Influence of holding times on the tempering mechanisms of bainitic microstructure

In this study, the effect of different tempering time in heat treatment of thick mold steel blocks was investigated. The tempering of bainitic microstructure evolution during short (30 min) and long (14 h) tempering at 620 °C, after austenitization at 900 °C was investigated. High-resolution dilatometry and microscopy revealed that the 10% retained austenite, present before tempering, decomposes directly during tempering and indirectly into fresh martensite during cooling after short tempering. In contrast, prolonged tempering results in the complete decomposition of the retained austenite. Short tempering maintains hardness, due to the formation of fresh martensite. In prolonged tempering, hardness is significantly reduced because of the martensite–austenite constituents tempering, and recovery of low-angle grain boundaries, as confirmed by nanoindentation and electron backscatter diffraction analyses.