Kinetics of niobium carbide precipitation in a low carbon austenitic steel

Abstract

An examination has been made of the kinetics of niobium carbide precipitation in a 18-10-1 austenitic stainless steel in the temperature range 650 to 750° C. Electrical resistance-time plots, thin film electron microscopy and hardness measurements have been employed to follow the ageing sequence. In these alloys the carbides precipitate on undissociated dislocations and in association with stacking faults; these processes are diffusion controlled and have an activation energy of ∼318 kJ mol⁻¹. Prior to the reaction beginning a clear incubation period existed, e.g. ∼30 h at 650° C and 20 min at 750° C. During the first 10% transformation the carbide nucleation rate increases and the associated faults nucleate and grow rapidly. The carbide nucleation rate appears to peak around this level and then falls away gradually to zero around 70% transformation. At this latter stage fault growth ceases, and transformation continues by a carbide growth process. The age-hardening peak occurs much beyond the end of the reaction by which time precipitate coarsening is in evidence. The precise effect mechanical deformation has upon stacking fault formation depends to a major extent on the niobium supersaturation in the quenched alloy.