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AMERICAN INSTITUTE O F MINING A N D METALLURGICAL ENGINEER8

Technical Publication No 766 (CLAYS C IRON AND STEEL DIVISION, NO 170)

DISCUSSION O F THIS PAPER IS INVITED D~scusslon In w1.1tlng mny be sent to the S e ~ r e - tary Amerlcan I n s t ~ t u t e of M ~ n l n g and Meta l lu~~g~cal Cng~neers 29 West 39th Street New York N Y Unless spec~al arrangement IS made d ~ s c u s s ~ o n of thls paper w ~ l l close . \pr~l 1937 Any dlscuss~on offered thereafter sl~ould prefer rbly be In the form of 1 new paper

Varlants Influencing Austenite Gram Slze as Determined by Standards Methods

(New P o l k M e e t ~ n g , February, 1936)

DURING thc past fear years, general ~nterest in the stcel-producing and stecl-consum~ng industrres has bcen centered on the so-called "inherent cl~aract,cr~stics" of stccls Wh~ le often vagucly described, these char- a c t e r~s t~cs arc known to ~nfluence the rcsponsc to heat-treatmcnt and the hardening character~strcs of the matcrial

Although most of the recent papers and discussions havc assoc~atcd the "inherent charactcr~st~cs" w ~ t h the austcnitic gram slze and empha- sized the importance of i t , con~parat~vcly little IS known of the variables that may affect the s ~ z c of the austeil~tc gram

The work to be dcscribed in t h ~ s paper was carr~ccl out during the coursc of a study 011 thc ~nhcrent characterist~cs of tool steel containing one pcr cent carbon The discrepancics encountered 111 the dc tc rm~nat~on and clnss~fication of the aus te r l~ t~c grain size lcd to an invest~gation of some of the variants ~nfluerlcing thc auste~litic gram s ~ z c as dctermincd by standard methods

The present methods used for thc determinat~on of the austen~tic grain srze may briefly be classrficd in two groups

1 E t ch~ng a t room temperature to re\ eal the austenrte gralil size priol to cool~ng

2 Etching a t clevated temperatures, cool~ng to roonl tcmpcraturc, and observ~ng the structural c o u d ~ t ~ o n s that ev~sted a t the temperature from which cool~ng occurred

Thc outs tand~ng mcthods of the first group alc (1) thc McQuaid-Ehn test, (2) optrmum ratc of coollng, (3) clucnching and etching

McQuazd-Ehn Test -Thrs cstabl~shcd and acccptcd test docs not necd much claborat~on It cons~sts of pack carburlzing a t a dcfinite temperature for a standard pcriod of time and slonly cooling in thc furnace The tcmperature IS 1700° F (925' C ) and thc t ~ m e is 8 hr The inter- pretat~on and evaluatron of t h ~ s test are fairly wcll defined in the A S T M

Manuscript recclved a t the officc of the Institute Dcc 2,1935, ~evised Fch 18,1936 * Mctallurglst, IIalcoilih Steel Co , Syra~use, N Y t Ass~starlt Mctallurglst, Halconib Steel Co

1

Copyright, 1937, by the Amerlcan I n s t ~ t u t e of M ~ n ~ n g and Metal lurp~~al Eng~neers, I n c P r ~ n t e d In U S A

METALS TELHNOLOGI January, 1937

2 VARIANTS INFLUENCING AUSTENITE GRAIN SIZE

standardization procedure and standard grain-size chart for the classification of steels, A.S.T.M. Designation E 1933, and does notlwarrant further discussion a t this time.

Optimum Rate of Cooling.-This method of determining the austenite grain size necessitates a careful regulation of the cooling rate and should only be resorted to on hypoeutectoid and hypereutectoid steels; that is, on materials that possess an excess constituent, either ferrite or cementite.

Davenport and Bainl describe this method of optimum cooling as follows:

Distinctly Hypereutectoid Steels: In most cases, particularly with plain carbon steels, i t suffices merely to cool the specimen in air after heating substantially according to the schedule contemplated or observed in the commercial operation under consideration. The specimen, which may vary from perhaps a quarter-inch section to a small billet, may be cooled a t any moderately slow rat.e, and about each austenite grain a continuous envelope of carbide will be formed which offers sufficient contrast to the pearlite to make grain size estimation quite feasible. .

If the steel contains considerable alloy or if it is only slightly hypereutectoid, it may be necessary to transfer the specimen, after heating and prior to final cooling, t o another furnace or bath a t a temperature just about A, for several minutes to insure the formation of the intergranular carbide network. The ideal cooling rate is then that which will form a black-etching pearlite of finest lamellae; in the larger sections, an oil quench is often effective. In some alloy steels martensite may form, in which case contrast between network and ground mass may be developed either by tempering and etching with alcoholic nitric acid or by etching in boiling sodium picrate solution.

Moderately Hypoeutectoid Steels: In many cases in which the volume of proeutectoid ferrite is inherently comparatively small the same general procedure as for hypereutectoid steels may be effectively employed. The network marking out the austenite grain will, in this case, be ferrite and the etching technique is appropriately selected. Likewise if the steel is only slightly hypoeutectoid a brief halt in the cooling a t a temperature just above A3 will aid in forming a more nearly continuous network of contrasting ferrite. When the carbon content of the steel is much lower with respect to the eutectoid composition good results may usually be secured by halting the cooling a t a temperature just below A3 and then cooling rather more rapidly than for eutectoid composition, e.g., by an oil quench; the network is thus rendered more narrow and clean cut.

Lowcarbon Steels: A moderately small sample is preferable; it is cooled from an appropriate heating temperature to somewhat below the A3 temperature where it is held for a few minutes to establish a ferrite network. I t is then quenched as rapidly as possible. If not too low in carbon a subsequent tempering will produce a dark etching ground mass, particularly if the boiling sodium picrate solution is used as an etching agent. I n some cases, when the carbon content is very low, only the texture of the etched surface serves to mark out the network and considerable patience and experience may be required to estimate the grain size.

Eutectoid Steels: With no proeutectoid constituent to form in the grain-boundaries of the austenite, the grain is often obliterated with the transformation, either t o pearlite or to martensite, or its tempered products. However, one characteristic of the transformation to fine pearlite serves to mark out the grain boundaries with

E. S. Davenport and E. S. Bain: General Relation Between Grain Size and Hard- enability and the Normality of Steels. Trans. Amer. Soc. Metals (1934) 20, 879.

R. SCHEMPP .AND C. L. SHAPZRO 3

remarkable clarity; the transformation invariably begins in the grain boundaries a t a number of points acting as nuclei. Accordingly, if the steel is cooled a t a rate just less than the critical quenching speed, the microscopic specimen shows clearly where the boundaries of the parent austenite lay. For any eutectoid carbon steel, a specimen of suitable size may be chosen for quenching so that a t some point in its section the cooling must have been a t the optimum rate. In rare cases of rather high alloy steels this type of demarcation of the grains may have to be secured by cooling the specimens quickly into a lead bath for a determined interval a t a temperature of some 100 to 200 degrees Fahrenheit (50 to 100' C.) below the Al to permit the desired degree of transformation to fine pearlite to occur, after which the specimen is quenched into water to insure that the remainder of the specimen becomes martensitic.

Quenching and Etching.-The procedure of determining the austenitic grain by quenching and etching is really but a modification of the preced- ing method. Dr. Grossmann and Miss Baeyertz were among the first to bring attention to the fact that the original austenitic grain boundaries may be observed in hardened steel after suitable etching. The procedure consists of quenching and etching in a manner by which the etching reagent will either stain or dissolve the austenitic grain boundaries that existed prior to cooling.

As most of the known methods of determining the austenitic grain size by this means did not give entirely satisfactory results when working with heat-treated high-carbon tool steels, the procedure developed to bring out the parent austenitic grain in the material under consideration consisted of etching in a solution of 1 per cent nitric acid and 1 per cent picric acid from 2 to 5 min., washing in alcohol and drying. To keep the contrast between grain and boundary a t a maximum, it was found desirable to wash in alcohol instead of water. Another method, 'of which the results were very consistent and satisfactory, was to etch for 10 to 20 sec. in a 10 per cent solution of nital (15 C.C. of nitric acid per 100 C.C. of ethyl alcohol), washing in alcohol and drying. This method seemed best in bringing out the austenitic grain in tempered material.

The differential attack of the etching reagents, which developed the required contrast for the examination of the austenitic grain, is illustrated in Figs. 1 to 4. Fig. 5 reveals an enormous austenitic grain (delineated by the cementite boundary) which, when examined a t a low magnification, creates the impression of being an agglomeration of a number of small grains.

Etching at Elevated Temperatures.-The second classification of determining the austenitic grain size of steels involves the etching of polished specimens a t elevated temperatures, using a suitable high-melting-point chloride compound as the etching reagent. The polished specimen is immersed in the molten compound (cadmium chloride) a t the desired temperature. After heating for a sufficient length of time, usually 5 to 30 min., this specimen is then water-quenched and washed in alcohol to remove the decomposition product caused by the etching attack. A


USTENITIC GRAIN SIZE IS S T E E L QUENCHED FROM 1700' F. (925 Etched 3 minutes in 1 per cent, nital-picric acid solution.

USTENITIC G R A I N DELINEATED I N STEEL ANNEACED FROM 1700' Etched 5 minutes in 1 per cent nital-picric acid solution.

.USTENITIC GRAIN IN STEEL QUENCHED FROM 1700' F. (925' C Etched 5 minutes in 10 per cent nital. All X 1000.

C.) I N OIL.

'. (925" C.).

IN WATER.

R. SCIlEMPP AND C. L. SHAPIRO 5

FIG. 4 - - A U ~ T E N I T I C GRAIN OUTLINED R Y C E h l E V T I T I C NETWORK I N STEEI , NORhfAT,IZED FROM 1700" F. (925" C.). X 1000.

Etched 1 minute in 1 per cent nital-p~cric acid. FIG. 5 . -~RUCTURAL VARIATIONS I N VERY COAR& SINGLE GRAIN. X 1000.

Etched 1 minute in 1 per cent nital-picric acid.


slight repolishing, which serves to intensify the contrast, then renders the specimen ready for microscopical examination.

The typical austenitic grain-boundary condition that may be obtained in this manner a t 1600' F. (870' C.) is shown in Fig. 6.

A careful study and comparison of the austenitic grain size as obtained by the various methods of determination showed the final results t o be in

FIG. 6.-AUSTENITIC GRAIN ~ I T ~ L I N E D B Y ETCHING IN CdC12 AT 1700' F. (925' C.). x 100.

Grain bounclaries in relief (white); grain depressed (black).

fairly close agreement, with the exception of the McQuaid-Ehn test grain-size ratings, which seemed to be markedly influenced by certain variables.

Material.-The material that was used in our efforts to develop the variables that may influence the grain-size ratings as interpreted by the McQuaid-Ehn test method was straight 1.00/1.10 per cent carbon basic electric tool steel. The compositioils of the heats most frequently used in this work are given in Table 1.

The so-called " inherent characteristics" of these heats as determined by the now fairly well

established P / F test, as developed by shepherd and described in his various papers, and the slab-hardenability test, together with the McQuaid-Ehn grain sizes of these heats, as determined in accordance with the mill's practice on hot-rolled billet material, are contained in Table 2.

TABLE 1.-Chemical Composition of Material

INFLUENCE O F PRIOR STRUCTURE ON ~\~IC&UAID-EHN TESTS

To determine the possible effect of various prior structural conditions upon the grain-size rating after the standard McQuaid-Ehn carburizing

Heat

A B C D E F

Residual Oxides

--

0.025 0.021 0.021 0.038 0.028 0.034

Compositiod, Per Cent

C

1.06 1.07 1.04 1.04 1.06 1.06

S

0.021 0.014 0.013 0.014 0.016 0.015

Mn

0.23 0.24 0.18 0.21 0.27 0.23

Si

0.21 0.23 0.24 0.22 0.25 0.22

P -----

0.015 0.010 0.008 0.007 0.013 0'.010

Ni --

0.10 0.09 0.10 0.09 0.09 0.09

Cr

0.05 0.03 0.03 0.04 0.04 0.06

A1203

0.005 0.007 0.006 0.008 0.006 0.008

Si00

0.004 0.007 0.005 0.004 0.005 0.005

R. SCHEMPP AND C. L. SHAPIRO 7

treatment, representative disks (2 in. round by % in. thick) and bars (10 in. long by 1 in. in diameter) of the various heats were heat-treated so

a Size of specimen: 3-in. long by y/, in. in diameter.

TABLE 2.-Characteristics of the Var ious Xteels

as to range from a completely spheroidized condition to a martensitic structure. The heat-treatments, together with the average hardnesses of the specimens before and after carburizing are recorded in Table 3.

Penetration (Case Depth), $64 In."

FIG. 7. FIG. 8. FIGS. 7-8.-EFFECT O F PRIOR STRUCTURE ON AUSTENITIC GRAIN SIZE.

Fig. 7. Brine-quenched and carburized. Fig. 8. Water-quenched and carburized.

Both etched in 1 per cent nital-picric acid solution. X 100.

The grain-size conditions after carburization-that is, 8 hr. a t 1700" F. (925" (3.)-are illustrated in Figs. 7 to 12.

Table 3 shows that although the hardness values of the specimens before carburizing are different, they are practically constant after the McQuaid-Ehn treatment.

Fracture Grain Size, No. (Shepherd

Standard Values)

Heat

A B C D E F

Hardenability, Core Depth, In.

McQuaid- Ehn Teat, Grain Size

% %

% % %

I Quenching Temperatures, Deg. F.

I Oil-treated 0,300.in. Disk

7J50.109-0.1150.093-0.093 8N7XO.094-0.0940.085-0.085

8~0.140-0 .1400.140-0 .140 0.040-0.040

No core

1450

6% 6 x 7 6 5 x 6 7 8

Oi,-treated and 0.343-in. Disk

0.109-0.1090.125-0.125

No core No core

1500 -----

7

635

8 9%

1600

11

835 9 9

16

1550

8 8 7% 7 8%

13

1450

934 9 % 9 % 9

9%

9% 9%

1500

834

9j4 9 % 9 % 9

9 9

1550 ----

8%

8%

9 8

1600

8%

8 7%

8 VARTANTS INFLUENCING AUSTENITE GRAIN SIZE

The inarlced influence of the structural conclitiol~, prior to carburization, upon the grain size of steel A is suminarized in Fig. 13.

FIGS. ~ - ~ ~ . - - E ; I ~ . P E c T O F PILIOR STRUCTT:RE OY ~ \ U S . ~ E i \ l l ' r l C G l t A l X SIZE.

Fig. 9. Oil-qt~enched anti carhurizccl. Fig. 10. Air-cooled and carbi~riaed.

Fig. 11. Furnace-cooled and carburized. Fig. 12. Sphcroidized (quenched and tempered) and cnrburiaed.

All et.chec1 in 1 per cent nital-picrir ncicl solubion. X 100.

It will be noted that a change from a martensitic to a fully spheroidized prior struc,ture effects a coilsiderable enlargeinel~t of the austenitic grain size after the standard carburizing treatment, generally, although it was observed that the austenitic grain size gradually increased from the prior martensitic to the spheroidized condition.

R.

SC

HE

MP

P A

ND

C

. L

. S

HA

PIR

O


The inherent characteristics of heats, which so markedly influence the general response of the material to heat-treatment, were also investi-

TABLE 3.-Effect oj Prior Structure and Carburizing on Hardness

I 1 Hardness, Rockwell C

Brine quenched. . . . . . . . . . . . . . . . . . . . . Water quenched . . . . . . . . . . . . . . . . . . . . Oil quenched.. . . . . . . . . . . . . . . . . . . . . . Air cooled.. . . . . . . . . . . . . . . . . . . . . . . . Furnace cooled.. . . . . . . . . . . . . . . . . . . . Quenched, tempered. . . . . . . . . . . . . . . .

Heat-treatment at 1600'' F.

Martensitic Martensitic Troostitic Sorbitic Pearlitic Spheroidized

Structure 1 Before 1 Aligr Carburizing Carburrzing

gated as to their effect upon the austenite grain size after the standard carburizing treatment. While these inherent characteristics do have some bearing on this matter, all of the results obtained indicate that the

FIGS. EFFEC EFFECT O F INHERENT CHARACTERISTICS ON AUSTENITIC GRAIN SIZE. Fig. 14. Steel C carburized 8 hr. at 1700" F. Fig. 15. Steel B carburized 8 hr. a t 1700" I?.

Both etched in 1 per cent nital-picric acid solution. X 100.

influence of prior structural conditions tend to overshadow the possible effects of such inherent characteristics. Figs. 14 and 15 are indicative of this, because both steels shown, while different in their inherent characteristics, respond more in accordance with their prior structural condition than with their inherent characteristics. (The inherent characteristics of these two steels, as measured by t,he Shepherd2 test

2 B. F. Shepherd: The P/F Characteristics of Steel. Trans. Amer. Soc. Metals (1924) 22, 979.


ofhardenability and penetration fracture grain size (P/F), are given in Table 4.)

Since the structural condition prior to carburizing yielded such marked variations in the austenitic grain size as obtained by the McQuaid-Ehn test, an effort was made to determine the possible effects of hot and cold deformation. The former (hot-working) was accomplished by forging

a Size of Specimen, 3 in. long by % in. in diameter.

TABLE 4.-Penetration and Fracture Grain Size of 8teels B and C

Penetration. Fracture Grain Size Temperature.

Deg. F. Steel B Steel C Steel B Steel C

1450 1500 9% 1550 8 7% 8%

material a t elevated temperatures, noting: (1) the influence of forging temperature (constant reduction), (2) the effect of reduction (constant temperature).

Eflect of Hot Work.-The influence of hot-working upon the austenitic grain size, as obtained after the McQuaid-Ehn treatment, is illustrated in Fig. 16, where A, B, and C demonstrate the effect of forging temperatures upon the condition of the austenitic grain size. The phenomena illustrated were found by upsetting bars (12 in. long by 3 in. dia.) at 1800" F. (980" C.), 2000" F. (1090" C.) and 2200" F. (1200" C.) to a constant size (3 in. long by 6-in. dia.). After cooling to room temperature, the bars were sectioned, surface ground, polished and carburized 8 hr. a t 1700" F.

With increasing forging temperature, the depth of carburization and the grain size increased progressively.

The effect of variation in reduction a t a constant hot-working temperature was recorded by heating bars (10 in. long by 2-in. dia.) to 2000" F. (1090" C.) and quickly forging them to a point. After cooling to room temperature, the bars were surface ground and car-

1600 1650 1700 1750 1800 1900

9% 11 13%

No core No core No core

2000 No core S o core , Coarse I Coarse'

8% 8 7 % 7?5 6% 5%

8% 7% 9 9%

10 12 18

7% 7 45

Coarse Coarse Coarse

R. SCHELMPP AND C. L. SHAPIRO 13

of some heats exhibited fine-grained zones in a coarse-grained matrix, while others showed coarse-grained circles ill a finc background. When- ever the circle grain structure was fine, the grain had a tendency to

FIG. 17.-INFLUENCE O F HOT MECHANICAL DEFORMATION (CONSTANT TEMPERATURE) ON T H E AUSTENLTIC GRALN SIZE. Forged a t 2000' F. (1090" C.)

coarsen slightly a short distance from the peripllery, indicating so-called critical grain growth, as observed by Professor Sauvcur"n his cold- work experiments.

T o obtain an i~~tonsificat,ion of' cold worlc, annealed bars of 1-in. dia.

Fro. 18. FIG. 1 9 .

FIGS. 18-19.-IXFLUENCE O F MEC'HANICAL DEFORMATION ON AUSTENX'l!IC GRAIN SIZE. Fig. 18. Austenitic grain size of unworked bar.

Fig. 19. Austenitic grain size of hot-worked bar. Both etcllecl in 1 per cent nital-picric acid solution. X 100.

were bent in a tensile machine and carburized S hr. a t 1700' F. After

' A. Sauveur: Meta,llography and Heat Treatment of Iron and Steel, Ed. 4. New York, 1936. McGraw-Hill Book Co.


The data presented in this paper clearly show that a number of variants may influence the austenite grain-size rating of steel: primarily, it is the prior structural condition of the material, which in turn is definitely dependent upon the heat-treatment of the material, the amount

carburizing, the bent bars were cut longitudinally through the center and polished. The results obtained are illustrated by one of the bars

6 B. L. McCarthy: Grain Size and Its Influence on the Manufacture of Steel Wire Iron Age (Oct. 10 and 17, 1935).

(Fig. 20), which shows excessive

FIG. 20.-INFLUENCE OF COLD WORK ON

grain growth during carburization. Fig. 20 also contains a photograph of a carburized disk showing the Brine11 marks.

Cold Drawing.-The results of the influence of cold drawing on the coarsening temperature, published by McCarthy5, were substantiated during the course of this investiga- tion. The conclusions obtained by carburizing cold-drawn wire (steel E), after various reductions, are presented in Table 5, and show that as the amount of reduction increases, the coarsening range and temperature automatically become lower.

Duplezing.-During the inves- tigation of the effect of cold work upon the austenitic grain size as developed by the McQuaid-Ehn treatment, i t was noted that the most severe amount of cold working yielded a uniform, fine grain size with rather coarse carbide boundaries: less severe distortion produced a maximum amount of duplexing, which progressively di-

AUSTENITIC GRAIN SIZE. minished with less deformation.

This indicates that duplexing is an inhibited grain-growth phenomenon, resulting from a " critical " amount of cold work. Similar " duplexing" results have more recently been obtained by localized temperature differences in "hot" deforming the material.


TABLE 5.-Effect of Cold Dmtcing upon Conrsetzittg T e m p ~ m t u r e

I Diamrtrr. In. Area. In.

I - - -

Time . Hr.

-- -

of hot or cold mechanirul deformation and the inherent characteristics of the melt, n-hieh detcrmincs the austenitic grain size after the stanc1:trd JIc.Quaid-E:hn c~arl)urizing treatment.

111 view of thcsc* results, it is recon~rnendecl 1 hat the artual service c.onditioiis to whicsh the ~iiateriul is to he sul)j(~ctccl r~iust he fully con- siderecl l~cforc a dcfiriite ttustcriitc. grain-size rztnge, as detcrniined l)y the JIcQuaicl-1:hri test, is spc.c.ificc1. An nustenit(, grain-size rating, if used and ititcrpreted ilitclligcritly uric1 with the proper precautiolis, cmuri be mad(. into n valual~le t est niet hod -both 11y t hc. st eeltnaker aricl consumer -to :iscertain, check and compare the uniformity of various steels a t definite stagr.8 during the proc.essing of the material. But it is obvious that :iny variutioris and differencc.~ in t he austeriite grain size, which may be 01)scrvcrl in the partly processed or finishocl-processed stock, are not necessarily inherent char:tcteristics of t he melt.

T h e authors desire to express their appreriation to thc! officers and members of the Crucible Steel (:ompany of America and its subsidiary plant, the Halcoml) Steel Ilivision, for the iritrrest and assistance that have rnade t hesc! investigations possil~lr. Partirularly, they wish to express their appreeiation and thanks to Mr. It. H. Ilougherty, Assistant t o t h e President of the (:rucil)le Steel Company, for his constructive criticism and pertinent suggestions.

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