Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Episode 6 pot

Fig 16 Effect of austempering heat treatment on the performance of 52100 steel bearings Source: Ref References cited in this section The Influence of Microstructure on the Properties of Case-Hardened Components, American Society for Metals, 1980 H Muro, Y Sadaoka, S Ito, and N Tsushima, The Effect of Retained Austenite on the Rolling Fatigue of Carburized Steels, in Proceedings of the Twelfth Japanese Congress on Materials Research (Kyoto, Japan), Society of Materials Science, 1969 K Nakamura, K Mihara, Y Kibayashi,and T Naito, Improvement on the Fatigue Strength of CaseHardened Gears by a New Heat Treatment Process, in Analysis and Design of Off-Highway Powertrains, SP522, Society of Automotive Engineers, 1982 N Tsushima, H Nakashima, and K Maeda, "Improvement of Rolling Contact Fatigue Life of Carburized Tapered Roller Bearings," Paper 860725, presented at the Earthmoving Industry Conference (Peoria, IL), Society of Automotive Engineers, 1986 C A Stickels and A.M Janotik, Controlling Residual Stresses in 52100 Bearing Steel by Heat Treatment, Met Prog., Sept 1981 H Schlicht, Materials Properties Adapted to the Actual Stressing in a Rolling Bearing, Ball Roller Bearing Eng., Vol 1, 1981 I Sugiura, O Kato, N Tsushima, and H Muro, "Improvement of Rolling Bearing Fatigue Life under DebrisContaminated Lubrication by Decreasing the Crack-Sensitivity of the Material" Preprint 81-AM-1E-2, American Society of Lubrication Engineers, 1981 G.E Hollox, R.A Hobbs, and J.M Hampshire, Lower Bainite Bearings for Adverse Environments, Wear, Vol 68, 1981 Bearing Steels Harold Burrier, Jr., The Timken Company Special-Purpose Bearing Steels When bearing service temperatures exceed about 150 °C (300 °F), common low-alloy steels cannot maintain the necessary surface hardness to provide satisfactory fatigue life The low corrosion resistance of these steels makes them susceptible to attack by environmental moisture, as well as aggressive gaseous or liquid contaminants Therefore, specialized steels are often applied when these service conditions exist Table lists the compositions of certain bearing steels suited for high-temperature service These steels are typically alloyed with carbide-stabilizing elements such as chromium, molybdenum, vanadium, and silicon to improve their hot hardness and temper resistance The listed maximum operating temperatures are those at which the hardness at temperature falls below a minimum of 58 HRC Figure 17 compares the hot hardness behavior of some high-carbon tool and bearing steels to AISI 52100 steel Table indicates the effect of extended exposure to elevated temperatures on the recovered (room-temperature) hardness of various steels, both carburized and through-hardened Table Nominal compositions of high-temperature bearing steels Steel Maximum operating temperature(a) Composition, % C Mn Si Cr Ni Mo V Other °C °F M50 0.85 4.10 4.25 1.00 315 600 M50-NiL 0.13 0.25 0.20 4.20 3.40 4.25 1.20 315 600 Pyrowear 53 0.10 0.35 1.00 1.00 2.00 3.25 0.10 2.00 Cu 205 400 CBS-600 0.19 0.60 1.10 1.45 1.00 0.06 Al 230 450 Vasco X2-M 0.15 0.29 0.88 5.00 1.50 0.5 1.50 W 230 450 CBS-1000M 0.13 0.55 0.50 1.05 3.00 4.50 0.40 0.06 Al 315 600 (a) Maximum service temperature, based on a minimum hot hardness of 58 HRC Table Room-temperature hardness of CBS-600 and CBS-1000 after exposure up to 540 °C (1000 °F) Steel type Exposure time, 103h Hardness as heat treated, HRC Minimum HRC after exposure for indicated time at °C ( °F) 205 (400) 260 (500) 315 (600) 370 (700) 425 (800) 480 (900) 540 (1000) Hardness of case layers (0.70-1.0% C) CBS-600 62 60 60 60 57 CBS-1000M 60 60 60 60 60 60 60 51 CBS-1000M(a) 60 59 59 59 58 58 9310 60 58 55 53 8620 60 58 56 53 47 52100 61 58 56 53 47 M50 62 62 62 62 62 62 60 52 Hardness of core regions(b) CBS-600 41 41 41 41 41 CBS-1000M 46 46 46 46 46 46 42 28 CBS-1000M(a) 44 44 44 44 44 44 40 27 (a) Oil quenched from 955 °C (1750 °F) rather than from the standard temperature of 1095 °C (2000 °F) (b) Core carbon 0.20% for CBS-600; 0.15% for CBS-1000M Fig 17 Hot hardness of homogeneous high-carbon steels for service above 150 °C (300 °F) The line at 58 HRC indicates the maximum service temperature at which a basic dynamic load capacity of about 2100 MPa (300 ksi) can be supported in bearings and gears Source: Ref An important application of the high-temperature bearing steels is aircraft and stationary turbine engines Bearings made from M50 steel have been used in engine applications for many years Jet engine speeds are being continually increased in order to improve performance and efficiency; therefore, the bearing materials used in these engines must have increased section toughness to withstand the stresses that result from higher centrifugal forces (Fig 18) For this reason, the carburizing high-temperature bearing steels, such as M50-NiL and CBS-1000M, are receiving much attention The core toughness of these steels is more than twice that of the through-hardening steels Figure 19 compares the case and core fracture toughness of some of the common through-hardening and carburizing bearing steels Figure 20 illustrates another benefit of the carburizing steels by comparing the compressive residual stress gradient present in carburized races with the tensile residual stresses found in through-hardened races The presence of compressive residual stresses may help to retard the propagation of radial fatigue cracks through the races cross-section Fig 18 Increasing section toughness of bearing materials used for jet engine applications (a) Trend in aircraft engine main bearing in units of DN, the bearing bore diameter in millimeters multiplied by the rotation of the shaft in revolutions per minute (b) Estimated inner race tangential stress versus bearing DN Source: Ref 10 Fig 19 Composite fracture toughness of carburizing and homogeneous high-carbon steels in slow bending Case depth is 0.76 to 0.89 mm (0.030 to 0.035 in.) to 0.50% C level Shaded areas indicate range of K values for cracks originating in core Cross-hatched areas indicate range of K values for cracks originating in case Charpy-sized specimens were carburized, hardened, tempered, and precracked to several depths in case and core regions before testing As cracks progress inward, the fracture resistance of carburized composites improves significantly Source: Ref Fig 20 Comparison of residual stresses in carburized versus through-hardened steel races The higher residual compression of carburizing M50-NiL provides greater resistance to fracture, fatigue damage, and stress corrosion Source: Ref 11 In general, high temperature carburizing steels require more care in the carburizing process than conventional low-alloy carburizing steels Because of the high content of chromium and silicon in the high-temperature steels, some precarburizing treatment, such as preoxidation, is always necessary to promote satisfactory carburizing Bearings that require the highest corrosion resistance necessitate the use of stainless grades with greater than 12% Cr At this time, no satisfactory carburizing technique has been developed for these grades Thus, all corrosion-resistant bearing steels are of the through-hardening type (Table 7) Steels such as the 440C modification, CRB-7, and BG42 also offer good high-temperature hardness Figure 21 compares the hot hardness and hardness retention properties of selected corrosion-resistant steels to those of 52100 and M50 steels Table Corrosion-resistant bearing steels Grade Composition, % C Mn Si Cr Mo W V Nb BG42 1.15 0.50 0.30 14.50 4.0 1.2 440C 1.00 0.40 0.30 17.00 0.50 440C modified 1.05 0.40 0.30 14.00 4.00 Fig 21 Hardness properties of selected bearing steels (a) Hot hardness values for several steels RT, room temperature (b) Rockwell C room-temperature hardness after exposure at 480 °C (900 °F) Source: Ref 12, 13 References cited in this section C.F Jatczak, Specialty Carburizing Steels for High Temperature Service, Met Prog., April 1978 10 E.N Bamberger, B.L Averbach, and P.K Pearson, "Improved Fracture Toughness Bearings," AFWALTR-84-2103, Air Force Wright Aeronautical Laboratories, AFSC, Jan 1985 11 T V Philip, New Bearing Steel Beats Speed and Heat, Power Transmission Des., June 1986 12 "LESCALLOY BG42," Data Sheet, Latrobe Steel Company 13 T.V Philip, A New Bearing Steel; A New Hot Work Die Steel, Met Prog., Feb 1980 Bearing Steels Harold Burrier, Jr., The Timken Company References The Influence of Microstructure on the Properties of Case-Hardened Components, American Society for Metals, 1980 H Muro, Y Sadaoka, S Ito, and N Tsushima, The Effect of Retained Austenite on the Rolling Fatigue of Carburized Steels, in Proceedings of the Twelfth Japanese Congress on Materials Research (Kyoto, Japan), Society of Materials Science, 1969 K Nakamura, K Mihara, Y Kibayashi,and T Naito, Improvement on the Fatigue Strength of CaseHardened Gears by a New Heat Treatment Process, in Analysis and Design of Off-Highway Powertrains, SP-522, Society of Automotive Engineers, 1982 N Tsushima, H Nakashima, and K Maeda, "Improvement of Rolling Contact Fatigue Life of Carburized Tapered Roller Bearings," Paper 860725, presented at the Earthmoving Industry Conference (Peoria, IL), Society of Automotive Engineers, 1986 C A Stickels and A.M Janotik, Controlling Residual Stresses in 52100 Bearing Steel by Heat Treatment, Met Prog., Sept 1981 H Schlicht, Materials Properties Adapted to the Actual Stressing in a Rolling Bearing, Ball Roller Bearing Eng., Vol 1, 1981 I Sugiura, O Kato, N Tsushima, and H Muro, "Improvement of Rolling Bearing Fatigue Life under Debris-Contaminated Lubrication by Decreasing the Crack-Sensitivity of the Material" Preprint 81-AM1E-2, American Society of Lubrication Engineers, 1981 G.E Hollox, R.A Hobbs, and J.M Hampshire, Lower Bainite Bearings for Adverse Environments, Wear, Vol 68, 1981 C.F Jatczak, Specialty Carburizing Steels for High Temperature Service, Met Prog., April 1978 E.N Bamberger, B.L Averbach, and P.K Pearson, "Improved Fracture Toughness Bearings," AFWALTR-84-2103, Air Force Wright Aeronautical Laboratories, AFSC, Jan 1985 T V Philip, New Bearing Steel Beats Speed and Heat, Power Transmission Des., June 1986 "LESCALLOY BG42," Data Sheet, Latrobe Steel Company T.V Philip, A New Bearing Steel; A New Hot Work Die Steel, Met Prog., Feb 1980 10 11 12 13 Bearing Steels Harold Burrier, Jr., The Timken Company Selected References • W.F Burd, A Carburizing Gear Steel for Elevated Temperatures, Met Prog., May 1985 • H.I Burrier, Jr., Alloy Substitution for Flexibility and Performance, in Proceedings of the Workshop on Conservation and Substitution Technology for Critical Metals in Bearings and Related Components for Industrial Equipment and Opportunities for Improved Bearing Performance, United States Bureau of Mines/Vanderbilt University, 1984 • C.F Jatczak, Hardenability in High Carbon Steels, Metall Trans., Vol 4, 1973 • J.D Stover and R.V Kolarik, Air-Melted Steel With Ultra-Low Inclusion Stringer Content Further Improves Bearing Fatigue Life, in Proceedings of the 4th International Conference on Automotive Engineering, SAE 871 20B, Society of Automotive Engineers High-Strength Structural and High-Strength Low-Alloy Steels Introduction HIGH-STRENGTH carbon and low-alloy steels have yield strengths greater than 275 MPa (40 ksi) and can be more or less divided into four classifications: • • • • As-rolled carbon-manganese steels As-rolled high-strength low-alloy (HSLA) steels (which are also known as microalloyed steels) Heat-treated (normalized or quenched and tempered) carbon steels Heat-treated low-alloy steels These four types of steels have higher yield strengths than mild carbon steel in the as-hot-rolled condition (Table 1) The heat-treated low-alloy steels and the as-rolled HSLA steels also provide lower ductile-to-brittle transition temperatures than carbon steels (Fig 1) Table General comparison of mild (low-carbon) steel with various high-strength steels Steel Chemical composition, %(a) Minimum yield strength Minimum tensile strength Minimum ductility (elongation in 50 mm, or in.), % C (max) Mn Si Other MPa ksi MPa ksi Low-carbon steel 0.29 0.601.35 0.150.40 (b) 170250 2536 310415 45-60 23-30 As-hot rolled carbonmanganese steel 0.40 1.001.65 0.150.40 250400 3658 415690 60100 15-20 HSLA steel 0.08 1.30 max 0.150.40 0.20 Nb or 0.05 V 275450 4065 415550 60-80 18-24 Normalized(b) 0.36 0.90 max 0.150.40 200 29 415 60 24 Quenched and tempered 0.20 1.50 max 0.150.30 0.0005 B 550690 80100 660760 95110 18 0.21 0.450.70 0.200.35 0.45-0.65 Mo, 0.001-0.005 B 620690 90100 720800 105115 17-18 Heat-treated carbon steel Quenched and tempered low-alloy steel (a) Typical compositions include 0.04% P (max) and 0.05% S (max) (b) If copper is specified, the minimum is 0.20% Figure shows the relationship between hardness and tensile strength for hardened and tempered, as-rolled, annealed, or normalized carbon and alloy constructional steels Because of the effect of cold working, this relationship may not apply to cold-drawn steels Figure shows the relation between tensile strength and yield strength The effect of tempering temperature on tensile strength and hardness is shown in Fig Fig Relation between tensile strength and Brinell hardness for steels in the as-rolled, normalized, or quenched and tempered condition The tensile strength in ksi is approximately one-half the Brinell hardness number and in MPa is approximately times the Brinell hardness number Fig Relation between tensile strength and yield strength for quenched and tempered steels Source: Ref Fig Effect of tempering temperature on tensile strength and hardness of hardened carbon and alloy steels with carbon contents of 0.50 and 0.30% An important exception to this similarity of properties is the relationship between tensile strength and the reduction in area Figure shows the direct relationship between ductility and hardness and illustrates that the reduction in area decreases as hardness increases and, for a given hardness, the reduction is higher for alloy steels than for plain carbon steels Figures 5, 6, 7, and further reinforce the contention that, despite some differences in certain properties, the major difference between carbon and alloy steels is hardenability Fig Relation between tensile strength and the reduction in area for quenched and tempered steels One other and sometimes important difference between carbon and alloy steels is that, for the same hardness levels, fully quenched alloy steels require higher tempering temperatures than carbon steels This higher tempering temperature is presumed to reduce the stress level in the finished parts without impairing mechanical properties Property relations given in Fig 5, 6, 7, and illustrate general correlations among mechanical properties of steels Normal variations in composition and grain size from heat to heat and even within one heat produce a considerable scatter of results in sections of the same size Changes in section size have a greater influence on the mechanical properties of carbon steels (particularly when quenched and tempered) than on the properties of alloy steels because of the lower hardenability of carbon steels The section size of a heat-treated section affects not only specific properties, but also the relation of one property to another As the section size increases, incomplete response to hardening lowers the ratio of yield strength to tensile strength Figure shows mechanical property relations for 1030 steel in both the as-rolled and quenched and tempered conditions, as a function of section size The tensile strength decreases as the section size increases for a given composition and heat treatment, and there is some lowering of the ratio of yield to tensile strength Fig Effect of processing variables on mechanical properties of 1030 steel Billets of 1030 steel were either forged to 25 mm (1 in.) or 57.15 mm (2.25 in.) in diameter, then quenched and tempered, or they were hot rolled to 25 mm (1 in.) in diameter and not heat treated Heat-treated specimens were water quenched from 870 °C (1600 °F) and tempered at 535 to 650 °C (1000 to 1200 °F) Specimens were taken from the center of 25 mm (1 in.) bars and at half-radius from the 57.15 mm (2.25 in.) bars Reference lines represent mean values for the forged and heat-treated 25 mm (1 in.) diam bars As the section size increases and the hardness and strength levels increase, the hardenability of carbon steels is no longer adequate, and alloy steels must be used The H-band alloy steels that should be considered for different design yield strength levels for highly stressed parts are given in Table for round sections up to 102 mm (4 in.) in diameter Either oil quenching or water quenching these alloys, as noted, should given 80% martensite at the indicated location within the section For moderately stressed parts, a 50% martensite structure at the center is frequently adequate The H-band alloy steels that can produce 50% martensite at the indicated location by oil or water quenching are given in Table for round sections up to 102 mm (4 in.) in diameter This information is intended to be a guide in the selection of an appropriate alloy steel; variations in equipment and techniques will greatly influence the final properties obtained The grades are listed in the approximate order of increasing alloy cost to further aid in the initial screening of candidate alloys Table Alloy steel selection guide for highly stressed parts Unless otherwise indicated in the footnotes, any steel in this table may be considered for a lower strength level or a smaller section, or both Required yield strength As-tempered hardness Steels to give 80% martensite, minimum, for indicated location in a round section of indicated diameter At center MPa ksi HRC HB ≤13 mm ( At midradius 13-25 mm in.) ( -1 in.) 25-38 mm (1-1 in.) At 38-50 mm (1 -2 radius 50-63 mm (2-2 in.) in.) 63-75 mm (2 -3 in.) 75-89 mm (3-3 in.) 89-102 mm (3 -4 in.) Oil quenched and tempered 620860(a) 90125(b) 23-30 241285 1330H 5132H 4130H 8630H 8601030(c) 125150 3036(d) 285341 1335H 5135H 4135H 8640H 94B30H 8740H 4137H 4142H 9840H 10301170(e) 150170 3641(f) 331375 1340H 5140H 4135H 8637H 94B30H 3140H 50B40H 4137H 8642H 8645H 8742H 4140H 94B40H 4145H 9840H 86B45H 4337H 11701275(g) 170185 4146(h) 375429 50B46H 5145H 50B40H 4140H 8640H 8642H 8645H 8740H 8742H 5155H 50B44H 5147H 94B40H 6150H 81B45H 4142H 4145H 8650H 8655H 4337H 86B45H 9840H 4147H 4340H 4150H >127(i) >185 46 min(j) 429 5150H 5155H 50B44H 5147H 9260H 81B45H 8650H 86B45H 6150H 5160H 50B50H 9262H 4147H 8655H 50B60H 51B60H 8660H 4150H Water quenched and tempered(k) 94B30H 4337H 4340H 4340H 9805H E4340H 9850H 620860(a) 90125 2330(b) 241285 5130H 5132H 4130H 8630H 5135H 8601030(c) 125150 3036(d) 285341 1330H 5135H 1335H 4135H(l) 8640H(l) 8740H(l) 3140H(l) 10301170(e) 150170 3641(f) 331375 1330H 1335H 5130H 5132H 5135H 4130H 8630H 4042H 4047H 1340H 50B46H 5140H 4135H 8637H 94B30H 3140H 11701275(g) 170185 4146(h) 375429 5140H 4037H 4042H 4137H 8637H 1340H 50B46H 3140H 5145H 50B40H 8640H 8642H 8740H >1275(i) >185 46 min(j) 429 5046H 50B46H 5145H 4047H 4142H 8642H 5147H 4145H 8645H 86B45H 50B44H Source: Ref (a) Tensile strength, 790 to 940 MPa (115 to 138 ksi) (b) As-quenched hardness, 42 HRC, or 388 HB (c) Tensile strength, 940 to 1100 MPa (136 to 160 ksi) (d) As-quenched hardness, 44 HRC, or 415 HB (e) Tensile strength, 1100 to 1300 MPa (160 to 188 ksi) (f) As-quenched hardness, 48 HRC, or 461 HB (g) Tensile strength, 1300 to 1530 MPa (188 to 222 ksi) 4135H 94B30H 1340H(m) 8637H(m) 50B40H 8642H 94B30H 4137H 4140H 94B40H 50B40H(l) 4137H(l) 8642H(l) 8745H(l) 8640H(m) 8740H(m) 50B44H 5147H 4140H 8645H 8742H 94B40H 81B45H 4142H 4337H 50B44H(l) 5147H(l) 81B45H(l) 94B40H(l) 4140H(m) 8645H(m) 8742H(m) 4142H 81B45H 4337H 4145H 4147H 86B45H 9840H 4340H E4340H 81B45H(m) 4147H (h) As-quenched hardness, 51 HRC, or 495 HB (i) Tensile strength, over 1530 MPa (222 ksi) (j) As-quenched hardness, 55 HRC, or 555 HB (k) Through steels with 0.47% C nominal (l) May be substituted for steels listed under the 50 to 63 mm (2 to 2 in.) column at same strength level or less (m) Not recommended for applications requiring 80% martensite at midradius in sections 38 to 50 mm (1 to in.) in diameter because of insufficient hardenability Table Alloy steel selection guide for moderately stressed parts Unless otherwise indicated in the footnotes, any steel in this table may be considered for a lower strength level or a smaller section, or both Required yield strength As-tempered hardness Steels to give 50% martensite, minimum, for indicated location in a round section of indicated diameter At center At midradius At radius HRC HB 13-25 mm 25-38 mm 38-50 mm 50-63 mm ( -1 (1-1 (1 -2 (2-2 in.) ksi ≤13 mm ( MPa in.) in.) in.) 4140H 94B40H 4142H in.) in.) 63-75 mm (2 -3 75-89 mm (3-3 89-102 mm (3 -4 in.) in.) 4337H 4340H Oil quenched and tempered 620860(a) 90125 2330(b) 241285 1330H 5132H 4130H 8630H 8737H 50B40H 8642H 94B30H 8740H 3140H 8601030(c) 125150 3036(d) 285341 1335H 4042H 4047H 5135H 4135H 8640H 94B30H 8740H 3140H 50B44H 5147H 4137H 8645H 8742H 1030- 150- 36- 331- 1340H 5140H 5150H 50B40H 5160H 50B50H 4142H 51B60H 4145H 4147H 86B45H 9840H 4145H 4147H 86B45H 4150H 1170(e) 170 41(f) 375 4135H 8637H 94B30H 3140H 4137H 8642H 8645H 8742H 4140H 94B40H 6150H 8655H 9840H 4337H 11701275(g) 170185 4146(h) 375429 5145H 50B40H 50B46H 4063H 4140H 8640H 8642H 8745H 8740H 8742H 5155H 50B44H 5147H 94B40H 6150H 81B45H 4142H 4145H 8650H 8655H 4337H 86B45H 9840H 4147H 8660H 4340H 4150H >1275(i) >185 46 min(j) 429 5150H 5155H 50B44H 5147H 9260H 81B45H 8650H 86B45H 6150H 5160H 50B50H 9262H 4147H 8655H 50B60H 51B60H 8660H 4150H 4340H 9850H E4340H 9850H Water quenched and tempered(k) 620860(a) 90125 2330(b) 241285 4037H 5130H 5132H 4130H 8630H 5135H 8637H(l) 5140H(m) 4135H 50B40H 8642H 94B30H 3140H 4137H 8601030(c) 125150 3036(d) 285341 1330H 5135H 1335H 4135H(l) 1340H(m) 8637H(m) 50B40H 8640H 8642H 94B30H 8740H 3140H 50B44H 5147H 4137H 8645H 8742H 4140H 94B40H 10301170(e) 150170 3641(f) 331375 1330H 1335H 5130H 5132H 5135H 4130H 8620H 4042H 4047H 1340H 50B46H 5140H 4135H 8637H 94B30H 3140H 50B40H(l) 4137H(l) 8642H(l) 8640H(m) 8740H(m) 50B44H 5147H 4140H 8645H 8742H 94B40H 81B45H 4142H 4337H 11701275(g) 170185 4146(h) 375429 5140H 4037H 4042H 4137H 8637H 1340H 50B46H 3140H 5145H 50B40H 8640H 8642H 8740H 50B44H(l) 5147H(l) 94B40H(l) 4140H(m) 8645H(m) 8742H(m) 4142H 81B45H 4337H 4145H 4147H 86B45H 9840H 4340H E4340H >1275(i) >185 46 429 5046H 50B46H 5147H 4145H 50B44H 81B45H(m) 4147H min(j) 5145H 4047H 4142H 8742H 8645H 86B45H Source: Ref (a) Tensile strength, 790 to 940 MPa (115 to 136 ksi) (b) As-quenched hardness, 42 HRC, or 388 HB (c) Tensile strength, 940 to 1100 MPa (136 to 160 ksi) (d) As-quenched hardness, 44 HRC, or 415 HB (e) Tensile strength, 1100 to 1300 MPa (160 to 188 ksi) (f) As-quenched hardness, 48 HRC, or 461 HB (g) Tensile strength, 1300 to 1530 MPa (188 to 222 ksi) (h) As-quenched hardness, 51 HRC, or 495 HB (i) Tensile strength, over 1530 MPa (222 ksi) (j) As-quenched hardness, 55 HRC, or 555 HB (k) Through steels with 0.47% C nominal (l) May be substituted for steels listed under the 50 to 63 mm (2 to 2 in.) column at same strength level or less (m) Not recommended for applications requiring 50% martensite at midradius in sections 38 to 50 mm (1 to in.) in diameter because of insufficient hardenability Increasing carbon content consistently increases tensile and yield strength and decreases elongation and reduction in area, regardless of whether the steel is as-rolled or quenched and tempered (provided the ranges of tempering temperatures are the same) However, there is one major disadvantage to increasing the carbon content: Carbon steels show an increasing tendency to crack on quenching as the carbon content increases above about the 0.35% level Consequently, parts to be made from steel having a carbon content greater than 0.35% should be tested for quench cracking before production is begun Variations in chemical composition within a specific grade contribute to the scatter of mechanical properties This is illustrated by the test data in Fig 10, where the properties for two heats of quenched and tempered 1050 steel are compared for a tempering range of 315 to 650 °C (600 to 1200 °F) Fig 10 Effect of composition and tempering temperature on mechanical properties of 1050 steel Properties are summarized for two heats of 1050 steel that was forged to 38 mm (1.50 in.) in diameter, then water quenched and tempered at various temperatures Open symbols are for heats containing 0.52 C and 0.93 Mn; closed symbols, for those containing 0.48 C and 0.57 Mn References cited in this section 1989 SAE Handbook, Vol 1, Materials, Society of Automotive Engineers, 1989 Republic Alloy Steels, Republic Steel Corporation, 1961 Hardenable Carbon and Low-Alloy Steels Revised by Eugene R Kuch, Gardner Denver Division of Cooper Industries Tempering Hardened steels are softened by reheating, although this effect may not be sought in tempering The real need is to increase the capability of the steel to flow moderately without fracture, and this is inevitably accompanied by a loss of strength The tensile strength is very closely related to hardness in this class of steels, as heat treated; thus, the effects of tempering can be followed by measuring the Brinell or Rockwell hardness Figure 11 shows the response to tempering of four carbon and alloy steels containing 0.45% C All steels were tempered for h at the temperatures indicated Somewhat shorter or longer intervals at temperature would affect hardness values to various degrees, depending on the tempering temperature Fig 11 Tempering characteristics of four 0.45% carbon and alloy steels tempered for h The general effect of alloying is to retard the tempering rate, and therefore alloy steels require a higher tempering temperature to obtain a given hardness than does carbon steel of the same carbon content However, the individual elements show significant differences in the magnitude of their retarding effect Nickel, silicon, aluminum, and, to a large extent, manganese, all of which have little or no tendency to occur in the carbide phase and merely remain dissolved in ferrite, have only a minor effect on the hardness of the tempered steel, as would be expected from the general pattern of solid-solution hardening However, the carbide-forming elements, chromium, molybdenum, and vanadium, retard softening, particularly at higher tempering temperatures These elements not merely raise the tempering temperature; when they are present in higher percentages, the rate of tempering is no longer a continuous function of tempering temperature That is, the tempering curves for these steels will show a range of tempering temperature in which the tempering is retarded or, with relatively high alloy content, in which the hardness may actually increase with an increase in tempering temperature This characteristic behavior is known as secondary hardening and results from a delayed precipitation of fine alloy carbides Secondary hardening is most often encountered in the higher-alloy tool steels As mentioned previously, the primary purpose of tempering is to impart plasticity or toughness to the steel, and the loss in strength is only incidental to this very important increase in toughness The increase in toughness after tempering reflects two effects of tempering: • • The relief of residual stress induced during quenching The precipitation, coalescence, and spheroidization of iron and alloy carbides, resulting in a microstructure of greater plasticity In addition to their effects on microstructure, the alloying elements have a secondary function The higher tempering temperatures for a given hardness, which has been determined to be characteristic of alloy steels (particularly those containing carbide-forming elements), will presumably permit greater relaxation of residual stress and thereby improve properties Furthermore, as discussed in the section "Alloying Elements in Quenching" in this article, the hardenability of these steels may permit the use of less drastic quenching practices, so that the stress level before tempering will be lower, permitting these steels to be used at a higher level of hardness; this is because higher temperatures are not required for relief of quenching stresses It should be noted, however, that this latter characteristic is only a secondary function of alloying elements in tempering; the effect primarily reflects the hardenability function of the alloying elements Another secondary function of alloying elements in tempering is to permit the use of steels with lower carbon content for a given level of hardness, because adequate tempering may be ensured by the retardation of softening caused by alloying This results in greater freedom from cracking and generally improved plasticity at any given hardness Here again, the function of alloying elements in tempering is a secondary function; their primary function is to increase hardenability sufficiently to offset the effect of a decreased carbon content The increase in plasticity upon tempering is discontinuous in those alloy steels that contain the carbide-forming elements; the behavior of notched specimens shows a characteristic irregularity at approximately 260 to 315 °C (500 to 600 °F) The quenched martensitic steel gains toughness, as reflected in a notched-bar impact test, by tempering at temperatures as high as 205 °C (400 °F) However, after tempering at higher temperatures, in the temper-brittle range, these types of steel lose toughness until they may be less tough than the same steels not tempered Still higher tempering temperatures restore greater toughness (see Fig 12) Fig 12 Hardness and notch toughness of 4140 steel tempered for h at various temperatures The mechanism of this behavior is not fully understood, but it seems to be associated with the first precipitation of carbide particles and is presumably a grain boundary phenomenon; fractures of steels tempered in this region tend to follow intergranular paths Thus, there is a range of tempering temperatures at about 205 to 370 °C (400 to 700 °F) never used for these steels; the tempering temperature is either below 205 °C (400 °F) or above 370 °C (700 °F) Although this phenomenon is common to all of these alloy steels, the alloying elements have a secondary function in this connection; a combination of carbon and alloy contents of suitable hardenability may be chosen that would permit tempering to the desired strength at temperatures outside this undesirable range Temper brittleness is another example of a discontinuous increase in plasticity subsequent to the tempering of steels containing the carbide-forming elements This phenomenonis manifested as a loss of toughness, observed after slow cooling from tempering temperatures of 575 °C (1070 °F) or higher or after tempering in the temperature range between approximately 375 and 575 °C (700 and 1070 °F) Thus, a steel that is susceptible to temper embrittlement may lose much of its plasticity, as indicated by a notched-bar impact test, during slow cooling from a high tempering temperature, although the same steel will be very tough if it is quenched from the same tempering temperature This expedient of quenching from the tempering temperature is often overlooked as a practical means for avoiding sever temper embrittlement in susceptible steels tempered at 575 °C (1070 °F) or higher In steels susceptible to temper brittleness, embrittlement will also be observed after tempering at 375 to 575 °C (700 to 1070 °F), particularly if the tempering times are protracted Under such circumstances, quenching from the tempering temperature will never restore the toughness High manganese, phosphorus, and chromium concentrations appear to accentuate the embrittling reaction; molybdenum has a definite retarding effect Here again, the carbon and alloying elements may be chosen so that the susceptibility to temper embrittlement is minimized or the desired strength level is obtained by tempering either below 375 °C (700 °F) or above 575 °C (1070 °F) and then quenching Temper brittleness is discussed in greater detail in the article"Embrittlement of Steels" in this Volume Hardenable Carbon and Low-Alloy Steels Revised by Eugene R Kuch, Gardner Denver Division of Cooper Industries Distortion in Heat Treatment Distortion during heat treatment may occur with almost any hardenable carbon or alloy steel, although distortion is usually more severe for carbon grades than for alloy grades of equivalent carbon content Carbon steels distort more than alloy steels mainly because carbon steels require a water or brine quench to develop full hardness (at least in sections thicker than about 9.5 mm, or in.) This often eliminates carbon steels from consideration for critical parts This distortion may be observed as a change in dimensions (size distortion) or a change in configuration or contour (shape distortion or warpage), or both A more complete discussion of these types of distortion and the factors that influence them may be found in Ref 4, 5, and Several factors contribute to the total distortion that occurs during heat treatment These include residual stresses that may be present as a result of machining or other cold-working operations, the method of placing in the furnace, the rate of heating, nonuniform heating, and the normal volumetric changes that occur with phase transformations However, the most important, single factor is uneven cooling during quenching, caused mainly by the configuration and by changes in cross-sectional area Symmetrical parts with little or no variation in section may have almost no distortion, whereas complex parts with wide variations in section may distort so much that they cannot be used (or at least so much that they require excessive finishing operations to make them suitable for use) Other factors being equal, the distortion in carbon steels will increase as the carbon content increases because of the gradual lowering of the martensite start (Ms) temperature with increasing carbon There is also a significant variation in the magnitude of distortion and direction of dimensional change among different heats of the same grade of steel, even though other variables are minimal This happens because of several factors, including minor variations in composition and grain size, but mainly because of the history of the steel with regard to hot working, cold working, and heat treatment Because of the different variables that contribute to total distortion, the prediction of distortion in actual parts is seldom reliable if it is based on the behavior of small test pieces The most practical approach is to make studies on pilot lots of actual pieces that have been heat treated under production conditions This procedure eliminates the shape variable so that the direction and magnitude of distortion can be plotted as ranges that incorporate most of the other variables After a quantity of such data has been secured, a series of guideposts is established, and it becomes possible to predict distortion for similar parts made from the same steel grade with reasonable accuracy However, it must be emphasized that any such study is accurate only when many parts made from different heats supplied by several mills are included References cited in this section B.S Lement, Distortion in Tool Steels, American Society for Metals, 1959 Properties and Selection of Tool Materials, American Society for Metals, 1975 J.A Ferrante, Controlling Part Dimensions During Fabrication and Heat Treatment, Met Prog., Vol 87 (No 1), Jan 1965, p 87-90; reprinted in Source Book on Heat Treating, Vol I, American Society for Metals, 1975 Hardenable Carbon and Low-Alloy Steels Revised by Eugene R Kuch, Gardner Denver Division of Cooper Industries Induction and Flame Hardening The relatively low hardenability of carbon steels is often a primary reason for choosing them in preference to alloy steels for parts that are to be locally heat treated by flame or induction hardening One of the oldest rules for selecting steels for heat treating is to choose grades that are no higher in carbon or alloy content than is essential to develop required properties This rule remains valid in the selection of steels to be heat treated by induction or flame processes When the peripheries of steel parts are heated rapidly and quenched, the tendency to crack depends mainly on a combination of four factors: • • • • Final surface hardness Temperature to which the surface has been heated Uniformity of heating Depth of hardened zone The optimum heat pattern for either induction or flame heating depends on the type of steel and on the mass and shape of the part The ideal heat pattern for any specific part will provide a hardened shell to a depth that will strengthen the part by establishing a favorable stress pattern However, if the hardened zone is too deep for the specific section thickness, high tensile stresses are established in the surface layers, and these may either cause cracking or adversely affect service life Excessive depth of the hardened zone can be caused by improper processing (overheating, for instance) or by the choice of a steel with excessive hardenability However, excessive carbon can aggravate other contributing factors and become the basic cause for cracking The Ms temperature decreases as the carbon content increases It is lowered further by higher austenitizing temperatures In general, as the Ms temperature is lowered, the probability of surface cracking increases Hardenable Carbon and Low-Alloy Steels Revised by Eugene R Kuch, Gardner Denver Division of Cooper Industries Fabrication of Parts and Assemblies Fabrication processes are usually performed on hardenable carbon and alloy steels in the unhardened condition, that is, prior to heat treating This is done primarily to avoid the high cost and difficulty of fabrication that are characteristic of high-strength materials However, even in the unhardened condition, there are differences among the various grades in respect to formability, weldability, machinability, and forgeability properties In many instances, difficulties arising during the fabrication of a given hardenable steel are directly related to the maximum hardness that can be developed and to hardenability Hardenable Carbon and Low-Alloy Steels Revised by Eugene R Kuch, Gardner Denver Division of Cooper Industries References 1989 SAE Handbook, Vol 1, Materials, Society of Automotive Engineers, 1989 S.L Semiatin and D.E Stutz, Induction Heat Treatment of Steel, American Society for Metals, 1986, p 24 Republic Alloy Steels, Republic Steel Corporation, 1961 B.S Lement, Distortion in Tool Steels, American Society for Metals, 1959 Properties and Selection of Tool Materials, American Society for Metals, 1975 J.A Ferrante, Controlling Part Dimensions During Fabrication and Heat Treatment, Met Prog., Vol 87 (No 1), Jan 1965, p 87-90; reprinted in Source Book on Heat Treating, Vol I, American Society for Metals, 1975 Hardenability of Carbon and Low-Alloy Steels Revised by Harold Burrier, Jr., The Timken Company Introduction HARDENABILITY OF STEEL is the property that determines the depth and distribution of hardness induced by quenching Steels that exhibit deep hardness penetration are considered to have high hardenability, while those that exhibit shallow hardness penetration are of low hardenability Because the primary objective in quenching is to obtain satisfactory hardening to some desired depth, it follows that hardenability is usually the single most important factor in the selection of steel for heat-treated parts Hardenability should not be confused with hardness as such or with maximum hardness The maximum attainable hardness of any steel depends solely on carbon content Also, the maximum hardness values that can be obtained with small test specimens under the fastest cooling rates of water quenching are nearly always higher than those developed under production heat-treating conditions, because hardenability limitations in quenching larger sizes may result in less than 100% martensite formation The effects of carbon and martensite content on hardness are shown in Fig Basically, the units of hardenability are those of cooling rate, for example, degrees per second These cooling rates, as related to the continuous-cooling-transformation behavior of the steel, determine the hardness and microstructural outcome of a quench In practice, these cooling rates are often expressed as a distance, with other factors such as the thermal conductivity of steel and the rate of surface heat removal being held constant Therefore, the terms Jominy distance and ideal critical diameter can be used ... 70 62 0-7 60 9 0-1 10 19 100, 100W 65 69 0 100 76 0-8 95 11 0-1 30 18 ASTM A 533 2 2 100, 100W 65 100 -4 62 0 90 69 0-8 95 10 0-1 30 16 20 8 0-1 00 19 -8 55 0 -6 85 8 0-1 00 20 69 0-8 25 10 0-1 20 17 2015 0 -6 69 0-7 90... 69 0-8 95 10 0-1 30 18 69 0 100 760 (min) 110 (min) 18 1 69 0 100 76 0-8 95 11 0-1 30 18 2 69 0 100 76 0-8 95 11 0-1 30 18 65 150 -6 62 0 90 69 0-8 95 10 0-1 30 16 Class (type A, B, or C) 300 12 345 50 55 0 -6 90 8 0-1 00... 9310 60 58 55 53 862 0 60 58 56 53 47 52100 61 58 56 53 47 M50 62 62 62 62 62 62 60 52 Hardness of core regions(b) CBS -6 0 0 41 41 41 41 41 CBS-1000M 46 46 46 46 46 46 42 28 CBS-1000M(a)