Effects of Cr and S on Ductility-dip Cracking Susceptibilities in the Reheated Weld Metals of Ni-Cr-Fe Ternary Alloys : Study on Microcracks in Multipass Weld Metals of Ni-base Alloys (Part 2)

概要

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Hot ductility testing of weld metal was conducted in order to rnake clear the mechanisrn of ductility-dip cracking in the weld metal of 70% Ni-Cr-Fe ternary alloys containing 0.007% S in which Cr contents were varied from 0 to 20%. The temperature of testing was varied from 873 to 1473 K. Microstructures and fracture surfaces were also examined. The ductility at 1173 K of Ni-10% Cr-20% Fe alloy was higher than those of Ni-30% Fe and Ni-20% Cr-10% Fe alloys. It follows that the ductility-dip cracking was related to the hot ductility. Fracture modes of testing samples were almost intergranular fracture. According to the microstructural examination, grain boundary carbides, M_<23>C_6 appeared in only Ni-20% Cr-10% Fe alloy weld metal at the temperature of 1173 K. Hardness in weld metal increased with an increase in Cr content. On the other hand, increasing holding time at 873 K decreased the ductility of. the base metal of Ni-30% Fe containing 0.007% S at 873 K. It follows that S easily segregated on grain boundary in low Cr alloy. It was concluded from these results that optimum Cr content in weld metal was necessary in order to reduce the ductility-dip cracking susceptibility because of contrary effects due to grain boundary carbide, hardness and grain boundary segregation of S.
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1993-10-01