Quantitative Characterization of the Microstructure of an Induction Heat-treated Low Alloy Steel using EBSD

Abstract

This study aimed to evaluate microstructural changes in various locations of an induction heat treated low-alloy steel in a quantitative manner. A commercially produced ring gear sample of a SNCM 439 steel was prepared. The specimens for experiments were obtained from the gear sample after a surface induction hardening heat treatment. Based on optical microscope images and corresponding hardness test results, the induction-hardened gear sample was found to be roughly divided into three main regions: the hardened surface region, transition region, and matrix region. Five different positions were selected for EBSD analysis to identify the evolution of the microstructure within the three regions. A quantitative examination of the microstructural differences between the five selected positions was conducted using the parameters obtained by the EBSD analysis, including image quality (IQ), kernel average misorientation (KAM), and misorientation angle (MA) values. The matrix region revealed traces of martensitic substructures in the prior austenite grain structure, indicating a tempered microstructure of low-temperature transformation (LTT) products. The hardened surface region had a fresh martensite microstructure, while the substructure size varied greatly depending on the position-dependent prior austenite grain size. In the transition region, tempered LTT microstructure and fresh martensite were both observed, but the fraction of fresh martensite decreased sharply with increasing distance from the surface. The differences in hardness within and between the three main regions can be explained by the results of the microstructural analysis.