Kinetics of γ → α Transformation of Retained Austenite in Maraging Steel after Double Annealing at Subzero Temperatures and Loads

Abstract

This paper studies the influence of mechanical tensile loads in the range of 0–300 MPa on the nature of γ → α transformation in cold-rolled maraging steel in the temperature range of –20…–60°С. Based on the kinetic curves of the electrical resistance, critical temperatures M_beg and M_end were determined and the diagrams of the isothermal decomposition of supercooled retained austenite annealed in the two-phase region after two cycles at a temperature of 650°C for 6 h were plotted. It has been established that mechanical loads in the range of 20–300 MPa affect the parameters of the gamma-alpha transformation ambiguously, shifting the isothermal diagrams in temperature and time and significantly changing the stability characteristics of austenite. Resistance of austenite to supercooling T_s varies from –25 to –55°С, incubation period τ_inc from 2 to 8 min, and decay period τ_dec with two extremes (50 and 100 MPa) from 51 to 518 min depending on the load level. The recorded maximum duration of the annealing austenite decomposition at low loads (20–100 MPa) is explained by the structural and chemical inhomogeneity of the phases, as well as the stress level of the initial state of the samples under study. The threshold stress is determined, loading up to which leads to a decrease by ∼15°С in the beginning of the γ → α transformation in the interval of subzero temperatures (from –40° to –55°C). It can be assumed that the shift of the diagrams to the range of low temperatures is due to a change in the mechanism of the γ → α transformation. It is shown that cyclic heat treatment in the two-phase region at a temperature of 650°C for 6 h at loads up to 100 MPa delays the isothermal decomposition of supercooled retained austenite in the steel under study.