Microstructural evolution and weldability of dissimilar welds between a super austenitic stainless steel and nickel-based alloys

Microstructural evolution and solidification cracking susceptibility of dissimilar metal welds between AL-6XN super austenitic stainless steel and two nickel-based alloys, IN625 and IN622, were studied using a combination of electron microscopy, differential thermal analysis, and Varestraint testing techniques. Welds were prepared over the entire dilution range (where dilution was determined with respect to AL-6XN as the base metal). The effect of processing parameters and filler metal chemistry on the fusion zone composition, microstructure, and resultant weldability was investigated. Iron additions to the weld (which occur with increasing dilution) increased the segregation potential of Mo and Nb. This behavior was attributed to a reduction in the solubility of Mo and Nb in austenite with increasing iron additions. as inferred from available binary phase diagrams. Welds prepared with IN622 formed a single interdendritic a phase at the end of solidification, and the amount of this secondary phase was not sensitive to changes in dilution. The a phase formed at a relatively high temperature and led to a relatively narrow solidification temperature range for welds prepared with IN622. In contrast, welds prepared with IN625 exhibited NbC and Laves phases in the interdendritic regions, and the total amount of secondary phase decreased with increasing dilution. Solidification of welds prepared with IN625 terminated with formation of the Laves constituent at relatively low temperature and, thus the solidification temperature range of welds involving IN625 was relatively wide. The solidification cracking sensitivity of welds prepared with IN622 was relatively low and independent of weld metal dilution level, while the cracking susceptibility of welds produced with IN62,5 was relatively high and increased with decreasing dilution. The dilution/cracking relation is controlled by the solidification temperature range and amount of secondary phase that forms at the terminal stages of solidification. The good cracking resistance of welds prepared with IN622 is attributed to the small amount of secondary phase and narrow solidification temperature range. The relatively poor cracking resistance of welds prepared with IN625 is attributed to a wide solidification temperature range and increasing amount of secondary phase that forms with decreasing dilution.