The grain size-dependent control of the phase composition in ion-plasma treated 316L stainless steel

We study the influence of different grain size (density of grain boundaries) on the way of phase transformations in the surface layers of 316 L-type austenitic stainless steel under ion-plasma treatment. Using thermomechanical treatments, we fabricated a series of specimens possessing a single-phase austenitic structure, close density of the defects of the crystal lattice and different grain sizes (fine-grained with d ≈ 3–6 μm and coarse-grained with d ≈ 55 μm). These specimens were subjected to ion-plasma surface treatment at 550 ± 10 °C in N2+C2H2+Ar gases mixture to provoke a precipitation hardening. Although fine-grained and coarse-grained specimens possess similar penetration depth of interstitial atoms (N, C) under ion-plasma treatment (≈40–48 μm), the distribution of interstitials and phase composition are different in them. After ion-plasma treatment, specimens with low density of grain boundaries (coarse-grained structure) maintain a high level of N, C atoms in the solid solution of austenite (a = 0.3653–0.3674 nm) with a strip-like arrangement of Fe4(N,C) particles within grains, while precipitation of Cr(N,C) phase is suppressed. For these specimens, tensile diagrams have the extended linear stages typical of nitrogen-bearing austenitic steels, and the loss of ductility assisted with ion-plasma treatment is the smallest among studied specimens. Ion-plasma treated specimens with high density of grain boundaries (fine-grained structure) are prone to a decomposition of Fe-γN,C phase with the formation of grain-boundary and intragranular Cr(N,C) and Fe-α phases and partial preservation of a solid-solution strengthening of austenite (a = 0.3597–0.3622 nm). Precipitation hardening is more characteristic of these specimens and their flow curves are parabolic. The complex fracture mode of the specimens subjected to ion-plasma treatment is caused by the surface solid-solution strengthening and precipitation hardening. In the surface-hardened region (where the concentrations of N, C atoms are the highest), brittle quasi-cleavage fracture occurs due to the presence of Fe-based and Cr-based precipitates and austenite oversaturated with interstitials.