Temperature Dependence of Mechanical Properties and Fracture Micromechanisms of Austenitic Stainless Steel with Surface Ion–Plasma Hardening

We investigated the mechanical properties, microstructure, and fracture micromechanisms of austenitic stainless steel with a composition of Fe-17Cr-13Ni-1.7Mn-2.7Mo-0.5Si-0.01C (in mass. %), subjected to an ion–plasma treatment at 550°С for 12 hours in a mixture of N2, C2H2 and Ar. The study was carried out under uniaxial tension in a temperature range from 77 to 293 K. During the ion–plasma saturation of the steel with nitrogen and carbon, a compound layer (10–15 μm, Fe4(N,C), γN,C-Fe, Cr(N,C), and α-Fe phases) and a diffusion sublayer (35–45 μm, a solid–solution hardening of austenite by interstitial atoms, aγN,C = 3.599–3.629 Å) form. It has been shown that surface solid solution hardening of steel with nitrogen and carbon increases thermal and athermal stress components and makes the temperature dependence of an yield strength stronger. Surface hardening changes a shape of “stress–strain” diagrams, increases strain hardening and decreases ductility of the steel in the whole temperature range. It has been established that compound layer undergoes transcrystalline brittle cracking in the early stages of elastic deformation. In the subsurface layer, dislocation pile–ups, dislocation arrays, microbands, microtwins, and stacking faults form during the plastic flow, which all contribute to high strain hardening of the ion–plasma–treated steel. During the room–temperature deformation, subsurface layer fractures by ductile mode. However, an extended region with quasi–cleavage fracture is characteristic of deformation at 77 K. Thus, a ductile-to-brittle transition is realized in the subsurface layer under straining in low–temperature regime. Despite this, the predominantly ductile fracture of the central (untreated) parts of ion–plasma–treated samples provides their high plasticity even at cryogenic temperature.