Microstructural Development and Fracture of SUS304 Jig under Gas Nitriding and Gas Nitrocarburizing

The microstructural change and fracture by repeated gas nitriding and gas nitrocarburizing of SUS304 jig were investigated in this study. It was found that an oxide layer which was formed owing to high-temperature oxidation of the microstructure near the surface, was easily detached from the surface layer. Also, many micro voids in the oxide layer were observed. Beneath the oxide layer, a compound layer containing mainly nitrides was identified. A carbon diffusion region was observed in the matrix adjacent to the interface between the compound layer and the matrix. It was considered that the fine voids and uneven surfaces induced under loading stress during both treatments were the crack initiation points, which coalesced for the development of cracking. The fractured surface exhibits a sequence of brittle and ductile fracture patterns as the microstructure changes from the surface to the interior.