Transformation and Deformation Texture Study in Friction Stir Processed API X80 Pipeline Steel
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INTRODUCTION
FRICTION stir welding/processing (FSW/P) has shown promising results as a solid-state joining process. Aluminum, magnesium, copper, and titanium alloys are commercially being friction stir welded for various engineering applications.[1,2] Among these materials, Aluminum has been by far the most widely investigated, especially with regards to deformation and microstructural evolution.[1,2] Although there is growing interest in FSW/P of steels, much of the research to date has been feasibility studies.[3–8] Recently, there have been several investigations focused on developing correlations between process parameters and post-weld microstructure.[8–10] In steels, Wei has shown linear correlations between heat input and microstructural features such as bainite lath and packet size.[10] Despite these correlations, deformation and microstructural evolution in friction stir welded steels are not well understood. Understanding deformation and microstructural evolution facilitates microstructureproperty-process parameter correlations. These correlations are essential in understanding and optimizing the process. Investigating deformation in friction stir welded steels is challenging. Compounding the complexity of nonuniform material flow is the allotropic phase transformation. Phase transformations not only induce internal MAJID ABBASI, Research Faculty, TRACY W. NELSON and CARL D. SORENSEN, Professors, are with the Mechanical Engineering Department, Brigham Young University, 435 CTB, Provo, UT 84602. Contact e-mail: [email protected] Manuscript submitted December 4, 2011. Article published online August 8, 2012 4940—VOLUME 43A, DECEMBER 2012
stresses, but they mask the thermal/mechanical history of the elevated temperature phase. The combination of these makes it difficult to characterize the deformation in FSW/P of ferritic steels because deformation mainly occurs at elevated temperatures where the microstructure is austenite which then transforms back to ferrite at room temperature. Texture has been a useful tool for investigating deformation in FSW/P. Field et al., and Sato et al., have studied textures in FSW of aluminum alloys.[11,12] Shear components such as {110}h001i and {114}h221i were observed after rotating data to the appropriate coordinate system. Nelson has observed {111}h110i, {112}h110i, {001}h110i shear components and rotated-cube texture in friction stir processed 304 austenitic stainless steel.[13] Friction stir processed pure iron is reported to show a well-defined {112}h111i shear texture.[14] The fact that different FCC shear texture components are active in friction stir processed Al and 304 austenitic stainless steel may suggest that the deformation in FSW/P is process and material dependent. Weak shear textures and the rotated-cube texture observed in FCC alloys suggest that extended plasticity mechanisms such as dynamic recrystallization are active during the process.[11,13] Both extended plasticity mechanisms and a phase change (in allotropic alloys) will affect the room tempera
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