Effect of heterogeneous structure on mechanical behavior of austenitic stainless steel subjected to novel thermomechanical processing

Introduction. The low yield strength of austenitic stainless steels is a factor significantly limiting their industrial applications. In turn, the formation of a heterogeneous structure is a promising method for achieving a synergy of mechanical properties. At the same time, an effective way to obtain a bulk heterogeneous structure is cold radial forging. However, the underlying mechanisms for the improved mechanical properties of materials with a heterogeneous structure formed in the process of cold radial forging are currently poorly understood. Purpose of the work is to investigate the effect of a heterogeneous structure obtained by deformation and heat treatment on the mechanical properties of austenitic stainless steel 0.08 C-17 Cr-13 Ni-2 Mn-Ti. Methods. Uniaxial tensile tests were performed on specimens obtained by cold radial forging followed by heat treatment at 600–700°C, using an Instron 5882 testing machine at room temperature with a strain rate of 1.15 × 10–3 s–1. A VIC-3D visual inspection system was used to measure elongation during testing. The fine structure was examined on perforated foils with a diameter of 3 mm using a JEOL JEM-2100 transmission electron microscope at an accelerating voltage of 200 kV. Results and discussion. It was shown that, after thermo-mechanical treatment, a twin-matrix austenite structure was obtained in the center of the rod, while an ultrafine-grained structure with isolated recrystallized austenite grains of approximately 1 μm in size was obtained at the edge. It was established that a two-component axial austenite texture <001>/<111> is formed in the center of the rod, which transformed into a shear texture B/B̅ towards the rod surface. It was determined that the formation of a heterogeneous structure led to additional strengthening due to back stresses. It was found that, after heat treatment at 700°C, the specimen with a heterogeneous structure exhibited the highest yield strength, equal to 1054 MPa, with a relative elongation of 16%. Thus, the employed thermo-mechanical treatment may be a promising method for obtaining large-sized rod stocks from austenitic stainless steel 0.08 C-17 Cr-13 Ni-2 Mn-Ti with high mechanical properties.