Influence of carbonitriding and sub-zero treatment on the microstructure and fatigue strength of JIS-SCM420 low-alloy steel

This study examines the influence of carbonitriding followed by sub-zero treatment on the microstructure, hardness, residual stress, and fatigue strength of JIS SCM420 low-alloy steels. Carbonitriding, followed by quenching and tempering treatment (NS), effectively enhanced surface hardness and induced compressive residual stress. The subsequent sub-zero treatment followed by quenching (S) further decreased the retained austenite (γ R ) content, leading to increased surface hardness and residual stress. Microstructural analysis revealed that sub-zero treatment not only reduced the γ R content but also facilitated the formation of martensite and fine carbides, contributing to higher compressive residual stress. Despite these microstructural improvements, the fatigue limits of the NS and S specimens remained identical, suggesting that the stability of γ R is a more critical factor for fatigue resistance than its volume fraction. • The microstructure and fatigue properties of carbonitrided JIS-SCM420 low-alloy steel were investigated. • Sub-zero treatment followed by quenching further reduced the retained austenite content and increased both surface hardness and compressive residual stress. • The size and stability of retained austenite proved to be more critical to fatigue strength than its volume fraction.