The kinetics of the thermal stabilization of austenite at sub-zero temperatures

Abstract

The kinetics of the thermal stabilization of austenite have been studied using an electrical resistance measurement technique in two 0.5% carbon steels containing 19% and 24% nickel after treatments at temperatures between −100 and −40°C. Stabilization has been shown to develop extremely rapidly in both alloys; the degree of stabilization, θ, increases with increase in ageing time, prior martensite content, and ageing temperature above −70°C, and is generally greater in the 19%Ni than in the 24%Ni steel. A significant difference in the temperature dependence of stabilization has been revealed. In the 19%Ni alloy, θ increases continuously with increase in ageing temperature, whereas a minimum θ value is observed at −80°C in the 24%Ni alloy. The kinetics of stabilization are otherwise similar in the two alloys studied, even though their martensites have different crystallography. Stabilization is dependent on a process which either occurs within, or is stimulated by the presence of, preformed martensite; however, the relationship is not a simple volumetric effect. The results show that stabilization at sub-zero temperatures cannot be explained by existing theories.