N rich nanocrystalline bainite in surface layer of carbon steel

A novel technology was proposed to synthesis nanocrystalline bainitic microstructures in the surface layer of medium carbon steel, which is based on nitriding and subsequent low temperature austempering. The nitrogen content in the nitriding layer increased to 1·2 wt-%, indicating that the martensite starting point was depressed. Nanocrystalline bainite formed in the nitrided layer and lath martensite in the centre by austempering at a low temperature, slightly higher than the martensite starting point of the surface layer. The N rich nanocrystalline bainite is composed of bainitic ferrite plates with uniformly distributed retained austenite and γ′-Fe4N. In addition, the hardness of the nitrided layer reaches 750 HV with the transformation of N rich nanocrystalline bainite.