Amelioration of Microcracking in Multipass Weld Metal of Alloy 690 by Adding Rare Earth Metals

The effect of the addition of rare earth metals (REM) to the weld metal on the microcracking susceptibility in the multipass welds of alloy 690 was examined by using the La or Ce containing filler metals. The amounts of the La and Ce in the filler metal were varied in several levels. The microcracking susceptibility of the reheated weld metal was evaluated by the spot and transverse-Varestraint tests using pre-welded specimens made by GTAW. The augmented strain levels were varied from 0.50–2.44%. Cracks that occurred in the reheated weld metal evaluated by the Varestraint test could be classified into three types; ductility-dip, liquation and solidification cracking. The ductility-dip cracking susceptibility could be significantly improved by adding 0.01–0.025mass%REM to the weld metal. Adversely, the excessive REM addition led to the liquation and/or solidification cracking in the weld metal. Microstructural analyses revealed that phosphide and sulphide of La or Ce were formed in the REM containing weld metals, and Ni-La or Ni-Ce intermetallic compound was additionally identified in the excessively REM containing weld metals. High temperature tensile test indicated that hot ductility of the weld metal was ameliorated by adding 0.01–0.03mass%REM, implying that the ductility-dip cracking susceptibility was decreased as a result of lowering the grain boundary segregation of impurity elements such as P and S due to the scavenging effect of REM. The liquation and solidification cracking in the excessively REM containing weld metals were considered to be due to the formation of liquefiable intermetallic compounds of Ni and 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.