Effect of Composition on the Formation of Sigma during Single-Pass Welding of Mo-Bearing Stainless Steels
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MANY high alloy stainless steels employ molybdenum additions to increase the corrosion resistance of the material. However, numerous studies[1–4] have demonstrated the susceptibility of Mo-depleted regions of the microstructure to preferential corrosive attack when the chemical distribution of the alloy is not homogenous. This can be particularly problematic when welding austenitic and superaustenitic stainless steels, where microsegregation during primary c-austenite solidification results in Mo-depleted dendrite cores and Mo-rich interdendritic regions.[5–7] This segregation phenomenon can promote the formation of eutectic d-ferrite at the end of solidification, which can slightly reduce the hot cracking susceptibility of the alloy by dissolving tramp elements such as P and S that can form low-melting-point phases. However, higher concentrations of Mo promote the formation of Mo-rich intermetallics such as r-sigma and v-chi at the termination of solidification, which degrade mechanical properties,[8–14] and can encourage hot cracking by extending the solidification temperature range. M.J. PERRICONE, Senior Member Technical Staff, and J.R. MICHAEL, and C.V. ROBINO, Distinguished Members Technical Staff, are with the Joining and Coatings Division, Sandia National Laboratories, Albuquerque, NM 87185, USA. Contact e-mail: [email protected] T.D. ANDERSON, Graduate Research Assistant, and J.N. DUPONT, Associate Professor, are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015, USA. Manuscript submitted September 26, 2006. Article published online July 31, 2007. 1976—VOLUME 38A, SEPTEMBER 2007
Conversely, duplex and ferritic stainless steels that exhibit primary d-ferrite solidification can dissolve a much larger amount of Mo, and the higher substitutional diffusivity in d-ferrite allows for homogenization during welding even at high cooling rates.[15] Not only does this reduce the solidification cracking susceptibility of these alloys, but fully ferritic solidification can produce a microstructure with a more uniform Mo distribution. Of course, the high magnetization and lower toughness of d-ferrite are traits that can eliminate such alloys from structural applications that require good toughness and the absence of any magnetic signature. Both effects can be ameliorated to some extent by selecting compositions that transform to c-austenite upon cooling due to its increased thermodynamic stability when compared to d-ferrite at lower temperatures.[16] However, the advent of super-duplex stainless steels[8,12,17] with increased additions of Mo and Cr for corrosion resistance may make the eutectoid decomposition of d fi c + r[8,12,18] much more likely in this class of materials. As such, controlling alloy composition and the welding process to avoid r-sigma formation (a brittle intermetallic) during cooling is only part of the solution. The extent and mechanism of d-ferrite decomposition controls material properties and depends on composition and cooling rate. While the literatu
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