Abstract: Effect of solution treatment temperature on the carbide evolution behavior and mechanical properties of a Cr-Co-Mo martensitic steel were investigated by SEM and TEM methods. The results show that as the solution treatment temperature increases, the M₆C carbides re-dissolve into the matrix, the yield strength decreases, while the impact absorbed energy at room temperature increases; concurrently, the prior austenite grains grow rapidly due to the lack of pinning by spherical M₆C carbides on the grain boundaries, which weakens the grain refinement strengthening effect, but improves the toughness by reducing the crack origins at the grain boundaries. When solution treated at 1120 ℃, the grain size is the largest, but the yield strength does not decrease significantly after grain growth, and the toughness is not reduced. The reasons for this is that the average particle size and the area fraction of nanoscale M₂C carbide are the smallest and the highest respectively, and the particle spacing is the shortest, so that the precipitation strengthening effect effectively makes up for the loss of the grain refinement strengthening effect; and simultaneously, the coherent precipitation of nano scale rod-like M₂C carbide with the matrix results in that both the deformation compatibility of the matrix and the toughness are improved.

Key words: Cr-Co-Mo martensitic steel, solution treatment temperature, carbide, mechanical properties