Isothermal Transformation Diagrams of Retained Austenite Formed after Annealing and Quenching in Maraging Steel

Abstract

In this paper, the γ → α transformation kinetics in cold-rolled maraging steel is studied in the temperature range of 0…–100°C. Based on electrical-resistivity kinetic curves R = f(τ), critical temperatures M_begin and M_end have been determined and isothermal transformation diagrams have been constructed for overcooled retained austenite formed during annealing and quenching with preliminary annealing. It is shown that the martensitic γ → α transformation of retained austenite formed during annealing in the two-phase (α + γ) region at the temperature of 500°C during 50 h, which is close to the critical temperature A_begin, takes place in the temperature range of –8…–16°C; the temperature of the minimal stability (the “nose” of the C-curve) is –12°C, at which the incubation period is 4 min and the transformation period is 11 min. The martensitic γ → α transformation of retained austenite formed during quenching with preliminary annealing takes place in the temperature range of –86…–96°C. In this case, the incubation period increases by more than a factor of 10 and the transformation period increases by more than a factor of 8. The maximal shift of the diagrams to the side of low temperatures is ~80°C and is not associated with the preliminary annealing influence since the stability characteristics of retained austenite of quenching with preliminary annealing and without it are similar. The shift of the diagrams to the side of low temperatures is due to the processes occurring at heating for quenching, and according to the resistometry results, preliminary annealing does not influence the above processes.