Comparison of three different techniques for measuring the residual stresses in an electron beam-welded plate of WASPALO
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DURING welding operations, the heating action of the torch (e.g., flame, arc, laser, or electron beam) causes thermal expansion of the weld, heat-affected zone, and parent plate.[1] Since the surrounding cold metal and any jigging arrangement provide a considerable amount of constraint, plastic deformation usually occurs in the vicinity of the weld and beyond it.[2] The residual stress field which is produced varies strongly with spatial position in the weldment.[3,4] In the vicinity of the weld centerline, it is common for the local yield stress to be exceeded.[5] The generation of residual stress in welded structures can be a cause for concern for at least two reasons. First, as is well known, the residual stress state influences the mechanical behavior of the joint and the fatigue properties in particular.[6,7] It follows that the residual stress state must be
H.J. STONE, Research Student, P.J. WITHERS, Lecturer, S.M. ROBERTS, Research Fellow, and R.C. REED, Assistant Director of Research, are with the Department of Materials Science and Metallurgy, University of Cambridge/Rolls-Royce University Technology Centre, Cambridge CB2 3QZ, United Kingdom. T.M. HOLDEN, Research Scientist, is with the National Research Council of Canada, Chalk River Laboratories, Chalk River, ON, Canada KOJ 1JD. Manuscript submitted September 11, 1998.
METALLURGICAL AND MATERIALS TRANSACTIONS A
accounted for when the structural integrity of any welded component is being assessed, so that any estimation of the safe working life is accurate. Second, the residual stress state gives rise to permanent distortion, which can be such that the dimensional tolerances associated with the design specification are exceeded.[8,9,10] This can be a major cause for concern since the cost associated with scrapping welded components can be very high, since these are often a long way down the manufacturing stream. The purpose of this article is to report measurements of the residual stress state in an electron beam–welded plate of WASPALOY, a high-strength nickel-based superalloy. The electron-beam welding process[11] is a popular welding technique, particularly in the aeroengine industry, because the heat-source intensity is at least two orders of magnitude greater than for processes involving electric arcs or plasmas.[12] Thus, rather thick sections can be welded at relatively modest values of heat input per unit length of weld, with only minimal distortion arising. Nevertheless, the magnitude of the residual stress state is significant, and it is important that this is acknowledged and quantified. In this work, the neutron diffraction, X-ray diffraction, and hole drilling methods have been employed. The results from each of these different techniques are compared, and the uncertainties and difficulties associated with the techniques are discussed.
VOLUME 30A, JULY 1999—1797
II. BACKGROUND A. Neutron Diffraction The determination of residual stresses using neutron diffraction (e.g., Reference 13) involves the measurement of interplanar spacings a
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