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FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 9 4 2 in part, to an increase in material ductility resulting from higher retained austenite content. Increased ductility delays crack growth, while at the same time, austenite transforma- tion due to mechanical stress can induce additional bene- ficial compression in the case [3] , further enhancing fatigue resistance. SUMMARY This study showed that fatigue life, residual stress, and hardness of 8620H were comparable when carburized for four, eight, and 12 hours. Carburization for two and four hours produced the lowest retained austenite content, while 24-hour carburization provided the best fatigue life, but the highest retained austenite content. Knowing how the carbu- rization process influences fatigue strength, retained austen- ite, case depth, and residual stress enables developing an optimized process that balances these characteristics to fit the specific engineering design parameters of a component. For example, a four-hour carburization process might be chosen for an application in which dimensional tolerance is tight, as it minimizes the metastable retained austenite content while also minimizing processing time. However, if dimensional stability and low retained austenite content are not as crucial, but contact fatigue life must be maximized, 24-hour carburization is optimal if the increased processing time can be tolerated. This investigation demonstrates howprocess optimiza- tion can be achieved when multiple variables are measured and studied as a whole. A wide array of material properties such as hardness, volume retained austenite, and compres- sive residual stress can be optimized through adjustments to the carburization process, ultimately providing the tools and knowledge to aid in the pursuit of optimal component performance. ~HTPro For more information: Perry W. Mason, Lambda Technol- ogies Group, 3929 Virginia Ave., Cincinnati, OH 45227-3411, 800.883.0851, info@lambdatechs.com. References 1. Online materials information resource: www.matweb. com, MatWeb LLC, date of visit: Feb 2016. 2. K. Genel and D. Mehmet, Effect of Case Depth on Fatigue Performance of AISI 8620 Carburized Steel, Int. J. Fatigue, Vol 21.2, p 207-212, 1999. 3. D. Herring, A Discussion of Retained Austenite, Industrial Heating, Vol 72.3, p 14-16, 2005. 4. C.F. Jatczak, J.A. Larson, and S.W. Shin, Retained Austenite and Its Measurements by X-ray Diffraction: An Information Manual, SAE Report SP-453, 1980. 5. N.K. Arakere, et al., Stress Field Evolution in a Ball Bearing Raceway Fatigue Spall (preprint), No. AFRL-RZ-WP- TP-2009-2211. Air Force Research Lab, Wright-Patterson AFB, OH, Propulsion Directorate, 2009. 6. W.A. Glaeser and S.J. Shaffer, Contact Fatigue, ASM Handbook, Vol 19, Fatigue andFracture, ASM International, 1996. 7. V.M.S. Naidu, Surface Characterization of Contact Fatigue Tested Ground Spur Gears, Chalmers University of Technology, Sweden, 2017. 8. Standard Test Method for Microindentation Hardness of Materials, ASTM E384-17, ASTM International, 2017. 9. A. Ahmad, P. Prevéy, and C. Rudd. Residual Stress Measurement by X-Ray Diffraction, SAE HS-784, 2003. 10. D.P. Koistinen, A Simplified Procedure for Calculating Peak Position in X-ray Residual Stress Measurements of Hardened Steel,” Trans. ASM, Vol 51, p 537-555, 1959. 11. G. Moore and W.P. Evans, Trans. SAE, Vol 66, p 340-345, 1958. 12. Properties and Selection: Irons and Steels, Vol 1 (9th edition), ASM Metals Handbook, p 538. 13. B.L. Averbach and M. Cohen, X-ray Determination of Retained Austenite by Integrated Intensities, Trans. Metall. Soc. AIME, Vol 176, p 401, 1948. 14. D. Glover, A Ball-Rod Rolling Contact Fatigue Tester, Rolling Contact Fatigue Testing of Bearing Steels, ASTM International, 1982. 15. B. Dodson, The Weibull Analysis Handbook, ASQ Quality Press, 2006. Fig. 7 — Weibull probability distributions for rolling contact fatigue tests. 1 10