EFFECT OF PROLONGED EXPOSURES OF 9CR-1MO-0.07C STEEL TO ELEVATED-TEMPERATURES

9Cr-1Mo steel in its commercially heat treated condition consists of a tempered martensitic microstructure with M2X and M23C6 precipitates. Exposure of the steel in this condition to high temperatures (823-1023 K) for prolonged durations results in the dissociation of martensite, formation of subgrains of ferrite and evolution and growth of secondary carbides. The sequence and kinetics of the above reactions are found to be dependent on temperature. The microchemical nature of the carbides suggest that though the type of carbide is the thermodynamically stable M23C6, the ratio of Cr and Fe varies systematically with ageing temperature. This behaviour can be attributed to an interplay of the abundance of Fe atoms and the diffusivity of Cr at the precipitate matrix interface, which influences the nature of M in M23C6.