Carbide precipitation in Nb-V-Ti microalloyed ultra-high strength steel during tempering

The microstructure, precipitates and mechanical properties of Nb-V-Ti microalloyed ultra-high strength steel under different tempering temperatures are investigated. With the increasing tempering temperature, the width of martensitic laths gradually increased with reduced dislocations within them, and the volume fraction of retained austenite first increased and then decreased. Furthermore, the Charpy impact toughness, reduction of area and elongation are enhanced at the sacrifice of the strength. Four kinds of precipitates are identified at different tempering temperatures, namely M3C, M2C, M23C6 and M7C3. M3C precipitated when tempering at 200–400°C, and it was replaced partially by M2C when the temperature is elevated to 500°C, at which M23C6 also precipitated. For a higher temperature, above 600°C, M3C disappeared, and some M2C transformed into M7C3. The other two types of MC precipitates are found to be the stable phases existing over different tempering temperatures. The correlation between the mechanical properties and microstructure was established, and the precipitation mechanisms of carbides at the different tempering temperatures are explained.