Assessment of Long-Term Creep Strength and Review of Allowable Stress of High Cr Ferritic Creep Resistant Steels

Abstract

Large drop in creep rupture strength in the long-term is a noticeable phenomenon for high Cr ferritic creep resistant steels, and the neglecting this phenomenon may result in overestimation of 100,000h creep rupture strength, and allowable stress. A committee at Japan Power Engineering and Inspection Corporation was organized to evaluate long-term creep strength and to review current allowable stresses of high Cr ferritic creep resistant steels. Life prediction method for high Cr ferritic creep resistant steels with tempered martensite microstructure is discussed. Appropriateness of current allowable tensile stress regulated in METI (Ministry of Economy, Trade and Industry) Thermal Power Standard Code has been assessed on a modified 9Cr-1Mo steel (ASME Gr.91) and KA-SUS410J3 type steels. KA-SUS410J3 is a material specification in METI Thermal Power Standard Code and corresponds to ASME Gr.122. The validity of existing allowable stress was shown on a modified 9Cr-1Mo steel. Those for KA-SUS410J3 type steels, however, have been revised to the lower values.