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Abstract
The elevated temperature creep properties of a ½Cr-Mo-V creep-resisting steel have been investigated as a function of heat treatment and microstructural condition. It was found that a wide range of creep properties could be obtained by controlling (i) the austenitizing temperature, (ii) the primary transformation temperature, and (iii) the subsequent tempering treatment. The creep strength was found to be mainly determined by the state of the VC precipitate dispersion, so that the best creep strengths were obtained by austenitizing at high temperatures and transforming at temperatures which are close to the bay in the T-T-T curve (600–650°C) in order to produce either granular bainite (which was subsequently tempered), or a strong directly transformed ferrite (which was not tempered). Apart from a general decrease of creep ductility with increasing creep strength, it was found that there was a strong dependence of reduction-in-area at fracture upon the grain morphology produced by primary transformation. In this respect, creep ductility could be improved by either transforming at high temperatures (750–775°C) to produce a weaker directly transformed ferrite, or transforming at temperatures in the range 500–600°C to produce a high-strength granular bainite which was subsequently tempered.