THE INFLUENCE OF STRAIN-INDUCED DEFECTS ON PHASE AND ELEMENTAL COMPOSITION OF HARDENED SURFACE LAYERS OF AUSTENITIC STAINLESS STEEL FORMED DURING ION-PLASMA TREATMENT

Austenitic stainless steels are demanded alloys in modern industry due to their physical and mechanical characteristics. Concurrently, they are not devoid of weaknesses - strength properties do not meet the performance requirements for their use in the manufacture of essential components. One of progressing way to solve this problem is ion-plasma saturation with interstitials (nitrogen and carbon) of materials surface. In this paper, authors investigated the influence of pre-deformation microstructure with different density of deformation-associated defects on phase and elemental composition of surface layers formed during ion-plasma treatment in stable austenitic stainless steel (316L-type). It was shown that thermal-mechanical treatment in two regimes facilities to the formation of grain-subgrain structure submicrometer scale in specimens, in which main differences lie in the density of deformation defects and fraction of low-angle boundaries. It has been shown that during ion-plasma treatment in the mixture of gases (Ar + N2 + C2H2) at 540 °С (12 hours) of stable austenitic stainless 316L-type steel independently of initial microstructure (deformation-induced grain-subgrain with high density of defects or annealed grain-subgrain) in specimens surface layers with the same phase compositions were formed - supersaturated with nitrogen and carbon austenite and ferrite (Fe-γ_{N, C} and Fe-α_{N, C}), nitrides and carbonitrides Cr(N, C), Fe₄(N, C). The high density of non-equilibrium crystal defects promoted to the intensive saturation of the surface layers with nitrogen and carbon in austenitic stainless steel. The developed defective grain-subgrain structure in specimens contributes accumulation of interstitials (nitrogen and carbon) during ion-plasma treatment in the surface layer (≈ 5 μm) and suppression of bulk diffusion of carbon compared to the annealed grain-subgrain structure. The experimental results provide support for significant role of deformation-assisted well-developed microstructure in accumulation and bulk diffusion of interstitials under ion-plasma treatment of austenitic stainless steel.

austenitic stainless steel, cold rolling, grain-subgrain structure, crystal structure defects, dislocations, ion-plasma treatment, X-ray diffraction, Auger-electron spectroscopy, solid-solution strengthening, precipitation hardening