High-Cycle and Impact Fatigue Behavior of Carburized Steels

Abstract

The influence of residual stress and retained austenite on the fracture behavior of bend test specimens of carburized steels was evaluated. A specimen designed to represent a simplified geometry of a gear tooth was used to evaluate high-cycle fatigue properties. A specimen having the same tooth geometry as that of the high-cycle fatigue specimen was employed to determine single-impact fracture stress. The ability of impact fracture stress to absorb repeated low-energy impacts was assessed, and both metallographic and x-ray diffraction techniques were utilized to evaluate retained austenite contents of the carburized cases. Test results, part of an ongoing research effort to compare relative fracture properties of alternate steel grades, showed that steels of equal hardenability and core carbon content generally exhibited similar fracture behavior. Compressive residual stress in the carburized cases significantly influenced impact fracture stress which, in turn, correlated with impact fatigue properties. High levels of retained austenite at the surface caused peak compressive residual stress to occur further into the case. This occurrence was not necessarily detrimental to impact fracture stress or impact fatigue properties. High-cycle fatigue limits were more dependent on processing variables than on differences in alloy content.