Microstructure and property evolution during tempering of a medium carbon bainitic steel

Tempering a medium carbon carbide-free bainite steel led to two distinct microstructures and properties below and above the bainitic start (Bs) temperature. Austenite decomposition during tempering, depended on carbon content, tempering temperatures and time, determining retained austenite (RA) stability. Up to 400 °C, most RA remained intact, but a significant reduction occurred above the Bs temperature. Film austenite primarily turned into carbides within the bainitic–ferrite matrix and form lines of carbide particles, while blocky austenite decomposed into troostite at 500 °C and sorbite at 600 °C. During tempering, on the other hand, bainitic ferrite lost tetragonality losing its carbon and resulting in the growth of bainitic–ferrite plates through grain boundary migration and misorientation annihilation. Above Bs, drastic microstructural changes caused significant alterations in tensile properties.