Transformation-Induced Diffraction Peak Broadening During Bainitic and Martensitic Transformations Under Small External
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90QL1 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. 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.[2,3] During welding or while applying heat treatments to steel, the material is subjected to various thermal cycles
R.K. DUTTA, Ph.D. Researcher, is with the Materials innovation institute M2i, Mekelweg 2, 2628CD Delft, The Netherlands, also with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628CD Delft, The Netherlands. Contact e-mail: [email protected], [email protected] R.M. HUIZENGA, Research Support, M.J.M. HERMANS, Assistant Professor, and I.M. RICHARDSON, Professor, are with the Department of Materials Science and Engineering, Delft University of Technology. M. AMIRTHALINGAM, Research Fellow, formerly with Materials innovation institute M2i, is now with the Department of Materials Science and Engineering, Delft University of Technology. A. KING, Scientist, formerly with European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex, France, is now with Synchrotron Soleil, l’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette Cedex, France. Manuscript submitted February 11, 2013. Article published online June 13, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
which generate strain associated with temperature and phase transformation.[4] These phase transformationinduced strains are commonly called transformation strains. 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 a bcc phase, either bainite or martensite, the transformation is described as displacive,[5] 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).[6] 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 manner so that the large shear strains that occur at the crystallographic level can cancel each other when viewed on a macroscopic scale.[7] If, however, the transformation takes place under the influence of macroscopic stresses (such as residual stresses), then the crystallographic variants that occur in the product phase may form in
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