In-Situ Synchrotron Diffraction Studies on Transformation Strain Development in a High Strength Quenched and Tempered St
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UCTION
S690QL1 is a high strength quenched and tempered structural steel, with a minimum yield strength of 690 MPa and a notch toughness of at least 30 J at 213 K (60 C).[1] These steels are increasingly used in welded constructions, such as machines for structural engineering, cranes, and bridges. The microstructure of this steel typically consists of tempered martensite in a ferritic matrix, but upon welding, solid state phase transformations occur, accompanied by changes in specific volume. Details of the phase transformation kinetics can be derived from in-situ synchrotron diffraction measurements.[2–7] During welding or while applying heat-treatments to steel, the material is subjected to thermal cycles which generates transformation strain associated with temperature and phase transformation.[8] For commercial exploitation and modeling purposes, it is necessary to R.K. DUTTA and H. GAO, Ph.D. Researchers are with the Materials innovation institute M2i, Mekelweg 2, 2628 CD Delft, The Netherlands and also with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Contact e-mail: [email protected], r.k.dutta@ tudelft.nl R.M. HUIZENGA, Research Support, M.J.M. HERMANS, Assistant Professor, J. SIETSMA, and I.M. RICHARDSON, Professors, are with the Department of Materials Science and Engineering, Delft University of Technology. M. AMIRTHALINGAM, Research Fellow, is with the Department of Materials Science and Engineering, Delft University of Technology. A. KING, Scientist, formerly with the European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex, France, is now with the Synchrotron Soleil, l’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France. Manuscript submitted May 29, 2013. Article published online September 25, 2013 218—VOLUME 45A, JANUARY 2014
thoroughly understand the phase transformation behavior under mechanical loading. For precise modeling of the residual stress distribution during welding of such steels, it is essential to fully characterize the transformation temperatures and the nature and magnitude of the strains during phase transformation. The nature and magnitude of transformation strains that manifest when austenite decomposes on cooling will depend strongly on the mechanism of transformation. It is known that when austenite transforms on cooling to bainite or martensite, the transformation is described as displacive,[9] which involves the formation of plate-like microstructures through the coordinated movement of atoms. At the crystallographic level, both of these displacive transformations have an associated strain that comprises a large shear component (~0.22) as well as a smaller dilatational component (~0.03).[10] Upon transformation, each austenite grain will produce some combination of the 24 possible crystallographic variants that may occur in the product phase. In the absence of macroscopic stresses, the crystallographic variants may form in an unbiased manne
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