Microcracking in multipass weld metal of alloy 690 Part 1 - Microcracking susceptibility in reheated weld metal

Microcracking behaviour in the gas tungsten arc multipass weld metal of alloy 690 was investigated. The majority of microcracks occurred within about 300 mu m from the fusion line of the subsequent weld bead and propagated along the solidification boundaries in the multipass weld metal. The morphology of the crack surface indicated the characteristic texture of ductility dip cracking. The microcracking susceptibility of the reheated weld metal was evaluated via the spot Varestraint test using three different filler metals having varying contents of impurity elements such as P and S. Microcracking occurring in the spot Varestraint tests consisted predominantly of ductility dip cracking, with a small amount of liquation cracking. The ductility dip cracking temperature range was about 1350 - 1600 K in the weld metal FF1, and narrowed in the order of weld metals FF1>FF3>FF5. The ductility dip cracking susceptibility was reduced with decreasing contents of impurity elements in the filler metal. It was concluded that the amount of (P+S) in the filler metal should be reduced as much as possible ( to about 30 ppm in total) to suppress microcracking in the multipass weldment.