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DEPARTMENT OF COMMERCE 
 
 Scientific Papers 
 
 OP THE 
 
 Bureau of Standards 
 
 S. W. STRATTON, Director 
 
 No. 395 
 
 relation of the high-temperature treat- 
 ment OF HIGH-SPEED STEEL TO SECONDARY 
 HARDENING AND RED HARDNESS 
 
 BY 
 
 HOWARD SCOTT, Assistant Physicist 
 Bureau of Standards 
 
 SEPTEMBER 16, 1920 
 
 -^*2.fc«H ^ 
 
 PRICE. 10 CENTS 
 
 Sold only by the Superintendent of Documents. Government Printing Office 
 Washington, D. C. 
 
 WASHINGTON 
 
 GOVERNMENT PRINTING OFFICE 
 
 1920 
 
 
 h 
 
 is- 
 
I>. e»f 2. 
 nov . 19 (923 
 

 RELATION OF THE HIGH-TEMPERATURE TREAT- 
 MENT OF HIGH-SPEED STEEL TO SECONDARY 
 HARDENING AND RED HARDNESS 
 
 By Howard Scott 
 
 CONTENTS Page 
 
 I. Introduction 521 
 
 II. Physical properties and structure of high-speed steel 522 
 
 1 . Effect of quenching temperature 524 
 
 2. Effect of tempering temperature 527 
 
 III. Significance of the physical characteristics of high-speed steel 535 
 
 IV. Summary 536 
 
 I. INTRODUCTION 
 
 The metallography of high-speed tool steel presents certain 
 anomalies, the explanation of which is not clear from the usual 
 conceptions of the mechanism of hardening in simple carbon 
 steels, as, for example, the familiar polyhedral structure of prop- 
 erly quenched high-speed steel, which is often called austenite, 
 although its physical properties are largely those of martensite. 
 The explanation of such anomalies and a better understanding of 
 the fundamental nature of high-speed steel is becoming more and 
 more important as the peak of its development is being reached. 
 It is even probable that future improvements will be largely in 
 the technique of treatment and the adjustment of compositions 
 to meet special requirements, in which cases fundamentals are of 
 highest importance. 
 
 From this angle the problem of developing high-speed steel is 
 one of constitution and not composition. In spite of a wide 
 variety of compositions, the general characteristics are very 
 similar. The most effective method of attack is, therefore, 
 through the study of a number of the physical properties of a 
 high-speed steel and their correlation with the corresponding 
 characteristics of simple carbon tool steel, the recognized reference 
 standard. The relations for carbon steels between heat treat- 
 ment, micros true ture, and physical properties have been rather 
 thoroughly studied and are summarized in another paper. 1 
 
 1 Scott and Movius, see forthcoming paper, Thermal and Physical Changes Accompanying the Heating 
 of Hardened Carbon Steels. 
 
 S 21 
 
522 Scientific Papers of the Bureau of Standards [Vol. 16 
 
 To make the proposed correlation, the microstructiire, hard- 
 ness, density, magnetic properties, and thermal characteristics of 
 a typical high-speed steel as affected by various treatments were 
 studied. The variety of properties investigated necessitated the 
 cooperation of several other laboratories of the Bureau of Stand- 
 ards; H. S. Rawdon prepared the micrographs, E. L. Peffer made 
 the density determinations, R. L. Sanford the magnetic tests, 
 F. H. Tucker the chemical analyses, Miss H. G. Movius prepared 
 the thermal curves, and several assistants aided in the work. 
 
 The previously published researches related to the present one 
 are those of Edwards and Kikkawa, 3 Carpenter, 3 Yatsevich, 4 
 Mathews, 5 and Andrew and Green. 8 
 
 Edwards and Kikkawa determined the effect of tempering 
 temperature on the Brinell hardness of two series of chrome- 
 tungsten steels of constant carbon content, chromium being vari- 
 able in one series and tungsten in the other. A constant harden- 
 ing temperature of 1350 C was used with one exception, and the 
 density changes of one representative composition were deter- 
 mined. This work represents the most important contribution to 
 date to the study of the physical properties of high-speed steels. 
 
 Carpenter investigated the effect of quenching and tempering 
 temperatures on the structure and etching time of some high- 
 speed steels. 
 
 Carpenter, Yatsevich, and Andrew and Green studied the critical 
 ranges of high-speed steels as affected by maximum temperature 
 and rate of cooling. 
 
 Mathews summarized his wide experience with cutting and 
 physical tests of high-speed steel and reviewed developments 
 since the classic experiments of Taylor and White. 
 
 These papers represent work on high-speed steel of a wide variety 
 of compositions and treatments, so that it is very difficult to 
 establish any relation between the various properties studied. 
 The present work is, therefore, confined to one representative 
 type of modern high-speed steel. 
 
 II. PHYSICAL PROPERTIES AND STRUCTURE OF HIGH- 
 SPEED STEEL 
 
 For the purpose of this investigation tests were made on a 
 standard type of high-speed steel, chosen because it shows sec- 
 
 2 Edwards and Kikkawa, Jour. Iron and Steel Inst., 92, p. 6; 1915. 
 
 3 Carpenter, Jour. Iron and Steel Inst., 67, p. 433, 1905; 71, p. 377, 1906. 
 
 4 Yatsevich, Rev. deMet., 15, p. 65; 191s. 
 
 6 Mathews, Proc. A. S. T. M., 19, p. 141; 19:9. 
 
 8 Andrew and Green, Jour. Iron and Steel Inst., 99, p. 305; 1919. 
 
Scientific Papers of the Bureau of Standards, Vol. 16. 
 
 Fig. i. — Hardening cracks in high-speed steel 
 
 Specimens of steel B, i by % by 3 inches, were hardened as follows; a, Quenched in oil from 
 1030 C; surface of fracture parallel to 1 by 3 inches surface; ft, cooled in air from 1050 C; cracks 
 perpendicular to 1 by 3 inches surface; c, quenched in oil from 1050 C; fracture parallel and at 
 45 to 1 by 3 inches surface 
 
Scott] 
 
 Heat Treatment of High-Speed Steel 
 
 523 
 
 ondary hardening definitely. Unfortunately, sufficient material 
 of one composition was not available, so that another composi- 
 tion was used also, this being purchased to duplicate the first as 
 nearly as possible. As seen from the chemical composition given 
 in Table 1, the two steels are very similar except in carbon con- 
 tent, which is low in the B steel for the brand used. As pointed 
 out later, the lower carbon content, apparently, is responsible for 
 a poorer steel. 
 
 TABLE 1.— Results of Chemical Analyses of High-Speed Steels 
 
 No. 
 
 C 
 
 Mn 
 
 SI 
 
 W 
 
 Cr 
 
 V 
 
 P 
 
 s 
 
 A 
 
 Per cent 
 
 0.77 
 .65 
 
 Per cent 
 0.25 
 
 .31 
 
 Per cent 
 
 0.47 
 
 .17 
 
 Per cent 
 
 17.8 
 17.6 
 
 Per cent 
 3.5 
 3.4 
 
 Per cent 
 
 0.74 
 .73 
 
 Per cent 
 
 0.020 
 
 .004 
 
 Per cent 
 
 B 
 
 
 
 
 For hardening, the specimens were placed in an electrically 
 heated alundum tube furnace in which charcoal or illuminating 
 gas was burned to prevent excessive oxidation. The specimens 
 were slowly brought up to temperature and held there 1 5 minutes 
 before quenching. The quenching medium was a light mineral 
 oil. Tempering consisted of heating for 15 minutes in an oil 
 bath for temperatures up to 250 C, in a nitrate bath up to 
 6oo° C, and in a chloride bath for higher temperatures. The 
 use of these baths assured a uniform temperature quickly reached. 
 All high-temperature measurements were made with platinum 
 thermocouples. 
 
 The specimens were cut from a i-inch square bar (steel A), 
 and a 1 by Y% inch bar (steel B), both factory annealed. The 
 magnetic test 'specimens were of 1 cm square cross section, the 
 hardness specimens were of 1 by % inch (steel A), and 1 by y% 
 inch (steel B) section, and the density specimens about 1 by 1 
 by % inch (Fig. 7) and about one-half inch face cubes (Fig. 5). 
 Specimens for micrographs were taken from the ends of the 
 hardened magnetic test pieces. 
 
 In hardening, both steels usually cracked when quenched from 
 the region of 1050 C. The cracks followed the contour of the 
 specimens, thus indicating that they were characteristic of the 
 steel and not due to inclusions. The cracking was particularly 
 severe in the case of the low-carbon steel B, photographs of 
 typical samples of which are shown in Fig. 1 (a, b, and c). This 
 steel cracked also on cooling in air from 1050 C. It was, how- 
 
524 
 
 Scientific Papers of the Bureau of Standards 
 
 [Vol. 16 
 
 ever, possible to obtain specimens on which the physical measure- 
 ments could be satisfactorily made in spite of the cracks. 
 
 The density measurements were made by the usual method of 
 weighing in air and in water, the specimens being dipped in 
 alcohol prior to immersion in water to insure the absence of 
 bubbles. The magnetic measurements were made in a long 
 solenoid, corrections being made by the use of shearing curves. 
 Other tests were made by the usual standard methods. 
 
 1. EFFECT OF QUENCHING TEMPERATURE 
 
 It has long been recognized that raising the quenching 
 temperature increases the cutting efficiency of a high-speed steel 
 
 J3/n 
 
 /tooo 
 
 /oooo 
 
 8000 
 
 t>000 
 
 vooo 
 
 2000 
 
 ~i 1 1 r 
 
 C.t>5, W /r.s, Cr J.V, r.Y3 
 ooa Quenched /n o// 
 • ■ a Same sj&ec/merrs after 
 dijbfo/ng /n //au/d a/r 
 
 300 
 
 900 
 
 /000 
 
 //00 tfOO 
 
 80 
 
 70 
 
 60 
 
 SO 
 
 Quer?c/?/r?g temperature 
 
 /JOO'C 
 
 Fig. 4. — Relation of quenching temperature and subsequent 
 treatment in liquid air to maximum induction, residual 
 induction, and coercive force of steel B 
 
 tool, so that the highest temperature short of fusion is the best. 
 Observations were therefore made to determine the effect of 
 quenching temperature on the properties under consideration to 
 obtain evidence as to the nature of the constitutional changes. 
 
Scientific Papers of the Bureau of Standards, Vol. 16. 
 
 
 a -/7s rece/Vec/ 
 
 W£k" 
 
 6- 900 °C 
 
 
 C-97J °C 
 
 '4 
 
 nA • V\" ■ 
 
 • 
 
 e - //£5~°C 
 
 c/-/oso°c 
 
 f-/£00°C 
 
 Fig. 2. — Microstructure of specimens of steel A quenched from temperatures noted. 
 X500. Etched with 2 per cent alcoholic HN0 3 
 
 a. Annealed; (6), 900° C; (c), 975° C; (rf), 1050 C; c, 1125° C; /, 1200 C 
 
Scientific Papers of the Bureau of Standards, Vol. 16. 
 
 g>~^ZO °t 
 
 /7-/£90°CGnfer,or) 
 
 /'- /Z90°C (r?ear surface) 
 
 Fig. 3. — Microstructure of specimens of steel A quenched from temperatures noted. 
 X 500. Etched with 2 per cent alcoholic HN0 3 
 
 g, 1220 C; It. 1290° C, interior; i, 1290° C, structure near surface 
 
Scott] 
 
 Heat Treatment of High-Speed Steel 
 
 525 
 
 The microstructure of steel A as quenched from several tempera- 
 tures is shown in the micrographs of Figs. 2 and 3, the magnetic 
 properties in Fig. 4 (lower curves of shaded areas), and the 
 density in Fig. 5. The effect of quenching temperature on the 
 Brinell and scleroscope hardness (recording instrument) of steel 
 B is also shown in Fig. 5. 
 roo r 
 
 ^600 
 
 
 8b5 
 
 k 
 
 5 
 
 ■Stee/ _S -Sri net/ hardness 
 *5tee/ J? - Sc Zeros cO/be hardness 
 ■Sfee/ /? -JDens/fy 
 
 800 900 /000 /I00 /ZOO 
 
 Quenching temperature 
 
 /joo r 1 
 
 Fig. 5. — Relation of quenching temperature to Brinell and 
 scleroscope hardness of steel B and to density of steel A 
 
 These data permit of a classification of the quenched specimens 
 into two groups according to the nature of their physical character- 
 istics. The properties of the first group, quenched from tempera- 
 tures up to about 1100 C, vary in a manner distinct from those 
 of the second group, quenched from above that temperature. 
 Besides the change in slope of the curves, the physical properites 
 of the two groups are affected differently on cooling below ordi- 
 nary temperatures. Thus the magnetic properties of the speci- 
 mens quenched from the high temperatures are markedly in- 
 creased (Fig. 4, upper curve of shaded area) by immersion in 
 liquid air, while the specimens quenched from the low temperatures 
 remain practically unchanged. This is indicative of a con- 
 stitutional difference, other than a continuously changing one, 
 between the specimens of the two groups and has an important 
 bearing on the anomalies of high-speed steel. 
 
526 Scientific Papers of the Bureau of Standards [vu. tt 
 
 To distinguish between these two groups, specimens quenched 
 from the lower temperature range will be referred to as given the 
 low-temperature treatment, and those from the upoer range as 
 given the high-temperature treatment. 
 
 The microstructure of the samples given a low-temperature 
 quench is obscured by the excessive amount of free carbide im- 
 bedded in the apparently structureless matrix. The micro- 
 structure of the specimens of this series will be called martensite 
 from their physical characteristics, but it must be recognized that 
 this may be a misuse of the term, depending, of course, on its 
 definition. The specimens quenched from the high-temperature 
 range show well-defined grain boundaries in a structureless matrix 
 containing little free carbide. This structure is typical of properly 
 quenched high-speed steel and will be called polyhedral. 
 
 The polyhedral structure, smaller volume change, constancy 
 of hardness, and more rapid loss in magnetization with quenching 
 temperature in this range are all suggestive of austenitization, 
 though the polyhedral structure is not necessarily proof of it. It 
 is seen by extrapolation of the magnetic properties that zero 
 magnetization, and hence complete austenitization, would be 
 attained for a quenching of about 1450 C if this temperature 
 could be reached without fusion. That partial austenitization 
 has occurred on quenching from the high-temperature range is 
 shown, however, by the changes in physical properties on cool- 
 ing below ordinary temperatures. This treatment, if carried 
 to a low enough temperature, completes the A 3 transformation 
 with a corresponding change in physical properties, direct evi- 
 dence of previous austenitization. The effect on the magnetic 
 properties of immersion in liquid air is shown in Fig. 4. The 
 volume also increases on cooling below ordinary temperatures, a 
 drop in density of 0.059 g/cm 3 being observed when a specimen of 
 steel A, quenched from 1300 C, was cooled to — 45 ° C. It must, 
 therefore, be concluded that the specimens quenched from the 
 high-temperature range are constitutionally different from those 
 given the low-temperature treatment in that in the former case 
 the steel is partially austenitic, but is not in the latter. 
 
 This characteristic of high-speed steel may appear peculiar to 
 it, but on reference to the work of Maurer 7 one will find that more 
 or less partial austenitization is common to simple high-carbon 
 steel quenched from a high temperature, the degree depending, 
 of course, on the carbon content and the temperature. This 
 
 7 Maurer, Rev. deMet., 5, p. 711; 1908. 
 
Scoli] 
 
 Heat Treatment of High-Speed Steel 
 
 527 
 
 phenomenon is revealed by the change in density and in other 
 physical properties on immersion oi the steel in liquid air. 
 
 A further analogy between carbon and high-speed steel may be 
 seen by comparison of the effect of the quenching temperature 
 on the magnetic properties of high-speed steel (Fig. 4), with the 
 effect of the same variable on those of a carbon steel (Fig. 6). 
 
 /5000 
 
 /0000 
 
 5000 
 
 Cc 
 
 7r6on / 
 C A0O- 
 
 
 ee/ 
 
 
 
 
 V 
 
 
 
 V^- 
 
 
 
 
 h-/so 
 
 V. 
 
 so 
 
 vo 
 
 JO 
 
 20 
 
 900 °C 
 Qaenc/?/r>^ fem/iero/'u/'e 
 
 Fig. 6. — Relation of quenching temperature to 
 coercive force, maximum induction, and residual 
 reduction of carbon tool steel. (Gebert) 
 
 The data for the carbon steel were taken from Gebert. 8 The 
 similarity of these two figures is striking when the lack of any 
 similarity in microstructure is considered. 
 
 2. EFFECT OF TEMPERING TEMPERATURE 
 
 The most interesting feature of the tempering of high-speed 
 steel is the so-called "secondary hardening," which is revealed 
 as an increase in hardness over the original (or a previous minimum) 
 of certain high-speed steels given the high-temperature treatment 
 and tempered in the neighborhood of 6oo° C. This, at first sight 
 and in view of its absence in the usual carbon tool steels, is often 
 considered a mysterious phenomenon. However, when the 
 original condition of partial austenitization resulting from the 
 high-temperature treatment is considered, the phenomenon 
 appears quite natural. 
 
 •Gebert. Proc. A. S. T. M., 19, Part II; p. 117; 1919. 
 
 2371°— 20 
 
528 
 
 Scientific Papers of the Bureau of Standards 
 
 \Vol.i6 
 
 The effect of increasing the quenching temperature is to increase 
 the amount of dissolved carbide which lowers Ar" (Ar 3 _, of 
 martensitic steels, which is long and continuous for high-alloy 
 contents) progressively, until, for a quenching temperature of 
 about noo° C, its end reaches room temperature. Any further 
 increment of the quenching temperature — that is, quenching from 
 the high-temperature treatment range — will cause the end of Ar" 
 to fall below ordinary temperatures, with the result of partial 
 austenitization already noted. This phenomenon is analogous 
 to the lowering of Ar 3 in iron-nickel alloys by increasing the nickel 
 content, the essential difference being that in the preceding case 
 the composition of the matrix can be changed by tempering, but 
 it can not be so changed in the latter. 
 
 From the foregoing analysis it is evident that on tempering the 
 partially austenitic, and consequently somewhat softened, steel, the 
 dissolved carbide of the matrix will be gradually precipitated until 
 a stage is reached at which Ar" is no longer stable for the then 
 
 am 
 
 d.&0 
 
 ZOO UOO t>00 800'C 
 
 T~err?jber/n<? femjberature 
 
 Fig. 7. — Change in density of steel A with tem- 
 pering temperature 
 
 existing temperature and composition of the matrix. Ar" will 
 then complete itself as in the case of the austenitic carbon steel 
 discussed in another paper. 9 The consummation of Ar" implies 
 martensitization, and consequently an increase in hardness, which 
 the physical property and microstructure data verify. 
 
 9 See footnote 1. 
 
Sco«l 
 
 Heat Treatment of High-Speed Steel 
 
 529 
 
 
 Fig. 8.- 
 
 200 WO bOO dOO°C 
 
 Tempering temperature 
 
 -Change in scleroscope hardness of steel A 
 with tempering temperature 
 
 200 400 &00 800°C 
 
 Temper//7g temperature 
 
 Fig. g. — Change in coercive force of steel A with 
 temperimg temperature 
 
530 
 
 Scientific Papers of the Bureau of Standards 
 
 [Vol. 16 
 
 The effect of tempering temperature on the physical properties 
 of steel A quenched from several temperatures is shown in Figs. 
 7 (density), 8 (scleroscope hardness), 9 (coercive force) , 10 (maxi- 
 mum induction) , and 1 1 (residual induction) . The reciprocals of 
 the density values have been plotted to show more clearly their 
 
 JSOOO\ 
 
 ZOO WO 4,00 800°C 
 
 Tempering temperature 
 
 Fig. 10. — Change in maximum induction of steel A 
 with tempering temperature 
 
 relation to the hardness values. In Fig. 12 are given Brinell 
 and scleroscope curves for steel B. 
 
 The hardness curves (Figs. 8 and 12) show the phenomenon of 
 secondary hardening only when the steel was quenched from the 
 high-temperature range, above 1100 C, thereby confirming its 
 relation to partial austenitization noted only in specimens 
 quenched in the same temperature range. The curves represent- 
 ing the results of the low-temperature treatment are similar to 
 those of carbon steels with the exception that tempering occurs 
 at a much higher temperature, thus shortening the temperature 
 range over which troostite is stable, with the result that the 
 change from martensite to sorbite, or complete softening, is quite 
 
Scott] 
 
 Heat Treatment of High-Speed Steel 
 
 53i 
 
 abrupt. The density change is very small, indicating a slight 
 solubility of the carbide in the low-temperature treatment range. 
 
 Br 
 /ZOOO 
 
 /OOOO^ 
 I 
 
 8000 ^ 
 
 $000 
 
 C rr, W/7a,Crj.s,frf/ 
 Quenched /n o/7 
 from : 
 
 o 900"C 
 
 • /OSO'C 
 
 t/JOO °c 
 
 t./Z90 °C 
 
 VOOO 
 
 iooo 
 
 O ZOO V00 (,0O 800% 
 
 Tempering temperature 
 
 Fig. 11. — Change in residual induction of steel A 
 with tempering temperature 
 
 Of chief interest is the tempering of specimens given the high- 
 temperature — that is, the most efficient — hardening treatment. 
 The density curves (Fig. 7) show very markedly a peak between 
 600 and 700 C, which is parallel to the hardness curves and cor- 
 responds to a large increase in volume. This is direct evidence of 
 martensitization, and the magnetic properties offer further con- 
 firmation. The increase in maximum and residual induction 
 (Figs. 10 and 11), normally indicative of softening, accompanies 
 the rise in hardness in the secondary range. 
 
 Secondary hardening, as defined here, has been looked upon as 
 a feature peculiar to high-speed steel. The work of Maurer, how- 
 ever, shows that hypereutectoid carbon steels quenched from the 
 neighborhood of A cem exhibit the same phenomenon, as revealed 
 by density measurements, to a degree dependent on the amount 
 of carbide retained in solution. The only difference between the 
 high-carbon and the high-speed steel is that the peak comes at a 
 
532 
 
 Scientific Papers of the Bureau of Standards 
 
 [VU. 16 
 
 much lower temperature in the former case, and the tempering 
 below the peak is naturally more evident. The increase in inten- 
 sity of the peak with an increasing degree of austenitization is 
 evidence of a general relation between secondary hardening and 
 partial austenitization. 
 
 200 ¥00 600 SOO'C 
 
 Temjber/ng fem/berature m 
 
 Fig. 12. — Change in Brinell and scleroscope hard- 
 ness of steel B with tempering temperature 
 
 The conclusions arrived at from an examination of the physical 
 changes on tempering as related to the phenomenon of secondary 
 hardening should be capable of verification by observation of the 
 accompanying microstructural changes. Micrographs of speci- 
 mens of steel A quenched from 900, 1050, 1200, and 1290 C are 
 given in Figs. 13, 14, 15, and 16, respectively, as tempered at 
 several temperatures. The first two figures show the structure 
 resulting from a low-temperature treatment and the second two 
 from a high one. 
 
Scientific Papers of the Bureau of Standards, Vol. 16. 
 
 A) - 400 °C 
 
 £ - 400 °C 
 
 (rl * N, '** <* * 
 
 - 600 °c 
 
 of- roo°c 
 
 Fig. 13. — Microstruclure of steel A 
 quenched in oil from goo° C and 
 t, mpcrcd as noted. X500. Etched 
 with 2 per cent alcoholic HN0 3 
 
 a, 300° C; 6, 400° C; c, 600° C; d, 700 C 
 
 Fig. 14. — Microstructure of steel A 
 quenched in oil from 1050° C and 
 li mpered as noted. X500. Etched 
 with 2 per cent alcoholic HNt > 3 
 
 a, 300° C; 6, 400 C; c. 600 ° C; d, 700° C 
 
Scientific Papers of the Bureau of Standards, Vol. 16. 
 
 Fig. 15. — Microstructure of steel A 
 quenched in oil from 1200° C and 
 tempered as noted. X500. Etched 
 with 2 per cent alcoholic HN0 3 
 a; 200 ° C; b, 400° C; c, 600° C; d, 7o°° C 
 
 Fig. 16. — Microstructure of steel A 
 quenched in oil from I2go° C and 
 tempered as noted. X500. Etched 
 u ith 2 pel cent alcoholic IIN0 6 
 a, 200 C; 6, 400° C; c, 600° C; d, 700° C 
 
Scientific Papers of the Bureau of Standards, Vol. 16. 
 
 C7 - Quenched /h 
 od from /290°C, 
 temjbered a/ 600 °C. 
 
 
 
 
 a 
 
 . 1 
 
 d"K% 
 
 
 & 
 
 .0 h 
 
 s. * *•» 
 
 x^ - Quenched /n 
 o/7 from /300 °C 
 and differ/ in 
 //gu/'<d a/r. 
 
 Fig. i-. — Microstructure of steel A. X500. Etched with 2 per cent alcoholic H\0 3 
 
 a. Quenched in oil from 1290° C and tempered at 800° C; b, quenched in oil from 1300 C 
 and dipped in liquid air 
 
Scoe] Heat Treatment of High-SpeedSteel 533 
 
 Micrographs a and b of Figs. 1 3 and 1 4 (low-temperature treat- 
 ment) show an immature martensitic pattern resulting from 
 tempering at 300 and 400 C . At 6oo° C the structure is apparently 
 that of homogeneous troostite, and at 700 C the decomposition of 
 troostite into sorbite is well advanced. It may be noted on 
 referring to the relations between physical properties and micro- 
 structure of carbon steels 10 that the same general relations exist 
 for the high-speed steel quenched from the low-temperature range. 
 
 The micrographs of specimens given the high-temperature 
 treatment (Figs. 15 and 16) show a rather astonishing phenome- 
 non. Tempered at 200 and 400 C (micrographs a and b) , a well- 
 developed, needle-like pattern, suggestive of martensite, is pro- 
 duced. When it is considered that the steel is still partially 
 austenitic, the propriety of calling this constituent martensite is 
 questionable. When tempered at 600° C (micrograph c), where 
 secondary hardening first appears, a definite martensitic pattern 
 quite similar to that of martensitic carbon tool steel is developed. 
 For a tempering temperature of 700 C (micrograph d) , the struc- 
 ture is that of the first stages of troostite (of tempering) , and for a 
 temperature of 8oo° C (Fig. 17,0) it is that of sorbite, the structure 
 here being practically identical for all quenching temperatures and 
 similar to that of the annealed steel. 
 
 The microscopic evidence is positive and confirmatory of the 
 physical, namely, that the constituent accompanying the appear- 
 ance of secondary hardening is martensite. The nomenclature of 
 the patterns developed at 200 and 400 C must, however, await a 
 more precise definition of the constituent martensite. 
 
 From the relations pointed out in a previous paper 10 between 
 the heat evolution (Ac t ) observed on heating hardened carbon 
 steels and the accompanying changes in physical properties and 
 microstructure, it might be supposed that similar relations exist 
 for the high-speed steel. Heating curves were, therefore, taken 
 (Fig. 18) of steel A, quenched from three temperatures. The 
 inverse-rate method was used, and the thermal characteristics 
 noted on the curves are given in Table 2. There is evidence of a 
 slight heat evolution (Ac t ) between 600 and 68o° C, but it is not 
 sufficiently intense to allow of any positive conclusions. This is 
 probably due to the rather limited solubility of the carbide in the 
 matrix and the sluggishness of its precipitation, the rapid heating 
 rate required by thermal analysis allowing insufficient time for 
 its consummation (note the loss in intensity of Ac, with increased 
 quenching temperature) . 
 
 10 See footnote i. 
 
534 
 
 Scientific Papers of the Bureau of Standards 
 
 \ya. 16 
 
 'C 
 
 90O 
 
 800 
 
 700 
 
 toOO 
 
 500 
 
 930 'C 
 I 
 
 H/gh-Speect sfee/ 
 Hardened from : 
 
 /050°C 
 
 /200°C 
 
 \-rJc t f£ndl l- 
 ''. -. --/icffMoxj' 
 
 -i°<r t fMaxJ -\- 
 
 *t ? I 
 
 
 S 
 
 Time interval in seconds 
 /S 20 IS 20 tt 20 
 
 J_i 
 
 Fig. i8. — Heating curves of quenched specimens, 
 steel A 
 
 TABLE 2. — Transformation Characteristics of Steel A on Heating Following Quenching 
 
 Quenching 
 tempera- 
 ture 
 
 Rate of 
 heating 
 
 Ac 
 maximum 
 
 Ac, 
 
 Beginning 
 
 Maximum 
 
 End 
 
 •c 
 
 930 
 1050 
 1200 
 
 • C/sec. 
 0. 10 
 
 .11 
 .11 
 
 •c 
 
 765 
 763 
 760 
 
 •c 
 
 810 
 811 
 
 °C 
 
 823 
 821 
 815 
 
 •c 
 
 843 
 844 
 852 
 
 
 These curves are in substantial agreement with Carpenter's u 
 observations from differential curves on a tungsten-chromium 
 high-speed steel, with the exception that he does not recognize 
 the transformation Ac,, this due to the lack of its characteristic 
 features in differential curves. 
 
 11 See footnote 3. 
 
Swtt] Heat Treatment of High-Speed Steel 535 
 
 HI. SIGNIFICANCE OF THE PHYSICAL CHARACTERISTICS 
 OF HIGH-SPEED STEEL 
 
 Inasmuch as the high-grade high-speed steels have much in 
 common in spite of the wide range of compositions represented, 
 it is permissible to generalize somewhat from the observations 
 made on the familiar type of high-speed steel used here. It must 
 be remembered, however, that the degree of secondary hardening 
 is a function of the carbon and chromium content and also, probably 
 to a less degree, of the other alloying elements. 
 
 The rationale of the high-temperature treatment becomes evi- 
 dent from an analysis of the physical characteristics. By defining 
 red hardness as the resistance to softening by tempering, one may 
 see from the hardness versus tempering-temperature curves that 
 the red hardness increases slightly with quenching temperature. 
 This is evidently one potent reason for a high quenching tempera- 
 ture. The initial hardness also increases with the quenching 
 temperature, at least in the low-temperature range, and this is a 
 further highly desirable characteristic. With the increase in 
 hardness there is a corresponding increase in volume and conse- 
 quently in volume change on quenching, conditions which favor 
 the formation of cracks. If, however, the steel be hardened from 
 the high-temperature range with partial austenitization, the density 
 change is not increased, and the steel, even if not less hard, is less 
 brittle, thus counteracting its tendency to crack and furnishing 
 the most satisfactory combination of properties. 
 
 There has been much discussion as to the value of tempering 
 for maximum hardness or secondary hardening. This is evidently 
 largely a matter of composition for a given treatment, as this 
 factor determines the degree of secondary hardening possible. 
 For a steel showing this phenomenon definitely, the increased 
 hardness, if not accompanied by greater brittleness, which hardly 
 occurs in this case, is certainly of value in a tool, no matter what 
 its use. The increase in volume on tempering for secondary 
 hardening deserves some consideration on selecting a treatment 
 for a given tool. It is apparent that heating in service may change 
 the dimensions of an untempered tool to a troublesome degree. 
 Tempering for secondary hardening is, therefore, of general advan- 
 tage, but it is certainly not of any value to temper at a lower 
 temperature where these advantages do not accrue and where 
 practically the same degree of effort is expended. 
 
 The comparatively long temperature range, about ioo° C, 
 in which secondary hardening may be obtained suggests that a 
 considerable constitutional difference between the product of the 
 
536 Scientific Papers of the Bureau of Standards ivoi. 16 
 
 high and the low end of the range may exist, probably correspond- 
 ing respectively to martensite and troostite. This being the case, 
 it should be of interest to determine whether the low or high 
 temperatures giving the same degree of hardness will give the 
 better cutting results. 
 
 The sharpness of the changes in the magnetic properties and 
 in the density would indicate that these properties furnish a 
 valuable test for determining without destruction whether a tool 
 has been properly tempered or not. Of the hardness tests, the 
 Brinell is of little value on account of the very high degree of 
 hardness involved. The scleroscope, however, if properly 
 handled, is quite useful, although it is not as sensitive as the 
 Brinell to the advancement of the change from martensite to 
 troostite. On the whole the physical properties determined for 
 a hardened steel, while of course not furnishing a direct criterion 
 of its cutting efficiency, do offer a valuable indication of the 
 constitution of a given steel, and this is the principal value of 
 most physical and mechanical tests. 
 
 IV. SUMMARY 
 
 Attention is called to the importance of fundamental research 
 applied to high-speed steel and the value of physical tests for 
 this purpose. 
 
 The effect of heat treatment on the density, hardness, micro- 
 structure, magnetic properties, and thermal characteristics of 
 a standard brand of high-speed steel was determined. The inter- 
 pretation of these data permits the following conclusions: 
 
 1. A high-speed steel susceptible to secondary hardening is 
 partially austenitic when quenched from a temperature high 
 enough to produce this phenomenon. 
 
 2. The microstructure of steels hardened and tempered above 
 200 C is similar to that of carbon steels, although the same 
 nomenclature in certain cases is not permissible. 
 
 3. The behavior of the physical properties of high-speed steel 
 on heat treatment is analogous to that of hypereutectoid carbon 
 steel. 
 
 4. The following reasons are given for the use of the high heat 
 treatment: (a) Increase of red hardness; (b) increase of initial 
 hardness ; and (c) reduction of brittleness. 
 
 5. High-speed steel should preferably be tempered for secondary 
 hardness. 
 
 Washington, April 27, 1920. 
 
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