Effect of composition on the solidification behavior of several Ni-Cr-Mo and Fe-Ni-Cr-Mo alloys
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ent corrosion resistance of superaustenitic stainless steels (SASS) makes these materials attractive for use in aggressive environments. However, conventional arc welding processes result in the redistribution of strategic alloying elements in the fusion zone, which compromises local corrosion resistance. The tendency of molybdenum (Mo) to segregate to the interdendritic regions in the fusion zone has been well demonstrated in the technical literature,[1–3] as has the corresponding susceptibility of the solute-depleted dendrite cores to preferential corrosive attack.[4–9] To combat this phenomenon, high-Mo nickelbase filler metals such as IN622 and IN625 are often employed during conventional arc welding of SASS alloys to increase the minimum solute content in the dendrite cores.[3,10] In some cases, high energy density welding processes (laser and electron beam)[11,12] can also be used to potentially restore fusion zone corrosion resistance without the need for filler material.[13–15] This technique relies on the accurate control of solidification parameters (solidification velocity and temperature gradient) to induce dendrite tip undercooling and concomitant dendrite core enrichment, which can often be difficult in many practical applications. Furthermore, the redistribution of Mo during solidification has been shown to be largely responsible for the performance and microstructural development of this class of materials,[1,2,16–19] but previous work aimed at interpreting microstructural development for conventional stainless steels[1,19–23] is based exclusively on the Fe-Ni-Cr ternary system. As such, these interpretations must be expanded to improve the understanding of the microstructural development of SASS welds, particularly in describing solute redistribution and resultant fusion zone microstructure as M.J. PERRICONE, former Doctoral Student, with the Department of Materials Science and Engineering, Lehigh University, is currently a Senior Technical Staff Member, with the Joining and Coatings Division, Sandia National Laboratories, Albuquerque, NM 87185. Contact e-mail: mperric@ sandia.gov J.N. DUPONT, Associate Professor, is with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted July 2, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
a function of composition in this alloy system. To that end, the objective of this research is to characterize the effect of multicomponent alloy composition on overall microstructural development in order to provide an avenue for control of fusion zone properties in SASS welds.
II.
EXPERIMENTAL PROCEDURE
A. Experimental Alloy Preparation The commercially available SASS alloy chosen as the basis for this study was AL-6XN, the composition of which can be found in Table I. To isolate the contribution of individual elements, simpler experimental alloy compositions were also examined that closely simulate the solidification behavior of dissimilar welds made between AL-6XN and Ni-base filler metals.[3] Each experimental all
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