Dilution and microsegregation in dissimilar metal welds between super austenitic stainless steel and nickel base alloys

Abstract

Super austenitic stainless steels are often welded using high Mo, Ni base filler metals to maintain the corrosion resistance of the weld. An important aspect of this processing is the weld metal dilution level, which will control the composition and resultant corrosion resistance of the weld. In addition, the distribution of alloying elements within the weld will also significantly affect the corrosion resistance. Dissimilar metal welds between a super austenitic stainless steel (AL-6XN) and two Ni base alloys (IN625 and IN622) were characterised with respect to their dilution levels and microsegregation patterns. Single pass welds were produced over the entire dilution range using the gas tungsten arc welding process. Microstructural characterisation of the welds was conducted using light optical microscopy, scanning electron microscopy, and quantitative image analysis. Bulk and local chemical compositions were obtained through electron probe microanalysis. The quantitative chemical information was used to determine the partition coefficients k of the elements in each dissimilar weld. The dilution level was found to decrease as the ratio of volumetric filler metal feedrate to net arc power increased. Reasons for this behaviour are discussed in terms of the distribution of power required to melt the filler metal and base metal. In addition, the segregation potential of Mo and Nb was observed to increase (i.e. their k values decreased) as the Fe content of the weld increased. This effect is attributed to the decreased solubility of Mo and Nb in austenite with increasing Fe additions. Since the Fe content of the weld is controlled by dilution, which in turn is controlled by the welding parameters, the welding parameters have an indirect influence on the segregation potential of Mo and Nb. The results of the present work provide practical insight for corrosion control of welds in super austenitic stainless steels.