An investigation of secondary carbides in the spray-formed high alloyed Vanadis 4 steel during tempering

The precipitation of secondary carbides in spray-formed high alloyed Vanadis 4 steel during tempering at 500, 550, and 700 °C has been characterized and the carbides identified in detail using transmission electron microscopy. It was observed that the very fine and dense nature of the secondary MC precipitates was responsible for the secondary hardening peak when tempered at 500 °C. When the tempering temperature was elevated to 550 °C, the hardness greatly decreased due to the precipitation of M3C carbide. Overageing at 700 °C resulted in transformation of the fine secondary carbides to coarse M7C3, M23C6, and M6C and a change in the matrix morphology from martensitic plates to recrystallized ferrite. The decomposition at this temperature initiates with the nucleation of fine M7C3 carbides preferentially at the martensite lath boundaries; prolonged 24 h tempering results in fully equiaxed, recrystallized ferrite and coarse precipitates.