Characterizing Phase Transformations and Their Effects on Ferritic Weld Residual Stresses with X-Rays and Neutrons
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NEAR-YIELD tensile residual stresses are commonly encountered in welded components. These may compromise structural integrity through reduced fatigue life or increased susceptibility to environmentally assisted failure mechanisms.[1] Life-limiting residual stresses can sometimes be reduced by postweld heat treatment, but this may be impractical with large or inaccessible components such as those, for example, used in the construction of power plants or submersibles. The complex interplay between the sharp thermal gradients and the transient thermomechanical properties of the alloy can profoundly influence the final state of stress. Furthermore, strains arising from phase transformations can drastically affect the picture, giving localized stress relaxation and local material properties, which are hard to predict a priori. Displacive transformations in particular, such as bainite or martensite, are associated with shape deformations characterized as invariant-plane strains with large shear components in H. DAI and J.A. FRANCIS, Postdoctoral Research Fellows, and P.J. WITHERS, Professor of Materials Science, are with School of Materials, University of Manchester, Manchester M1 7HS, UK. Contact e-mail: [email protected] H.J. STONE, Assistant Director of Research, and H.K.D.H. BHADESHIA, Professor of Physical Metallurgy, are with Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, UK. This article is based on a presentation given in the symposium entitled ‘‘Neutron and X-Ray Studies for Probing Materials Behavior,’’ which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee. Article published online July 17, 2008 3070—VOLUME 39A, DECEMBER 2008
addition to dilatation normal to the invariant plane. The volume expansion due to transformation upon cooling is normally advantageous in reducing the effect of constrained thermal contraction as the weld zone cools. The large shear strain manifests on a macroscopic scale when the microstructure becomes non-random, i.e., when certain crystallographic variants are favored during phase transformation upon cooling. By engineering the temperature regime over which phase changes occur, the volume and shear strains may be exploited to mitigate the development of residual stresses. The work by Jones and Alberry[2,3] suggests that tensile residual stresses are best avoided by suppressing the transformation temperature such that the phase change can continue to compensate for the accumulation of contraction strains down to ambient temperature. Indeed, recent work in Japan using welding consumables with low transformation temperatures has shown that it is possible not only to reduce the residual tensile stresses, but also to introduce residual compression into the weld region with consequential increases in fatigue life.[4] Unfortunately, those w
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