Effects of Carbon and Nitrogen on the Microstructure and Mechanical Properties of Carbonitrided Low-Carbon Steel

Various carbonitriding processes have been applied to low-carbon steel. The carbon and nitrogen contents in the case of carbonitrided low-carbon steels have been evaluated, and their influences on microstructure, distortion, hardness, and strength have been investigated. The factor limiting the ferrite and pearlite formations with increasing nitrogen content was a mechanism that resulted in more retained austenite in the case of low-carbon steel treated using a high flow rate of ammonia. The high-nitrogen content and high amount of retained austenite in the case of carbonitrided low-carbon steel slightly reduced the hardness, case depth, and strength. However, the elongation was improved, and the distortion was reduced. In the case zones, fractures occurred along the grain boundaries, i.e., intergranular fractures, whereas the fracture surfaces in the core regions were typical ductile fractures, i.e., dimples were observed.