Effects of extended heat treatment on carbide evolution in Cr-Mo steels

• Cr-Mo steel mechanical properties were analyzed by studying carbide evolution.
• Heat treatment duration of steel was determined by Larson-Miller parameter fitting.
• Hardness decreased after heat treatment at 690 °C for 10 h.
• Increase in carbide Cr and Mo concentration deteriorated solid solution strengthening.
• Coarse carbides on grain boundaries deteriorated mechanical properties.

Cr and Mo are known to be effective steel alloying elements for high-temperature applications, and they affect steel mechanical properties via carbide formation. Mechanical properties of Cr-Mo steels affected by the carbide evolution were investigated by performing heat treatments, and the treatment duration was determined by the Larson-Miller parameter fitting. The hardness value decreased after heat treatment at 690 °C for 10 h with the decomposition of the bainitic microstructure. The quantified chemical composition results for the carbides were plotted on the Fe-Cr-Mo ternary and Fe-Cr binary composition diagrams to classify the types of carbides formed. Most of the carbides were identified as Fe-rich M3C-type carbides at the initial stage of heat treatment. The Fe fraction in the carbides decreased with time while the Cr and Mo fractions increased. The Cr- and Mo-rich carbides were identified as M23C6- and M2C-type, respectively. An increase in the Cr and Mo fractions in the carbides resulted in the decrease in the alloying element concentration in the matrix, which in turn resulted in the deterioration of the solid solution strengthening. Further, it was determined that coarse carbides, observed on the grain boundaries after extended heat treatment, deteriorated the mechanical properties of the alloy.