Effect of W on recovery of lath structure during creep of high chromium martensitic steels

Abstract

Effect of W on creep strength of martensitic steels was investigated paying special attention to microstructural degradation during creep. Though tempered martensitic lath structure is stable at the elevated temperatures without stress, its recovery takes place substantially during creep. After the recovery, lath width and dislocation density in lath interior reached the stationary values determined by creep stress. There was no difference in the stationary values between the two steels with (TAF650 steel) and without (Mod.9Cr–1Mo steel) W. However, recovery processes of the lath structure are significantly different between the two steels. The growth of lath width and the annihilation of dislocations in lath interior are slower in W containing TAF650 steel than those in Mod.9Cr–1Mo steel. Accumulation of creep strain is suppressed in TAF650 steel because of the slow recovery of its lath structure. The retardation of the recovery of lath structure results in the lower creep rate and the higher creep rupture strength of the W containing steel. The slow recovery of lath structure in the W containing steel is ascribed to pinning effect of M23C6 and Laves phase (Fe2W) precipitated on lath boundaries.