Effect of prior martensite on the thermal stability of nanobainitic steels

Two types of the nanobainitic microstructures which consist of plate-like bainitic ferrite, retained austenite with and without martensite were obtained by quenching to Ms-12 degrees C and ausforming by about 30% before isothermal bainitic transformation at 300 degrees C. The evolution of hardness and nanobainitic microstructure of both tested steels during the tempering were investigated through scanning electron microscopy and transmission electron microscopy. Results show that the hardness of both tested steels were stable at 540 HV when tempered at low and medium temperatures (200 degrees C-500 degrees C), higher than that of the untempered steel, and decreased when the tempering temperature reaches 500 degrees C or over. The nanobainitic microstructures of the two nanobainitic steels were not sensitive to tempering temperature before 350 degrees C. When temperature was increased to 450 degrees C, the bainitic ferrite coarsened obviously and the retained austenite film decomposed to fine carbides, which increased the hardness of the martensite-free tested steel. However, the supersaturated carbon was discharged from prior martensite into retained austenite at low temperature, decayed the decomposition of the later, and enhanced the thermal stability of the martensite tested steel when the tempering temperature was over 450 degrees C.