The influence of prior deformation on phase composition and strength properties of austenitic stainless steel in ion-plasma treatment

The effect of pre-deformation by cold-rolling on phase composition and nanohardness of a surface layer and resultant tensile properties of Fe-17Cr-13Ni-2.7Mo-1.7Mn-0.65i-0.01C (wt.%, 316L-type) austenitic stainless steel subjected to an ion-plasma treatment was investigated. The ion-plasma treatment facilitates a formation of inhomogeneous surface layers of approximate to 18 - 25 mu m in thickness in steel specimens. Independently of type of initial microstructure, coarse-grained or highly defective deformation-associated one, the surface layers of the steel specimens undergo similar phase transformations under ion-plasma treatment. Solid-solution strengthening of austenite (Fe-gamma(N,C)) and dispersion hardening by different phases (Fe-4(N, C), Cr(N, C), Fe-alpha(N,C)) both increase surface nanohardness and tensile strength characteristics of austenitic stainless steel. X-ray diffraction data show that morphology and distribution of dispersed phases in the surface layers could be strongly dependent on prior microstructure of the steel. In ion-plasma treatment, specimens with coarse-grained structure are prone to accumulate and save interstitials in austenite (solid-solution). After surface treatment, higher strength properties (nanohardness) of the composition layer and more extended diffusion zone both provide higher tensile strength characteristics of pre-deformed specimens as compared to coarse-grained one. The experimental results clearly show that surface hardening of specimens of 316L-type austenitic stainless steel during ion-plasma treatment strongly depends on its initial microstructure.