EXAMINATION OF DEFORMATION MECHANISM MAPS IN 2.25CR-1MO STEEL BY CREEP TESTS AT STRAIN RATES OF 10(-11) TO 10(-6) S(-1)

The deformation mechanism map of 2.25Cr-1Mo steel was examined by cree p data obtained over a wide range of creep rates down to 10(-11) s(-1) . The stress dependence of minimum creep rates of the steel is similar to that of particle strengthened materials: low, high, and low stress exponent, respectively, in high (H), intermediate (I), and low (L) st ress regions. The stress exponent and activation energy for creep rate suggest dislocation creep controlled by lattice diffusion as the defo rmation mechanism in regions I and L, including service conditions of the steel. Transition to diffusion creep occurs at a lower creep rate than what is expected in the deformation mechanism maps. Region H appe ars above athermal yield stress. During loading in this region, atherm al plastic deformation takes place by dislocation glide mechanism, and then dislocation creep starts. The dislocation creep in region H is d ifferent from the one in regions I and L due to the plastic deformatio n during loading. A modified creep mechanism map of 2.25Cr-1Mo steel i s proposed on the basis of the experimental results. (C) 1997 Elsevier Science S.A.