Abstract
Hot ductility testing of weld metal was performed in order to make clear the mechanism of ductility-dip cracking in 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 range 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 followed that these ductilities were related to the ductility-dip cracking. Fracture modes of this testing samples were almost intergranular fracture. According to the microstructure examination, grain boundary carbides, M23C6 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 in holding time at 873 K decreased a ductility of the base metal of Ni-30%Fe containing 0.007%S at 873 K. It followed that S easily segregated on grain boundary in low Cr alloy. These results meant 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.
