TS 320 
 .S4 
 Copy 1 
 
 DEPARTMENT OF COMMERCE 
 
 Scientific Papers 
 
 OF THE 
 
 Bureau of Standards 
 
 S. W. STRATTON, Director 
 
 No. 396 
 
 THERMAL AND PHYSICAL CHANGES ACCOMPANY- 
 ING THE HEATING OF HARDENED 
 CARBON STEELS 
 
 BY 
 
 HOWARD SCOTT, Assistant Physicist 
 H. GRETCHEN MOVIUS, Assistant Physicist 
 Bureau of Standards 
 
 SEPTEMBER 20, 1920 
 
 PRICE, S CENTS 
 
 Sold only by the Superintendent of Documents, Government Printing Office,', 
 Washington, D. C. 
 
 Washington 
 government printing office 
 
 1920 
 
DEPARTMENT OF COMMERCE 
 
 Scientific Papers 
 
 OF THE 
 
 Bureau of Standards 
 
 S. W. STRATTON, Director 
 
 No. 396 
 
 thermal and physical changes accompany- 
 ing the heating of hardened 
 carbon steels 
 
 BY 
 
 HOWARD SCOTT, Assistant Physicist 
 
 H. GRETCHEN MOVIUS, Assistant Physicist 
 
 Bureau of Standards 
 
 SEPTEMBER 20, 1920 
 
 PRICE, S CENTS 
 
 Sold only by the Superintendent of Documents, Government Printing Office, 
 Washington, D. C. 
 
 WASHINGTON 
 GOVERNMENT PRINTING OFFICE 
 
 1920 
 
tp 
 
 
 Digitized by the Internet Archive 
 in 2011 with fidrftfirftf from 
 
 OCT 29 1929, 
 
 The Library of Congress 
 
 http://www.archive.orig/details/thermalphysicalcOOscot 
 
^ 
 
 THERMAL AND PHYSICAL CHANGES ACCOMPANY- 
 ING THE HEATING OF HARDENED CARBON STEELS 
 
 By Howard Scott and H. Gretchen Movius 
 
 CONTENTS Page 
 
 I. Introduction 537 
 
 II. Experimental method 539 
 
 III. Heat evolution Ac, 540 
 
 1 . Effect of rate of heating 544 
 
 2. Effect of tempering temperature 545 
 
 3 . Effect of time at tempering temperature 546 
 
 4 . Effect of composition 548 
 
 5. Effect of austenitic structure 549 
 
 IV. Relation of changes in physical properties to heat evolution 551 
 
 1 . Martensitic steel 551 
 
 2. Austenitic steel 554 
 
 V. Summary 555 
 
 I. INTRODUCTION 
 
 The widespread interest which has been recently expressed in 
 the properties of steel in the "blue-heat" range and in the subject 
 of "temper brittleness" makes it highly desirable to study in 
 detail the transformations in steel below the Aj change. In a 
 previous paper 1 the authors have pointed out certain thermal 
 characteristics of the magnetic change in cementite as observed in 
 annealed steels by means of thermal analysis. In this paper the 
 subject under investigation is the thermal change observed in 
 hardened steels on heating below Ac t . 
 
 Outside of the possible bearing of such information on the low- 
 temperature properties mentioned, there remains the desirability 
 of establishing fundamental characteristics of steel. The one in 
 question is of particular value in that it may furnish a practical 
 basis for defining the natural boundary between martensite and 
 the troostite of tempering, which from present information is very 
 indefinite. 
 
 A survey of the changes in some of the physical properties of 
 carbon steels on tempering would, on account of certain incon- 
 sistencies, lead one to doubt the existence of a sharp demarcation 
 between the constituents — martensite and troostite. Heating 
 
 1 Chemical and Metallurgical Engineering, 22, p. 1069; June 9, 1920. 
 
 537 
 
538 Scientific Papers of the Bureau of Standards Ivoi. 16 
 
 curves of hardened steels, however, have shown a well-marked 
 heat evolution ending around 300 C. Such heat evolution would 
 be expected from the usual conception of the formation of mar- 
 tensite; namely, that one or more of the transformations occurring 
 on slow cooling are suppressed by quenching. The consummation 
 of the suppressed transformation (or transformations) is a mani- 
 festation of the completion of the constitutional change and, 
 therefore, evidence of a boundary between two constituents, pre- 
 sumably martensite and troostite. Whether the end of this heat 
 evolution should be used to define those constituents the future 
 will decide ; the present work seeks only to establish its nature in a 
 variety of carbon steels and its relation to accompanying changes 
 in some of the physical properties. 
 
 In the literature some work has appeared on this heat evolution 
 in hardened carbon steels. Osmond 2 and Maurer 2 have given 
 inverse-rate heating curves; Heyn and Bauer 3 and Portevin 3 
 have given differential curves showing the phenomenon. 
 
 The temperature values for the transformation are somewhat 
 higher than those obtained here. In general, the curve inflections 
 are neither prominent enough nor the statement of operating 
 details sufficient to allow of a precise definition of the transforma- 
 tion characteristics. Also the effect of important variables has 
 not been determined. This phenomenon has been observed also 
 by continuous measurement of the changes in some physical 
 properties on heating. Grenet * detected an inflection in the ex- 
 pansion and electric -resistance curves of a high-carbon steel, 
 Chevenard 5 in expansion curves, and Honda 6 in magnetic- 
 induction curves. The magnetic curves are the only ones which 
 seem to follow closely the progress of the heat change. Brush 7 
 has made extensive observations on the heat evolution at ordinary 
 temperatures in recently hardened steels. He noted a heat evo- 
 lution, greatest immediately after hardening, gradually diminishing 
 in rate with time and becoming imperceptible after several weeks. 
 The physical changes accompanying this spontaneous evolution 
 were very small in comparison with those accompanying even 
 slight tempering. 
 
 While the present research is confined to carbon steels because 
 of their fundamental importance, it is being extended to alloy 
 
 2 Osmond, J. Iron and Steel Inst., p. 38; No. 1, 1890. Maurer, Rev. de Met., 5, p. 711; 1908. 
 
 3 Heyn and Bauer, J. Iron and Steel Inst., 79, p. 109; 1909. Portevin, Rev. de Met.. 13, p. 9; 1916. 
 * Grenet, Rev. de Met., 1, p. 353; 1904. 
 
 6 Chevenard, Rev. de Met., 14, p. 610; 1917. 
 
 a Honda, Sci. Reports Tohoku Imp. Univ., 6, p. 149; 1917. 
 
 7 Brush, Bull. A. I. M. M. E. No. 153, p. 2389; 1919. 
 
Scot! "1 
 MoTiusj 
 
 Thermal Changes of Hardened Steels 
 
 539 
 
 steels in order to obtain further light on the effect and function of 
 the alloying elements. 
 
 II. EXPERIMENTAL METHOD 
 
 The inverse-rate method of obtaining thermal curves has been 
 used at the Bureau of Standards as the most effective and satis- 
 factory method for studying the transformations in steel. Used 
 in connection with the apparatus already described 8 excellent 
 curves can be obtained at the low temperatures where the heat 
 evolution under examination is found. The details of mounting, 
 size of sample, and operation are given in the above reference. A 
 temperature interval corresponding to 20 microvolts on a platinum, 
 platinum-rhodium thermocouple was used in this investigation. 
 
 TABLE 1. — Results of Chemical Analyses of Steels Investigated 
 
 c 
 
 Mn 
 
 Si 
 
 P 
 
 S 
 
 Per cent 
 0.40 
 
 .44 
 a. 46 
 a. 73 
 
 .95 
 1.01 
 1.94 
 
 Per cent 
 
 Per cent 
 0.01 
 .02 
 ,06 
 .01 
 .24 
 .01 
 .01 
 
 Per cent 
 
 Per cent 
 
 1.00 
 .35 
 .38 
 .22 
 
 
 
 0.02 
 .04 
 .02 
 
 0.05 
 .05 
 .01 
 .005 
 .005 
 
 
 
 
 
 a Furnished by courtesy of Carnegie Steel Co. 
 
 The materials studied were seven steels of the compositions given 
 in Table 1. Heating for quenching, as noted on the curves and 
 in the tabulated results, was carried out on the prepared samples 
 by introduction into an electrically heated alundum-tube furnace 
 wound with resistance wire. Charcoal was present to reduce oxi- 
 dation, and a platinum, platinum-rhodium thermocouple was used 
 for the measurement of temperature. In tempering, the specimen 
 was heated 30 minutes in an oil or nitrate bath, as required by the 
 temperature. Temperatures below 300 C were measured with a 
 mercury thermometer. 
 
 All samples of the quenched 0.95 per cent C steel not receiving 
 subsequent tempering were run within from 1 to 3 days after 
 quenching. All the other steels not tempered were run within 
 from 6 to 16 days after treatment, excepting the 1.01 per cent C 
 and 1 .94 per cent C steels (curve j) , which were run the day follow- 
 ing treatment. 
 
 8 Scott and Freeman, Bull. A. I. M. M. E. No. 152, p. 1429; 1919. Also B. S. Sci. Papers, No. 348. 
 
54-0 Scientific Papers of the Bureau of Standards 
 
 III. HEAT EVOLUTION Ac t 
 
 {.Vol. 16 
 
 The principal phenomenon under consideration here, the heat 
 evolution on heating hardened carbon steels, will be designated as 
 "Ac t ," a notation used by one of the authors • for the same phe- 
 nomenon in a high-alloy steel. 
 
 °C 
 500 
 
 WO 
 
 300 
 
 200 
 
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 \ 0.75 Per cent C Stecf 
 
 Quenched from 800 °C 
 
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 Time interval in seconds 
 
 /O IS 10 IS IS 20 20 2S ¥5 SO SS 
 
 FlG. I. — Inverse-rate heating curves of hardened steel, showing effect of rate of heating 
 
 on Act 
 
 The thermal curves, taken to show the effect of several variables 
 on the transformation Ac t , are shown in Figs. 1, 2, 3, 4, and 5. 
 For the reduction of the thermal-curve data to tabular form, the 
 
 • Scott, Bull. A. I. M. M. E. No. 146, p. 157; Feb.. 1919. Also B. S. Sci. Papers, No. 335. 
 
Scett 1 
 Meviusl 
 
 Thermal Changes of Hardened Steels 
 
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 Fig. 2. — Inverse-rate heating curves of hardened steel, showing effect of previous tempering 
 
 for jo minutes on Act 
 
542 
 
 Scientific Papers of the Bureau of Standards 
 
 Wot. r« 
 
 temperatures of the principal curve bends caused by the Ac t trans- 
 formation were taken as denoted on the curves by B, M, and E, 
 beginning, maximum, and end, respectively. The rate of heating 
 given is that just before the beginning of the transformation. The 
 values in the column of Table 2 marked "Intensity" represent 
 
 SOO 
 
 WO 
 
 3O0 
 
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 /oo 
 
 0.95 Percent C Steel 
 Oaenchec/ /r> o/7 from <SOO"C 
 ~femperea e?t ^00°C Tempered at £JO°C 
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 Ftg. 3. — Inverse-rate of heating curves on hardened steels, showing effect of duration of 
 
 previous tempering on Act 
 
 the difference in seconds between the time at the maximum and 
 at the end of Ac t , except in the case of the austenitic steel, where 
 they represent temperature drop. 
 
 The temperature of the maximum of A^ (maximum temperature 
 before decalescence when that phenomenon was observed) is also 
 
ScoU "1 
 Mtrvius} 
 
 Thermal Changes of Hardened Steels 
 
 543 
 
 
 
 Carbon Steels 
 
 
 
 
 Quenched from above /?c a 
 
 
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 Fig. 4. — Inverse-rate heating curves of quenched steel, showing effect of composition 
 4218°— 20 2 
 
544 Scientific Papers of the Bureau of Standards Ivoi. 16 
 
 given, but the heating curves are not plotted to show Aci, in order 
 to avoid excessive reduction of the curves on reproduction. 
 
 1. EFFECT OF RATE OF HEATING 
 
 Rate of heating has a considerable effect on the temperature 
 and form of Ac t for the comparatively fast rates required by ther- 
 
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 500 
 
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 300 
 
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 Fig. 5. — Inverse-rate heating curves of austeniiic iron-carbon alloy {1.Q4 P er cent O 
 
 mal analysis, as may be seen from the curves of Fig. i , which were 
 taken on the 0.95 per cent C steel. The principal data taken from 
 these curves are plotted with rate of heating as the abscissas, in 
 Fig. 6. It will be noted from this figure that the temperature 
 characteristics of Ac t for zero rate are 155, 250, and 260 C, respec- 
 tively, for the beginning, maximum, and end of the transforma- 
 tion. This appears to represent the progress of the transformation 
 
Scott 1 
 Mvvius} 
 
 Thermal Changes of Hardened Steels 
 
 545 
 
 for a tempering time approximating normal tempering conditions, 
 probably about 30 minutes. From the sharpness of the begin- 
 ning of the transformation it would appear that the quenched steel 
 is the equivalent of a steel instantaneously cooled and then drawn 
 in the neighborhood of 150 C. Fig. 6 illustrates this interesting 
 point: That for the size of specimen used in this case there is no 
 appreciable difference between the thermal characteristics of an 
 oil-quenched and of a water-quenched specimen. 
 
 j 
 2. EFFECT OF TEMPERING TEMPERATURE 
 
 The heating curves represent the progress of tempering for a 
 necessarily very short time at any temperature in the Ac t range. 
 To show the effect of hold- 
 ing for a definite time at 
 several tempering temper- 
 atures on the characteris- 
 tics of Ac t , heating curves 
 were taken on specimens 
 of the 0.95 per cent C steel 
 quenched in oil from 8oo° 
 C and tempered 30 min- 
 utes at the temperatures 
 given in Fig. 2 and Table 2. 
 
 From a consideration of 
 these data it may be seen: 
 (1) That the beginning of 
 Ac t is from 10 to 17 C 
 higher than the tempering 
 temperature when that is 
 above 200 C ; (2) that the 
 transformation is com- 
 pleted at a temperature 
 between 250 and 270 C; 
 and (3) that for each tem- 
 perature up to 250 C there 
 is a definite and character- 
 istic form of curve. The 
 estimated temperature of the end of Ac t for zero rate (260 C) is, 
 therefore, from (2) in practical agreement with the end of the 
 transformatior for a tempering period of 30 minutes. From (1) 
 and (3) it is evident that the heating curves might be used to esti- 
 mate the previous tempering temperature within certain limits. 
 
 Rate of heating 
 
 .30 X/sec. 
 
 Fig. 6. — Effect of rate of heating on temperature 
 and intensity of heat evolution of 0.95 per cent 
 C steel 
 
546 Scientific Papers of the Bureau of Standards ivoi. 16 
 
 It may be of interest to note that, for a tempering temperature 
 of 270 and 300 C (Fig. 2) , there is a slight deflection of the curves 
 to the right, indicating an absorption of heat over the range of 
 about 350 to 450 C, which is in conformity with the observations 
 of Heyn and Bauer 10 under similar conditions. 
 
 3. EFFECT OF TIME AT TEMPERING TEMPERATURE 
 
 It has long been recognized that the time of holding at a tem- 
 pering temperature has a very considerable effect on the resulting 
 physical properties, and it is even held that a long time at a low 
 temperature is equivalent to a short time at a higher temperature. 
 The thermal curves of steels tempered for different lengths of time 
 in the Ac, range should, therefore, throw some light on the validity 
 of this much discussed proposition. 
 
 In Fig. 3 heating curves are given to show the effect of main- 
 taining a steel for different lengths of time at the tempering tem- 
 perature. The 0.95 per cent C steel, hardened by being quenched 
 in oil from 8oo° C, was used. Specimens were maintained for 5, 
 30, and 60 minutes at each of the two tempering temperatures, 
 200 and 230 C, chosen because they represent temperatures at 
 which tempering is well in progress, but not to such an extent as 
 to eliminate the thermal effect. 
 
 It may be noted from these curves and the compiled data of 
 Table 2: (1) That the beginning of Ac t is higher for a long than 
 for a short exposure at the tempering temperature; (2) that the 
 intensity of the transformation is less for a long tempering period 
 than for a shorter one; and (3) that the rate of progress of the 
 transformation is greater at the higher tempering temperature 
 than at the lower one. 
 
 From (1) and ^(2) it is apparent that time has a decided effect 
 on the transformation characteristics. The third conclusion is 
 evident from the fact that at 200 C an exposure of 60 minutes is 
 necessary to reduce markedly the intensity of Ac t , while at a tem- 
 perature only 30 C higher the intensity is much more strongly 
 reduced by a 30-minute exposure. This is in agreement with the 
 tempering experiments of Barus and Strouhal, 11 whose measure- 
 ments of electrical resistance and thermal emf show the rate of 
 transformation to be much greater at the higher tempering tem- 
 peratures in the Ac t range. This indicates further that the tem- 
 pering time of 30 minutes used in the preceding section represents 
 actual equilibrium or zero-rate conditions at the temperature of 
 the end of Ac t , though, of course, not at lower temperatures. 
 
 10 See footnote 3. "Barus and Strouhal. Bull. U. S. Geological Survey. No. 14; 1885. 
 
Scott "I 
 Moviusi 
 
 Thermal Changes of Hardened Steels 
 TABLE 2. — Thermal Characteristics of Hardened Carbon Steels 
 
 547 
 
 Composition 
 
 Mn 
 
 Si 
 
 Heat treatment 
 
 Quench- 
 ing 
 temper- 
 ature 
 
 Quench' 
 ing 
 me- 
 dium 
 
 Tem- 
 pering 
 tem- 
 pera- 
 ture 
 
 Time 
 
 at 
 tem- 
 pering 
 tem- 
 pera- 
 ture 
 
 Rate 
 of 
 
 heat 
 ing 
 
 Act temperature 
 
 Be- 
 gin- 
 ning 
 
 Maxi- 
 mum 
 
 End 
 
 Intensity 
 ot heat 
 evolution 
 
 Aci 
 max- 
 imum 
 
 Per 
 cent 
 
 Per 
 
 cent 
 
 Per 
 cent 
 
 .95 
 
 .24 
 
 .40 
 .46 
 .44 
 .73 
 .95 
 1.01 
 
 .35 
 1.00 
 .38 
 .22 
 
 .01 
 .06 
 .02 
 .01 
 .24 
 .01 
 
 800 
 800 
 800 
 
 800 
 
 (") 
 
 800 
 800 
 800 
 800 
 800 
 800 
 800 
 800 
 
 800 
 800 
 800 
 800 
 800 
 800 
 
 900 
 900 
 900 
 900 
 
 1,100 
 1,100 
 1,100 
 1,100 
 
 Water. 
 ..do... 
 
 ..do. 
 Oil... 
 
 Oil... 
 
 ..do.. 
 ..do. 
 ..do. 
 ..do. 
 ..do.. 
 ..do. 
 ..do. 
 ..do.. 
 
 Oil... 
 ..do. 
 ..do. 
 ..do. 
 ..do. 
 ..do.. 
 
 Water. 
 ..do... 
 Oil-'... 
 
 Water. 
 Oil.... 
 Water. 
 
 Water. 
 ..do... 
 ..do... 
 ..do... 
 
 200 
 230 
 250 
 270 
 300 
 350 
 400 
 
 200 
 200 
 200 
 230 
 230 
 230 
 
 Min- 
 utes 
 
 °C 
 
 /sec. 
 
 Seconds 
 
 (1) Effect of heating at different rates 
 
 0.23 
 
 183 
 
 290 
 
 319 
 
 .16 
 
 178 
 
 285 
 
 308 
 
 .05 
 
 162 
 
 261 
 
 282 
 
 .22 
 
 167 
 
 273 
 
 295 
 
 .23 
 
 
 
 
 
 
 
 
 7 
 14 
 8.5 
 
 
 733 
 736 
 
 (2) Effect of tempering at different temperatures 
 
 0.12 
 
 167 
 
 273 
 
 295 
 
 .13 
 
 215 
 
 275 
 
 300 
 
 .13 
 
 240 
 
 282 
 
 307 
 
 .15 
 
 267 
 
 
 310 
 
 .16 
 
 
 
 
 .15 
 
 
 
 
 .12 
 
 
 
 
 .17 
 
 
 
 
 . 15 
 
 
 
 
 
 
 
 
 8.5 
 
 9.0 
 
 1. 5 
 
 0.5 
 
 
 
 
 
 
 
 
 
 
 
 733 
 733 
 733 
 734 
 734 
 733 
 733 
 733 
 735 
 
 (3) Effect of tempering for different periods of 
 time 
 
 0.15 
 
 204 
 
 276 
 
 300 
 
 .13 
 
 215 
 
 275 
 
 300 
 
 .16 
 
 224 
 
 273 
 
 300 
 
 .14 
 
 224 
 
 271 
 
 296 
 
 .13 
 
 240 
 
 282 
 
 307 
 
 .13 
 
 244 
 
 289 
 
 350 
 
 9 
 
 9 
 
 4 
 
 5.5 
 
 1.5 
 
 0.5 
 
 (4) Results for different compositions 
 
 0.12 
 
 167 
 
 259 
 
 278 
 
 .12 
 
 174 
 
 270 
 
 286 
 
 .13 
 
 174 
 
 276 
 
 297 
 
 .12 
 
 175 
 
 271 
 
 292 
 
 .12 
 
 167 
 
 273 
 
 295 
 
 .10 
 
 167 
 
 261 
 
 284 
 
 731 
 731 
 727 
 731 
 733 
 728 
 
 (5) Results for austenitic structure 
 
 .08 
 
 174 
 
 308 
 
 319 
 
 .11 
 
 182 
 
 347 
 
 361 
 
 .15 
 
 184 
 
 353 
 
 366 
 
 .12 
 
 179 
 
 302 
 
 317 
 
 ' C Temp, 
 drop 
 
 2 
 
 24 
 
 731 
 
 a Annealed. 
 
 6 Air cooled. 
 
548 
 
 Scientific Papers of the Bureau of Standards 
 
 {Vol. 16 
 
 It may be inferred from the nearly identical characteristics of 
 Ac t following tempering for 60 minutes at 200 C and 5 minutes at 
 a temperature 30 C higher, that these two treatments produce 
 the same structural condition, but, because the rate of transforma- 
 tion changes with temperature, it does not follow that this par- 
 ticular relation holds quantitatively for any other temperatures in 
 the Ac t range. 
 
 In general, however, the effect of time may be regarded as 
 
 equivalent to that of temperature within limits as far as the 
 
 characteristics of Ac t are a criterion of the constitutional changes 
 
 in the steel. 
 
 4. EFFECT OF COMPOSITION 
 
 For the sake of comparison of the several martensitic steels 
 investigated, the temperature values of Ac t taken from Table 2 
 have been given a small correction on the basis of Fig. 6 to reduce 
 them to a constant rate of heating of o.io°C per second; these 
 values are given in Table 3. By comparing the synthetic steels 
 in the first group, low in manganese, or the commercial steels in 
 the second group, containing 0.20 to 0.40 per cent manganese, 
 with respect to the variable carbon, one may see that the maxi- 
 mum and end of Ac t are somewhat higher for the higher carbon 
 contents and that the transformation intensity is approximately 
 proportional to the carbon content. 
 
 TABLE 3. — Transformation Characteristics of Martensitic Steels for Rate of Heating 
 
 of 0.10° C per Second 
 
 SYNTHETIC STEELS 
 
 C 
 
 Mn 
 
 SI 
 
 Ac, temperature 
 
 Intensity 
 
 Beginning 
 
 Maximum 
 
 End 
 
 Per cent 
 
 0.40 
 
 1.01 
 
 <H.94 
 
 Per cent 
 
 Per cent 
 
 0.01 
 
 .01 
 
 .01 
 
 °c 
 
 165 
 
 167 
 177 
 
 °C 
 
 255 
 261 
 298 
 
 °C 
 
 274 
 284 
 313 
 
 Seconds 
 
 2 
 
 13 
 
 32 
 
 
 
 
 COMMERCIAL STEELS 
 
 .46 
 .73 
 .95 
 
 .35 
 .38 
 .22 
 
 .06 
 .01 
 .24 
 
 172 
 173 
 165 
 
 266 
 267 
 269 
 
 282 
 288 
 291 
 
 3 
 
 7 
 8.5 
 
 
 S' 
 
 fNTHETIC 1 PER CENT MN ST 
 
 EEL 
 
 
 .44 
 
 1.00 
 
 .02 170 270 
 
 291 
 
 6 
 
 a Martensitic by immersion in liquid air; curve 4, Fig. 5. 
 
jVfori,„] Thermal Changes of Hardened Steels 549 
 
 The effect of an increase of carbon on the heat evolution is to 
 augment correspondingly the rate at a given stage in the progress 
 of the transformation for a given furnace rate. This increase in 
 rate from the effect noted in Section III— 1 , on rate of heating, will 
 raise the temperature of the maximum and end of Ac t . The in- 
 crease in temperature of the maximum and end of Ac t with in- 
 creasing carbon, being small, is probably due entirely to the 
 augmented rate of heating. This factor being ineffective for very 
 slow or zero rate of heating, it may be stated that, for this case, 
 the carbon does not materially affect the maximum and end of 
 Ac t . Likewise, the rate at the beginning is unaffected by the 
 subsequent heat evolution, so that the constancy of that point 
 verifies the conclusion that the temperature of Ac t is practically 
 independent of carbon content under conditions which render the 
 effect of intensity impotent. 
 
 From Table 3 it may be of interest to note further that Ac t is 
 slightly higher in the commercial than in the pure synthetic steels 
 of the same carbon content, and that the characteristics of Ac t 
 for the steel containing 0.73 per cent carbon and 0.38 per cent 
 manganese are practically identical with those for the steel con- 
 taining 0.44 per cent carbon and 1.0 per cent manganese. 
 
 S. EFFECT OF AUSTENITIC STRUCTURE 
 
 For obtaining the data presented in preceding sections, the 
 material used was a martensitic steel, but it was believed to be 
 of some interest to study also the thermal changes accompanying 
 the decomposition of an austenitic matrix in a carbon steel. 
 With this in mind, a steel of 1.94 per cent C content was quenched 
 in water, the resulting structure being uniformly austenitic 
 (Fig. 7 a). 
 
 The curves taken on specimens given such treatment are 
 shown in Fig. 5, curves /, 2, and 3, and the data taken from 
 them, are shown in Table 2. The transformation observed was 
 very intense, and in one case (Fig. 5, curve 3) the heating ad- 
 vanced so rapidly that the operator could not follow it. In 
 every case the heat evolution increased the specimen temperature 
 at the end of the transformation so that it exceeded the normal 
 furnace temperature, and a drop of temperature was then re- 
 corded. This amounted to 2° C in curve 1 and 24 C in curve 2; 
 the drop for curve 3, though not recorded, was very considerable. 
 
 In these austenitic steels the effect of rate of heating upon the 
 temperature of Ac t is much more pronounced than in the marten- 
 
550 Scientific Papers of the Bureau of Standards ivot. 16 
 
 sitic steels, though with slow rates the difference between the 
 two types of steel is small, if any. Thus by comparison of 
 curve i for the austenitic steel (rate of heating, 0.08 C per 
 second) with the curve for the 0.95 per cent C steel (rate of 
 heating, 0.16 C per second), in both of which cases the rate is 
 approximately the same at the maximum, one may note that 
 the maxmum for the austenitic steel is 23 C higher and the end 
 only ii° C higher than for the martensitic steel, even though an 
 actual temperature drop occurred in the former case. This 
 would indicate that for very slow rates there would be little 
 temperature difference between Ac t for a martensitic structure 
 and Ac t for an austenitic one. Here again the fact that the 
 beginning for both is practically the same verifies the conclusion 
 that the heat evolution of the transformation materially affects 
 its observed temperature for sensible rates of heating. 
 
 Curve 4 of Fig. 5 shows the thermal characteristics of one of 
 the austenitic steels after exposure for 30 minutes in liquid air. 
 This treatment rendered the steel partially martensitic in struc- 
 ture (Fig. 76). Comparison of the heating curve 4 for this 
 sample with curve 2, the rate of heating being essentially the 
 same in both cases, shows that treatment in liquid air causes (1) 
 a lowering of the maximum and end of the transformation by 
 about 45 C, and (2) an evolution of heat much less intense, 
 without any recorded drop of temperature. This indicates a 
 marked structural change, evidently from austenite to martensite, 
 on immersion of the austenitic steel in liquid air. 
 
 Further examination of the heating curves of the austenitic 
 steels reveals another significant phenomenon. An inflection in 
 these curves may be noted at 273 C for curve 2, 285 C for curve 
 3, and somewhat lower for curve 1, at which temperature there is 
 an augmentation of the rate of heating. It is evident from 
 curves 2 and 4 that the heat evolution of the latter (marten- 
 sitic) steel starts to drop off, while the former (austenitic) steel 
 is considerably intensified just above the inflection temperature 
 of 2 73 C. This indicates two stages in the decomposition of the 
 austenitic steel — namely, the low-temperature stage, probably a 
 manifestation of the simple carbide precipitation, and the high- 
 temperature stage, which is the same intensified by the A 3 and 
 A, transformation. In the case of the martensitic steel the 
 change designated by Ac t is very probably due only to the car- 
 bide precipitation. 
 
Scientific Papers of the Bureau of Standards, Vol. 16 
 
 to 
 
 to 
 Fig. 7. — Microstructure of 1.94 per cent C steel quenched in water from 
 1100° C. Etched with 2 per cent alcoholic HN0 3 
 
 (a) As quenched. X500 
 
 (b) Same as (a), but dipped in liquid air for 30 minutes. X2co 
 
M° "ms] Thermal Changes of Hardened Steels 551 
 
 IV. RELATION OF CHANGES IN PHYSICAL PROPERTIES TO 
 
 HEAT EVOLUTION 
 
 The thermal curves presented here show a rapidly increasing 
 heat evolution (Ac r ) from 155 C to 250 C (ending abruptly at 
 about 260 C) for a very slow heating rate. This last tempera- 
 ture, 260 C, very probably represents the completion of the 
 change from martensite into troostite. A consideration of the 
 changes in physical properties on tempering quenched steels 
 through the Ac t range should, therefore, assist materially in 
 determining other characteristics of this change. 
 
 1. MARTENSITIC STEEL 
 
 In order to make a reliable comparison of the characteristics 
 of Ac t with the changes in the standard scleroscope and Brinell 
 hardness numbers, measurements were made on samples of the 
 0.95 per cent C steel used for the majority of the thermal curves. 
 The results are shown in Figs. 8 and 9, the recording scleroscope 
 being used in the first case and the usual Brinell equipment in the 
 latter. A fresh surface of the ball was taken for every impression. 12 
 
 For the purpose of comparing some of the other physical proper- 
 ties with the heat evolution, Fig. 10 was prepared. Curves 1 
 and 2 of this figure were obtained from the magnetic data of 
 Burrows and Fahy 13 and are expressed in gausses per square 
 centimeter; curves 3 and 4 were obtained from the electric re- 
 sistance and thermal emf data of Campbell, 14 and curve 5 was 
 obtained from the density values of Schulz. 15 In all cases marten- 
 sitic carbon steels were used of approximately the same carbon 
 content as the 0.95 per cent C steel used here. 
 
 Upon returning to the hardness curves of Figs. 8 and 9 one 
 may see that the scleroscope hardness does not drop off abruptly 
 until slightly above the temperature (260 C) of the end of Ac t , 
 and that the Brinell hardness begins to drop linearly imme- 
 diately above the beginning of Ac t . Thus there exists in both the 
 hardness curves an inflection closely related to fundamental tem- 
 perature characteristics of the heat evolution. 
 
 Consideration next of curves 1 and 2, Fig. 10, for coercive force 
 and maximum induction, respectively, will show that these 
 
 12 Since these Brinell data were obtained, the work of Chevenard (see footnote 5) appeared, containing a 
 curve for an 0.85 per cent C steel practically identical in form with Fig. 9, with the exception that his values 
 are somewhat lower. 
 
 13 Burrows and Fahy. Trans. A. S. T. M., 19, part II, p. 5; 1919. 
 » Campbell, J. Iron and Steel Inst., 94, p. 36S; 1916. 
 
 15 Schulz, Forschungsarbeiten, No. 161, p. 1; I9r4. 
 
552 
 
 Scientific Papers of the Bureau of Standards 
 
 IVol. 16 
 
 properties change with an increasing rate over the range 20 to 
 300 C in the same manner as the heat evolution. The tempering 
 temperature steps were not taken sufficiently close to define the 
 end point of this change, but it coincides substantially with the 
 end of Ac t . The curves 3 and 4, for thermal emf against pure iron 
 and specific resistance, respectively, are practically parallel and 
 may, therefore, be considered as a unit. It may be observed 
 too 
 
 90 
 
 80 
 
 70 
 
 5 
 
 ! 
 
 * 
 
 v. so 
 
 vo 
 
 JO 
 
 
 — ■ ( 
 
 > 
 
 CarSd/7 Too/ S/ee/ 
 
 Q.95; /Vs?,.ZZ;S/,.ZV 
 
 
 
 " 
 
 ~°*% 
 
 
 Quenc/ied m water 
 from 300 "C 
 
 
 
 
 
 
 
 
 
 
 
 
 
 ov. 
 
 
 
 
 
 Sa/7 
 • Sam 
 
 i/b/e r7 " 
 
 b/e £ . 
 
 \/i>yZby 
 
 
 
 
 
 
 /6"> 
 
 
 
 
 
 
 
 
 
 
 
 
 /00 ZOO 300 WO 500 
 
 7~err7fier/ny temperature 
 
 (,00 
 
 700 
 
 FlG. 8. — Effect of tempering temperature on scleroscope hardness of o.Q$ per cent C steel 
 
 that the change is about 85 per cent complete at 300 C, or in the 
 Ac t range. The density curve 5 is somewhat irregular, but the 
 maximum rate of change occurs in the vicinity of the end of Ac<. 
 From the foregoing analysis it is evident that the changes in 
 the physical properties considered are related very closely to the 
 heat evolution Ac t , particularly in the case of the magnetic proper- 
 ties, maximum induction, and coercive force. These relations 
 are forceful indications of a natural boundary between martensite 
 and the troostite produced at about 260 C on tempering. Such 
 a boundary should be detectable also by the changes in micro- 
 
Scott "I 
 Movnts} 
 
 Thermal Changes of Hardened Steels 
 
 553 
 
 structure. Authorities, however, differ on the temperature of 
 this boundary for simple steels, and place it anywhere in the 
 range from 250 C to 400° C. Careful observers have studied 
 this change, and while not suggesting an end point, have made 
 observations indicative of one in the region of the end of Ac t . 
 
 Howe and Levy, 16 after quenching eutectoid carbon steel from 
 1100 C to water, find that on a 5-minute exposure to 300 C 
 
 700 
 
 600 
 
 \ 
 1 
 
 *soo 
 
 <o 
 to 
 &VOO 
 
 \ 
 
 ^JOO 
 ($£00 
 
 /OO 
 
 
 
 
 
 Carbon Too/ Sfee/ 
 
 
 u— 
 
 ■c< 
 
 C..95; Mn,.22; 3/..2V 
 J3or J6y'/ z by6/n. 
 
 
 ( 
 
 !\ 
 
 
 
 
 
 
 oV> 
 
 
 
 
 
 
 
 
 X 
 
 
 
 
 
 Quenched in wafer 
 from <900 " C 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 /OO 
 
 200 J00 400 500 600 700 "C 
 Tempering temperature 
 
 Fig. 9. — Effect of tempering temperature on Brinell hardness of 0.Q5 per cent C steel 
 
 the original white martensite needles are almost completely 
 broken up. Heyn 17 notes a coarsening of the needle structure at 
 275° C. 
 
 These observations are indicative of a structural change in the 
 vicinity of the end of the heat evolution. The region under 
 investigation is thus narrowed down, and future observers should 
 have little difficulty in denning precisely the nature of the accom- 
 panying changes. 
 
 16 Howe and Levy, Trans. A. S. T. M.. 16, part II, p. 7; 1916. 
 
 " Heyn quoted by Sauveur, The Metallography and Heat Treatment of Steel, p. 304. 
 
554 
 
 Scientific Papers of the Bureau of Standards 
 
 IVol. 16 
 
 2. AUSTENITIC STEEL 
 
 In a foregoing section (p. 550) attention was called to a sharp 
 change in direction of the heating curves of the austenitic steel. 
 This inflection, denoting an abrupt increase in the rate of heat 
 evolution, was noted to start at about the temperature at which 
 
 /00 200 J00 VOO 500 
 
 Te/nfoer/na temperature 
 
 too 
 
 700 'C 
 
 Fig. 10. — Change of physical properties with tempering tem- 
 perature of martensitic carbon steels 
 
 the heat evolution in a martensitic steel begins to disappear. It 
 may therefore be of some interest to compare this thermal 
 behavior of the steel with the density changes in similar steel. 
 
 In Fig. 1 1 are plotted density values given by Maurer 18 for a 
 1.66 per cent C steel quenched in water from 1,050° C. The 
 resulting structure is not completely austenitic, but nearly so. 
 
 u See footnote 2. 
 
Scott "| 
 Moviusl 
 
 Thermal Changes 0} Hardened Steels 
 
 555 
 
 The curve shows three distinct regions: (1) 20 to 150 C, in 
 which the density increases as in a martensitic steel; (2) 150 to 
 250 C, in which a drop occurs which recalls the second stage of 
 the heat change of the austenitic steel; and (3) above 250 C, in 
 which it follows the normal course of a martensitic steel. Since 
 martensitization implies a decrease in density (see the black 
 circles of Fig. 1 1 , representing the density change on immersion 
 in liquid air), the second step with a density drop is evidently 
 attributable to completion of the change from austenite to mar- 
 tensite, which is more or less transformed into troostite. The 
 
 
 
 
 
 
 
 
 
 
 
 
 
 G/.66; A/r?,.09; S/, ./O 
 
 Quenc/iet? from /050"C 
 (flanrer) 
 
 
 
 
 
 
 'cm 3 
 V.75 
 
 \7.70 
 
 7.05 
 
 -ZOO +200 VOO 600 800 'C 
 
 Te/nfoer/hg femjberafure 
 
 Fig. 11. — Change in density with tempering temperature of 
 semiaustenitic carbon steel (Maurer) 
 
 augmentation of the heat evolution of the austenitic steels, as 
 previously explained, is therefore definitely verified. 
 
 V. SUMMARY 
 
 The transformation, observed as an evolution of heat, on 
 heating curves of hardened steel has been designated here as Ac t , 
 and its characteristics as revealed in carbon steels have been 
 investigated. The effect of several variables was noted with the 
 following conclusions: 
 
 1. An increase in the rate of heating raises markedly the tem- 
 perature of Ac t for a 0.95 per cent C martensitic steel and has a 
 yet more marked effect for an austenitic carbon steel. For zero 
 rate of heating there appears, however, to be little, if any, difference 
 between the principal temperatures, whether the steel is of high 
 or low carbon content or whether it is martensitic or austenitic. 
 The principal temperatures for the 0.95 per cent C martensitic 
 
556 Scientific Papers of the Bureau of Standards Wot. 16 
 
 steel were found to be 155, 250, and 260 C, respectively, for the 
 beginning, maximum, and end. 
 
 2. The results obtained for specimens tempered at different 
 temperatures before taking heating curves confirm substantially 
 the temperature of the end of Ac t just given. 
 
 3. Tempering for a short time at a temperature within the Ac t 
 range has an effect on the transformation characteristics similar 
 to tempering for a longer time at a somewhat lower temperature. 
 
 4. The heat evolution of the austenitic steel takes place in two 
 steps, the second being probably connected with the transition 
 from austenite to martensite. 
 
 5. A survey of the changes in some physical properties of mar- 
 tensitic carbon steels through the tempering range leads to the 
 conclusion that these changes are all directly related to the heat 
 evolution observed, but only in the case of the magnetic proper- 
 ties, coercive force, and maximum induction is the change of the 
 same type. 
 
 6. The change in density of a semiaustenitic carbon steel pro- 
 ceeds in steps similar to the heat evolution of the austenitic steel. 
 
 7. The changes in microstructure on tempering martensitic 
 steels are unquestionably related to the heat evolution, but 
 further study is necessary to establish fully this relation. The end 
 point (260 C for zero rate) of Ac t may very properly be taken as 
 the natural boundary between martensite and the troostite of 
 tempering, representing as it does the end of the transformation 
 suppressed on rapid cooling. 
 
 The competent assistance of H. A. Wadsworth has greatly 
 facilitated this investigation. 
 Washington, April 22, 1920. 
 
LIBRARY OF CONGRESS 
 
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