Residual Stress Development during Carburizing Heat Treatments of Aerospace Gear Steel

The magnitude and depth of residual stresses in AISI 9310 gear steel are known to be impacted during vacuum carburizing and subsequent heat treating processes. Residual stress distribution can have adverse effects on part distortion during the final machining step and potentially cause premature part failure while in service. By understanding how residual stresses develop after each step in the heat treatment process, the final residual stress state can be better controlled. Microstructure, mechanical, and thermal properties are characterized and compared to residual stress measurements. Samples of AISI 9310 steel underwent a representative gear manufacturing sequence that includes normalizing, vacuum carburizing to four different case depths, austenitizing, oil quenching, cryo-treatment, and tempering. Results indicate residual stress state has a partial correlation to the case depth after each thermal processing step. At great case depths more carbon has diffused into the material, resulting in a larger region near the surface with a significant fraction of retained austenite and a larger transition region of martensite. At shallower case depths, there is a steeper hardness gradient from case to core. Residual stresses at the surface were dependent on the microstructural changes of each case depth in the intermediate heat treatment steps. The final tempering step however, did not show a large variation in magnitude of surface residual stresses. The data generated in the current study can be applied to a finite element model describing and predicting residual stress distribution in similarly heat-treated parts.