The Effect of Prolonged Annealing on the Structural Stability of Nanoparticle-Hardened Low-Carbon 9% Cr–3% Co Steel

Abstract—The effect of long-term annealing on the structure of low-carbon 9% Cr–3% Co steel hardened with nanoparticles has been studied. Annealing was performed for 100, 500, 1000, and 3000 hours at a temperature of 650°C. After heat treatment, the width of martensitic laths in the steel structure was about 300 nm and the dislocation number density inside the laths was high. The lath structure of tempered martensite was stabilized by (Ta,Cr)X carbonitrides with an average size of 11 nm. The Vickers microhardness decreased by 16% after 3000-h annealing compared to that of the initial state. The decrease in microhardness was accompanied by structural changes in the steel upon annealing, such as a decrease in the dislocation density, a decrease in the content of tungsten and copper in the solid solution, and the enlargement of carbonitrides (Ta,Cr)X and martensitic laths. In general, the structural stability of the investigated steel during long-term annealing is quite high compared to that of other high-chromium martensitic steels.