Kota Sawada

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

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.

Examination of deformation mechanism maps in 2.25Cr—1Mo steel by creep tests at strain rates of 10−11 to 10−6 s−1

Abstract The deformation mechanism map of 2.25Cr—1Mo steel was examined by creep 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) stress regions. The stress exponent and activation energy for creep rate suggest dislocation creep controlled by lattice diffusion as the deformation 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 appears above athermal yield stress. During loading in this region, athermal plastic deformation takes place by dislocation glide mechanism, and then dislocation creep starts. The dislocation creep in region H is different from the one in regions I and L due to the plastic deformation during loading. A modified creep mechanism map of 2.25Cr—1Mo steel is proposed on the basis of the experimental results.

Creep Rupture Ductility of Creep Strength Enhanced Ferritic Steels

Creep rupture strength and ductility of creep strength enhanced ferritic steels of Grades 23, 91, 92, and 122 was investigated with particular emphasis on remarkable drop in the long-term. Large difference in creep rupture strength and ductility was observed on three heats of Grade 23 steels. Remarkable drop of creep rupture strength in the long-term of T91 was comparable to those of Grades 92 and 122. Remarkable drop in creep rupture ductility in a stress regime below 50% of 0.2% offset yield stress was observed on Grade T23 steel, however, that of Grade P23 steel did not indicate any degradation of creep rupture ductility. Higher creep rupture ductility of Grade P23 steel was considered to be caused by its lower creep strength than that of T23 steels. Creep rupture ductility of Grades 92 and 122 steels indicated rapid and drastic decrease with decrease in stress at 50% of 0.2% offset yield stress. Stress dependence of creep rupture ductility of Grades 92 and 122 steels was well described by a ratio of stress to 0.2% offset yield stress, regardless of temperature. On the other hand, large drop in creep rupture ductility of Grade 91 steel was observed only in the very low-stress regime at 650 °C. Alloying elements including impurities and changes in precipitates may influence on creep rupture ductility, however, remarkable drop in ductility of the steels cannot be explained by chemical composition and precipitates. High ductility in the high-stress regime above 50% of 0.2% offset yield stress should be provided by easy plastic deformation, and it has been concluded that a remarkable drop in ductility in the low-stress regime is derived from a concentration of creep deformation into a tiny recovered region formed at the vicinity of grain boundary.

Improvement of creep rupture life by high temperature pre-creep in magnesium–aluminum binary solid solutions

Abstract Effects of high-temperature pre-creep on creep life in magnesium–aluminum solution hardened alloys have been investigated. Creep life, at 0.55 T m ( T m is the absolute melting temperature) are drastically improved by pre-creep treatment, which is given at higher temperature, 0.7 T m . The samples show almost the same minimum creep rate, although creep rate in tertiary creep is affected by the pre-creep treatment. Improvement of creep life by pre-creep is attributed to suppression of linkage of inter-granular cavities at non-straight grain boundaries formed by high temperature pre-creep. The significance of pre-creep treatment on creep life of the alloys is presented and the high-temperature pre-creep treatment is considered as a promising method to improve the creep life of magnesium alloys.