Microstructure-property relationship in bainitic steel: The effect of austempering

The bainite/martensite multiphase microstructure was studied in 0.22C-2.0Mn-1.0Si-0.8Cr-0.8(Mo+Ni) (wt%) bainitic steel subjected to austempering between 325 °C and 400 °C to elucidate the toughening mechanism and the determining role of martensite/austenite (M/A) constituents obtained through various heat treatment. Steel austempered at higher temperature had lower strength and poor toughness, because of the wide bainitic ferrite lath and blocky M/A constituents. Bainite with similar crystallographic orientation to lath martensite was formed and exhibited a typical K–S relationship. The samples austempered below martensite-start (Ms) temperature exhibited an excellent impact toughness at room temperature, due to the superfine sub-plates in lower bainite blocks. Meanwhile, air-cooled toughness at low temperature was superior to the steel austempered at 325 °C, because of its finer effective grain size. The relationship between microstructure and mechanical properties was studied using a combination of SEM, TEM and crystallographic analysis.