An Investigation of the Massive Transformation from Ferrite to Austenite in Laser-Welded Mo-Bearing Stainless Steels
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UCTION
THE massive phase transformation is a compositioninvariant, interface-controlled diffusional phase transformation.[1] Sometimes referred to as ‘‘partitionless,’’[2] massive transformations require only short-range diffusion across the massive/parent interface to change one phase to another.[1–10] The cooling rate must be sufficiently high to quench the material into a two-phase field below T0 while suppressing competing diffusional transformation mechanisms that can occur above T0, thereby accumulating sufficient driving force to facilitate this mechanism. Massive transformations have been observed in multiple materials systems[1–10] and growth rates exceeding 10 mm/s have been reported,[1] though much lower speeds are also possible.[4,5] The absence of long-range diffusion causes the massive product phase to inherit the composition and chemical distribution of the M.J. PERRICONE, formerly Senior Member Technical Staff, Sandia National Laboratories, Albuquerque, NM 87185, is Senior Scientist, RJ Lee Group, Inc., Monroeville, PA 15146. Contact e-mail: [email protected] J.N. DuPONT, Professor, is with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. T.D. ANDERSON, formerly Graduate Research Assistant, Department of Materials Science and Engineering, Lehigh University, is Senior Research Engineer, Exxon Mobil Corporation, Irving, TX 75039. C.V. ROBINO, Distinguished Member Technical Staff, is with the Joining and Coatings Division, Sandia National Laboratories. J.R. MICHAEL, Distinguished Member Technical Staff, is with the Materials Characterization Department, Sandia National Laboratories. Manuscript submitted June 19, 2007. Article published online November 19, 2010 700—VOLUME 42A, MARCH 2011
parent, a characteristic useful for identifying phases formed by this phenomenon. The compositional invariance that distinguishes massive transformations from other diffusional solid-state transformations may be particularly advantageous in austenitic stainless steels, which rely on the distribution of critical alloying elements for their corrosion resistance. Weldable commercial austenitic stainless steels often have compositions that solidify as primary ferrite[11] to avoid the solidification cracking susceptibility typical of primary austenite solidification,[12] followed by a solid-state transformation to austenite. However, as current trends continue to increase alloy content (e.g., superaustenitics and superduplexes) to maximize corrosion resistance in aggressive environments, the challenges associated with welding such alloys becomes a major source of concern. It is well known that alloying additions can change the primary solidification mode for a given alloy, and the non-uniform redistribution of critical alloying elements (especially Mo) during primary austenite solidification[13–15] results in depletion in localized regions of the fusion zone microstructure, leaving them susceptible to preferential corrosive attack.[16–19] Furthermore, the concomitant localized buildup of solute i
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