Clarification and evaluation of high temperature deterioration in high efficiency weld joint

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1) In the case of the FCA weld metal (which contained large amounts of bismuth, i.e., >0.02%), bismuth segregated as a single substance at the grain boundaries to a depth of about 15 A. The presence of this bismuth led to intergranular fracturing and very marked reductions in creep fracture elongation.2) There was no large difference in creep fracture elongation between the SMA and FCA weld metals, both of which contained no bismuth.3) Creep crack growth rate of the high Bi content FCA weld metal was extremely rapid compared with that of the SMA and FCA weld metals containing no bismuth and in addition to adversely affecting creep elongation properties, the bismuth which segregated at the grain boundaries was also highly deleterious with regard to creep crack growth properties.4) It has been found that the creep crack growth rate tends to be higher (about 2.5 to 3-fold) in cases where sigma phases have adequately precipitated as a result of high-temperature aging than in cases where they have not. This is because creep cracks propagate preferentially through the austenite-sigma interfaces at the dendrite boundaries.5) Accordingly, both the bismuth which segregates at the grain boundaries and the sigma phase which precipitates at the grain boundaries serve as important factors that increase creep crack growth rate. The grain boundary segregation of bismuth is a larger influence factor than sigma phase precipitation in increasing the creep crack growth rate. Therefore, large amounts of bismuth (approximately 0.02%) should be avoided in connection with Type 308 stainless welds and the delta ferrite content of the weld metal should be suppressed as much as possible for elevated temperature service.