Stress-Free Reference for Neutron Diffraction Measurement of Residual Stress in Butt-Welded Joints of Austenitic Stainless Steel Pipes

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Stress-free lattice spacing d0 has the most influence on reliability of neutron stress measurements made using an angle dispersive method. However, it is hard to evaluate the lattice spacing of welded structures and ductile materials such as stainless steel accurately. In this study, suitable measurement conditions for d0 of welded pipe joints of austenitic stainless steel were discussed. The d0 values derived from {311} and {111} reflections, which are often used in austenitic stainless steel for residual stress measurement, were examined. Comparison of the residual strains and stresses evaluated using the obtained d0 and the finite element analysis showed that the way the d0 values were chosen affected the measurement accuracy significantly. The stress measurement accuracy was remarkably improved when the {311} reflection was used and the proper d0 value was chosen in the respective neutron diffraction measurements. For instance, for the axial diffraction measurements using the {311} reflection, it was recommended that only the axial d0 value of the {311} reflection be used; the measurements using the {111} reflection were less accurate due to the large Young's modulus. Additionally, a lower diffraction angle was judged to be one of the factors leading to a decrease of the strain measurement accuracy.