Effects of Carbon Concentration Change on Growth of Compound Layer in Surface of Nitrided Steel

In order to clarify the effects of carbon concentration change on the growth of the compound layer in nitrided steel, the variation with time of the concentration of alloy elements on the surface layer and the phases of the compound layer were investigated in nitrided steels containing various amounts of carbon in the matrix. It was found that the variation with time of the carbon concentration in the compound layer was mainly responsible for the variation with time of the compound layer microstructure. Furthermore, it was discovered that the variation with time of the carbon concentration of the compound layer was caused by both the migration of carbon from the matrix to the compound layer and from the surface of the compound layer to the atmosphere. Thus, the gradient of the chemical potential of carbon in the through-thickness direction of the compound layer and the compound layer microstructure changed with nitriding time.