In situ investigation of structure and stability of niobium carbonitrides in an austenitic heat-resistant steel

The structure and stability of Nb(C,N) precipitates were direct observed during reheating of an austenitic heat-resistant steel using in situ hybrid observations. The coarsened δ-Nb(C,N) and finer ε-Nb(C,N) were present in as-received steel. During the coarsening process, the d-spacing of cubic δ-Nb(C,N) exhibits a sharp decrease with increasing temperature, while that of hexagonal ε-Nb(C,N) maintains a steady increase. Compared to δ-Nb(C,N), the ε-Nb(C,N) was quickly dissolved into the austenite during heating, and it would no longer reprecipitate in the subsequent cooling.