Residual Stress and Bending Fatigue Strength in Carburized and Quench Hardened Pyrowear 53 Steel Gears

Carburization of alloy steels promotes the formation of compressive residual surface stress upon quenching, and that compressive surface stress enhances fatigue life.  To build upon these established facts, a project was undertaken to assess the role of residual stress magnitude on bending fatigue life of a spur gear through innovative quenching and the achievement of deeper compressive surface stress. Under US Army Sponsorship, DANTE Solutions demonstrated the feasibility of improving the bending fatigue life of Pyrowear 53 steel gears by achieving deeper compressive residual stress in carburized and quench hardened parts. 

Computer simulations of two different quenching processes, conventional oil quenching and intensive quenching, were conducted using the DANTE heat treatment simulation software to predict differences in final residual stress state.  Although similar hardness profiles were predicted for both processes, the predicted surface stresses at the center of the gear root were -600 MPa and -300 MPa, with the intensive quenching producing higher compression.  These predictions agreed with XRD measurements.  The stresses predicted at the tooth fillet were even more compressive and maintained similar separation between the two quenching methods. The simulations showed that timing and sequence of martensite formation that occurred during the quenching process was related directly to the magnitude of compressive residual surface stress.

Tooth bending fatigue tests were conducted by Gear Research Institute, and these tests showed an endurance limit difference of 15% between the two quenching methods, with higher surface compression yielding higher fatigue life.  Scatter in the data was significant, even with surface conditions within product specification.  Isotropic surface finishing increased the endurance limit, and the difference between quenching methods was maintained.