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.

Microsegregation and Precipitates in Inconel 625 Arc Weld Overlay Coatings on Boiler Pipes / Mikrosegregacja I Wydzielenia W Powłokach Ze Stopu Inconel 625 Napawanych Łukowo Na Rury Kotłowe

“…It is worth noting that the obtained results are close to the data found in the relevant literature k Nb =0.49; k Mo = 0.89; k Ni =1.04; k Fe =1.14; k Cr =1.05) [17]. Banovic [7] and DuPont [2] showed that, in the case of Ni-based weld overlays, the content of Fe in a particular alloy has a high impact on the value of the partition coefficient k for Nb and Mo. The higher the content of Fe in a weld overlay, the lower the value of the partition coefficient k for Nb and Mo.…”

“…It means that the segregation of these elements is stronger. The increasing value of the partition coefficient k for Nb and Mo, coupled with an increase of the Fe content in the alloy, likely results from a substantial difference in solubility of Nb and Mo in Ni and Fe-γ [2,7]. The highest solubility of Nb in Ni reaches the value of about 18 wt % (at 1286 °C), while the maximum solubility of Nb in Fe-γ is only 1.5 wt % (at 1210 °C).…”

“…Precipitation of carbides and the Laves -type phases is also disadvantageous, because ductility and fracture toughness decrease, as the amount of these phases increases. Moreover, susceptibility to solidification and liquation cracking increases [7,8].…”

Phase Transformations and Microstructural Evolution of Mo-Bearing Stainless Steels

“…Such concentration gradients have been previously confirmed via EPMA composition traces. [2] Alloys that solidified entirely as austenite could also show another type of microstructure due to the occurrence of the shear-type solid-state (Table II, type B). Because all of the alloys in this study were cooled under relatively the same conditions, the composition of the alloy button was the major factor in the initiation of such a transformation.…”

Analysis of Phase Transformations in Inconel 625 Alloy during Annealing

“…Such a situation occurs in alloys containing niobium and carbon. At the end of solidification NbC or other carbides as well as the Ni 3 Nb phase can be formed in interdendritic spaces [11][12][13]. The tendency to segregation increases for silicon, the content of which should be limited in the alloy.…”