Temperature and stress dependence of creep life of welded joints in strength enhanced high Cr ferritic steels

Overestimation of creep life is a serious problem that happens in long term creep of welded joints made of 9–12%Cr ferritic steels. Creep rupture data of welded joints made of grades 91, 92 and 122 steels were subjected to multiregion analyses paying attention to change in activation energy Q for creep rupture life. A region with a low Q value was found in long term creep of the welded joints. Therefore, long term creep rupture life of the welded joints can be overestimated when it is evaluated by temperature acceleration tests based on a high Q value of short term data. The decrease in activation energy was closely related to the change in fracture location from base metal to heat affected zone. Activation energy and stress exponent for creep rupture life take similar values both in base metal and in the welded joint, though the transition to the low Q region starts at shorter time in cross-welded joints. This fact suggests that creep rupture life can be formulated in a unified manner irrespectiv...