Fractogrphy on Microcracks in Ni-base Multipass Weld Metals : Study on Microcracks in Multipass Weld Metals of Ni-base Alloys (Part 1) :

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Fractographic investigation on microcracks in the multipass weld metals of Ni-base alloys was carried out in order to find out the factors which caused microcracks in this study. Two types of hot cracking, viz., HAZ liquation cracking and the ductility-dip cracking were observed in the reheated zones of the multipass weld metal. According to EDX and AES analyses on microcrack surface, not only alloying elements such as Nb, Mo and Fe but also impurity elements such as P, Si and S were rich on the crack surface. The cross-bead varestraint test was conducted in order to make clear the effects of impurity elements such as P, Si and S on the hot cracking susceptibility of the 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 not so high, they increased with an increase in impurity elements. On the other hand, in the case of the ductility-dip cracking, the cracking susceptibilities of materials containing S were much higher than those of materials containing P and Si. Especially the cracking susceptibilities of Ni-10% Cr-20% Fe alloys were minimum among Ni-30% Fe, Ni-5% Cr-25% Fe and Ni-20% Cr-10% Fe alloys containing 0.007% S. It follows that the ductility-dip cracking susceptibility of the reheated weld metal of 70% Ni-Cr-Fe alloy strongly dapended on Cr and S contents in weld metal.

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