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

1)在FCA焊接金属（含大量铋，即>0.02%）的情况下，铋在晶界处以单一物质分离，深度约为15a。这种铋的存在导致了晶间断裂和蠕变断裂伸长率的显著降低。2)不含铋的SMA和FCA焊缝金属的蠕变断裂伸长率差异不大。3)与不含铋的SMA和FCA焊缝金属相比，高Bi含量FCA焊缝金属的蠕变裂纹扩展速度非常快，除了对蠕变延伸性能产生不利影响外，晶界处偏析的铋对蠕变裂纹扩展性能也非常有害。4)已经发现，当σ相由于高温时效而充分析出时，蠕变裂纹的扩展速度往往比没有析出σ相的情况下要高（约为2.5至3倍）。这是因为蠕变裂纹优先通过枝晶边界的奥氏体-西格玛界面扩展。5)因此，晶界处偏析的铋和晶界处析出的sigma相都是增加蠕变裂纹扩展速率的重要因素。铋的晶界偏析比sigma相析出对蠕变裂纹扩展速率的影响更大。因此，在308型不锈钢焊接中应避免使用大量铋（约0.02%），并且在高温应用中应尽可能抑制焊缝金属中的δ铁素体含量。

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.