Impact of case depth & hardening heat treatments on Residual Stresses in gear steel

Residual stress significantly impacts the fatigue life of carburized low alloy steels resulting in premature failure, making it essential to understand and accurately predict the relationship between carburization case depth, heat treatment processes, and residual stress to optimize performance in gear steel. The current study investigates conventional low pressure gear carburizing at four case depths (0.014”, 0.026”, 0.050” and 0.066”) applied to AISI 9310 steel. The sin²Ψ X-ray diffraction technique is used to determine residual stress depth profiles on samples exposed to different heat treatments and after different steps within the heat treatment sequences including carburizing, oil quenching, cryo-treating, and tempering steps. Hardness distributions and microstructures are evaluated using Knoop micro indentation testing and light microscopy. Results show that greater case depths result in more gradual hardness transitions from case to core and more uniform hardnesses throughout the case regions. Greater carburization depth induces compressive residual stress after martensite formation, but at intermediate heat treatment steps (e.g., carburizing, oil quenching) very high retained austenite is present at the sample surfaces due to incomplete martensite transformation. Subsequent heat treatments convert retained austenite to martensite and develop additional compressive stress. Some samples show moderate tensile stresses, largely due to incomplete carbide dissolution during austenitizing, resulting in lower martensite content and hardness. Surface residual stresses are influenced by microstructural changes at each carburization case depth during intermediate treatments, whereas final tempering exerts minimal impact on the magnitude of these stresses. Our study provides useful data for finite element model validation and design of heat treat processes to better withstand premature failure, significantly extending their service life for a wide range of industries such as automotive, aerospace, and heavy machinery.

Keywords: Residual Stress; Case Depth; Vacuum Carburizing; Hardness; Retained Austenite

Distribution A. Approved for public release: distribution unlimited. (AFRL-2024-5142) Date Approved 09/19/24