Structural characteristics of low-temperature plasma-nitrided layers on AISI 304 stainless steel with an α′-martensite layer

The characteristics of the structure formation of a low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel with a pre-existing α′-martensitic deformation layer were studied in detail by glancing-angle X-ray diffraction (XRD). The results show a two-step formation process for the plasma-nitrided layer. The α′-martensitic layer gradually reverts to the original γ-austenite structure at first, followed by the transformation of γ→γN as the nitrogen content increases with treatment time. Within the same nitriding time, the sample with an α′ layer shows much less lattice expansion than the sample without an α′ layer. The volume fractions of α′ and γ phases transformed during the plasma-nitriding process were calculated by quantitative XRD. The transformation of α′→γ is dependent on the nitrogen content dissolved in α′, apart from that resulting from heating at the nitriding temperature.