Characterization of Phase Transformations and Stresses During the Welding of a Ferritic Mild Steel
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TRODUCTION
OVER the past 25 years, much progress has been made in the development of models for the prediction of the temperature and stress fields produced during fusion welding, with particular emphasis on the prediction of residual stresses.[1–3] The residual stress state produced in and around the weld depends on the alloy and phase transformations that occur, the size and shape of the weld pool, and on the welding speed. Unfortunately, it is difficult to predict, a priori, the weld pool shape due to the large no. of variables involved. The situation is complicated by a lack of knowledge of the fundamental physical constants relating to the arc and molten pool, and the way in which the arc couples with the material, for example, the arc pressure and electromagnetic stirring of the pool by the arc. In spite of these difficulties, some models capable of predicting weld pool shapes have been successfully created.[4,5] To ascertain both the residual stress state and microstructure around the weld, a modeling-based approach is attractive, since complete experimental characterization is typically both time-consuming and expensive. However, such models demand knowledge of the temperature dependence of the thermophysical and mechanical properties of the material along with the
D. DYE, Reader, is with the Department of Materials, Imperial College London, London SW7 2AZ, U.K. Contact e-mail: david. [email protected] H.J. STONE, Assistant Director of Research, is with the Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, U.K. M. WATSON, Technical Officer, and R.B. ROGGE, Senior Research Officer, are with the Canadian Neutron Beam Centre, National Research Council, Chalk River Laboratories, Station 18, Building 459, Chalk River, ON K0J 1J0, Canada. The following pertains only to authors M. Watson and R.B. Rogge: Reproduced by permission of Minister of Supply and Services Canada. Manuscript submitted April 17, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS A
distribution and magnitude of heat input into the material. Obtaining these data is again usually timeconsuming and expensive, but has been performed in very many successful weld models in a range of materials, for example, in the welding of steels,[6,7] aluminum,[8] and nickel[9,10] alloys. Such weld models are often verified by reference to measured residual stress distributions and to the final distortion found in the workpiece, and are calibrated using the temperature evolution in the heat-affected zone measured using thermocouples. Among residual stress measurement techniques, neutron or synchrotron diffractions are often preferred, since these techniques are non-destructive, capable of characterizing the spatial distribution of the stress field rather than performing measurements at individual points, and can measure stresses at depth. The ability to perform measurements at depth is particularly advantageous compared to the leading alternatives of incremental hole drilling and laboratory X-ray diffraction using
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