The effect of aging at 700°C on the microstructure and mechanical properties of high-nitrogen austenitic steel

Problem. Prolonged annealing of high-nitrogen austenitic steels leads to the precipitation of secondary phases, which significantly affects the structure and properties of the material. However, the nature of phase precipitates and their influence on the mechanical properties of VNS-53-Sh steel have been insufficiently studied. Aim. The aim of this work is to investigate the effect of long-term annealing at 700 °C on the microstructure and mechanical properties of VNS53-Sh high-nitrogen austenitic steel and to determine the phase composition and characteristics of the formed secondary phases. Methods. VNS-53-Sh steel samples were subjected to annealing at 700 °C for 100 hours. Microstructural investigation was carried out using optical metallography, scanning and transmission electron microscopy with a quantitative assessment of the secondary phase fraction. Mechanical properties were determined by uniaxial tensile testing followed by analysis of changes in strength and ductility characteristics. Results. After annealing, grain-boundary and intragranular precipitates of M23C6-type carbides were revealed, formed by the discontinuous austenite decomposition mechanism. Estimates of the area occupied by carbides give values of less than 5 %. Meanwhile, the proportion of a pearlite-like structure represented by lamellar particles and a chromium-depleted matrix is approximately 23 %. The yield strength decreases by 111 MPa, the ultimate tensile strength – by 62 MPa, and the relative elongation increases by about 7 %. Conclusions. Long-term annealing of VNS-53-Sh steel at 700 °C leads to the formation of M23C6 carbide precipitates and partial relaxation of the dislocation structure, which is accompanied by a moderate reduction in strength properties while simultaneously increasing the ductility of the material.