High toughness and multiphase microstructure transition product of carburizing steel by a novel heat treatment cooling process

A novel heat treatment cooling process was applied to a Fe-0.27C-1.50Mn-1.7Si-0.35Cr-0.64Mo-1.50Ni-0.10 V (wt%) carburizing steel. The cooling process was designed based on the continuous cooling transformation (CCT) and time-temperature-transformation (TTT) analyses of the carburizing steel. Mechanical property results of the heat treatment samples show that the carburizing steels exhibit excellent toughness (118 J) which is superior to that of the traditional quenching process. Microstructural characterization indicates that high toughness of the carburizing steel subjected to the cooling heat treatment process results from a proper multiphase microstructure transition from the surface to the matrix of the steel. The key factors which greatly improve the toughness are the lower bainite consisting of fine ferrite laths and retained austenite films along the laths at the transition region and the carbide-free lower bainite at the matrix.