CALORIMETRIC STUDY OF THE AUSTENITE : PEARLITE TRANSFORMATION*

W. C. HAGELf, G. M. POUND:, and R. F. MEHLt$

A high temperature, continuously recording, constant heat-flow calorimeter was used to measure the specific heats of austenite and pearlite in eutectoid plain carbon and alloy steels and the enthalpy of the pearlite-austenite transformation as a function of alloy content. The precision of the present method is estimated to be *5 per cent.

The specific heats of austenite and pearlite of eutectoid composition are reported from 400 to 850°C. The addition of 0.80 per cent carbon causes an increase in specific heat relative to pure iron. Small additions of manganese, molybdenum, or cobalt, and variation of pearlite spacing, have no appreciable effect on specific heat.

The enthalpy of the austenite : pearlite transformation for a plain carbon eutectoid steel was found to be 19.6 cal gm-’ at the eutectoid temperature of 727°C. This is in good agreement with the value of 20.0 cal gm-’ calculated from the thermodynamic tables of Darken and co-workers. Additions of manganese (1.85 per cent) or molybdenum (0.51 per cent) lower the enthalpy of transformation to 18 cal gn-i, and cobalt (1.91 per cent) increases it to 23 cal gm-i.

The free energy of the transformation was calculated from the calorimetric data. The free-energy change is decreased by manganese and molybdenum and increased by cobalt. The effect of these alloying elements is in accord with Zener’s prediction, and is of some consequence in the theory of pearlite growth.

ETUDE CALORIMETRIQUE DE LA TRANSFORMATION AUSTENITE-PERLITE

Un calorimetre pour hautes temperatures it enregistrement continu a Bte utilise pour la mesure de la chaleur specifique de I’austenite et de la perlite dans les aciers eutectoide et allies et it la mesure de I’enthalpie de la transformation perlite-austenite en fonction de la composition. La precision de cette methode est estimee Zt f 5%. La chaleur specifique de l’austenite et de la perlite Q l’eutectoide est donnee entre 400 et 850”. L’addition de 0,s de carbone accroit la chaleur specifique du fer pur. De petites addi- tions de manganese, de molybdene, de cobalt ou des variations dans la periodicite de la perlite n’ont aucun effet appreciable sur la chaleur specifique. L’enthalpie de la transformation austenite-perlite pour un acier eutectoide au carbone est de 19,6 calg-i a la temperature eutectoide de 727”. Ceci est en bon accord avec la valeur de 20,O cal g-r calculee a partir des tables de Darken et de ses callaborateurs. L’addition de manganese (185%) ou de molybdene (0,51%) abaisse l’enthalpie de la transformation a 18 cal g-l, celle de cobalt (1,91%) I’augmente a 23 cal g-i.

L’energie libre de la transformation a et& calculee It partir des mesures calorimetriques. Le change- ment de l’energie libre est diminue par le manganese et le molybdene et accru par le cobalt. Cet effet des elements d’addition est conforme aux previsions de Zener et est important pour la theorie de la croissance de la perlite.

KALORIMETRISCHE UNTERSUCHUNGEN DER AUSTENIT-PERLIT UMWANDLUNG

Zur Messung der spezifischen W&men von Austenit und Perlit in eutektoiden Kohlenstoffstahlen und in legierten Stahlen, sowie zur Bestimmung der Enthalpie der Perlit-Austenit Umwandlung in Abhiingig- keit vom Legierungsgehalt wurde ein Hochtemperaturkalorimeter mit kontinuierlicher Schreibvor- richtung und konstantem Warmefluss herangezogen. Die Genauigkeit der Methode betrligt schatzungs- weise * 5%.

Die spezifischen Warmen des Austenits und des Perlits bei der eutektoiden Zusammensetzung werden fur einen Temperaturbereich von 400 bis 850°C angegeben. Bezogen auf reines Eisen verursacht die Zugabe von O,S% Kohlenstoff eine Erhohung der spezifischen Warme. Geringe Zusiitze von Mangan, Molybdan oder Kobalt und eine Veriinderung des Lamellenabstandes des Perlits haben keinen wesent- lichen Einfluss auf die spezifische Wllrme.

Bei der Temperatur des Eutektoids von 727°C hat die Enthalpie der Perlit-Austenit Umwandlung eines reinen Kohlenstoffstahles einen Wert von 19,6 cal gm- i. Der Wert zeigt gute Ubereinstimmung mit dem aus den thermodynamischen Tabellen von Darken und Mitarbeitern berechneten Wert von 20,O cal gn-1. Zusatze von Mangan (1,85%) oder Moybdan (0,51%) erniedrigen die Enthalpie der Umwandlung auf 18 cal gm-1, wiihrend Kobalt sie auf 23 cal gm-i erhdht.

Aus den kalorimetrischen Werten wurde die freie Energie der Umwandlung berechnet. Die Anderung der freien Energie bei der Umwandhmg wird durch Mangan und Molybdan erniedrigt, durch Kobalt erhiiht. Der Einfluss dieser Legierungselemente ist in Ubereinstimmung mit den Vorhersagen von Zener und hat gewisse Auswirkungen auf die Theorie des Perlitwachstums.

* Received 21 April, 1955. This paper is from part of a t Materials and Process Laboratory, General Electric thesis presented by W. C. Hagel to The Committee on Graduate Company, Schenectady, N.Y.; formerly with Department Studies of the Carnegie Institute of Technology, Pittsburgh, of Metallurgical Engineering, Carnegie Institute of Tech- Penn., in partial fulfillment of the requirements for the degree nology, Pittsburg, Penn. of Doctor of Philosophy in Metallurgical Engineering, June $ Department of Metallurgical Engineering, Carnegie 1954. Institute of Technology, Pittsburgh, Penn.

ACTA METALLURGICA, VOL. 4, JANUARY 1956 37

38 ACTA METALLURGICA, VOL. 4, 1956

1. INTRODUCTION

Thermodynamic information on alloy systems is of obvious general importance. It is of especial import- ance to metal systems that exhibit transformation in the solid state, for, whatever the mechanisms of these transformations, t.hermodynamic data must be in- volved in theory. In the case of the austenite : pearlite transformation, for example, a transformation which, in either direction, proceeds by a nucleation andgrowth process, the free-energy change is currently an essential part of the theory of the rate of nucleation and of the rate of growth. These matters have recently been extensively reviewed for this particular transfor- mation.(l)

The present paper reports an experimental attempt to measure the free-energy changes accompanying the austenite : pearlite transformation, for a simple-carbon eutectoid steel and for Mn-, MO-, and Co-eutectoid steels.

2. EXPERIMENTAL METHOD

2.1. Principle of Constant Heat-.ow Calorimeter

A constant heat-flow calorimeter similar to that described by C. S. Smitht2) was used to obtain the specific heats of austenite and pearlite and the latent heat of the transformation. The basic technique con- sists of surrounding the metal specimen with a refractory container of low thermal conductivity and placing the assembly in a furnace, the temperature of which is maintained at a constant level above the specimen temperature. The specimen temperature is measured by a thermocouple extending to the speci- men’s interior; for the determination of specific heat, a differential thermocouple with junctions inside and outside the container is used to control the furnace temperature. In this manner a constant temperature- difference is maintained across the container walls. Heat-flow through the refractory is then constant at a given temperature, since thermal conductivity of refractory remains constant; the heat gained by the specimen in a given time-interval is also constant. Therefore, the time taken to increase the specimen temperature a given interval is proportional to the heat absorbed. In practice, heat-flow varies slightly with temperature owing to changes in refractory thermal conductivity and e.m.f. difference of opposed thermocouples, but the variation can be determined by use of a specimen of known specific heat to give reproducible results.

The steady-state transmission of heat by conduction through the container walls can be expressed by the Fourier relationship

q=kS,AT (1)

where q = heat-flow,

k = thermal conductivity of container,

8, = container-shape factor,

AT = temperature difference between inner and outer surfaces.

For a given calibrated container, K and 8, are constant, and AT is held constant experimentally. The resulting constant heat-flow serves to supply specimen specific heat, heat of transformation, and the specific heat of part of the container as follows

qAt, = C,W,AT, + W,AH, + CRWRATl (2)

where AT, = temperature change in time At,,

AH, = enthalpy of transformation occurring within AT,,

C, and C, = specific heats of specimen and container,

WI and W, = masses of specimen and container.

When the container is empty, equation (2) reduces to

qbt, = C,W,AT,. (3)

If experiments are made with specimens of the same dimensions but of different weights and specific heats, omitting any heats of transformation, equations (2) and (3) can be combined as follows

The quantities fi are the slopes AT

of time-

temperature heating curves for the empty container (subscript R), the calibration specimen (subscript 2), and the specimen under study (subscript 1). If the specific heat of the calibration specimen is known over a wide range, then the specific heat of the specimen under study can be determined at any temperature within that range.

When a heat of transformation is absorbed at a single temperature or within a very small temperature interval, it can be given by the equation

qAt’ = AHIWl (5)

where At’ = arrest time when AT = 0.

HAGEL, POUND, AND MEHL: AUSTENITE: PEARLITE TRANSFORMATION 39

Combination with the expression for the calibration specimen gives

AH1 Wl p_

q = At,

or (6)

Whenever a transformation occurs over a temperature range, its heat will appear as apparent specific heat, and a graphical integration is required for a complete solution.

2.2. Furnace Assembly

A diagram of the interior arrangement of specimen within the wire-wound furnace is presented as Fig. 1, and Pig. 2 shows the same parts photographed in a dismantled condition. The Nichrome-wire furnace is constructed by winding 20-gauge wire around a 1.5- foot collapsible mandrel. End losses are compensated by spacing turns more closely near the ends and using an excess of insulation. By taking temperature read- ings at 0.5-inch intervals within the furnace throughout the range of operation (400 to 850°C), it is eventually possible to arrive at a winding spacing which gives a 6-inch uniform heating zone completely surrounding

DlFFERENTIAL COUPLE TO CONTROLLER-

SPECIMEN /

THERMOCOUPLE LEADS TO COLD JUNCTION AND 2 POTENTIOMETER

+sAUEREISEN

rALUNDUM

TOTAL LENGTH _OF NIGHROME

y ‘w”~:N”o’s 12. *

UMFORM HEATING ZONE. 6’

i-lNERT GAS FLOW

FIG. 1. Interior arrangement of apparatus for measuring specific and latent heats.

FIG. 2. Apparatus photographed in a dismantled condition.

the container. The life of a well-constructed furnace is about thirty heating and cooling cycles.

The specimen has a diameter of 0.688 in., is 1.562 in. long, and weighs about 70 to 80 g, depending on material density. To give the same emissivity to the calibration specimens as the steel specimens, all specimen surfaces are rough-machined and evenly coated with colloidal graphite. The container, with wall 0.625 in. thick, is shaped by hand from a block of Armstrong LW-20 insulating brick. Careful shaping makes it possible to replace the specimen, container, and furnace in the same position over many heating cycles.

The specimen thermocouple, encased in thin ceramic tubing, is led up through the pedestal base to the center of the specimen. The differential thermocouple is positioned near the mid-portion of the container, and its leads are brought out the top of the furnace.

Prior to calorimeter operation, the specimen is placed on the pedestal, the container is positioned over the specimen, and the furnace is centered around the junction. An insulating cap is placed directly above the container, through which the differential thermo- couple wires lead to the controller. The specimen

40 ACTA METALLURGICA, VOL. 4, 1956

thermocouple remains intact until another container and pedestal must be used. Return of parts to their positions of original calibration is accomplished by means of fiducial markings, so that specimens can be inserted and removed with a minimum of disturbance to calibra~d su~oun~gs. Inert gas-flow is main- tained under slight pressure from the center of the pedestal to the semi-sealed insulating cap. A large amount of lagging is placed around the furnace; though furnace interior approached 900°C, the exterior of the lagging is at room temperature.

The thermocouples used for temperature measure- ment and furnace control were made from 28-gauge chrome1 P-alumel wires, and calibrated by standard methods, It appeared that these thermocouples were accurate to -+0.5”C.

A Brown electronic two-second stripchart potentio- meter is used to record time-temperature curves for slope determinations. For most experiments, a chart speed of 240 in. per hour, a span of 1.6 millivolts, and suppressions in steps of 1.60 millivolts are used. Since its 12-inch chart is evenly graduated from 0 to 100, a temperature calibration must be made against a potentiometer for millivoltage-temperature conversion of chart readings.

Sat~facto~ control of ~rnper~t~e difference is obtained by use of a Brown ~~erential recording controller combined with an electronic position- proportioning relay with automatic reset. The circular chart is evenly graduated from 0 to 100% in tempera- ture difference, and makes one revolution every four hours, which is approximately the time required for recording one thermal curve, Differential temperature actuation is from chrome1 P-alumel thermocouple junctions positioned inside and outside the refractory container. Most experiments are conducted at temperature differences of 30 to 5O”C, and maximum variation for short times from control temperatu~ is of the order of f0.2YX

2.4. Gas Pwiyic4xtio?l

To prevent specimen oxidation and decarburization, a purified nitrogen gas atmosphere is maintained within the calorimeter at a pressure slightly greater than atmospheric. The gas is passed through a drying tower containing calcium chloride, and then over copper chips heated at 6OO”C, to the pedestal from which it rises out through the top of the furnace. Microscopic examination of specimen surface showed no evidence of decarburization or oxidation.

2.5. Specimen Preparation

Simple carbon and alloy eutectoid steels were received from Vanadium-Alloy Steel Company in the form of annealed l-inch round bars. These materials were east as 3-inch square ingots weighing abont 30 lb. The ingots were annealed at 9OO”C, hammer-eo~ged to 1.75-inch square billets from 112O”C, rolled to l-inch rounds from 1040°C, and annealed at 900% for twenty hours to minimize banding. The chemical analyses of heats found suitable for use are as follows:

Com.position- Weight per cent

o.“,o 4’

0% Mn

d16 P MO co

0.27 0.011 - - 0.79 0.22 1.85 0.011 0.020 - - 0.79 0.19 0.12 0.005 0.005 0.51 0.79 0.22 0.12 0.014 0.005 - 1,9r

Ad~tional heat treatment, microscopic examination, and thermal analyses have shown these steels to be very close to eutectoid composition. The presence of ferrite or cementite is not observed outside the eutectoid structure, and temperature arrests take place over a narrow temperature range. The isothermal transformation techniques used to obtain suitable interlamellar spacings for these steels have long been standard procedure.

The oil-quenched 0.75inch rounds were further machined to oalorimeter specimen size, and a 0.25inch endpiece was removed for metallographi~ examinat.ion and in~rlamellar spacing determination. Any traces of decarburization were removed by this machining operation. All the metallographic specimens were electropolished and given a light Mital etch. The pro- cedure developed by Pellissier et uZ.(~) for determining interlamellar spacing was modified to avoid needless photography, and readings were made directly with a stage microscope at 850 diameters. Considering possible errors, t,hese interlamellar spacing measure- ments are probably accurate from 55.0 to flO.0 per cent. The photomicrographs in Figs. 3 and 4 show the appearance of pearlites in the various eutectoid steels, and their spacing variation from coarse to fine structures.

2.6. Operationd Procedure

The description which follows will outline the steps which must be taken to determine the specific and transformational heats of one steel specimen. A com- plete measurement cycle consists of first heating the furnace assembly to about 500°C to remove moisture held by the insulation brick. Time-temperature curves are then obtained (1) ~thout a specimen in t,he apparatus, (2) with a copper calibration specimen, and (3) with the steel specimen under investigat,ion.

(i) Fe-C Spec. No. 79, 19,000 8, (ii) Fe-C Spec. No. 22, 12,000 A

HAGEL, POUND, AND MEHL: AUSTENITE: PEARLITE TRANSFORMATION 41

(iii) Fe-C Spec. No. 6, 4440 8, (iv) Fe-C Spec. No. 8, 2500 A

FIG. 3. Photomicrographs of iron-carbon eutectoid steel electropolished x 1500 nital etch.

The rate of heating can be varied by changing con-

tainer-wall thickness, container material, or tempera-

ture difference between the inside and outside of the

container. Heating too rapidly is avoided in order to

obtain a constant temperature of transformation, but

too slow a rate causes spheroidization of the pearlite

and decreases the sharpness of the transformation.

In order to employ specimens of known interlamellar

spacings, all reported measurements were made on a

heating cycle .

2.7. Determination of Thermal Values from Calorimetric Data

Referring to equations (4) and (6), Kelleyc4) reports

that the specific heat of solid copper can be expressed

as C, = 8.52 x 10-s + 2.36 x lop5 T calories

gram-l “K-l. The masses WI and W, of the steel and

calibration specimens were weighed on an analytical

balance. The ratios( , ( g)2 ,and(gjRare

(i) Fe-C-Co Spec. No. 33, 11,000 8, (ii) Fe-C-Co Spec. No. 29, 3600 .A

ACTA METALLURGICA, VOL. 4, 1956

(iii) Fe-C-MO Spec. No. 70, 10,000 A (iv) Fe-C-Mn Spec. No. 42, 3300 8,

FIG. 4. Photomicrographs of alloy eutectoid steels electro-polished x 1500 nital etch

HAGEL, POUND, AND MEHL: AUSTENITE : PEARLITE TRANSFORMATION

FIG. 6. Section of recorder time-temperature chart for a blank run.

3. EXPERIMENTAL RESULTS

3.1. 8pecific Heats

Fig. 8 is an example of an inverse heating curve for

a steel specimen of mean interlamellar spacing equal to

4400 A. One can see how the heating curve for pearlite

increases prior to a transformation in accord with the

increasing specific heat of the ferrite as it approaches

its ferromagnetic transformation at 760°C. The arrest

time for this steel is 1030 seconds at a temperature of

737°C. A heating rate of approximately 3°C minute-l

was obtained by a differential-controller temperature

difference of 45°C. Based on equation (4), a sample

specific heat calculation for a specimen (number 6) at

510% is as follows:

83.7837 c,=--- .

72.4320

x o 1o36 x (83.5 - 28.0)

(68.0 - 28.0,

= 0.166 calories gram-l “C-l

The calculation of about twenty specific heat curves

for plain-carbon and low-alloy eutectoid steels was

averaged to give the pearl& and austenite specific

heats plotted in Fig. 9. Within the error of the calori-

meter (&5 per cent, as established by a specific heat

FIG. 7. App~rance of the beginning of a pearlite-austenite transformation.

o ) j BLANK

CALIBRATION

I d R-C14400i)

0 8

60 70 60 so lb0

TIME TO HEAT 0.200 MILLIVOLTS IN SECONDS

FIG. 8. Inverse heating curve for a carbon eutectoid steel.

44 ACTA METALLURGICA, VOL. 4, 1956

0

t

H 0 c ‘oooloo T --600-- 700 800

TEMPERATURE.

Exnerimental sDecific heat of ueerlite and sustenite for plain-carbon and low-alloy eutectoid &eels. (Best curves for all points of present work. Maximum deviation = * 5 per cent).

determination on pure nickel), variation in inter- lamellar spacing and low alloy content appears to cause no significant deviation from these average curves. The line connecting the pearlite specific heat curve to the austenite specific heat curve was not drawn for this average curve, since different trans-

Fe-C

Fe-C-Mn

Fe-C-MO

Fe-C-Co

Specimen No.

57

6: 12

I 13

4

6: 9

16

4: 22 79

1,400 18.0 735.9 18.2 19.1 18.9 2,500 10.1 735.8 18.1 19.0 19.2 3,300 7.74 735.6 18.4 19.3 19.3 3,400 7.4 737.0 18.1 19.1 19.3 3,500 7.2 736.0 18.1 18.9 19.3 3,800 6.8 737.5 17.9 19.0 19.3 4,200 6.1 738.0 18.4 19.5 19.3 4,400 5.8 737.2 18.3 19.3 19.3 5,000 5.1 738.6 17.8 19.0 19.4 5,900 4.3 737.7 18.2 19.3 19.4 6,800 3.8 739.4 18.4 19.7 19.4 8,100 3.2 738.6 18.5 19.7 19.4

10,500 2.4 740.8 18.5 19.9 19.5 12,400 2.1 740.7 18.4 19.8 19.5 19,400 1.3 741.6 17.8 19.3 19.5

42 3,300 38 3,600 45 3,600 44 4,000 46 5,100 74 5,700

71 56

z 67 70

54

6,5:: 7,300 9,300 9,900

29 32 35 34

;: 33 64

3,600 3,800 3,900 5,600 7,000 7,600

11,200 11,600

formation temperatures were observed for specimens of various spacings and alloy content. Comparison of the present data for steels with the specific heat of pure iron, as compiled by Darken and R. P. Smith,t5) shows that a carbon content of 0.80 per cent does slightly increase specific heat, as first observed by Esser and Baerlecken.(6) Little effect is caused by the presence of manganese, molybdenum, or cobalt on the specific heat. This might be expected, in view of the similarity in thermal properties of the latter elements to pure iron. Any heat effects caused by alloy par- titioning were not observed. The present specific-heat data for plain-carbon eutectoid steel are in fair agree- ment with the results of Awbery and Snow(‘) and Esser and Baerlecken,c6) except that the new pearlite specific-heat data are somewhat lower at temperatures (700-730°C) just preceding the transformation.

3.2. Enthalpies of Transformation

Table 1 gives the experimental results. All values refer to one gram of material. As an aid in explanation,

TABLE 1. Experimental results

cr-Fe,C A

X 10-3cma

7.7 7.2

::: 5.0 4.5

z 6:5 4.5

33.36 2:3 2.2

722.8 16.8 17.4 17.7 720.8 17.1 17.6 17.8 723.4 17.4 18.1 17.8 723.8 17.0 17.7 17.8 725.6 16.8 17.6 17.8 725.5 17.2 18.0 17.9

734.4 16.8 17.2 17.5 737.8 16.9 17.6 17.5 738.2 16.9 17.6 17.5 741.0 16.8 17.8 17.5 742.7 16.6 17.8 17.6 740.6 16.2 17.3 17.6

745.4 21.4 22.6 22.6 748.0 20.8 22.3 22.6 747.4 21.5 22.9 22.7 748.4 21.4 22.9 22.7 748.7 21.2 22.8 22.8 749.8 20.8 22.5 22.8 748.8 20.9 22.5 22.8 749.5 21.5 23.2 22.8

AHT cal g-1

AHT,

C&l g-1

--._

b-0

c II

II II

46 ACTA METALLURGICA, VOL. 4, 1956

Fro 11. Free-energy change of austenite-pearlite trans- formation versus temperature for plain-carbon and alloy eutectoid steels.

(1) Fe-C (bulk) (2) Fe--&Co (bulk, 1.91 per cent Co) (3) Fe-C-Mn (bulk, 1.85 per cent Mn) (4) Fe-C-MO (bulk, 0.51 per cent MO)

The results for representative specimens are plotted in Pig. 11. The values for the alloy steels represent the bulk free-energy changes for the formation of pearhte obtained by isothermal transformations under the conditions given in the section on Experimental Method. Thus, although the degree of alloy par- titioning is unknown, the results are definite for these specific pearlites.

The free-energy change is increased by Co (1.91 per cent Co) and decreased by Mn (1.85 per cent Mn) and by MO (0.51 per cent MO). This is in accord with Zener’s prediction. ~0) Zener argued that one-half of the bulk free-energy change for the pearlite reaction must be expended in providing the energy for the ferrite-cementite interface, snd the rest dissipated by the accompanying diffusional processes; on this basis it is the latter half that would be obtained from calori- metric measurements on fine pearlites. Assuming all this, then there is more free energy available for the

ferrite : cementite interface in Co-steels than in the others, and accordingly the interlamellar spacing in Co-steels should be less and the rate of growth more than in the other steels, as is indeed the case. This relationship as derived should be one of simple pro- portionality, but there is no simple proportionality between the interlamellar spacing and the rate of growth,(ll) and accordingly the principle proposed remains but a qualitative one.(l)

ACKNOWLEDGMENT

This work was supported by the U.S. Army Office of Ordnance Research under Contracts No. DA-36-061-ORD-205 and -350. Thanks are due Dr. G. A. Roberts of the Vanadium-Alloy Steel Co., who supplied the plain-carbon and alloy eutectoid steels.

The authors also wish to express their thanks to Mr. J. J. Kramer, who helped in making calorimeter measurements, and to Mr. Y. T. Chou, who assisted in specimen preparation.

1.

2.

3.

4. 5.

6.

7.

9”:

10.

11.

REFERENCES

R. F. MEHL and W. C. HAGEL, Progress in Metal Physics, Vol. VI (1955). C. S. SMITH. Trans. Amer. Inst. Min. Met. Em.. 137. 236 (1940). ’

“_ I

G. E. PELLISIER, M. F. HAWKES, W. A. JOHNSON, and R. F. MEHL, Truns Amer. Sot. Metals, 30, 1049 (1942). K. K. KELLEY. Bureau of Mines Bulletin 476 (1946). L. S. DARKEN and R. P. SMITH. Industr. enaia. &em.. 43, 1815 (1951).

” I

H. ESSER and H. BAERLECREN, Arch. Eieenhiittenwesen, 14, 617 (1941). J. AWBERY and A. SNOW. Snec. Renort No. 24-Second Report of the Alloy Steels Researoh*Committee, p. 216 (1939). H. ESSER and H. GRASS, Stahl und E&en 53, 92 (1933). J. C. FISHER, Eutectoid decomposition in Thermodynamics in Physical Metallurgy, American Society for Metals, Cleveland (1949). C. ZENER, Trans. Amer. Inst. Min. Met. Eng., 167, 550 (1946). R. W. PARCEL and R. F. MEHL, Trans. Amer. Inst. Min. Met. Eng., 194, 771 (1952).