Investigation of the influence of thermo-mechanical treatments on the structural-phase state and mechanical properties of VZH171 alloy

An analysis of the influence of thermomechanical treatment (TMT) (rolling to a strain degree of 50%, annealing at a temperatures of 600–1200°C, 1 h) on the microstructure and phase composition of a VZh171 alloy in the initial state and after its bulk nitriding is performed. It is shown that nitriding leads to an increase in the thermal stability of the alloy microstructure. While a primary recrystallization and formation of finegrained structure occurs in an non-nitrided alloy quite intensively at the temperature above 1000°C, the process of collective recrystallization does not gain any significant development in the nitrided alloy even at an annealing temperature of 1200°C, and the microstructure remains ultrafine-grained (grain size 5–10 μm). The mechanical properties of the nitrided and non-nitrided alloy are investigated. It is established that nitriding has no significant effect on the alloy strength at room temperature. In the tests at 800°C, the maximum strength and the minimum plasticity are demonstrated by the nitrided alloy after rolling and subsequent annealing at 600°C. The optimum ratio of the ultimate tensile strength (480 MPa) to plasticity (elongation to fracture 23%) is observed in the nitrided alloy after TMT and annealing at 800°C.