Analysis of nonisothermal transformation kinetics; tempering of iron-carbon and iron-nitrogen martensites

Abstract

A powerful method for the analysis of solid-state transformation kinetics can be based on non-isothermal analysis where the temperature corresponding to a fixed stage of transformation is measured as a function of heating rate. Adoption of a specific kinetic model is not required for valid application of this method. This removes constraints imposed unnecessarily on so-called Kissinger-like procedures. The method was applied to study and to compare the tempering kinetics of iron-carbon martensite and iron-nitrogen martensite (about 1.1 wt pct interstitials). Dilatometry and differential scanning calorimetry were employed. This combination of experimental techniques allowed distinction between preprecipitation processes as segregation and clustering of interstitials. In contrast with iron-carbon martensite, no indications were obtained for clustering of interstitials in iron-nitrogen martensite. The preprecipitation stages have activation energies of 75 to 85 kJ/mole, which are ascribed to volume diffusion of interstitials. The precipitation of the transition carbide/ nitride (first stage of tempering) occurs with an activation energy of 110 to 125 kJ/mole, which is ascribed to pipe diffusion of iron. The precipitation of the equilibrium carbide/nitride (third stage of tempering) is associated with an activation energy of 195 to 205 kJ/mole which is ascribed to combined pipe and volume diffusion of iron.