Abstract
Fractographic investigation of microcracks in a real multi-pass weld metal of Ni-base alloys was carried out in order to find out the factors which caused microcracks in this study.
Two modes of microcracks, viz., liquation crack and ductility-dip crack were observed in reheated zones of weld metal. According to the EDX and AES analyses on fracture surface of these cracks, not only Nb and Mo in microconstituents but also P, Si and Senriched on thefracture surface.
The cross-bead varestraint test was performed in order to make clear the effects of impurity elements such as P, Si and S on hot cracking susceptibility of reheated weld metal of Ni-Cr-Fe ternary alloys. Base metals used were basically 70%Ni-Cr-Fe ternary alloys in which Cr contents were varied from 0 to 20%. P, Si and S contents were independently varied in the ranges of 0.002-0.032%, 0.05-1.02% and 0.001-0.02%, respectively.
Although the liquation cracking susceptibilities of all materials used were too low, they increased with an increase in impurity elements. On the other hand, in the case of ductility-dip cracking, cracking susceptibilities of materials containing S were much higher than those containing P and Si. Especially cracking susceptibilities of Ni-30%Fe and Ni-20%Cr-10%Fe alloys containing 0.007%S were higher than those of Ni-5%Cr-25%Fe and Ni-10%Cr-20%Fe alloys. It followed that the ductility-dip cracking susceptibility of reheated weld metal of 70%Ni-Cr-Fe alloy was affected as a function of interaction of Cr and S contents in weld metal.
