Gas Diffusion Hybrid S Phase: Influence of Treatment Time

The incorporation of carbon and nitrogen elements concurrently during low-temperature gas diffusion process on the austenitic stainless steel surface leads to the formation of S phase layer. However, findings on the characterization studies of gas diffusion hybrid S phase layer are still unprecedented. This study examines the effect of treatment time on the S phase layer forming behavior of AISI 316LVM (ASTM F138) stainless steel through low-temperature hybrid gas diffusion process. The hybrid heat treatment process performed at 475 °C, with the gas composition of N₂, CH₄ and NH₃ at 10%, 10% and 80% respectively. The variation of treatment time tested from 3 to 18 h. The outcome demonstrates the treatment time proportionally influence the expanded layer thickness. However, when the duration of treatment exceeds 15 h, the surface quality of the S phase layer decreases due to the possible high residual stress that causes the layer to crack. There are no traces of chromium precipitates at 18 h of treatment time which indicates the duration of heat treatment does not influence the formation of the precipitate. Thus, the treatment time plays significant roles in the development of the hybrid S phase layer other than heat treatment temperature and gas composition for the hybrid gas diffusion process.