Effect of Si Concentration on Carbon Concentration in Surface Layer after Gas Carburizing

Abstract

The mechanisms for the decrease in the carbon concentration in the surface layer after gas carburizing that occurs with increasing the Si concentration was investigated using three kinds of steel having compositions based on JIS SCr420 with varying Si concentrations (0.25%Si steel, 1%Si steel, 2%Si steel). The carbon concentration in the surface layer after gas carburizing decreased with the increase in the Si concentration, and the carbon concentration was particularly low in 2%Si steel. The carbon concentration in the surface layer after gas carburizing of 1%Si steel was nearly equal to the thermodynamic calculation value for a simulated gas carburizing reaction, whereas that of 2%Si steel was much lower. This substantial change seems to be due to the oxide formation on the surface, with 2%Si steel showing the different oxide formation on the steel surface compared to the other steels. The oxides layer of the surface of 2%Si steel densely covered the surface. By removing the oxide layer, the carbon concentration in the surface layer of 2%Si steel increased to the nearly calculated value. These results reveal that the mechanisms for the decrease in the carbon concentration in the surface layer change depending on the Si concentration and are the thermodynamic interaction between Si and carbon for 1%Si steel and 2%Si steel and the oxide layer’s inhibitory effect on the carburizing reaction for 2%Si steel.