were hardened throughout their cross

sections. The relations of tensile strength,

yield strength, elongation, and reduction of area to hardness are shown in figure 27. The

tempering temperatures necessary to secure

certain hardness levels in steels are affected by

alloying elements, and two different steels may

have to be tempered at two different temperatures to secure the same hardness. When

this has been accomplished, however, the

strength and ductility of the two steels will be

quite similar. It must be emphasized that

these relationships are valid only if the heat

treated structures are tempered martensite. A

steel that has been incompletely hardened (that

is, quenched at too slow a rate to prevent the

formation of some fine pearlite) may have,

after tempering, a hardness and tensile

strength equal to that of a completely hardened

and tempered steel, but its yield strength and

ductility will be inferior.

70

60

50

40

30

20

10

25 HARDNESS .ROCKWELL C

30 35 40 45

1 1

1 1 1 1 1 1 1 1 1 1 1 ; 1 1 1 r 1

REDUCTION OF AREA

ELONGATION

1 1 1 1 1 J 1 ''

1

1 -J L. 1 1 - 220

I 180

»-

tDi

160

toa 140

_iUJ^ '20

US

UJS '00 z

UJ 80

1 1 1 1 1 1 1 1 1 1 1 I 1 1 1 1 1 1 1 1

TENSILE STRENGTH^

'lELD STRENGTH

1

1 1 1 11 1 1 1 1 1 1

1 _1 L 1 > 1 240 280 320 360 400

BRINELL HARDNESS NUMBER 440

Figure 27. Tensile strength, yield strength, elongation,

and reduction of area as a function of hardness in struc- tural steels. Valid only for steels originally hardened throughout and then tempered. Although drawn as lines, the relationship between these properties Is not

exact, and some deviations may be expected. The modulus of elasticity of steel is the same

as that of iron (about 29,000,000 Ib/in.^). It

is not affected by heat treatment or by the addition of alloying elements. Since stiffness, or

the resistance to deformation under load, is a

function of the modulus of elasticity, it follows

that the stiffness of steel cannot be changed by

heat treatment or by alloying elements, provided that the total stress is below the elastic

limit of the steel in question. Either heat

treatment or alloying elements can raise the

elastic limit and thus apparently improve the

stiffness in that higher allowable unit stresses may be imposed on the steel.

a. Plain Carbon Structural Steels The plain carbon steels are the least costly

and may be used for a variety of purposes.

The lower carbon grades (up to about 0.25% of

carbon), frequently termed machinery steels, are used in the hot- and cold-worked conditions

and for carburizing.Except when carburized,

they are not very responsive to hardening by

heat treatment because of the low carbon content. The medium carbon steels (about 0.30 to 0.60%) are the forging grades and are com- monly used in the heat-treated condition. The

higher carbon grades are the spring and tool

steels.

38
Carbon steels are essentially shallow-hardening, usually requiring a water or brine

quench, although very small sections may be

successfully hardened in oil. Even with the

most drastic quench, they cannot be hardened

throughout even in moderate (about %-in.)

cross sections. Large cross sections (about

4 in.) cannot be fully hardened even on the

surface except by induction or flame hardening.

b. Alloy Structural Steels From the viewpoint of heat treatment, the

advantages accruing from the use of alloy

steels are threefold: (a) Increased hardenability, (b) retention of fine-grained austenite,

and (c) retardation of softening during

tempering.

All of the alloying elements commonly used

in structural alloy steels are effective to varying degrees in displacing the S-curve to the

right as compared with plain carbon steels of

similar carbon contents. Consequently, they

can be hardened throughout in larger sections

and, generally, by means of a milder quenching

medium, such as oil. Because of the slower

rates of quenching that can be employed in

hardening these steels, they are less likely to

distort or crack during hardening than the

plain carbon steels. The hardenability of alloy steels varies

widely, depending on the composition. Certain elements and combinations of elements are very effective in increasing hardenability. Of

the five alloying elements most generally used,

and within the amounts normally present in

the structural steels, nickel enhances hardenability but mildly, vanadium, chromium, manganese, and molybdenum moderately to strongly, depending upon the amount of alloy

dissolved in the austenite at the hardening temperature. Combinations of chromium, nickel,

and molybdenum, such as are present in the

4300 series (table 1), result in very deep hardening steels. The alloy elements that form stable carbides,

principally vanadium and molybdenum (and

chromium to a lesser degree), are very effec- tive in inhibiting grain growth in austenite.

These steels may be overheated to a considerable degree during heating for hardening without suffering grain growth. This is desirable

since martensite formed from fine-grained

austenite is tougher than that formed from

coarse-grained austenite.

Alloying elements, particularly those that

form the more stable carbides, tend to retard

the softening effects of tempering so that alloy

steels must frequently be tempered at higher

temperatures than carbon steels in order to secure the same hardness. This is beneficial

in conferring increased toughness, i.e., resist- ance to shock.

It is pertinent to note that manganese and

nickel lower the annealing and hardening temperatures, and that chromium, molybdenum,

and vanadium raise these temperatures. Combinations of these alloys may therefore, raise,

lower or have no effect on the heat-treating

temperatures, depending on the combinations

of elements and their amounts.

The low carbon (up to about 0.25% of car- bon) alloy steels are used mainly as carburizing steels. The medium carbon (about 0.30

to 0.60% of carbon) alloy steels find wide use

as stressed members in an almost infinite variety of structural parts. Although some of

these alloy steels containing high carbon are used for certain special applications, the maximum amount of carbon present in the alloy

structural steels in generally about 0.5 percent.

11.2. Tool Steels

Practical experience has shown that in a majority of instances the choice of a tool steel

is not limited to a single type or even to a particular family for a working solution to an

individual tooling problem. It is desirable to

select the steel that will give the most economical overall performance; the tool life obtained

with each steel under consideration should be

judged by weighing such factors as expected

productivity, ease of fabrication, and cost. Hardness, strength, toughness, wear resist- ance, and resistance to softening by heat are prime factors that must be considered in select- ing tool steel for general applications. Many

other factors must be considered in individual

applications; they include permissible distortion in hardening, permissible decarburization,

hardenability, resistance to heat checking, machinability, grindability, and heat treating

requirements.

The straight carbon tool steels can be used

for a variety of purposes, depending upon the

carbon content and heat treatment. The lower

carbon ranges are used for tools where toughness and resistance to shock are of primary

importance; these steels are usually tempered

at temperatures of 500 to 700 °F or even

higher. The higher carbon ranges are used

where the main requirements are hardness, re- sistance to abrasion, or ability to hold a keen

edge; these steels are usually tempered at temperatures of 300 to 500 °F.

An addition of 0.20 to 0.50 percent of chromium or of vanadium is frequently made to the

carbon tool steels. The chromium-bearing car- bon steels have a greater depth of hardening

and are slightly more wear-resistant than the

chromium-free steels of the same carbon content. Vanadium decreases slightly the depth

of hardening but increases toughness. (This

may appear contradictory to previous state39
ments that alloying- elements increase hardenability. In the case of the carbon-vanadium

tool steels, the normal hardening temperatures

are too low to allow the solution of vanadium

carbide in the austenite. The presence of undissolved carbides decreases hardenability)

The cutting ability, or the ability to hold an

edge, is closely related to the hardness. For

most purposes, therefore, cutting tools are used

in a highly hardened condition. Excessive

heat, whether caused by heavy cutting or careless grinding, will "draw the temper" (that is, decrease the hardness) and ruin the tool, necessitating rehardening and retempering. Since

hardened carbon steels soften rapidly under the

influence of heat, the carbon steels cannot be used

as cutting tools under conditions where an appreciable amount of heat is generated at the

cutting edge. Their uses are limited to conditions entailing light cuts on relatively soft

materials, such as brass, aluminum, and un- hardened low carbon steels. The ranges for the carbon content and some

typical applications of carbon tool steels are

as follows:

0.60 to 0.75 percent of carbon—Hot forming

or heading dies for short runs, machinery

parts, hammers (sledges and pinch bars), concrete breakers and rivet sets. 0.75 to 0.90 percent of carbon—Hot and cold

sets, chisels, dies, shear blades, mining drill

steel, smiths' tools, set hammers, swages, and

flatteners.

0.90 to 1.10 percent of carbon—Hand chisels,

small shear blades, large taps, granite drills,

trimming dies, drills, cutters, slotting and milling tools, mill picks, circular cutters, threading

dies, cold header dies, jewelers' cold striking

dies, blanking, forming and drawing dies, and

punches.

1.10 to 1.40 percent of carbon—Small cutters,

small taps, drills, reamers, slotting and planing tools, wood-cutting tools, turning tools, and

razors. Suitable tempering temperatures for carbon

tool steels are as follows:

300 to 375 °F—Lathe tools and milling cutters, scrapers, drawing mandrels, dies, bonecutting tools, engraving tools, gages, and

threading dies.

375 to 500 °F—Hand taps and dies, hand

reamers, drills, bits, cutting dies, pen knives,

milling cutters, chasers, press dies for blanking

and forming rock drills, dental and surgical

instruments, hammer faces, wood-carving tools,

shear blades, and hack saws. 500 to 700 °F—Bending dies, shear blades,

chuck jaws, forging dies, tools for wood cutting, hammers and drop dies, axes, cold chisels,

coppersmith tools, screwdrivers, molding and

planing tools, hack saws, butcher knives, and

saws.

The high-speed steels are intended for use under heavy cutting conditions where considerable heat is generated. These steels are very

complex, containing large amounts of alloying

elements. The high-speed steels, which harden

from a very high temperature (2,150 to 2,400 °F), develop what is termed "secondary

hardness" after tempering at about 1,050 to 1,100 °F and maintain their cutting edge at considerably higher temperatures than do car- bon tool steels. Dies, depending on their use, can be made

of a variety of steels. When intended for use

at low temperatures, even carbon steel may be

satisfactory sometimes. For use at elevated

temperatures, certain minimum amounts of al- loying elements are necessary and for the

higher ranges of hot-working temperatures,

steels of the high-speed steel type are frequently used.

The manufacture of dies usually involves

such considerable expense in machining that

cost of material and heat treatment form but

a relatively small proportion of the total. It

is imperative, therefore, that die steels be se- lected carefully and properly heat treated.

The so-called "nondeforming" tool steels (type

0).. are favorites for dies that are not required

to operate at elevated temperatures. These

steels usually contain about 0.9 percent of car- bon and 1.2 percent to 1.6 percent of manganese. With the lower manganese content they

also contain about 0.5 percent each of chromium and tungsten. The expansion of these

steels during hardening is much less than is experienced with carbon steels, and the shrinkage that occurs during the initial stage of

tempering is almost suflficient to return the

steel to its original size when it has been tempered at the proper temperature.

The high carbon-high chromium steels (type

D) are frequently used for dies. This type

contains about 12 percent of chromium and 1.5 or 2.2 percent of carbon, the lower carbon steel being air hardening. These steels also are non- deforming, are more resistant to wear, and

may be used at higher temperatures than the

manganese nondeforming steels. They have

the disadvantage of requiring higher hardening

temperatures (about 1,800 °F).

11.3. Stainless and Heat Resisting Steels Steels for high-temperature service must

resist scaling and have high creep strength,

that is, resistance to deformation under prolonged stress at elevated temperatures. Resistance to scaling is aided by the presence of

chromium, aluminum, or silicon, and heatresistant steels invariably contain one or more

of these elements. Resistance to creep is aided

by elements that form stable carbides, such as tungsten, vanadium, molybdenum, and chromium.

40
The stainless steels, often called corrosionresistant steels, contain appreciable amounts of

chromium (12<~r or more) , either with or without other elements, the most important of

which is nickel. The heat-resistant and cor- rosion-resistant steels all belong to the same

family, the terminology frequently being dictated by the temperature of use. By common

consent, this dividing temperature is taken at about 1200 °F.

The three general groups of stainless steels

have markedly diiferent characteristics, as

follows:

a. Group I—Hardenable Chromium Steels (Martensitic

and Magnetic)

The significant characteristic of these is that

the steels respond to heat treatment in a manner similar to most lower alloy steels. The

steels possess a pearlitic structure on slow cooling and a martensitic (hardened) structure on

rapid cooling from within the austenitic field. The chromium content varies from about 12 to 18 percent and the carbon content from less than 0.1 to over 1.0 percent; the higher chromium content demands the higher carbon.

The mechanical properties of this group of

steels are markedly improved by heat treat- ment and heat treatment is essential to realize

their corrosion-resistant properties. The only

exceptions are the low carbon, low chromium

steels (types 403, 410, 416) which are cor- rosion resistant in the annealed condition,

although heat treatment decidedly improves

mechanical properties.

Finishing of the martensitic steels has an

important effect on corrosion resistance, since

maximum corrosion resistance is obtained only

with highly finished surfaces. Grinding should

be done carefully, using a sharp wheel, light

cuts, and plenty of coolant. Grinding burrs

and embedded scale serve as focal points for

corrosion.

As a group, the martensitic steels resist

many types of corrosive environments, including the atmosphere, fresh water, mine water,

steam, food acids, carbonic acid, crude oil, gasoline, blood, perspiration, ammonia, and

sterilizing solutions.

Type 410 is a general purpose steel suitable

for numerous applications where severe cor- rosion is not a problem. Types 416 and 416L

are free-machining varieties of type 410. The

higher hardness of type 420 makes it suitable

for such uses as cutlery, surgical instruments,

valves, ball bearings, magnets, etc. Type 420F

is its free-machining counterpart. Where still greater hardness is required, type 440A, B, or C may be used; the last is the hardest of all the

stainless steels. Types 420 and 440 should be

used only in the hardened and stress-relieved

conditions. Type 431 is a structural steel capable of being used in the strength range of

200,000 psi.

b. Group II—Nonhardenable Chromium Steels (Ferritic and Magnetic)

The distinguishing characteristic of this

group is that these steels are ferritic at all temperatures and, therefore, incapable of being

hardened by heat treatment. They can, however, be strengthened by cold working.

The ferritic steels develop coarse grained

structures when subjected to temperatures

above 1650 °F for varying lengths of time.

This grain growth, frequently objectionable

because of concomitant brittleness, cannot be

eliminated by heat treatment alone. It can be

corrected to some extent by cold working, fol- lowed by annealing.

The ferritic steels have lower strength at elevated temperatures than the martensitic

steels, but resistance to scaling and corrosion

is generally better. As a class, the ferritic

stainless steels resist corrosion from food products, organic acids, salt water, nitric acid,

many fused salts and molten nonferrous

metals.

Type 430 finds extensive use in automobile

trim, chemical equipment, food containers, and

furnace parts. If ease of machining is important, type 430F or 430F Se may be substituted. Where greater resistance to corrosion

or oxidation is required, type 446 may be used.

c. Group III—Nonhardenable Chromium-Nickel and

Chromium-Nickel-Manganese Steels (Austenitic and

Nonmagnetic)

The steels in this group are austenitic at room temperature and, hence, both nonhardenable by heat treatment and nonmagnetic.

They can, however, be strengthened appreciably by cold work. Depending upon the

amount of cold work, the tensile strength

varies from about 80,000 psi for fully annealed

material to as high as 300,000 psi for severely

work hardened steel; the highest strengths can

be secured only with small cross sections. A

stress-relieving treatment of about 1 hr at 750

to 800 °F after severe cold working improves

elastic properties without any adverse effect

upon ductility. Cold work causes partial trans- formation of austenite to ferrite, with conse- quent appearance of magnetism, in some of

these steels. The austenitic steels have excellent coldforming properties and are extremely tough

and ductile, even at low temperatures. Their

toughness makes machining of all but the freemachining varieties difficult. As a group, the austenitic steels are the most

corrosion resistant of all the stainless steels. They have excellent resistance to acetic, nitric,

and citric acids (liquid), foodstuffs, sterilizing

41
solutions, most of the organic chemicals and

dyestuffs, and a wide variety of inorganic

chemicals. Typical applications are for out- door trim, kitchen equipment, dairy utensils,

soda fountains, and light weight transportation

such as aircraft.

This group of steels should not be discussed

without some mention of intergranular embrittlement. If maintained in the temperature

range of about 800 to 1,400 °F, or if slowly

cooled through this range after annealing, some

of these steels precipitate carbides at the grain

boundaries. This is not deleterious unless the

steel is either simultaneously or subsequently

exposed to acidified corrosive conditions, caus- ing intergranular corrosion. When this occurs, the steels lose their metallic ring and become

quite brittle.

Type 302, with its higher carbon content,

is more prone to carbide precipitation than is 304, which in turn is more susceptible than

304L.

Prolonged exposure of these steels at temperatures within the range of 1000 to near

1600 °F may also promote the formation of a sigma phase constituent, resulting in increased

hardness and decreased ductility, notch toughness and corrosion resistance.

Type 301, because of its lower alloy content,

is quite susceptible to work hardening and is used largely in the cold rolled or cold drawn

form of sheet, strip, and wire. Types 201 and

202 are similar to types 301 and 302 in properties. The former were developed primarily

to conserve nickel.

Types 305, 308, 309, and 310 contain progressively higher amounts of chromium and

nickel and are, therefore, superior to the lower

alloy steels in corrosion and heat resistance.

Types 316 and 317 contain molybdenum.

The incorporation of this element increases

resistance to corrosion, particularly of the "pitting" type.

Types 321 and 347 were developed specifically to resist intergranular corrosion. This

property of the titanium-bearing steel (type

321) is generally enhanced by a stabilizing

heat treatment. This treatment is not particularly necessary for the niobium-bearing steels

(types 347 and 348).

d. Precipitation-Hardenable Stainless Steels The precipitation-hardenable stainless steels constitute a group of proprietary steels for

which standard specifications have not been

published by AISI. They may be grouped into

three types; martensitic, semi-austenitic and

austenitic. These steels are of increasing commercial importance because they can be

strengthened by heat treatment after fabrication without resorting to very high temperatures or rapid quenches. They possess properties that make them particularly useful for

structural parts and jet-engine components in

high-performance aircraft as well as many

non-military applications. Desirable characteristics include ease in hot working, machining, forming, and joining. High strength can

be obtained by relatively-low elevated-temperature aging treatments or by refrigeration

plus aging treatments. Ultrahigh strengths can be produced by a combination of cold working and aging. These steels usually possess

good properties at ambient and moderately ele- vated temperatures (temperatures below those

of the aging treatments) along with good cor- rosion and oxidation resistance. The austenitic

types also retain very good impact properties

at subzero temperatures.

Martensitic Types—The martensitic types

are austenitic at annealing temperatures but

transform to martensite of fairly low hardness on cooling to room temperature, either by air

cooling, or by oil or water quenching. Elements contributing to the precipitation-hardening phases remain in supersaturated solid-solution in the steels on cooling. Precipitation occurs on the subsequent aging of the steels

within the temperature range of 800 to 1100 °F

(Figure 28). The martensitic matrix is also

tempered by the aging treatment. The cooling

method, aging temperatures and times at temperature depend upon the specific composition

of the steel. Manufacturer's literature should

be consulted for specific recommendations.

The ductility and impact strength of these

hardened martensitic steels are relatively low

compared to those of either the semi-austenitic

or austenitic types. Increasing the aging temperature raises the ductility and impact

strength with an accompanying loss in the

yield and tensile strengths. These steels, in

the annealed condition, lack formability as compared with that of the semi-austenitic or

austenitic types of precipitation-hardenable

stainless steels. Semi-Austenitic Types—The semi-austenitic

steels are austenitic as annealed within the

temperature range of 1850 to 1950 °F and air

cooled to room temperature (large sections

may be water quenched). They are normally

supplied by the mill in the annealed condition or in an annealed plus severe cold-worked condition. By careful control of composition and

annealing treatment, an austenite can be produced which has a stability intermediate between that of the martensitic and austenitic

types of precipitation-hardenable stainless

steels. These steels are readily amenable

to forming operations after the annealing

treatment.

Some representative heat treatments for

these steels are shown in figure 28. The series

of treatments (temperatures and times at tem42
MARTENSITIC TYPES

Anneal 1850-1950 °F

Air cool

900-1 150

Air

°F, V2-4 hr cool

Anneal 1900 °F

Oil or water quench

900-1 100 °F, 1 hr Air cool

SEMI-AUSTENITIC TYPES

Anneal 1875-1950 °F

Air cool

1375-1400 °F, 1 V2-3 hr Air cool to 60 ° F

850- 11 00 °F, 1 V2-3 hr Air cool

1710-1750 °F, 10 min

Air cool to 60 ° F Severe cold work

at mill

-100 °F, 3-8 hr 850-900 °F, 1-3 hr

Air cool

850-1 100 "F, 1-3 hr Air cool Cold

850 °F, 3 hr Air cool

AUSTENITIC TYPES

Anneal 1650, 1 800,

or 2050 °F

Oil quench

1350 °F, 16 hr Air cool

Anneol 2050 °F Water quench

1300 °F, 24 hr Water quench

1300 °F, 1 2 hr Water quench

1200 "F, 24 hr Water quench

FlGxniE 28. Heat treating ranges'-"-'' for precipitation-hardenable stainless steels.

(a) Ranges of temperatures, heating periods and cooling rates (quenching mediums) shown include those for several commercial steels. Heat treatment procedures and values depend upon the composition of the specific steel selected. These steels are generally provided

in the annealed ( solution-treatd) condition by the mills, in the annealed (solution-treated) condition by the mills.

perature) to be followed depend upon the specific composition of the steel and the desired

final properties. After the forming operations,

these steels are usually given a destabilizing

treatment at temperature ranges of 1375 to 1400 °F or 1710 to 1750 °F in which some of

the carbon is rejected from the solid solution

as a carbide. This facilitates the transformation of the austenite to martensite during the

cooling to 60 °F or -100 °F and results in a strengthening of the matrix. However, the

final high strengths of these steels are obtained

by the subsequent aging treatments at temperatures ranging form 850 to 1100 °F. The

aging treatments temper the martensite and

increase the strength of the steels by precipitation of solute elements from solid solution as hardening phases (usually nickel-aluminum

compounds). Cold working of the steel fol- lowing the transformation of the austenite to martensite and prior to the aging treatment

produces very high strengths.

If these semi-austenitic steels are severely

cold-worked (usually at the mill) following the

annealing treatment most of the austenite is transformed to martensite by the cold working. Then no destabilizing treatment is needed

prior to the aging treatment which produces

very high strengths. However, the retained

ductility is usually very low.

43
These initially semi-austenitic steels through

the hardening and aging treatments develop

the highest room-temperature tensile properties (tensile strengths up to nearly 300,000

psi) of the three types of precipitation-hardenable stainless steels. Manufacturer's literature

should be consulted for heat treatment procedures as they depend very strongly on the

specific composition of the steel and its in- tended use. Austenitic Types—The austenitic steels are

austenitic in both the annealed and hardened

conditions. They are normally supplied by the

mill in the annealed condition. Annealing

temperatures (Figure 28) vary from 1650 to 2050 °F, depending upon the composition of

the steel. The steels are cooled rapidly from

the annealing temperatures either by oil or water quenching. These steels are subjected

to the forming or cold working operations

prior to hardening. The hardening treatment

(Figure 28) consists of aging within the temperature range of 1200 to 1400 °F which precipitates carbides and other hardening phases.

Cold working of these steels prior to aging

increases the strength with the properties

depending upon both the degree of cold-working and the aging treatment. As the heat

treatment procedure for these steels depends

upon the composition of the steel and its in- tended use, the manufacturer's literature

should be consulted for details of chemical composition, recommended annealing and aging

treatments, and typical properties and uses. In general the austenitic steels have lower

room-temperature properties than either the

martensitic or semi-austenitic types. However, they retain very good tensile and impact

properties at subzfro temperatures. Moreover, they retain be^ ter tensile, creep and rupture properties at temperatures up to their

aging temperatures than the other two types

of precipitation-hardenable stainless steels.

11.4. Nickel Maraging Steels A series of high-strength iron-base alloys of

very low carbon content (0.03%) with yield

strengths of 250,000 psi or greater in combination with excellent fracture toughness have

been developed in recent years. These are the

nickel maraging steels which do not require

rapid quenching, as the hardening and

strengthening are obtained by an iron-nickel

martensite transformation and an age-hardening reaction. The iron-nickel martensite is only moderately hard (approximately Rockwell

C 25) and very tough compared to that of un- tempered carbon steel martensites. The transformation of austenite to martensite is not

sensitive to cooling rates. Thus rapid quenching is not required and section-size effects are

small. No significant tempering occurs upon

the reheating of the iron-nickel martensite.

Moreover, no appreciable reversion of martensite to austenite occurs during the reheating of

the steel for the aging reaction (precipitation

of intermetallic compounds* in the presence of

suitable alloying elements)

The maraging steels are proprietary steels

for which standard specifications have not been

published by AISI. Four grades are commercially available: 25 Ni (25% Ni, 1.3-1.6% Ti);

20 Ni (20% Ni, 1.3-1.6% Ti); 18 Ni (17-19%

Ni, 7-9.5% Co, 3-5% Mo, and 0.2-0.8%Ti); and

12 Ni (9-15% Ni, 3-5% Cr, 2-4% Mo, and

0.08-0.55% Ti) . All of these steels contain appreciable amounts of aluminum. The 25 Ni

and 20 Ni grades have a niobium content of

0.30 to 0.50%.

The maraging steels can be heat treated in

conventional furnaces without protective atmospheres. They can be formed or machined to

final dimensions in the annealed condition and

will retain close tolerances during the aging

treatment. Oxidation during aging is usually

insignificant. Heat treating ranges are shown

in figure 29. As the heat treatment procedures depend upon the specific composition of

the steel, the manufacturer's literature should

be consulted for the chemical composition and

recommended heat treatment of the selected

steel.

a. 25 Ni Maraging Steel The 25 Ni steels remain austenitic upon air

cooling to room temperature after annealing

(solution-anneal) at 1500 °F. The steels in

this condition are relatively soft and readily

fabricated. As indicated in figure 29, two procedures are available for conditioning the steels

for transformation to martensite. The steels may be given an aging treatment at 1300 °F

which reduces the stability of the austenite,

presumably by precipitation of nickel-titanium

compounds in the austenite. This raises the

martensite transformation temperature so that

the austenite will transform to martensite on

cooling to room temperature. In the other

procedure, the martensitic transformation can

be induced by cold working the annealed steels to a reduction of 25% or more. After either

treatment, cooling at —100 °F for several

hours secures complete transformation. Full

strength properties (250,000-270,000 psi yield

strength) are then obtained by the aging

treatment at 900 °F for 1 to 3 hours.

b. 20 Ni Maraging Steel The annealed 20 Ni steels have a martensitic

transformation temperature above room temperature and hence they transform to martens-

" Currently, there is a question as to whether or not ordering

occurs during the aging treatment and contributes to the strength- ening of the steel.

44
25 Ni MARAGING STEEL

Anneal 1500 °F., 1 hr per in. thickness

Air cool

1300 °F, 4 hr Air cool

— 100 °F, Several hours

900 °F, Air 1-3 hr cool 20 Ni MARAGING STEEL

Anneal 1500 °F, 1 hr per in. thickness

Air cool

Cold work, 50%

-100 °F, Several hours

900 °F, 1-3 hr Air cool

1 8 Ni MARAGING STEEL

Anneal 1500 °F, 1 hr per in. thickness

Air cool

1 Cold work, 50%

1 900 °F, 1-3 hr Air cool

12 Ni MARAGING STEEL

Anneal 1500 °F, 1 hr per in. thickness

Air cool

800- 1000 °F, 1-6 hrs

Air cool

Figure 29. Heat treating ranges for Ni maraging steels

(a) Usually recommended, but may be omitted.

ite upon cooling to room temperature from the

annealing or hot working temperatures (figure

29). Refrigeration at —100 °F for several

hours is usually recommended for assurance of

complete transformation prior to the aging

treatment at 900 °F for 1 to 3 hrs. For en- hanced strength (yield strength up to 260,000

psi) the steels may be cold worked following

the annealing and prior to refrigeration at — 100 °F and subsequent aging.

c. 18 Ni Maraging Steel The heat treatment procedures for the 18 Ni

steels are the same as those for the 20 Ni steels

except that refrigeration of the steels is not

necessary; the complete transformation of the

austenite to martensite occurs during air cooling of the steels from the hot working or an- nealing temperatures to room temperature.

Full strength properties are obtained by the

aging treatment at 900 °F. For the highest

strength properties (yield strength of 300,000

to 310,000 psi) the steels are cold worked to reductions of 5()% or more following the annealing and prior to the aging treatment.

d. 12 Ni Maraging Steel The 12 Ni steels combine intermediate

strengths (150,000-215,000 psi yield strength)

with good ductility and high toughness as measured by Charpy V-notch impact resistance

at room temperature. These steels usually re- tain good impact resistance at subzero temperatures down to —100 °F. The heat treating

procedures are indicated in figure 29. They

are very similar to those for the higher

strength 18 Ni steels. In preparing this monograph, the authors

have drawn liberally from the field of common

knowledge and published information, especially from the Metals Handbook, published by

the American Society for Metals, and the Steel

Product Manuals of the American Iron and

Steel Institute. Grateful acknowledgment is due to all metallurgists, both known and un- known, whose work and study have transformed the art of heat treating into a science.

The various photomicrographs used were prepared by Carolyn R. Irish, a former member

of the staff of the National Bureau of

Standards.

12. Selected References

The following is a selected, but by no means complete, list of books that contain information pertaining

to the subject of this Monograph. In addition, many

manufacturers of steels and of equipment related to heat treating have available excellent pamphlets and

books. American Foundrymen's Society, The cast metals

handbook, Am. Foundrymen's Soc, Chicago, 111. (1957) 316 pages. American Iron and Steel Institute Steel Products

Manuals, as follows: Carbon steel: Semi-finished for forging; Hot rolled and cold finished bars; Hot rolled deformed concrete reinforcing bars. May 1964; Alloy

steel: Semi-finished; hot rolled and cold finished bars, Feb. 1964; Tool steels, Apr. 1963; Stainless and heat

resisting steels, Apr. 1963; Alloy steel wire, Aug. 1958; Wire and rods, carbon steel, Dec. 1963, American Iron and Steel Inst. New York, N.Y.

45
American Society for Metals, Metals handbook

(1948) and Vol. 1 and 2 (1961 and 1964, respectively),

Am. See. Metals, Metals Park, Ohio. Edgar C. Bain, Functions of the alloying elements in

steel, Am. Soc. Metals, Metals Park, Ohio (1939) 312

pages.

R. M. Brick and Arthur Phillips, Structure and properties of alloys (McGraw-Hill Book Co., Inc., New

York, N.Y., 1949) 485 pages. Malcolm S. Burton, Applied metallurgy for engineers

(McGraw-Hill Book Co., Inc., New York, N.Y., 1956)

407 pages.

J. M. Camp and C. B. Francis, The making, shaping

and treating of steel, 7th edition U.S. Steel Corp., Pitts- burgh, Pa. (1957) 1048 pages. Walter Crafts and John L. Lament, Hardenability

and steel selection (Pitman and Sons, New York, N.Y.,

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