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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.

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The solidification reaction sequences of experimental superalloys containing systematic variations in Fe, Nb, Si, and C were studied using differential thermal analysis (DTA) and microstructural characterization techniques. The reaction sequences responsible for microstructural development were found to be similar to those expected in the Ni-Nb-C ternary system and commercial superalloys of comparable composition. The solute-rich interdendritic liquid generally exhibited two eutectic-type reactions at the terminal stages of solidification: L → (␥ ϩ NbC) and L → (␥ ϩ Laves). The Nibase alloys with a high C/Nb ratio represented the only exception to this general solidification sequence. This group of alloys terminated solidification with the L → (␥ ϩ NbC) reaction and did not exhibit the ␥/Laves constituent. At similar levels of solute elements (Nb, Si, and C), the Fe-base alloys always formed more of the ␥/Laves eutectic-type constituent than the corresponding Ni-base alloys. Silicon additions also increased the amount of the ␥/Laves constituent that formed in the assolidified microstructure, while C additions promoted formation of ␥/NbC. The influence of Nb was dependent on the C content of the alloy. When the C content was low, Nb additions generally promoted formation of ␥/Laves, while Nb additions to alloys with high C led to formation of the ␥/NbC constituent. The results of this work are combined with quantitative analyses for developing ␥-Nb-C pseudoternary solidification diagrams in a companion article.

Equilibrium distribution coefficients and pseudoternary solidification surfaces for experimental superalloys containing systematic variations in Fe, Nb, Si, and C were determined using quenching experiments and microstructural characterization techniques. In agreement with previous results, the distribution coefficient, k, for Nb and Si was less than unity, while the ''solvent'' elements (Fe, Ni, and Cr) exhibited little tendency for segregation (k Ϸ 1). The current data were combined with previous results to show that an interactive effect between k Nb and nominal Fe content exists, where the value of k Nb decreases from 0.54 to 0.25 as the Fe content is increased from Ϸ2 wt pct to Ϸ47 wt pct. This behavior is the major factor contributing to formation of relatively high amounts of eutectic-type constituents observed in Fe-rich alloys. Pseudoternary ␥-Nb-C solidification surfaces, modeled after the liquidus projection in the Ni-Nb-C ternary system, were proposed. The Nb compositions, which partially define the diagrams, were verified by comparison of calculated amounts of eutectic-type constituents (via the Scheil equation) and those measured experimentally, and good agreement was found. The corresponding C contents needed to fully define the diagrams were estimated from knowledge of the primary solidification path and k values for Nb and C.

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