Phase Transformations and Microstructural Evolution of Mo-Bearing Stainless Steels
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SUPERAUSTENITIC stainless steel (SASS) alloys with high Mo concentrations are used in a wide variety of applications that require good toughness and corrosion resistance. Unfortunately, the corrosion resistance of welds is never as good as the corresponding wrought base metal.[1] The microstructural development and resultant corrosion resistance of austenitic alloys in this compositional regime has been shown to be strongly controlled by the microsegregation of Mo to the intercellular regions.[2] The ensuing depletion of Mo in the cell core results in preferential corrosive attack.[3] Alloys that solidify as primary austenite are also susceptible to solidification cracking caused by the accumulation of tramp elements (such as P and S) and concomitant formation of low melting point liquid films in the intercellular regions.[4] Previous studies have also shown that the microsegregation of Mo will proceed over a wide range of welding conditions and compositions.[2,5,6] Although high Mo nickel-base filler metals can be used to help mitigate this problem, microsegregation is not eliminated, and careful control of welding variables must be exercised for close control over weld metal composition. In contrast, it is well known that stainless steels that solidify as ferrite exhibit significant T.D. ANDERSON, Graduate Research Assistant, J.N. DUPONT, Associate Professor, and A.R. MARDER, Professor, are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015, USA. Contact e-mail: tda2@ lehigh.edu, M.J. PERRICONE, Senior Member of Technical Staff, is with the Joining and Coatings Department, Sandia National Laboratories, Albuquerque, NM, USA. Manuscript submitted: July 21, 2006. 86—VOLUME 38A, JANUARY 2007
backdiffusion that can minimize or eliminate residual microsegregation, and have good resistance to solidification cracking due to the high solubility of tramp elements.[4] Thus, identifying high Mo SASS alloys that solidify first as ferrite and subsequently transform to austenite via a solid-state reaction may help eliminate problems associated with solidification cracking and microsegregation-induced corrosion. The variety of stainless steel microstructures was previously outlined by Elmer et al.[7,8] for the Fe-Ni-Cr ternary system. Twelve different morphologies were identified as a function of composition (Cr/Ni ratio) and cooling rate. In this current research, 20 different microstructural development sequences are proposed for the case of Fe-Ni-Cr-Mo stainless steels under cooling rate conditions typical of casting and welding. The compositional variables have been extended to include variations in Fe and Mo content in addition to Cr/Ni ratio. The effect of cooling rate will be discussed in a future publication.[9] A wide range of microstructures are possible for neareutectic Mo-bearing stainless steels, owing to a variety of solidification modes and possible solid-state phase transformations that occur from the ferrite to the austenite phase. Multicomponent phase diagrams provide an
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