Microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel using La added filler metals

The microcracking susceptibility in dissimilar multipass welds of alloy 690 to type 316L stainless steel was investigated by the Varestraint test and the multipass welding test using the four different stainless steels varying the amounts of impurity elements such as P and S, and using the nine different filler metals of alloy 690 varying the La content. In order to simulate the dissimilar weld metals, stainless steels were gas tungsten arc (GTA) welded using alloy 690 filler metals with varying the dilution ratio. Microcracks occurring in the reheated weld metals were classified into ductility dip, liquation and solidification cracks. The ductility dip cracking susceptibility decreased with increasing the La content in the weld metal, while the liquation and solidification cracking susceptibilities increased contrarily when the La content in the weld metal exceeded ∼0·03 mass-%. The relation among the microcracking susceptibility, the (P + S) and La contents in every weld pass of the multipass welding was investigated. Ductility dip cracks occurred in the compositional range (atomic ratio) of La/(P + S)<0·17 and solidification/liquation cracks occurred in that of La/(P + S)>0·76, while any cracks did not occur at La/(P + S) being between 0·17–0·76. The ductility dip cracking susceptibility could be improved by adding La due to the scavenging of the impurity elements. The excessive La addition to the weld metal resulted in the solidification/liquation cracking alternatively attributed to the formation of Ni–La intermetallic compounds with low eutectic point. The optimal filler metal for the dissimilar multipass welding of alloy 690 to stainless steel was selected as La/(P + S) would be 0·17–0·76 for every weld pass. Microcracking could be completely prevented in the dissimilar multipass weld metal of alloy 690 to type 316L stainless steel by using two kinds of filler metals containing 0·03 mass-%La for the weld passes adjacent to the stainless steel base metal and containing 0·01 mass-%La for other weld passes.