Effect of molybdenum on hardness of low-temperature plasma carburized austenitic stainless steel

The effect of molybdenum in hardening on low-temperature plasma carburized layer of austenitic stainless steel was investigated. This compared a low-temperature carburized layer of AISI 316 (Fe–17Cr–10Ni–2Mo) with that of AISI 304 (Fe–18Cr–8Ni) to evaluate the influence of molybdenum on carburizing. Samples were plasma carburized using DC plasma apparatus under 667 Pa of mixed gas flow of 5% CH4 + 45% H2 + 50% Ar at 673 K or 723 K for various durations. Depth profiles of hardness and micro-structural layer thickness were measured; GDOES, TEM, and XRD were subsequently used to characterize their microstructures. Surface hardness of 316 steel carburized at 673 K for 8 h reached 800 HK, but that of 304 steel remained at 530 HK. Results show no carbide formation in either steel treated at 673 K. The degree of lattice expansion by carburizing in the presence of molybdenum, 316 steel, is higher than that of 304 steel. Furthermore, the diffusion rate of carbon in the 316 steel is higher than that in the 304 steel. The higher hardness of carburized 316 steel is inferred to result from carbon super saturation enhanced by the effect of large molybdenum atoms, which can widen the octahedral sites.