Anelastic effects connected with isothermal martensitic transformations in 24Ni4Mo austenitic and 12Cr9Ni4Mo maraging steels

Abstract

Anelasticity of the austenitic steel 24Ni5Mo and the stainless steel 12Cr9Ni4Mo has been investigated in the austenitic state after quenching with respect to isothermal martensitic transformation during cooling and subsequent heating. Maxima of anelasticity due to isothermal transformation at ≈200 K (24Ni5Mo, 0.002% C) and ≈250 K (12Cr9Ni4Mo; 0.01% C) coincided well with C-curve noses obtained by methods based on magnetic properties and electric resistivity. Corresponding internal friction maxima were found to be dependent on cooling or heating rate, quenching temperature and the frequency of oscillation and may therefore be described using the Delorme approximation. The activation energy of isothermal martensitic transformation calculated from the lower part of the C-curves estimated using the Borgenstam–Hillert and Arrhenius methods (3–8 kJ/mol for 24Ni5Mo and 15–20 kJ/mol for 12Cr9Ni4Mo) are comparable with the energy of impurity–dislocation interaction (≈10 kJ/mol) and interpreted as too low to be caused by diffusion processes: the activation energy for carbon diffusion in austenitic steel 24Ni5Mo is found to be ≈135 kJ/mol and ≈145 kJ/mol for austenite in two-phase 12Cr9Ni4Mo steel. An estimation showed that the activation energy for the isothermal martensitic transformation for the 24Ni5Mo alloy with so-called binary martensitic kinetics was higher in the vicinity of the nose of the C-curve, became lower with a decrease in temperature range and approached zero in the vicinity of the athermal martensitic point. A similar effect was not observed in the 12Cr9Ni4Mo steel.