Effect of Cooling Rate after High Temperature Nitriding on Transformation Microstructure in Low Carbon Steel

Abstract

High temperature nitriding was applied to a low carbon steel, and then the transformation microstructure formed from the high nitrogen austenite was observed. The microstructure was greatly influenced by cooling rate after the nitriding. The austenite transformed to martensite when the cooling rate was fast (water-cooling) though a certain amount of austenite was retained in the martensite. On the other hand, a diffusional transformation product such as (ferrite + Fe4N) eutectoid structure was formed when the cooling rate was slow (gas- or furnace-cooling). The hardness profile obtained in these specimen were related with microstructure depending on the nitrogen concentration profile and cooling rate.