Deterministic Analysis of Crack Growth Rates in Carburized Steels

In view of the need to understand low cycle fatigue (LCF)-microstructure relationships in carburized steels, bending fatigue tests and fatigue modeling were conducted on carburized 4320 steel. The starting case microstructure of the specimens, directly quenched and tempered after carburizing, consisted of plate martensite and retained austenite. This paper compares linear elastic fracture mechanics (LEFM) based models that predict crack growth rates in the transgranular crack growth region with an effective stress intensity reduction due to stress/strain induced transformation of austenite to martensite in front of the crack tip. These models more accurately predict fatigue lives than a simple linear elastic fracture mechanics model without a reduction in the stress intensity. A sensitivity analysis on the model results as a function of input parameters was also performed.