NATIONAL BUREAU OF STANDARDS REPORT

10 015

EFFECT OF REFRIGERATION AND STRAIN ON THE CHARPY V-NOTCH

IMPACT PROPERTIES OF TYPE 431 STAINLESS STEEL

Sponsor

Materials and Processing Branch

Naval Air Systems Command

Department of the Navy

U.S. DEPARTMENT OF COMMERCE

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

3120410 10 015

EFFECT OF REFRIGERATION AND STRAIN ON THE CHARPY V-NOTCH

IMPACT PROPERTIES OF TYPE 431 STAINLESS STEEL

By

I. J. Feinberg

Engineering Metallurgy Section

Metallurgy Division

Sponsor

Materials and Processing Branch

Naval Air Systems Command

Department of the N avy

IMPORTANT NOTICE NATIONAL BUREAU OF I ;ress accounting documents intended

for use within the Governmen Approved for public release by the ' s subjected to additional evaluation ana review, ror inis reason, whole or in part, is not autt Bureau of Standards, Washin

the Report has been specitical October 9 2015 II copies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Effect of Refrigeration and Strain on the Charpy V-Notch

Impact Properties of Type 431 Stainless Steel

By

I . J . Fe i nber

g

The Naval Air Systems Command has requested an investigation of

the effects of refrigeration after hardening and subsequent straining

on the notch toughness of type 431 stainless steel.

It is known that an entirely martensitic structure cannot be

obtained by heat treatment alone, even with refrigeration, and there

will always be some retained austenite. It is also known that retained

austenite may transform on straining to untempered martensite and this

phase is generally considered to be embrittling. Little data are available from which to ascertain the effects of refrigeration treatments

and straining on the properties of 431 stainless steel. Data are presented, obtained from tensile and Charpy V-notch tests, which show that

either refrigeration after hardening or straining after hardening or both, exert a strong influence on notch toughness. The observed effects

on properties are related to mi crost ructu ra 1 changes produced in the

steel by the treatments applied.

Specimens for this investigation were machined from one inch diameter

bar stock. The chemical composition of the material in percent by weight

was as follows: carbon, 0 . 151 ; manganese, 0.59; phosphorus, 0.021; sulfur,

0.018; silicon, 0.30; chromium, 1 6

.

1 Y ; nickel, 211; molybdenum, 0. IT; nitrogen, O.O85 . The composition complies with that specified for type

431 in specification. Mi 1 -S- 1 8732C . The bar stock was furnished in the

annealed condition with an austenitic grain size of 6-8, classified in

accordance with'ASTM designation E - 1 9

Specimens and Procedure :

Notch impact tests were conducted on four groups of specimens

identified as follows:

Mater i a 1

NRUS

NRS

RUS

RS

(Not refrigerated, unstrained)

(Not refrigerated, strained)

(Refrigerated, unstrained)

(Refrigerated, strained)

2

Figure I is a flow diagram describing the specimen preparation sequence.

Standard Charpy V-notch specimens were used in the impact tests. The

loads at which Charpy specimens were strained were determined from data

obtained in tensile tests on 0.252 inch diameter subsize control specimens. The heat treatment sequence in Figure 1 that included refrigeration is essentially the same as that specified for heat treating A I S

1

431 to the HT-200 condition, as given in paragraph 3-4.1, Mi 1 -S- 1 8732C

.

In fabricating the NRUS and RUS impact specimens, blanks with a 0.50 in. by 0 . 50 in. square cross-section were first rough machined from

the as-received stock. These blanks were first heat treated and then

ground to finished dimensions and notched. The procedure for making NRS

and RS specimens involved first producing tensile test bars with a 0.50

in. by 0.50 in. square reduced cross section. These bars were heat

treated, then strained by stressing at approxi mate 1 y 1 38 percent of

their yield strength at 0.20 percent offset as determined in the tensile

control tests. The stress was the maximum attainable before the onset

of the accelerated strain (necking) characteristic of the material prior

to fracture. Impact specimens cut from the strained tensile bars were

ground to finished dimensions and notched.

Retained austenite contents were determined by X-ray diffraction

measurements using the Ogi 1 vi e-Bechtol dt technique . This technique

compares the integrated peak intensities of the ( 1 1 0 ) , (200), (211) and

(220) martensite lines and the (ill), (200), (220) and (311) austenite

lines and is claimed to give an accuracy of ± ]%.

Results and Discussion :

Table 1 indicates that longitudinal tensile properties that were obtained for RUS control material complied in all respects with that

specified for the HT-200 condition in Mi 1 -S- 1 8732C . All of the

longitudinal properties of NRUS control material complied with that

specified for the HT-200 condition except yield strength which at 145.3

ksi was slightly low. Tempering the NRUS material at a temperature

slightly lower than 550°F would probably raise the yield strength

sufficiently to meet Mi 1— S— I8732C requirements without adversely affecting

the other tensile properties. However, the NRUS control material, as heat-treated with a 550°F temper, had a better combination of tensile

strength and ductility than the RUS control material similarly tempered.

' Technique described by Robt. E. Ogilvie, Massachusetts Inst, of

Technology in Norelco Reporter, Vol. VI, page 60 and adapted to

computer by C. J. Bechtoldt, National Bureau of Standards.

- 3

-

Table 2 gives the approximate retained austenite, untempered

martensite and tempered martensite contents of the test material. Untempered and tempered martensite contents are differences obtained after

retained austenite determinations were made.

Strains at 1 38 percent of the yield stress at 0.2 percent offset of

the NRS material reduced the retained austenite content of this material

from 23.1 percent to 10.6 percent. Strains at 1 38 percent of the yield

stress of the RS material reduced its retained austenite content from

4.5 to 3-6 percent. These values imply that a high stress (greater than

the yield stress) will readily convert austenite to martensite when the

initial austenite proportion is either high or low. However, when the

ratio of austenite to martensite is initially low, the proportion of

austenite converted to martensite on high stress application is less.

From Table 3 which lists Charpy V-notch impact properties obtained

and Figure 2 which presents percent losses in notch impact strength the

following observations may be made:

1. The room temperature notch impact strength of the NRUS

material is more than twice that of the RUS material.

2. Marked losses in the impact strength of both NRUS and RUS

material occurred on testing at -40°F. However, the tests

at this temperature indicated a far greater loss of notch

toughness in the RUS material.

3. The notch toughness of both NRS and RS materials is reduced

by strain as indicated in room temperature tests.

4. Refrigeration reduced notch toughness to a greater extent

than strain.

5. The reduction in notch toughness is most severe in RS

material as indicated in tests at -40°F.

Macroscopic Features :

The fracture surfaces of broken impact test specimens are shown in

Figure 3- Ductile fracture is indicated by large shear lips and large

fibrous areas in a fracture surface while in brittle fracture shear

lips are either very small or absent and flat cleavage areas predominate

in the fracture surface. From the standpoint of fracture surface

appearance the specimens that were not refrigerated and not strained

fractured in the most ductile manner while those that were refrigerated

and strained fractured in the most brittle manner. These observations

coincide with the impact test results.

4

Microscopic Features :

Figure 4 presents the mi c ros t r uctu ra I features that influenced

impact test results. Austenite bands are revealed in the NRUS

material. Figure 4a, that are not evident in the RUS and RS material.

Figures 4c and 4d, respectively. The absence of austenite banding

in the refrigerated material is attributed to the martensite generation

occurring within band areas of this material on refrigeration. Untempered

martensite formed in the austenite bands of non-ref r i gerated material on straining is visible in Figure 4b. However, the structure of RUS material,

Figure 4c, is quite similar to that of the RS material, Figure 4d,

evidently due to the very small amount of untempered martensite formed

on straining the latter material.

Both austenite and tempered martensite have relatively high notch

toughness in this steel in comparison to any untempered martensite that

may be present. It is apparent, particularly at lower temperatures,

that the austenite is less notch sensitive than the tempered martensite.

Obviously, the three phases will vie as factors affecting the notch

toughness of 431 depending on their concentration in the material.

Austenite is the important factor in imparting superior notch

toughness to unrefrigerated 431 at room temperature and at -40°F. As

the unrefrigerated material was strained, retained austenite was converted

to untempered martensite. The loss in notch toughness of NRS material

on straining is attributed to the decrease in retained austenite occurring

on straining and to the presence of strain-induced untempered martensite.

Because of its high initial retained austenite content NRS material is

capable of undergoing more plastic deformation before rupture than RS

material with a lower retained austenite content. Both theory and

experiment suggest that the volume fraction of untempered martensite

formed during deformation is a continuous function of plastic strain.

Thus under progressively higher strains the untempered martensite in an NRS material may increase to a point where the NRS material will have a very low notch toughness.

Cone 1 us i ons :

431 stainless steel that has been heat treated to the HT-200

condition using refrigeration in the heat treating sequence is con- siderably more notch sensitive than 431 heat treated to the HT-200

condition without refrigeration. The superior notch toughness of the

unrefrigerated material is due to the large retained austenite content

resulting from the omission of refrigeration.

- 5

-

Plastic strain reduces the notch toughness of both refrigerated

and pon-ref r i gerated 431. The loss in notch toughness from plastic

strain is due to a decrease in the retained austenite content and an accompanying increase in the untempered martensite content in the

material. After being subjected to severe strains, strains approaching

necking in specimens of both refrigerated and non-ref ri gerated material,

the non-ref ri gerated 431 continued to exhibit better notch toughness.

Refrigeration severely reduces the notch toughness of 431 and

subsequent plastic strain will greatly add to the notch sensitivity of

the material. From these considerations it appears that any advantage

refrigeration might impart would be nullified if part geometry included

notches or other areas of stress concentration where plastic strain

could occur, and/or if parts were subjected to inpact loading. Therefore, refrigeration is not recommended for 431 stainless steel used in

such app 1 i cat i ons

.

USCOMM-NBS-DC
pit:'
Table 1. Average Longitudinal Tensile Properties of Heat Treated

431 Control Material Used in Notch Impact Tests

Spec i men GroupHardness Yield

Strength

at 0.2$

Offset

Ultimate

Tens i 1

e

Strength

E longat i on

i n 1 i nch

Reduct i on

of Area

Rc ks i ks i i Jo

RUS 47 158.4 224.4 17.16 58.67

NRUS 45 1^5.3 215.5 20.57 59.53

Spec

long

ified HT -200

i tudi na

1

150.0 (min.) 200.0 (min.) 10.00 (min.) 40.00 ( m i

r

propert i es

,

Mi 1-S- 18732C

Table 2. Approximate Retained Austenite, Untempered Martensite and

Tempered Martensite Contents of Heat Treated 431 Test

Material Used in Notch Impact Tests.

(Strained Material Stressed at 138$ of Yield Strength at

0.2$ Offset.)

Mater i a 1 Condi t i on

\lo 1 ume Percent

NRUS

/Not refrigerated

\ unstrained

1 ^

^Not

NRS

ref r i gerated

\

strained ]

RUS

[Ref r i gerated\

\ unst ra i ned /

/

RS

/ Ref r i gerated

\ strained

Reta i ned

austen i te 23.1 10.6 ^.5 3.6

Untempered

martens i te 0 12.5 0 0.9

Tempered

martens i te 76.9 76.9 95-5 95.5

Table 3. Charpy V-notch Impact Properties of Type 431 Stainless

Steel; Stress Relieved 1/2 Hour at 1200 F, Austenitized

1/2 Hour in Air at 1900 F, Quenched in Oil at 100-105 F and Refrigerated for 2 Hours at -140 F as Noted. Tempered

Twice at 550 F for 2 Hours. Specimens Strained by Stressing at I 38 percent of Yield Stress at 0.2$> Offset, as Noted. Specimens Ground to Finished Dimensions After Heat

Treatment or After Heat Treatment and Strain.

Specimen Charpy V-notch, ft. lbs.

Condi t ioni ng No. R. T. -40 F

Stress re 1 i eve NRUS - 1 55.5 30.0

Austeni t i ze 2 52.5 27.5

Quench 3 52.5 29.0

T emper Avg 53-5 28.8

Stress re 1 i eve NRS - 1 25 .O 11.5

Austeni t i ze 2 26.5 13.0

Quench 3 30.5 14.0

T emper

Strain

Avg 27.3 12.8

Stress re 1 i eve RUS - 1 22.5 7.0

Austeni t i ze 2 21 .0 5-5

Quench 3 28.5 8.5

Ref r i gerate

T emper

Avg 24.0 7-0

Stress re 1 i eve RS 1 18.5 J+.5 Austeni ti ze 2 19.5 5.5

Quench 3 15.0 4.0

Ref r i gerate

Temper

Strain

Avg 17.7 ^.7

Austenitize

in

Air

1/2

Hour

at

1900 F,

Quench

into

Oil

at

100-105

F.

Then

Cool

to

Room

Temperature

in

Cold

Water ooo

CO

CL

to z

L-3 ®\ o •- co

ZT. u_ CL

OJ

OLO

L- LT\ o

4- 4-» fD

<DO c0) 2 CL CO }— E ' IS) CL <u o ZD z c. (- O CL

CD o Z

** Q. J=

CO e o c. ZD <U <0 — co

CL (— LU < ZD z sc

to °!o

of

co oo 4-> c m CO -o JZ

03 >~ o —

4-» cn

_J Q> c 4-*

4-i a> a> >- CO L- (/> -O E M 4—

LO 4-

C

X o — o X)

(T3 c.

—

1- CL 0) CM

4-> CL •— • co C >- O

TD

a> (S) -C CL

in — to c c CO — o CD

Ll_ oc o c

0) CO E a:

XJ

— z c o

CjO 13 ZD

CL

to

CL

CO

ZD

CL

CM

4. U_ o

4-

O

0) •—

•M I fD

4-» a) ID CD •— to

L_ 1- 4- 3

a) o

CL I

Figure

1.

Procedure

for

Preparing

Tensile

Control

and

Notch

Impact

Specimens

for

Tests.

90

80

70

60

50

40

30

20

10

0

Room Temperature

-40 °F

00

TO

oo

TO GO

CO03

TO

03

I OO

CO

03

JVJ

Not Refrigerated

Unstrained

N R U S

Not Refrigerated

Strained

N R S

Refrigerated

Unstrained

R U S

Refrigerated

Strained

R S

Relative Embrittlement of AISI 431 Test Material as Measured by Percent Loss

n Notch Impact Strength in Tests at Room Temperature (72 °F) and at -40 °F.

Effects of Various Combinations of Heat Treatment and Strain Compared.

Not Refrigerated

RT -40 F

Ref r i gerated

RT -40 F

Stra i ned

Unstrai ned

Figure 3* Fracture surfaces of notched impact test specimens.

Specimens broken at room temperature (RT) and at

-40 F. Superior notch toughness of non-ref r i gerated

specimens is indicated by larger shear lips and

fibrous fracture areas. These two criteria also

indicate a loss in notch toughness or embrittlement

in both non-ref r i gerated and refrigerated specimens

as a result of strain. X 2.

-O

0) szu •

*-> o UJ O

LCN

• X -O

0)c • o

LTA

4-1 o •— OJ "O </1 co

o

•\ >- CD

CD LCA

ID • L. o CD> •4 •— o -O OJ JO

CD , 0) E

4->

l/l OUA

4-1 ID *\ 0) —

4-> CJ

zrz

CD *\ SZ —

3

4-1 E

>4— O

o

'•—

/

ID

d> 4-> i_ c 3 CD

4-1 CD u CD 3 CD

s_ S_

4—1 IDO

• o o

•

2

E

ID

03 d> C jr •— CD “D i_ 3 <D

4-> m

CD SZ C 4-1 o —

—j 2 dj _Q O T)

0) l_3

CD

Hardened,

tempered,

unstrained

('IRUS).

Hardened,

tempered,

strained

(NRS).

Hardened,

refrigerated,

tempered,

unstrained

(RUS).

Hardened,

refrigerated,

tempered,

strained

(RS).


'I