STRESS DEPENDENCE OF DEGRADATION AND CREEP RUPTURE LIFE OF CREEP STRENGTH ENHANCED FERRITIC STEELS

Overestimation of long-term creep strength of creep strength enhanced ferritic steels is caused by change in stress dependence of creep rupture life with decrease in stress. Creep rupture strength of those steels has been re-evaluated by a region splitting analysis method and allowable tensile stress of the creep strength enhanced ferritic steels regulated in METI (Ministry of Economy, Trade and Industry) Thermal Power Standard Code in Japan has been reduced in December 2005 and July 2007. A region splitting analysis method evaluates creep rupture strength in the high stress and low stress regimes individually, which is separated by 50% of 0.2% offset yield stress. Change is stress dependence of the minimum creep rate is observed at 50% of 0.2% offset yield stress, which roughly corresponds to 0% offset yield stress, on ASME Grade 122 type steels. Stress dependence of the minimum creep rate in the high stress regime is equivalent to that of flow stress observed in tensile test, and a magnitude of stress exponent, n, in the high stress regime decreases with increase in temperature from 20 at 550 degrees C to 10 at 700 degrees C. On the other hand, n value in the low stress regime is 4 to 6 for tempered martensite single phase steel, however, remarkably small value of 2 to 4 is observed in the low stress regime of the dual phase steel containing delta ferrite. Large stress dependence of creep rupture life and minimum creep rate in the high stress regime is caused by contribution of considerable plastic deformation due to applied stress higher than a proportional limit. Creep deformation in the low stress regime is considered to be governed by diffusion controlled phenomena and dislocation climb as a rate controlling mechanism.