Abstract
In order to estimate the long term stability of the creep rupture strength in a 23Cr-18W-Ni alloy, which is developed for the use in a nuclear steel making process, the effect of grain size on creep behaviour has been studied on samples produced by the conbination of various kinds of heat treatment and cold work. Undissolved α-W particles were liable to prevent grain growth and this caused increase in creep rate at 1000°C under low stresses.
The steady state creep rate, ε, with usual grain sizes and at 1000°C was found to be propor-tional to σ6 at high stresses and σ2.5/dd2 at low stresses, where σ is initial applied stress and d is average grain diameter. Grain boundary migration and/or recrystallization were observed in the specimens crept to secondary or tertiary stage at 1000°C. However, no evidence of grain boundary cracking or grain boundary sliding was obtained. These results suggest that the obvious and strong grain size dependence of the creep rate is related to recovery or recrystallization process near the grain boundary.
When the grain size is larger than the critical value, the grain size dependence of the creep rate becomes ambiguous. This critical value increases with decreasing applied stress. This observation, in the engineering point of view, leads to the conclusion that the optimum creep strength around 1000°C at low stresses should be obtained when the grain size is about ASTM No. -1 ( grain dia-meter ≅400μ).
