Effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature of 9CrWVTiN steels

摘要

The effects of Ti and a twice-quenching treatment on the microstructure and ductile brittle transition temperature (DBTT) of 9CrWVTiN steels have been studied. The results show that Ti addition reduces austenite grain size and martensitic lath, and moderate Ti (＜0.018%) content reduces the precipitates size. Microstructure, especially the coarse M_(23)C_6 precipitates is remarkably refined by twice quenching and by consuming C through preferential precipitation of MX precipitates in the furnace cooling period, thus contributing to the decrease of DBTT compared with quenching-tempering process. However, DBTT increases with increasing Ti content, and the increased DBTT reaches to as high as 36 ℃ compared with steel without Ti regardless of the refinement of micro-structure. Through precipitate analyses, we find that, Ti strongly interferes with the precipitation of V(C,N). Ti(C,N) hardly exists in matrix alone. Instead, it acts as the core of quadrate (Ti,V)(C,N) particle. Big stress concentrations at the corners of coarse quadrate (Ti,V)(C,N) precipitates make them as crack initiators during impact tests, thus deteriorating the toughness. Also, formation of complex (Ti,V)(C,N) particles reduces the amounts of V, N, and C available for adequate fine V(C,N) particle precipitation. These two factors are the main reasons for high DBTT generated by Ti addition.