Influences of phosphorus and sulphur on ductility dip cracking susceptibility in multipass weld metal of alloy 690

The influence of P and S on ductility dip cracking susceptibility in the reheated weld metal of alloy 690 was evaluated by the spot Varestraint test using different alloy 690 filler metals, while varying the contents of P and S. The ductility dip cracking susceptibility was reduced with a decrease in the content of P and S in the filler metal; the amount of (P+1·2S) in the weld metal should be limited to 30 ppm in order to prevent microcracking in the multipass weld metal. A numerical simulation of cosegregation behaviour of P and S revealed that both elements were segregated at the grain boundary in the ductility dip temperature range during multipass welding. A molecular orbital analysis has suggested that ductility dip cracking can be attributed to grain boundary embrittlement due to grain boundary segregation of P and S.