Effect of Retained Austenite on Spalling Behavior of Carburized AISI 8620 Steel Under Boundary Lubrication

The objective of this study is to investigate the effect of retained austenite (RA) on the macropitting or spalling behavior of carburized AISI 8620 steel under boundary lubrication condition. Samples with RA ranging from approximately 0 to 70% were prepared using specific carburizing schemes. Macropitting tests were carried out at maximum contact pressure 1.9 GPa using a benchtop test rig. Samples were subsequently analyzed using noncontact white light profilometry and micro X-Ray Diffraction to observe the evolution of contact surface and RA respectively. Results indicated that increasing RA and minimizing pre-existing sub-surface cracks in the material help to enhance macropitting life. The failure mechanism for all samples was from spall formation due to sub-surface crack initiation and propagation with clear evidence of RA transforming to martensite. Pre- and post-experiment residual stress showed the impact of phase transformation on enhancement of residual stress during RCF experiments, however residual stress was not a significant factor in impacting the life.