Investigating the effect of retained austenite on wear and fatigue behavior of AISI 8620 steel

Enhancing the operational life and reliability of drivetrain components via heat treatment routes is a long-studied topic of research. By varying parameters using heat treatment, the resulting attributes of the material such as hardness, residual stress, amount of retained austenite (RA), microstructure, grain size, grain orientation etc. can be varied. While the effect of hardness and residual stress on the resulting tribological properties of the steel is well known, the effect of RA is not as clearly understood yet. In this study, AISI 8620 steel samples were subjected to several heat treatment schemes using conventional and laser-based surface treatment routes to vary the amount of RA. The samples were subsequently used to conduct wear and rolling contact fatigue tests where samples are compelled to fail due to abrasive wear, micropitting, macropitting and white etching crack (WEC) formation. By conducting these systematic experiments, the effect of RA on those failure modes was elucidated using several surface based and microstructure based experimental techniques. It was observed that at higher contact pressures, a higher hardness and higher RA% result in superior wear resistance. This is attributed to the higher hardness resulting from the heat treatment route to generate higher RA% as well as martensitic transformation of austenite during sliding. Generated RA% due to laser treatment was higher at lower scanning velocity and with air and nitrogen as a shielding gas compared to argon but was limited to below 10%. Higher RA had significant effect on micropitting but less significant effect on macropitting since sub-surface cracks observed to have a compelling impact on macropitting life. Higher RA samples showed far more branching WEC networks, and the white etching area adjacent to the crack networks was less defined. The study establishes the role of RA on tribomechanical behavior in the context of other mechanical and microstructural phenomena as well as the correlation of behavior to heat treatment routes for preferred wear and fatigue life. Overall, the findings will provide valuable input for the design and manufacturing of drivetrain components for a wide range of applications ranging from agricultural equipment to the wind energy sectors