Creep deformation in a modified 9Cr-1Mo steel

The short term creep properties and microstructural changes associated with creep were studied for constant stress specimens of a modified 9Cr-1Mo steel tested at 923, 873, and 823 K. The variation in the minimum creep rate with stress could be well described by the Norton equation (E=Asigma(II)exp(-Q/RT)) and many of the variations of this equation described in the literature. The values for the activation energy and stress exponent were comparable to values reported previously. The Monkman-Grant equation was found to describe well the relationship between the minimum creep rate and the time to rupture. The microstructural changes have been considered in terms of precipitates previously identified in these steels. A coarse structure (size greater than or equal to 200 nm), comprising M23C6, M2X and a Nb rich MX type precipitate, was observed using SEM. A finer scale structure (size < 200 nm) was observed, using TEM, to be predominantly comprised of M23C6, M2X and a V rich MX type precipitate, with smaller amounts of M6X, V4C3, etc. After creep the coarse M23C6 and M2X precipitates were found to coarsen and the fine V rich MX type precipitates were found to increase in number. It is considered that precipitation of the V rich MX precipitates compensates for the coarsening of the M23C6 and M2X, thus offsetting the reduction in creep strength that would have been produced by the coarsening.