Influence of chemical segregation on bainitic microstructures in a carburized bearing steel

Abstract

Bainite to austenite reversal is one of the grain refinement techniques employed in carburized steels. However, chemical segregation influences the homogeneity of the bainitic structure, which is seminal to exploit the advantages associated with austenite reversal. It is therefore important to understand the influence of chemical segregation on bainite formation, which is investigated in this work. Characterizations were performed on the microstructures obtained from the case and core regions of a carburized steel after 30 h of bainite treatment at 320 °C for two carbon compositions: 0.85 wt% C (z_case) and 0.16 wt% C (z_core). The microstructure of z_case is shown to contain bands with bainite in alloy-lean regions and martensite/austenite in alloy-rich regions. For z_core, although the chemical bands are not composed of different phases, the alloy-rich regions have a fraction of martensite-austenite (MA) islands that is twice the fraction in alloy-lean regions. Despite this difference, the austenite phase fractions in the chemical bands of z_core are low and almost similar, indicating that the MA islands are mostly martensite. From experimental results and thermodynamic and kinetic simulations, it is elucidated that a different rate of phase transformation in the chemical bands is the cause for the observed microstructural inhomogeneities.