Microstructure Evolution in an Advanced 9Cr–1.5Mo–1Co–VNbBN Alloy during Heat Treatment and High Temperature Aging

Abstract

The 9Cr–1.5Mo–1Co–VNbBN steel is the most successful cast steel developed so far in the COST program. Its microstructure evolution influenced by austenitizing and tempering are investigated in this paper. The microstructure characterization using an optical microscope and transmission electron microscope is carried out after different heat treatments and long‐term aging at 620 °C. After austenitization at 1100 °C, the steel exhibits a martensite structure with Fe‐rich M3C particles. The M23C6 particles are detected in the sample tempered at 500 °C for 2 h, as well as their sizes continue to grow as the tempering temperature raise. The formation of MX phase is identified after tempering at 500 °C, while the size of MX phase remains stable within the tempering temperature 760 °C and below. The structure evolution of martensitic laths shows that tempering reduces the density of dislocation and forms subgrains and dislocation networks. The microstructure of long‐term aged sample still reveals martensite lath structure with increasing width. In addition to M23C6 and MX phases, Laves phase nucleates and grows rapidly as a result of long‐term aging.