Fatigue Design and Behaviour of Carburized Steel

Abstract

Steel is an essential component used in infrastructure, automobiles, machines and many other areas due to its low cost and high tensile strength. Carburization is a heat treatment process commonly used to harden and strengthen the metal. Carburizing a steel component produces a case layer with high strength and hardness and a core layer with adequate ductility and toughness. Residual stresses are generated in the component during the carburization process, which improves the fatigue behaviour of the component under certain loading conditions. However, no complete design theory has been established for carburized steel components due to the complexity of the microstructure transformation and the residual stress relaxation phenomenon under cyclic loading.

This research aims to improve our understanding of the fatigue behaviour of carburized steel components, considering the microstructural and the residual stress changes during cyclic loading. A methodology for estimating the fatigue life for carburized steel components is proposed in this study.

Extensive experimental and analytical work was performed to study the stress-strain behaviour and the fatigue performance of carburized steels.

Monotonic and fatigue tests were conducted on carburized case-hardened samples, through-carburized case samples and simulated core samples to obtain the model input parameters and to acquire data for simulation results validation. Subsurface crack initiation was observed in the long-life fatigue tests in the case-hardened axial samples. The fracture surfaces of the samples with subsurface failure were examined using Scanning Electron Microscopic (SEM). X-ray diffraction (XRD) was employed to measure the retained austenite (RA) content and the magnitude of residual stresses at different depths of the case-hardened samples before and after various loading histories.

A finite element model (FEM) and a compatibility model were developed to estimate the initial residual stress formed during the carburization process. The amount of RA transformed under various cyclic loading amplitude was determined. Models for predicting the crack initiation and propagation life were developed in Fortran. A three-layer model was employed in the crack initiation and propagation simulations for the case-hardened axial and notched samples under constant amplitude (CA) and variable amplitude (VA) loading histories. The residual stress in each layer, including the RA transformation effect, was accounted for in the fatigue life prediction.

The three-layer model can sufficiently predict the crack initiation location in the casehardened sample at different CA and VA load levels. Reasonable accuracy was obtained with the total fatigue life prediction for the case-hardened axial and notched samples.