Influence of long term strain hold on creep-fatigue life of Mod. 9Cr-1Mo steel
Ogata Takashi
Abstract:Modified 9Cr-1Mo steel is the principal candidate material of a steam generator in a fast breeder reactor because of its superior high temperature property. In this study, the influence of long term strain hold on the failure life of Mod. 9Cr-1Mo steel was investigated and the applicability of existing creep-fatigue life evaluation methods was discussed based on the experimental results. Creep-fatigue tests with hold time up to 10 hours per cycle at either tension or compression maximum strain were conducted under strain control of 0.5% and 1.0% at 550 deg.. Although failure life reduction occurred by introducing short hold period in the compression hold tests, the degree of reduction became smaller with increasing hold period. On the other hand, the failure life reduction became larger with increasing hold period in the tension hold tests. It was found that the failure life reduction in the compression hold tests was mainly due to the accumulation of tensile strain at the center of a specimen and that in the tension hold tests it was caused by intergranular damage in terms of creep cavity nucleation and growth from observation of the failure specimens. The time fraction rule adopted in a design code gave longer life compared to the experimental life, especially at low strain level. The failure life was well predicted by the ductility exhaustion method and the nonlinear damage accumulation model. Key Words:Mod.9Cr-1Mo steel, creep-fatigue, strain hold, intergranular failure, transgranular failure, creep cavity, life prediction