Microcracking behaviour and mechanism in multipass weld metal of alloy 690?

Abstract

Microcracking behaviours in the multipass welds of alloy 690 were investigated. The effect of impurity elements such as P and S on ductility-dip cracking susceptibility in the reheated weld metal was evaluated by the spot-Varestraint test using different filler metals varying the contents of P and S. The ductility-dip cracking susceptibility was reduced with a decrease in the content of impurity elements in the filler metal, and the amount of (P+1.2S) in the weld metal should be limited to 30ppm in order to prevent the microcracking in the multipass weld metal. Numerical calculation of microsegregation and molecular orbital analysis have suggested that ductility-dip cracking was attributed to the grain boundary embrittlement due to the grain boundary segregation of P and S. FEM analysis of thermal strain predicted that ductility-dip cracks would occur during multipass welding when the plastic strain-temperature curve intersected the ductility-dip temperature range (DTR). The effect of the addition of rare earth metals (REM) to the weld metal on the microcracking susceptibility was examined by using the La or Ce containing filler metals. The ductility-dip cracking susceptibility could be significantly improved by adding 0.01-0.025mass%REM to the weld metal. Microstructural analyses revealed that the ductility-dip cracking susceptibility decreased as a result of lowering the grain boundary segregation P and S due to the scavenging effect of REM. The multipass welding test confirmed that microcracks in the multipass welds of alloy 690 were completely prevented by using the filler metals containing approx. 0.03mass%REM.