Effect of CO concentration on paraequilibrium gas carburization of 316L stainless steel

Surface strengthened layers were prepared by paraequilibrium gas carburization in mixture of CO/H₂/N₂ with different CO concentration. The effect of CO concentration on microstructure, carbon profile, surface nano-hardness and residual stress of the carburized layer on 316L stainless steel surface was investigated. The results show that the paraequilibrium carburized layer consists of single expanded austenite. With increase of the CO concentration, the thickness, surface nano-hardness and compressive residual stress of the carburized layer increase when the CO concentration is less than 30%, but when the CO concentration continues to increase to 35%, the surface strengthening effect decreases because of lower carbon activity of the carburizing atmosphere. After the paraequilibrium carburization at 470 ℃ for 20 h, the thickness of carburized layer can reach 30 μm, the surface carbon concentration increases to 2.6 mass%, leading to an increase in surface nano-hardness from 3 GPa to 9 GPa and compressive residual stress more than 1300 MPa.