Characterization of Microstructural Susceptibility by Color Tint Etching in a Friction Stir Welded Trip-780 SteelCharact
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Characterization of Microstructural Susceptibility by Color Tint Etching in a Friction Stir Welded Trip-780 Steel 1
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G.Y. Perez-Medina , P. Zambrano , F.A. Reyes-Valdés , H.F. López
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Corporación Mexicana de Investigación en Materiales. Saltillo, Coah. México. Universidad Autónoma de Nuevo León. FIME. San Nicolás de los Garza Nuevo León.México. 3 Materials Department, University of Wisconsin-Milwaukee, Milwaukee WI 53201, USA Email: [email protected]
ABSTRACT The effect of friction stir welding (FSW) on the resultant microstructures in the welded nugget (WN), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base metal (BM) of a TRIP-780 steel was investigated in this work. Color tint etching was used in the welded regions to disclose the exhibited microstructural constituents. In addition, significant fine grain size material was found in the WN regions. It was found that is considered to have experienced severe plastic deformation due to interaction with the welding tool pin lead to a drop in mechanical properties. Lap shear tensile testing indicated that the steel shear tension strength in the welded condition dropped compared with the BM. Microhardness profiles of the welded regions indicated that the hardness in both the WN and TMAZ were relatively elevated suggesting the development of martensite in these regions. In particular, the WN was found to shear fracture with uniformly distributed elongation shear dimples. Keywords: X-ray diffraction, steel, scanning electron microscopy, welding, toughness. INTRODUCTION In an attempt to reduce automobile vehicle weight and enhance passenger safety, multiphase advanced high strength steel (AHSS) such as transformation-induced-plasticity (TRIP)-aided steels have been developed. In recent years these steels have been subject of extensive interest because of their unique combination of mechanical properties, combine high strength with good formability. Their microstructure consist bainite (B), ferrite (F), martensite (M), and retained austenite (RA). Is difficult to distinguish the retained austenite because has minor than 13 vol. % [1-3]. Therefore phase identification is frequently possible with color micrographs. In the TRIP steel, it is important to know the volume fraction of retained austenite in WN, HAZ and TMAZ when a TRIP is welded by friction stir welding (FSW) because the local operating temperature generated by both friction and deformation needs to be at 900-1000 oC [46], as a result the retained austenite in the strain concentration region will transform to martensite. At the onset of microscopic instability, the increased stress in the area of localized deformation generates the austenite to martensite transformation, since the carbon hardening is much higher for the martensite than for the austenite phase and the volume expansion due to this transformation result in plastic deformation and work hardening of the surrounding ferrite, a localized strengthening is obtained [7,8]. These effects postpone further deformat
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