Coarsening of vanadium carbide,carbonitride,and nitride in low-alloy steels

Abstract

Detailed particle coarsening experiments have been done on a series of 0.25% vanadium steels containing various amounts of carbon and nitrogen. The relative rates of particle coarsening of the three vanadium compounds in ferrite can be ranked in the ascending order VN ≃ V(CN) < VC, with vanadium carbide coarsening approximately fifty times more rapidly than vanadium carbonitride and nitride. A possible explanation for this result is given. During the aging of all five alloys investigated, a changeover from (time)½ to (time)⅕ particle coarsening kinetics occurred. This is interpreted as a changeover from interface reaction controlled coarsening to dislocation coarsening (controlled by pipe diffusion), a view supported by detailed electron microscopy of the alloys. A mechanism for this coarsening process is suggested. Estimates of the relative activation energies indicate that this is lower for dislocation coarsening than for interface reaction controlled coarsening. The effect of increasing the dislocation density (by 5% cold deformation) was to greatly increase the coarsening rate of the vanadium carbonitride particles by a factor of about thirty. Particle size distributions resulting from coarsening were generally found to be broader than predicted by theory. The coarsening reactions were also followed by hardness measurements.