NBSIR 76-1068 i

Evaluation of A New Hardening

Process for Steel Transfer

Materials

I. J. Feinberg

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Failure Analysis Report

May, 1976

Issued August 1 976

Prepared for Office of Engraving

Bureau of Engraving and Printing

Washington, D. C.

NBSIR 76-1068

EVALUATION OF A NEW HARDENING

PROCESS FOR STEEL TRANSFER

MATERIALS

I. J. Feinberg

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Failure Analysis Report

May, 1976

Issued August 1 976

Mention of the name of a commercial process for the hardening of steel

is in no way intended to imply an endorsement of the process by the

National Bureau of Standards, nor is it intended to suggest that it is the best or only process to use for hardening steel. The process

is identified in order to describe specifically what was done in this investigation.

Prepared for Office of Engraving

Bureau of Engraving and Printing

Washington, D. C. 20234

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

SUMMARY

1 . INTRODUCTION 1

1.1 Some Requirements of a Substitute

Hardening Process 1

1.2 Possible Substitute Procedures 1

2 1.2.1 Description of a New Hardening Process...

2. PURPOSE 2

3. EXPERIMENTAL WORK 3

3.1 Materials 3

3.1.1 Characteristics of Finished Transfer

Process Components 3

3.1.2 Experimental Test Material 4

3.1.2.1 Chemical Analysis of

Test Material, As-Received 4 3.1.2.2 Hardness of Test Material,

As-Received 5

3.1.2.3 Microstructure of Test

Material, As -Received 5

4. TEST PROCEDURES 5

4.1 Test Equipment 5

4.1.1 Equipment Used at Manufacturer's

Laboratory 5

4.1.2 Equipment Used at NBS 5

4.2 Preparation of a New Process Salt Bath 6

4.3 Experimental Hardening Procedure Variables....... 6

4.4 Analysis of Exhaust Air from New Case

Hardening Process

..

7

i
TABLE OF CONTENTS (continued)

Page

5. RESULTS AND DISCUSSIONS 7

5.1 Chemical Composition of As-Received

Material 7

5.2 Hardness Characteristics of Heat Treated

Material 7

5.3 Condition of Polished Surfaces After

Hardening 8

5.4 Case and Core Microstructures Obtained

in Experimental Tests 8

5.4.1 Observation of Microstructures of

Hardened AISI 1031 Specimens 9

5.4.2 Observation of Microstructures of

Hardened AISI 1045 Specimens 9

5.4.3 Observation of Microstructures of

Hardened W2 Specimens 9

6. CONCLUSIONS 10

7. RECOMMENDATIONS 12

ACKNOWLEDGMENT 12

REFERENCES 13

TABLES

1. Chemical Analysis of Test Material, As-Received.

2. Hardness of Test Material, As-Received.

3. Hardness Characteristics of Selected AISI 1031 Specimens

After Heat Treatment.

4. Hardness Characteristics of Selected AISI 1045 Specimens

After Heat Treatment.

5. Hardness Characteristics of Selected W2 Specimens After

Heat Treatment

FIGURES

1. Microstructures in Transfer Process Test Material Specimens

As-Received.

ii
FIGURES (continued)

2. Schematic Representation of Salt Pot and Stirrer

Arrangement For Experimental Case Hardening Tests at

Manufacturer's Laboratory.

3. Salt Pot Arrangement for Experimental Case-Hardening

Tests at NBS

.

4. Equipment Arrangement for Exhaust Air Analysis.

5. Appearance of Polished Surfaces of Hardened Specimens.

6. through 23. Structure and Properties of Hardened

Specimens

:

6. Specimen 1032- 2

7 . Specimen 1031- 3

8 . Specimen 1031- 7

9 . Specimen 1031- 8

10. Specimen 1031-9

11. Specimen 1031-•11

12 . Specimen 1031-•15

13. Specimen 1045- 2

14 . Specimen 1045-•3 15. Specimen 1045-7

16. Specimen 1045-•8 17 . Specimen 1045-•12

18. Specimen 1045-•14

19. Specimen 1045-•15

20. Specimen W2-2

21. Specimen W2-3

22 . Specimen W2-5

23. Specimen W2-6

APPENDIX. Analytical Report, "The Sampling and Analysis for

Cyanide in the Exhaust Air from a Steel-Hardening Process,

Authors: Barry C. Cadoff and John K. Taylor, Air Pollutio

Analysis Section, Analytical Chemistry Division, National

Bureau of Standards.
SUMMARY

A new molten salt hardening process has been evaluated for

hardening steels used in the three stage transfer process for printing

stamps. Evaluation of the new process was undertaken because virtually

all cyanide salts are eliminated as ingredients in the process. It was found that complete elimination of cyanide emissions is not achieved in

the new process, however, the level of cyanide emitted does meet OSHA

requirements. A case-hardening capability was exhibited in experimental

tests on Bureau of Engraving and Printing materials, but case depths

and hardness did not meet Bureau of Engraving and Printing requirements.

Further development of the new process is needed before it can be

considered for the Bureau of Engraving and Printing.

iv
1. INTROUUCTION

Cyaniding of steel transfer materials (dies, rolls, transfer plates)

has been employed by the Office of Engraving of the Bureau of Engraving

and Printing (Bu E^P) for many years. Cyaniding has been adapted to

both case and through hardening of selected steel materials used in the

three stage transfer process for printing stamps and has been found to

be consistently satisfactory in these applications. Nevertheless,

Bu E^P has deemed it advisable to investigate the practicability of

shifting to some other hardening process, because of a concern with

health hazards, and has requested an NBS investigation of such

processes. The investigation of one substitute process is reported

herein. [1]*

1.1 Some Requirements of 1a Substitute Hardening Process

'fhe following may be considered as requirements for a suitable

replacement process.

a. Freedom from cyanide toxicity.

b. Capability of obtaining acceptable case hardnesses and case depths

for components austenitized at 1450F to 1600F, the temperature

range in which presently-used transfer process steels are austenitized.

c. Capability of the equipment used to accommodate the dimensions

and the configurations of the material to be hardened.

d. Adequate protection of engraved surfaces from damage.

e. Cleanliness of material surfaces after hardening and a good

product from an engraver's standpoint.

f. Acceptable costs of the initial installation and of subsequent

operation from the standpoint of- a small production output.

1.2 Possible Substitute Processes

Consideration was given to the following as substitute hardening

processes

:

a. Gas carburizing [2a]

b. Pack carburizing [2b]

c. Gaseous atmosphere carbonitriding [2c]

Figures in brackets indicate references at the end of this report.

- 1 -
d. Induction hardening [2d]

e. Flame hardening [2e]

All of the processes listed above were reviewed and found to involve

the installation of costly new equipment and extensive shut down time

during installation. Thus, they were given limited consideration.

An interest was found in a newly developed molten salt hardening process [1

This hardening process appeared to be adaptable to the case-hardening of

the transfer process materials currently hardened by cyaniding at Bu

E^P. Moreover it appeared to require less expensive equipment for a change-over than that needed by other replacement processes.

1.2.1 Description of the New Hardening Process

The salt bath used in the new case hardening procedure is prepared

from three ingredients named by the manufacturer as follows:

a. NOCY base salt

b. LOCYN pellets

c. NOCY additive

A detailed description of the composition of the ingredients was not

available from the manufacturer for proprietary reasons. However,

the manufacturer explained that the NOCY base salt is composed of

carbonates and chlorides, the LOCYN pellets have an organic base and

are high in nitrogen and the NOCY additive is synthesized graphite.

According to the manufacturer, graphite reacting with sodium carbonate,

Na2 CO3, in the bath forms carbon monoxide, CO, which is the active

carburizing agent. Nascent nitrogen, N, entering the case is derived

from breakdown of the LOCYN pellets.

Salt pot equipment such as that used for cyaniding at Bu E§P is

suitable for hardening by the new process. However, provision must be

made for stirring (agitation) of the molten salt bath during the

hardening operation. The process was reported by the manufacturer

to be suitable for operation at temperatures of 1450 to 1600F, the

range at which hardening is done at Bu Ei^P

.

This report gives results of an NBS evaluation for Bu E^P of

the new process applied to the hardening of steel transfer materials.

Purposes of this investigation were to learn characteristics of

the new process and to determine its suitability as a replacement for

the molten cyanide salt process currently used in heat treating

2. PURPOSE

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transfer materials at the Bureau of Engraving and Printing by

performing laboratory tests on these materials. A requirement in

heat treating is operation at a temperature range of 1450 to 1600F at

which certain materials are case-hardened and others through

hardened

.

3. EXPERIMENTAL WORK

Experimental work included heat treatment of transfer process

test material supplied by the Bu E^P. ITie treatments were carried

out at the heat treating laboratory of the manufacturer in Detroit,

Michigan and at NBS in Gaithersburg, Maryland. Case depth and

hardness determinations were made. Metallographic examinations were conducted to determine the effects of the hardening procedure

on case and core microstructures

.

Replenishment of salt pot additions necessary for continuous

salt bath operation was studied. Tests were conducted under the

direction of the NBS Air and Water Pollution Section to determine

whether toxic cyanide fumes were evolved during hardening.

3.1 Materials

3.1.1 Characteristics of Transfer Process Components

Dies - Dies are engraved in the first stage of the transfer processes.

A modified AISI 1045 steel is specified for transfer dies. The

dies are flat plates with dimensions of 0.25 inch x 4.00 inch x 4.50 inch. One surface of a die plate is polished. After

engraving on the polished surface, die plates are through

hardened with the intention of achieving a desired hardness of

80 HR30N on the polished, engraved surface with a tolerable

gradient to about 77 HR30N at a depth of 0.006 inch from this

surface

.

Rolls - In the second stage of the transfer process rolls are revolved over dies to "pick-up" impressions from the engraving

on the dies. Rolls are presently made from W2 tool steel

because AISI 1031, a preferred material, is currently

unavailable for this application. The rolls are 2.75 inch

in diameter by 1.125 inch in length, have a tapered drilled

hole at the center and a polished cylindrical surface. After

pick-up of impressions on the cylindrical surface, the rolls

are heat treated to obtain a case 0.004 inch to 0.006 inch

deep adjacent to the cylindrical surface, with a hardness of

80 HR30N on the roll surface.

- 3 -
Transfer Plates - In the third stage of the transfer process,

impressions on rolls are transferred (rolled) onto the

finely polished surfaces of transfer plates. Transfer plates

are presently made from AISI 1030 steel (AISI 1031 modified,

when available. Transfer plates start as flat pieces with

dimensions of 0.25 inch x 19.5 inch x 24.5 inch. After

transfer of impressions on the rolls to the polished surface

of the transfer plates, the plates are reduced to a thickness

of 0.180 inch by grinding the opposite surface of the plates.

Next, the plates are shaped into half cylinders of 6.5 inch

radius. Heat treatment follows to produce a hardness of

77 - 80 HR15N* on the impressed polished surface of the plates

and a case with a depth of 0.004 inch to 0.006 inch adjacent

to this surface.

3.1.2 Experimental Test Material

Tests were conducted on 0.25 inch x 1.0 inch x 1.0 inch specimens cut

from AISI 1031 and AISI 1045 steel transfer process plates, supplied

by the Office of Engraving of the Bu E^P. Tests were also conducted

on small wedge-shaped specimens cut from W2 tool steel rolls, also

supplied by the Office of Engraving. Each roll was cut to provide

10 specimens. Given below in section 3.1.2.1 are property data obtained

for specimens 1031-5, 1045-6 and W2-6. These data are considered to be

representative of that for all test specimens of a simlar composition.

3.1.2.1 Chemical Analysis of Test Material, As-received

The chemical composition of samples of as-received test material

as determined from analyses by spectrometric or combustion methods are given in Table 1.

Table 1. Chemical Analysis of Test Material, As-Received

Samples

Chromium

Molybdenum

Copper

Vanadium

Carbon

Manganese

Phosphorus

Sulfur

Silicon

Nickel

1031-5

0.33%

0.53

0.005

0.013

0.21

<0.05

<0.05

<0.05

<0.05

1045-6

0.46%

0.10

0.005

0.007

<0.05

0.05

0.08

<0.05

<0.05

W2-6

1.06%

0.31

0.006

0.008

0.25

0.05

0.10

<0.05

<0.05

0.18

* HR15N is the accepted ASTM notation for hardness on the Rockwell

15N scale.

- 4 -
3.1.2.2 Hardness of Test Material, As-Received

The hardness obtained on specimens of the as-received test

material are given in Table 2.

Table 2. Hardness of Test Material, as-received

Hardness of Specimens

Area 1051-1 1045-1 W2-1

Polished face 77 HRB* 77 HRB 64 HRB

Longitudinal edge 74 HRB 75 HRB

Transverse edge 74 HRB 74 HRB 85 HRB**

3.1.2.3 Micro structure of Test Material, As-Received

The microstructures of specimens 1031-1, 1045-1 and W2-1, as received, are shown in Figures la, lb and Ic, respectively, and

are described in the legends for these figures.

4. TEST PROCEDURES

4.1 Test Equipment

4.1.1 Equipment Used at Manufacturer's Plant

Specimens were heated for hardening at the manufacturer's

plant in a cast iron pot having an inside diameter of approximately

9 inches and a depth of approximately 12 inches. Electric power

served as a heat source. A thermocouple was inserted in the pot

during operation and temperatures were monitored on the laboratory

console. Salts in the pot were stirred by a 3.5 inch diameter

propeller driven by an air motor. Figure 2 is a schematic

representation of the salt pot and stirrer arrangement. After

hardening, specimens were tempered in a convection type tempering

furnace.

4.1.2 Equipment Used at NBS

At NBS specimens were heated for hardening in a cast iron pot,

6 inches in diameter by 12 inches deep; also using electric power

HRB is the accepted ASTM notation for hardness on the Rockwell B scale

.

Value is for a reading obtained at a depth of 0.25 inch normal to

the cylindrical polished surface of this W2 roll specimen. W2-1

contained a decarburized zone approximately 0.005 inch deep adjacent

to the cylindrical polished surface. In this zone, the hardness

ranged from HRB 42 at 0.002 inch to HRB 76 at 0.005 inch from the

polished surface. All W2 rolls supplied have a decarburized surface

zone

.

- 5 -
as a heat source. Temperature readings were obtained from a fixed

thermocouple inside the salt pot and monitored on the laboratory

console. Temperatures were also observed, for greater accuracy,

from potentiometer readings obtained from an auxiliary thermocouple

inserted in the pot during operation. Figure 3 is a view of

equipment used in the NBS case-hardening tests. A recirculating

air furnace was used in tempering.

4.2 Preparation of a Salt Bath

The following is the manufacturer's recommended procedure for

preparation of salt baths that was used in all hardening evaluation

tests.

a. Fill base salt, which is in powder form, to near the top of

the pot, weighing the amount added. Melt down at 1700F.

b. With the temperature still at 1700F add the NOCY additive,

which is in powder form, stirring until a foamy cover

is attained. Record the weight of additive supplied. Use

a wire test* to determine whether case hardening is attained.

More of the additive is required if hardening is not attained.

Drop the temperature to 1500F after hardening is achieved.

c. Add LOCYN pellets equal to 2 1/2 percent by weight of the

base salt additive mixture. Use a wire test to determine

whether case hardening is obtained. If the wire test is

negative, more of the pellets are required.

4.3 Experimental Hardening Procedure Variables

Initially, the standard treatments for hardening at Bu E§P

were followed quite closely with respect to temperature, holding

times and quenchants. Later, certain modifications in these factors were found to be desirable. A superficial hardness test on a polished surface was used as a control in determining whether a given procedure was suitable. When the hardness was found to be

unacceptable, the procedure was modified in the test that followed.

The condition of polished surfaces was closely observed and the

procedure was also modified if the appearance of a polished

surface after hardening was considered to be unsatisfactory. Criteria

resulting in modification were the presence of an adherent black

deposit and/or extensive staining on the polished surface of a specimen

after hardening. Sixteen AISI 1031, thirteen AISI 1045 and four W2

specimens were hardened in the course of the investigation. Hardening

procedures employed for selected specimens are described in the

descriptive information with Figures 6 through 23.

* The wire test involves the insertion of a 16 or 18 gage mild steel

(AISI 1010 or 1015) wire into the molten salts for one-half hour.

After quenching the heated end of the wire in water, case-hardening

is indicated if this end will break in a brittle manner before it can be bent back 180 degrees on itself.

- 6 -
4.4 Analysis of Exhaust Air from the New Hardening Process

A test for cyanide released into the exhaust air during hardening

was conducted by the NBS Air Pollution and Analysis Section. Details

of the procedure used in the analysis and results obtained are given

in an analytical report entitled "The Sampling and Analysis for

Cyanide in the Exhaust Air from a Steel Hardening Process," which

is included as an Appendix to this report. Equipment used in the

exhaust air analysis is shown in Figure 4.

5. RESULTS AND DISCUSSIONS

5.1 Chemical Composition of As-Received Material

The chemical composition of the AISI 1031 sample, which was analyzed, complied with the requirements in the Bu E^P Specification

for Steel Engraving Plates, dated December 17, 1970. The 0.10

manganese content of the AISI 1045 sample is lower than the 0.60 to

0.90 percent manganese content specified for this material. The

composition of the W2 sample is similar to that required for AISI W2.

The chemical analyses confirmed that the compositions of the materials

used in the NBS evaluation tests are the same as those used in Bu E^P

production runs.

5.2 Hardness Characteristics of Heat Treated Material

The hardness of polished surfaces and the hardness variations

observed in microhardness traverses conducted on specimens after

heat treatment are given in Table 3 for selected AISI 1031 specimens,

in Table 4 for selected AISI 1045 specimens and in Table 5 for

selected W2 specimens. The traverses were made on a Tukon hardness

tester using a Knoop indenter and a 500 gram load. Indentation

measurements were made using a 20 X objective. Rockwell HR15N or HR30N hardness values given are the approximate equivalent values

derived from Knoop hardness numbers (KHN) . Knoop hardness readings

were made at 0.001 inch intervals starting at 0.001 inch from the

polished surface and continuing into the thickness of each specimen.

Tables 3, 4 and 5 give only the hardnesses observed in cases to a maximum depth of 0.004 inch, which is considered to be sufficient in

evaluating the depth of hardening capability of the new process in

relation to Bu E^P requirements.

An acceptable polished surface hardness and acceptable case hardness and depth of case were sought in the experimental hardening

tests. The selected hardness test results given in Tables 3, 4 and

5 are considered to be representative of results attained in these

tests. Table 3 shows that the polished surface hardness of one

- 7 -
AISI 1031 specimen after hardening was considerably higher than

required, one was within the required hardness range and the rest

below. The surface hardnesses of the AISI 1045 specimens were all

lower than the required hardness. A high surface hardness was obtained on one W2 specimen while that of the others was lower than

required.

Most of the AISI 1031 specimens exhibited acceptable hardness

values in case areas immediately adjacent to polished surfaces, as shown in Table 3. However, the cases were shallow with hardness

generally decreasing rapidly to unacceptable values with increasing

distance from the surfaces. The hardnesses obtained in the cases of

the AISI 1045 specimens were all lower than that required. The cases

in the AISI 1045 specimens did not exhibit the marked hardness gradients

observed in the cases of the AISI 1031 specimens. Insufficient hardness

was obtained in the cases of the W2 specimens.

5.3 Condition of Polished Surfaces After Hardening

The surfaces of specimens hardened at the manufacturers

laboratory were clean after hardening. However, some of the

specimens hardened at NBS had a black, adherent coating after hardening.

The coatings observed on the polished surfaces of hardened specimens

1031-14 and 1045-12 are shown in Figures 5a and 5b, respectively. It was found that the coatings could be removed by immersion of the

specimens in a 10 percent solution of sulfamic acid (NH2SO3H) at

room temperature for from fifteen to thirty minutes, followed by

swabbing with a soft bristle brush and a clear water rinse. The

appearance of the polished surfaces of hardened specimens 1031-14

and 1045-12 after the sulfamic acid cleaning treatment are shown in

Figures 5c and 5d, respectively. The black, adherent coating was observed on some of the specimens with low surface hardness reported

in 5.2. It is uncertain as to whether the coating formed on specimens

while they were in the salt bath; or was picked -up by specimens from

a residual, substance on the surface of the bath, when specimens were being removed for quenching.

5.4 Case and Core Microstructures Obtained In Experimental Tests

Figures 6 through 22 contain photomicrographs showing the

structures of representative specimens hardened in the experimental

hardening tests. Hardening procedures and microstructures are described in the figure legends.

- 8 -
5.4.1 Observation of Microstructures of Hardened AISI 1031 Specimens

Fine grained cases varying in depth from 0.003 inch to 0.006

inch were observed adjacent to the polished surfaces of hardened

specimens. The cases were followed by a coarser structural transition zone and finally by the core which had the coarsest structure.

The cases of specimens 1031-2, 7, 8, 11 and 15, which were oil quenched

in hardening, contained tempered martensite with ferrite in the grain

boundaries and in the transition zones. The cores of these specimens

contained ferrite in a matrix of tempered martensite. The microstructures of the case, transition zone and core of specimen 1031-9,

Figure 10, which was brine-quenched in hardening, appeared to be

predominantly tempered martensite. The variation in austenitizing

temperatures of oil quenched specimens, ranging from 1550F to 1575F,

and the variation in holding time at temperatures ranging from 36 to

45 minutes, produced no apparent effect on case and core structures.

The case depths of oil quenched AISI 1031 specimens were apparently

unaffected by variation in hardening temperature or time at temperatures.

No increase in case depths through the use of fresh salt baths or salt replenishment was observed metal lographically.

5.4.2 Observation of Microstructures of Hardened AISI 1045 Specimens

Specimens were austenitized in the salt bath at temperatures

ranging from 1480F to 1550F and generally held in the bath for

30 minutes. All of the AISI 1045 specimens were quenched in brine.

Tempered martensite predominated in the three structural zones of

these specimens. Troostite was observed at high magnification in

grain boundaries in the cores of all hardened 1045 specimens. Case

depths, observed metallographically , ranged from 0.003 inch to

0.004 inch. Variations in the austenitizing temperature or salt

bath replenishment had little effect on case depths.

5.4.3 Observation of Microstructures of Hardened W2 Specimens

W2 specimens were austenitized in the salt bath at temperatures

ranging from 1500F to 1575F and held in the bath for 60 minutes.

All of the W2 specimens were quenched in brine. All exhibited a five-grained structural transition zone between case and core. The cases contained carbides in a ferrite matrix while cores were essentially tempered martensite. The structures in transition

zones were mixed, containing some ferrite and carbide particles

and tempered martensite. Case depths, observed metallographically,

ranged from 0.003 to 0.006 inch. No apparent affect on case depths

from salt bath replenishment was observed.

- 9 -
6. CONCLUSIONS

The two principal criteria in judging the suitability of a process to be substituted for the cyanide hardening process currently

in use are considered to be (a) freedom from cyanide toxicity and

(b) capability of obtaining acceptable component case hardness and case depth.

With regard to cyanide toxicity, the analyses described in the

appendix were conducted on samples of the base salt, additive

and pellet ingredients used in the process. Analyses were conducted

on samples of quench water. The air above the salt pot was analyzed

for cyanide emissions from the molten salts in the pot during

operation of a hardening test. This air which is vented from the

salt pot during operation of hardening tests is hereinafter referred

to as exhaust air.

As reported in the appendix, no cyanide was found in the base

salt or in the additive. Cyanide, 0.004 percent by weight, found in

a pellet sample, can be considered to be a trace contaminant.

Quench water analyzed after the hardening tests was found to

contain 3.4 mg. of cyanide in 2500 ml. of quench water. This

relatively high concentration of cyanide in the quench water was evidently derived from molten salts adhering to specimens that

were quenched. Avoidance of physical contact with the quenchant

would be desirable. The dangerous cyanide radical in the

quenchant can be rendered harmless by the controlled addition of

chlorine or sodium hypochlorite into the quenchant, using suitable

precautions [3]

.

Cyanide, 0.17 parts per million, was found in the exhaust air

in one of the four sampling periods during the hardening test

described in the appendix. This amount is less than 2 percent

of the OSHA permissible concentration level for HCN in an eight

hour day. The appendix speculates that no cyanide is emitted into

the exhaust air in gaseous form, but that the cyanide is most likely

in a particulate state.

It is further speculated that cyanide in the concentrations

reported in the Appendix, may be derived from a chemical reaction

of the ingredient salts during hardening.

The new process exhibited a hardening capability for all

three materials of interest, as demonstrated by comparing hardnesses

in Table 2 with hardnesses in Tables 3 through 5. Hardness tests

and microstructural examination revealed the formation of cases adjacent

to the polished surfaces of specimens. It seems that the case

- 10 -
hardnesses observed stem from two interrelated factors: (1) 'iTie fundamental hardness increase resulting from quenching from

austenit izing temperatures; (2) 'Hie diffusion of carbon and

nitrogen from the hardening salts. 'Die diffusioii of carbon into

specimens is indicated by the increased martensite content in case

areas. It is assumed, because of the nitrogen content of the salts,

that there was also some diffusion of nitrogen into the cases although

there was little micro structural evidence of this. The hardness

obtained in a case is significantly greater in carbonitriding where

nitrogen is added, than when the same steel is carburized only [4]

.

Oil quenching resulted in the presence of ferrite in the grain

boundaries of the AISI 1031 specimens. ITie ferrite evidently

contributed to the low case hardness. Too slow a cooling rate,

evidenced__by troostite in core grain boundaries, may account for

the generally low case hardness of the AISI 1045 specimens. Low case hardness of W2 specimens may be attributed to an insufficient

diffusion of carbon and possibly nitrogen into the decarburized

zones of these specimens. As stated earlier in this report W2

rolls, as received, contain a decarburized zone adjacent to their

cylindrical surface.

The holding time and temperatures levels employed for case hardening, in the currently used cyanide process at Bu E^P, suffice

to provide adequate case depths and hardness. However, in applying

these levels in experimental tests of the new process, shallower

cases and lower case hardnesses were obtained. It may be that the

ingredients used in the new process do not respond, at these levels,

to provide an adequate supply of carbon and nitrogen for a sufficient

case. Case compositions and depth could possibly have been

improved by longer times at temperature and/or higher temperatures

plus added agitation. However, higher temperatures or longer holding

times in these tests could have led to serious grain growth or other

microstructural changes deleterious to the mechanical properties of

the hardened steels.

From observations made in this investigation, the new process

appears to have some potential as a substitute for currently-used

cyaniding processes. It was demonstrated that the process has a case-hardening capability and that cyanide toxicity is not a matter

of great concern. The question as to the nitriding efficacy of the

process in case-hardening merits further investigation and probably

further development.

7. RECOMMENDATIONS

Evaluation tests at NBS were made in a small salt pot, with

small specimens and with perhaps inadequate bath agitation.

Further evaluation tests might involve larger salt pots, more sophisticated bath agitation and test material approaching the

size of the material case hardened at Bu E^P.

- 11 -
ACKNOWLEDGEMENT

The author wishes to express his appreciation for the helpful

information supplied during the investigation by Walter Olson and

Paul Krasley of the Office of Engraving of the Bureau of Engraving

and Printing. Thanks are due to Dr. Bruce Christ, Acting Chief

of the NBS Mechanical Properties Section, for helpful suggestions.

Leonard C. Smith of the Mechanical Properties Section obtained the

hardness data and prepared the metal lographic illustrations.

- 12 -
REFERENCES

[1] LOCYN Process , Park Chemical Co., 8074 Military Ave., Detroit,

Michigan 48204, developer and manufacturer.

[2a through 2e] ASM Metals Handbook, Vol. 2, 8th Edition, 1964,

American Society for Metals, Metals Park, Ohio 44073.

[2a] Gas Carburizing , p. 93.

[2b] Pack Carburizing, p. 115.

[2c] Gaseous Atmosphere Carbonitriding, p. 119.

[2d] Induction Hardening, p. 167.

[2e] Flame Hardening, p. 189.

[3] Cyanide , Wastewater Control and Treatment, A. Edward Ward,

Electroplating Engineering Handbook, 3rd Edition, A. Kenneth Graham,

Editor, Van Nostrand Reinhold Co., publisher. New York, N.Y.,

1971, p. 357.

[4] Hardenability of Case , ASM Metals Handbook, Vol. 2, 8th Edition,

p. 121.

- 13 -

Table 3. Hardness Characteristics of

Selected AISI 1031 Specimens

After Heat Treatment

Polished Surface

Hardness (1)

Characteristics of Case

Specimen No. Average HR15N HR15N Range to Depth of

1031-2 (3) 79. 5 83 - 80.5

jl031-3 (3) 73 79. 5 - 76

!l031-7 74 - 75.5 80. 5 - 75

1031-8 71. 5 78 76.5

1031-9 86 85 86

1031-11 68 91 90 R15T

1031-15 68 78 . 5 - 74

(1) 77-80 HR15N required.

(2) 77-80 HR15N required to depth of 0.004 inch, minimum.

(3) Hardened at manufacturer's laboratory.
Table 4. Hardness Characteristics of

Selected AISI 1045 Specimens

After Heat Treatment

Specimen No

1045-2 (3)

1045-3 (3)

1045-7

1045-8

1045-12

1045-14

1045-15

Polished Surface

Hardness (1)

Average HR3 0N

75.5

76

60

69.5

75

76

75.5

Characteristics of Case

Adjacent To Polished Surface (2)

HR30N Range to Depth of 0.004 incf"

75.5 - 75

74

67 - 72

75 - 71.5

74.5 - 73

75 - 73.5

75 - 74

(1) 8 0 HR30N required.

(2) 80 HR30N required to depth of 0.004 inch, minimum.

Material to be through hardened.

(3) Hardened at manufacturer's laboratory.
ipimen No

I (3)

^

(3)

5

Table 5. Hardness Characteristics of

Selected W2 Specimens After

Heat Treatment

Polished Surface

Hardness (1)

Average HR3 0N

75

74.5

87

78

Characteristics of Case

Adjacent To Polished Surface (2)

HR30N Range to Depth of 0.004 inch

F4.5 - 6f

74

71.5 - 71

66.5 - 68 .5

80 HR30N required.

80 HR30N required to depth of 0.004 inch, minimum,

Hardened at manufacturer's laboratory.


Figure 1. Nicrostructures in Transfer Process Test

Material Specimens, as-received. Etched

with 4 percent nital. X 200

la. Specimen 1031-1. Partially spheroidized

carbides in a ferrite (white) matrix. ASTM

grain size, 8-9. No decarburization is present

adjacent to polished surface.

lb. Specimen 1045-1. Lamellar pearlite (dark)

in a ferrite (white) matrix. ASTN grain size,

7. No decarburization is present adjacent to

polished surface.

Ic. Specimen W2-1. Decarburized zone (white

ferrite grains) adjacent to polished surface.

Mixture of lamellar pearlite and ferrite in

transitional zone between decarburized zone

and core. Core structure consists of pearlite

and ferrite.
Figure 1.

Figure 2. Schematic representation of salt pot and stirrer

arrangement for case-hardening tests at

manufacturer ' s laboratory

.

Figure 3. Salt pot arrangement for experimental casehardening tests at NBS.

a. Air motor

b. Stirrer drive shaft

c. Conduit for auxiliary thermocouple lead wires

d. Quench tank

u

tdH

(d

U

•H -p

rO

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w

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w o 0

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c d

Figure 5. Appearance of polished surfaces of hardened

specimens

.

a. Coating on specimen 1031-14 before cleaning

b. Coating on specimen 1045-12 before cleaning

c. 1031-14 after cleaning

d. 1045-12 after cleaning


Figures 6 through 23

Microstructure and Properties of selected AISI 1031, AISI 1045!

and W-2 Specimens.

Structures and hardness properties obtained from specific

treatments and salt bath conditions are described.

Each figure shows microstructures revealed by a four percent

picral etchant:

a. Case and core microstructure at 2^0 X

b. Microstructure of case (insert area in a) at 1000 X
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(U-«$) APPENDIX

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

WASHINGTON. D.C. 20234

REPORT OF ANALYSIS

Requested by I. J. Feinberg

Div. Req. No. 5125 Project No. 3120541

Section 312.01

The work performed is described in Analytical Report "The

Sampling and Analysis for Cyanide in the Exhaust Air from a Ste^l -Hardening Process." A copy of which is attached.

John K. Taylor

Chief

Air and Water Pollution

Analysis Section

Analytical Chemistry Division

J^mds I. Shultz

ir/ice Analysis Coordinator

Analytical Chemistry Division

November 15, 1974

75-43

ANALYTICAL REPORT

The Sampling and Analysis for Cyanide in the

Exhaust Air from a Steel -Hardening Process

Barry C. Cadoff and John K. Taylor

Air Pollution Analysis Section

Analytical Chemistry Division

Requested by

Irving J. Feinberg

Mechanical Properties Section

Metallurgy Division

The Sampling and Analysis for Cyanide in the

Exhaust Air from a Steel -Hardening Process

This report describes the collection and analysis for

cyanide of air samples taken from the exhaust air of a steelhardening process. The sample collection was performed on

October 3, 1974.

Sampling Procedure and Equipment

The steel -hardening process under investigation

involves the use of a proprietary molten salt mixture, contained

in a salt pot, in which is immersed the steel item that is

to be hardened. Positioned above the salt pot is an exhaust

hood and duct that vents gases or particulates produced during

the process.

Air samples were collected by inserting a sampling tube

into a hole drilled in the duct. The sampling tubes contained

4 g of Ascarite, 20-30 mesh, a formulation consisting of

sodium hydroxide on asbestos. This material traps acidic

gases including any HCN which might be formed during the process.

The sampling tube was connected to a vacuum sampling system

capable of maintaining a constant flow rate. (This system

has been described in NBSIR 74-527, "Development of Solid

State Samplers for Work Atmospheres", B. Greifer, B. C. Cadoff,

J. Wing and J. K. Taylor", June 1974.) The flow was adjusted

to 1.0 liter/min using a calibrated rotameter. The sampling

times were measured with a stopwatch.
Sample Collection

The use of a high-capacity sampling tube and a constantflow sampling system made it feasible to use a single sampling

tube for periods of one or more hours, without having to

I periodically check the flow rate. A total of four sampling

tubes were used to monitor the steel-hardening process.

Descriptions of the process are presented along with data on

the duration of sampling.

Air Sample #1

The salt pot was charged with the commercial product

No-Cy Base Salt and the heating was begun at 8:45 A.M. Sampling

was started at 9:45 A.M. when the pot temperature had reached

500°F. At 11:10 the salt bath was molten and more No-Cy Base

Salt was added. Stirring of the salt bath was begun at 11:30 A.M

Sampling was stopped at 11:46 A.M.

Total sampling time: 121 min. , 10 sec.

! Air Sample #2 i Sampling was started at 12:13 P.M. (temp. 1550°F) at which

time the No-Cy Additive (black powdery material) was added.

Sampling was stopped at 12:28 P.M.

Total sampling time: 15 min.

Air Sample #3

Sampling was begun at 12:30 P.M., at which time the Lo-Cy N

Pellets were added. At 1:35 P.M. two steel samples designated

1031-15 and 1031-16 were placed in the molten salt to test its

2
steel-hardening characteristics. Air sampling was stopped at

1:51 P.M.

Total sampling time: 81 min. , 5 sec.

Air Sample #4

Sampling was started at 1:54 P.M. Steel samples 1031-15

and 1031-16 were removed from the salt pot at 2:15 P.M. and

quenched in a bath containing 2500 ml distilled water. At 2:24

P.M. steel samples 1031-17 and 1031-18 were dipped in the salt

pot and were removed at 3:04 P.M. and quenched in the water

bath. Air sampling was stopped at 3:30 P.M.

Total sampling time: 96 min., 10 sec.

Water Sample "B"

A sample of the quenching water, Water Sample "B", was

taken after the four steel samples had been quenched.

Water Sample "A"

Before any of the steel samples had been quenched, a sample of the quenching water was taken and labeled, Water

Sample "A".

Analysis and Results

To analyze for cyanide, the cyanide ion-selective electrode

was used. For the calibration of this procedure a series of

standard cyanide solutions, covering the range from 10 ^ to

_ 3 10 M were prepared. The contents of each sampling tube was

extracted with hot water, filtered and diluted to exactly 100 ml.

The ion-selective electrode was then used to determine the

quantity of cyanide. A blank sampling tube was also analyzed.

3
Water samples "A" and "B" were prepared for analysis by adding

15 ml of the water sample to 15 ml of 1 M NaOH. The analytical

technique for cyanide in the water samples was the same as for

the air samples.

No cyanide was found in air samples #1, 2 and 4 and Water

Sample "A". Air Sample #3 gave a solution containing 5.5 x 10'^

cyanide. This is equivalent to a concentration in the sampled

air of 0.18 yg/1 (0.17 ppm) cyanide. Water Sample "B" gave a solution containing 2.6 x 10 ^ M cyanide which is equivalent to

a total of 3.4 mg cyanide in the 2500 ml quenching water.

Other Observations

The ingredients that comprise the steel -hardening formulation were briefly examined.

No-Cy Base Salt

Two g of No-Cy Base Salt was dissolved in 50 ml water, to

give a solution with a pH = 12.4. This solution did not

contain cyanide.

No-Cy Additive

One g of No-Cy Additive was added to a beaker containing

50 ml water. A black powdery material floated to the top of

the water and a white crystalline material sank to the bottom

of the beaker. The white material dissolved upon heating.

Solution contained no cyanide and had pH = 11.3.

Lo-Cy N Pellets

One pellet (1.80 g) was dissolved in 50 ml water, ph = 11.9

Fifteen ml of this solution was added to 15 ml of 1 M NaOH and

4
this solution contained 2.7 x 10 M cyanide. This is equivalent

to 0.004 percent cyanide, by weight, in the pellet.

Conclusions

Throughout most of the steel -hardening process no cyanide

is emitted into the exhaust air. In the case of air sample #3

a small amount of cyanide was found; this level of cyanide,

0.17 ppm (averaged over the entire sampling period) is less than

2 percent of the OSHA permissible concentration level for HCN

in an eight hour working day. Most likely the cyanide is nnt

in the gaseous HCN form, since the salt mixture is strongly

basic and HCN would not be expected to be formed, but rather

the cyanide may be in the particulate state and may have been

formed and emitted during the addition of the Lo-Cy N pellets,

at which time splattering was observed. This speculation is

supported by the fact that a relatively large amount of cyanide

was found in the quench water, due to transfer of some of the

molten salt mixture to the water during quenching. The cyanide

found in the Lo-Cy N Pellet, 0.004 percent by weight, can be

considered as a trace contaminant.

5
i

ii
I

||4A (REV. 7-73)

KS. DEPT. OF COMM.

BLIOGRAPHIC DATA

SHEET

1. PL'BI ICATION OR RFPOUT MO

NBSIR 76-1068

2. Ciov*t Access !on No.3 . Recipient's Ac cc ss 1 on No

.

IFLE AND SUBTITLE

lALUATION OF A NEW HARDENING PROCESS FOR STEEL TRANSFER

fTERIALS

5. Publ ication Date May 1976

6* F'^erformin^ Or^ani^.iition C.c)cie

Whorisi

J. Feinberg

8. Performing Or^an. lU'porc No. NBSIR 76-1068

"R FOR MIN'f", ORGANIZATION NJAMT AND ADHR F^<? NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projc-ct/Task/Work Unit No. 3120541

11. Contract/Grant No.

won ponbUiiiig c /\r 1 n a C\r o an i "7 3 t i r\r\ a m a n iH I /*» m \ ^ r n /A /A r c c /"^ t i t\t ^ t a t ^ 71 \ v^i ^ till a L I (ji 1 i>i diiitr diiu ^uuipicic *»(j(jrt:as (•Jircci, ^\\y , ^loiG, ) rfice of Engraving

ireau of Engraving and Printing

ishington, D.C.

13. Type of Report & Period

Covered

Failure Analysis Report

14. Sponsoring Agency Code

UPPLEMENTARY NOTES

ABSTRACT {A 200-word or less factual summary of most significant information. If document includes a significant

nbliography or literature survey, mention it here.)

A new molten salt hardening process has been evaluated for hardening steels used

I the three stage transfer process for printing stamps. Evaluation of the new

'ocess was undertaken because virtually all cyanide salts are eliminated as igredients in the process. It was found that complete elimination of cyanide

nissions is not achieved in the new process, however, the level of cyanide emitted

)es meet OSHA requirements. A case-hardening capability was exhibited in experimental

3sts on Bureau of Engraving and Printing materials, but case depths and hardness did

3t meet Bureau of Engraving and Printing requirements. Further development of the

ew process is needed before it can be considered for the Bureau of Engraving and

rinting.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

name; separated by semicolons)

Carburizing; case; cyanide; emissions; hardenability; nitriding;

substitution; toxicity

AVAILABILITY {^2 Unlimited

fjT f^or Official Distribution. Do Not Release to NTIS

I

^ Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

I

^ Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

22. Price

USCOMM.DC 29042-P74