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

Abstract

The mechanisms of decrease of carbon concentration on the surface layer after gas carburizing with increasing Si concentration was investigated using three kinds of steel whose compositions were based on JIS SCr420 varying the Si concentration (0.25%Si steel, 1%Si steel, 2%Si steel). The carbon concentration on the surface layer after gas carburizing decreased with the increase of the Si concentration, and especially that of 2%Si steel was substantially low. The carbon concentration on the surface layer after gas carburizing of 1%Si steel was nearly equal to the thermodynamic calculation value simulating the gas carburizing reaction, whereas that of 2%Si steel was much lower. This substantial change seemed to come from the oxide formation at the surface, that is, 2%Si steel was different from the other steels in oxide formation at the surface, and the oxide of 2%Si steel densely covered the surface. These results reveal that the carbon concentration on the surface layer decreased according to the effect of the inhibition of the carburizing reaction by the oxide layer in 2%Si steel in addition to the effect of the thermodynamic interaction between Si and carbon.