1、 SURFACE VEHICLE INFORMATION 400 Commonwealth Drive, Warrendale, PA 15096-0001 REPORT Submitted for recognition as an American National Standard J1975 REV. NOV1997 Issued 1991-06 Revised 1997-11 Superseding J1975 JUN91 CASE HARDENABILITY OF CARBURIZED STEELS 1. ScopeThis SAE Information Report summa
2、rizes the characteristics of carburized steels and factors involved in controlling hardness, microstructure, and residual stress. Methods of determining case hardenability are reviewed, as well as methods to test for freedom from non-martensitic structures in the carburized case. Factors influencing
3、 case hardenability are also reviewed. Methods of predicting case hardenability are included, with examples of calculations for several standard carburizing steels. A bibliography is included in 2.2. The references provide more detailed information on the topics discussed in this document. 2. Refere
4、nces 2.1 Applicable PublicationsThe following publications form a part of this specification to the extent specified herein. Unless otherwise indicated, the latest issue of SAE publications shall apply. 2.1.1 SAE PUBLICATIONSAvailable from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001. SAE
5、J403Chemical Compositions of SAE Carbon Steels SAE J404Chemical Compositions of SAE Alloy Steels SAE J406Methods of Determining Hardenability of Steels SAE J417Hardness Tests and Hardness Number Conversions SAE J1268Hardenability Bands for Carbon and Alloy H Steels 2.2 Other Publications 1. R.F. Tho
6、mson, “Summary,“ Fatigue Durability of Carburized Steel, ASM International, Metals Park, Ohio, 1957, p. 110. 2. D.H. Breen, “Fundamentals of Gear Stress/Strength RelationshipsMaterials,“ SAE Technical Paper 841083, 1984. 3. J.M. Hodge and M.A. Orehoski, “Relationship Between Hardenability and Percen
7、tage of Martensite in Some Low Alloy Steels,“ Trans. AIME, 1946, Vol. 167, pp. 627642. 4. M. Atkins, Atlas of Continuous Cooling Transformation Diagrams for Engineering Steels, ASM International and British Steel Corporation, 1980. 5. A. Rose and H. Hougardy, Atlas zur Waermebehandlung der Stahle, V
8、.2, 1972, Max-Planck-Institut fuer Eisenforschung; Verlag Stahleisen m.b.H., P.O. Box 8229, D-4000, Dusseldorf, West Germany. Summarized in English by Rose and Hougardy in “Transformation Characteristics and Hardenability of Carburizing Steels,” in the proceedings of the Symposium Transformation and
9、 Hardenability in Steels, Climax Molybdenum Co., 1967, pages 155-167. SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for
10、 any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your written comments and suggestions. QUESTIONS REGAR
11、DING THIS DOCUMENT: (724) 772-8512 FAX: (724) 776-0243 TO PLACE A DOCUMENT ORDER; (724) 776-4970 FAX: (724) 776-0790 SAE WEB ADDRESS http:/www.sae.org Copyright 1997 Society of Automotive Engineers, Inc. All rights reserved. Printed in U.S.A. -2 SAE J1975 Revised NOV1997 6. C.A. Siebert, D.V. Doane
12、and D.H. Breen, The Hardenability of SteelsConcepts, Metallurgical Influences, and Industrial Applications, ASM International, 1977, pp. 163176. 7. “Modern carburized nickel alloy steels,” Reference Book No. 11005, Nickel Development Institute, Toronto, Ontario, Canada M5H 3S6, 1990, pages 19-22. 8.
13、 A.E. Gurley and C.R. Hannewald, “Development and Applications of Iso-Hardness Diagrams,“ Metal Treating, V. 7, May-June 1956, p. 2. 9. J. A. Halgren and E.A. Solecki, “Case Hardenability of SAE 4028, 8620, 4620 and 4815 Steels,“ SAE Technical Paper 149A, 1960. 10. Atlas, Hardenability of Carburized
14、 Steels, Climax Molybdenum Co., 1960. 11. D.E. Diesburg, C. Kim and W. Fairhurst, “Microstructure and Residual Stress Effects on the Fracture of Case-Hardened Steels,“ Proceedings of Heat Treatment 81, The Metals Society, London, September, 1981. 12. R.F. Kern, Metal Progress, Oct. 1972, p. 172. 13.
15、 G.T. Eldis and Y.E. Smith, “Effect of Composition on Distance to First Bainite in Carburized Steels,“ Journal of Heat Treating, V. 2, No. 1, June 1981, pp. 6272. 14. I.R. Kramer, S. Siegel and J.G. Brooks, “Factors for the Calculation of Hardenability,“ Trans. AIME, 1946, Vol. 167, p. 670. 15. C.F.
16、 Jatczak, “Hardenability in High Carbon Steels,“ Met. Trans., 1973, V. 4, p. 2272. 16. “New CCT Diagrams for Carburizing Steels,“ Molybdenum Mosaic, 1987, V. 10, No. 1, AMAX Metal Products, Bridgeville, PA, p. 11. 17. D.V. Doane, “Softening High Hardenability Steels for Machining and Cold Forming,“
17、Journal of Heat Treating, V. 6, No. 2, 1988, pp. 97109. 18. R.J. Love, H.C. Allsopp and A.T. Weare, “The Influence of Carburizing Conditions and Heat Treatment on the Bending Fatigue Strength and Impact Strength of Gears Made from EN352 Steel,“ MIRA Report No. 19.59/7. 19. J.A. Burnett, “Prediction
18、of Stresses Generated During the Heat Treating of Case Carburized Parts,“ Residual Stresses for Designers and Metallurgists, ASM International, 1981, pp. 5169. 20. C. Kim, D.E. Diesburg and G.T. Eldis, “Effect of Residual Stress on Fatigue Fracture of Case-Hardened SteelsAn Analytical Model,“ Residu
19、al Stress Effects in Fatigue, ASTM Special Technical Publication 776, 1982, pp. 224234. 3. GeneralThe typical carburized steel component can be modeled as a composite material with a high- hardness, carbon-rich surface layer on a lower carbon base that is lower in hardness but higher in toughness. T
20、he continuous nature of the transition between the high-carbon case and the low-carbon core, combined with the sequence of transformation events occurring throughout the component during quenching result in the development of a microstructural gradient and a favorable residual stress profile. These
21、factors define the overall fatigue and fracture properties of the carburized component. Failure modes of carburized components influence the choice of case depth and microstructure. To illustrate the nature of the stresses developed in a carburized component, and how they can be effectively used, Fi
22、gure 1 shows the stresses in a carburized bar subjected to bending fatigue 1.1 In this situation, the applied stress is highest at the surface and zero at the centerline. The hardness gradient of the carburized and hardened bar indicates the probable gradient in endurance limit (or fatigue limit) wh
23、ich is highest at the surface, and drops through the case-core interface to the lower fatigue limit of the core. 1. Numbers in brackets are references cited in 2.2. -3 SAE J1975 Revised NOV1997 During quenching, the core material transforms first because its lower carbon content has a higher martens
24、ite- start temperature. The case material transforms somewhat later because its higher carbon content has a lower martensite-start temperature. Since the strength of the core resists the expansion of the case during its martensite transformation, compressive stresses develop in the case that are bal
25、anced by tensile stresses in the core. These residual stresses (curve A) add to, or subtract from, the inherent microstructural strength (curve B), resulting in the net effective fatigue limit (or endurance limit) shown by the dashed curve. Note that in this properly designed and loaded beam, the ef
26、fective fatigue limit level is always greater than the applied stress. The diagram is over-simplified, of course, to demonstrate the principles involved. Breen has discussed modes of failure in gears 2 and showed that the applied stresses at the root of the tooth decrease nonlinearly with depth. The
27、 high stress level at the surface is a result of the cantilever loading of the gear tooth, intensified by the stress concentration caused by the root radius and surface finish. Thus, for a carburized gear, it is quite important that the effective fatigue limit be as high as possible at the surface.
28、For failure at and below the contact or pitch line of a gear tooth, the applied stress curve is yet a different shape, as described in Breens article 2, and illustrated in Figure 2. Hertzian stresses are greatest below the surface, the depth depending on the profile of the surfaces in contact. If th
29、e net fatigue limit curve, the critical strength curve B shown in the figure, coincides with the applied stress curve A at some depth X below the surface, e.g., at the case-core interface, then subcase (spalling) fatigue can occur. This failure mode emphasizes the need to provide adequate case depth
30、 and optimum microstructure at all carbon levels. The ratio of the volume (or cross-sectional area) of case to core defines the magnitude of compressive stress at the surface. Thus, for a given part, the magnitude of the compressive stress in the case tends to decrease as the case depth increases. W
31、hen the design is correct, the critical shear strength will remain above the applied stress curve. 3.1 Hardness versus Carbon ContentFor a given carbon level there is a systematic relationship between hardness and structure in hardened steel, as shown in Figure 3, from the work of Hodge and Orehoski
32、 3. The curves not only show the differences due to microstructure, but also the variability in measurements. The spread in hardness at 99.9% martensite is due primarily to measurement errors; the greater spread at 50% martensite is attributable to the variability in the non-martensitic structure. B
33、reen 2 has stated that to resist fatigue failure due to cyclic bending stresses at the root fillet of gears, the optimum case structure is a mixture of high carbon martensite and retained austenite, with enough martensite to assure a hardness of at least 57 HRC. The microstructure in the core should
34、 comprise only martensite and bainite. For most alloy carburizing steels, transformation to at least 50% martensite assures that the balance of the structure is bainite 4,5. To maintain high case hardness, retained austenite must be restricted. Data from Rose and Hougardy 5 on microstructure and har
35、dness of several carburized steels show that alloy content and alloy interactions influence the range of case carbon contents within which a suitable hardness and a martensite/austenite microstructure can be achieved. 3.2 HardenabilityA certain minimum hardenability is necessary to develop the requi
36、red strength in a carburized part. The hardenability of the base composition governs the capability of developing high strength martensite in the core and in the medium carbon portion of the case. Hardenability in the high carbon region controls the capability of a steel to develop sufficient hardne
37、ss and an appropriate microstructure at the case surface. The conventional Jominy end-quench test can provide much of the needed information, if case hardenability is considered as well as base, or core, hardenability. -4 SAE J1975 Revised NOV1997 For certain applications, shallow carburized cases m
38、ay be employed to improve wear resistance under light to moderate load conditions. For such applications, high surface hardness is the important criterion. A fully martensitic structure at the surface provides highest hardness and best resistance to wear. Section size dictates the cooling rate that
39、can be achieved at the surface, especially in parts which are oil quenched (Figure 7 of SAE J406). Cooling rate, expressed as distance from the quenched end of the Jominy hardenability bar, can define the hardenability required. Hardenability requirements for carburized components are discussed in s
40、ome detail in an ASM monograph 6, including consideration of section size in terms of “Jominy equivalent,“ carbon gradient, and surface oxidation. An example uses a gear to demonstrate the engineering approach to steel selection, and the steps involved in reaching a cost-effective choice of steel wh
41、ich meets design requirements. Processing requirements are also included. 4. Methods of Determining Case HardenabilityThe end-quench method for determining hardenability is described in SAE J406. The method has been used to determine case as well as core hardenability of carburized steels. Figure 4
42、shows the core and case hardenability of a heat of SAE 4620 steel, containing nominally 0.2% C, 0.6% Mn, 1.8% Ni, and 0.25% Mo. A common criterion for evaluating the hardenability of a steel is the “ideal critical diameter, DI.“ It is defined as the diameter of a bar which exhibits an acceptable mic
43、rostructure when subjected to an “ideal“ quench (a quench of infinite severity, defined in more detail in 6). For carbon contents in the core and transition regions of a carburized steel, a microstructure of 50% martensite, balance bainite, is often chosen. This microstructure is characteristic of t
44、hat found in the inflection region of the hardenability curve. This “50% martensite“ criterion is indicated by the dashed line in Figure 4, and relates to the DI for each carbon level. In the carburized case, however, a microstructure containing at least 90% martensite and retained austenite is cons
45、idered necessary to resist fatigue failure. This “90% martensite“ criterion is indicated by another dashed line in Figure 4, and relates to the DI for case hardenability. 4.1 Jominy End-Quench TestThe test can be used to determine hardness at various carbon levels in the carburized case, as a functi
46、on of cooling rate, expressed as the distance from the quenched end of the test bar. Figure 4 is one example. The method for determining case hardenability from Jominy end-quench bars is described in detail in Appendix A. Data showing case hardenability can be found in several references 710 present
47、ed either as standard hardenability curves or as isohardness diagrams. 4.2 Distance to First Appearance of Bainite in the Carburized CaseData suggest that the as-quenched microstructure must be substantially free from bainite or pearlite to obtain the greatest resistance to impact 11. The presence o
48、f very small amounts of bainite in the case has also been reported to reduce fatigue resistance 12. Eldis and Smith reported the results of a detailed study of the occurrence of bainite in carburized end-quench hardenability specimens 13. In the study, specimens of 81 alloys were carburized at 925 C
49、 (1700 F), cooled to 845 C (1550 F) and end-quenched. Companion bars were carburized to provide carbon gradient data. Flats were ground on the bars to a depth corresponding to 0.9% carbon in the case and hardness profiles were determined. Those flats were then metallographically polished, etched, and examined using quantitative metallographic techniques to determine the amount of bainite as a function of distance from the quenched end of the bar. The data for percent bainite were plotted and extrapolated to determine the
