Bending Fatigue of Carburized Steels: A Statistical Analysis of Process and Microstructural Parameters

A large set of bending fatigue data on carburized steels has been statistically analyzed to quantitatively describe the effects of process and microstructural variables. Increasing demands on gear steels require a broad examination of past bending fatigue research to reveal the primary factors that determine fatigue performance and guide future gear steel design. Fatigue performance was correlated to specimen characteristics such as retained austenite content, case and core grain size, extent of intergranular oxidation, surface roughness, and the case profiles of residual stress, hardness, and carbon content. Prior austenite grain size in the case and surface residual stress were found to most strongly influence bending fatigue endurance limit. A multiple regression model to predict endurance limit achieved an R-squared value of 0.56. Fatigue performance as measured by the high-stress portion of the S-N curve showed significant correlations to surface residual stress and endurance limit; however, a satisfactory regression model could not be constructed. Case grain refinement was most effectively achieved through reheat treatments, while shot peening was found to be the only means to significantly increase compressive surface residual stresses.