Creep damage analysis of mod.9Cr-1Mo steel welds considering void mechanics modeling

Since creep strength reduction of mod.9Cr-1Mo steel welds in long-term creep was confirmed experimentally, residual life prediction for Type IV creep failure has been an important theme in Ultra Super Critical (USC) plants after long-term service. The mechanism of Type IV creep failure within the fine-grained heat affected zone (FGHAZ) is twofold; an increase in number density of creep voids of a size comparable to FGHAZ grain size (5 mu m), and strain softening in the last stage of creep life. In this paper, a creep damage analysis scheme considering the above two mechanisms is applied to welded joints with non-uniform weld metal materials, heat affected zone (HAZ) and base metal, such as those found in large uniaxial cross-weld specimens. A void mechanics model expresses the increase of number density of creep voids (creep void density) in terms of a strain for void nucleation, which is strongly influenced by the multiaxial state of stress. A critical value of creep void density corresponding to the initiation of a micro-crack is determined based on the results of a random-fracture-resistance model of grain boundaries by the authors of this paper. Both the creep void density distribution and the failure process with crack initiation, propagation and final rupture predicted by the analysis are compared with experimental results at 650 degrees C.