Discussion on Accuracy of Weld Residual Stress Measurement by Neutron Diffraction -Influence of Strain Free Reference-
Hiroshi SUZUKI and Koichi AKITA
Abstract:It is required to evaluate a strain-free reference, a0, to perform accurate stress measurement using neutron diffraction. In this study, accuracy of neutron stress measurement was quantitatively discussed from a0 evaluations on a dissimilar metal butt-weld between a type 304 austenitic stainless steel and an A533B low alloy ferritic steel. A strain-free standard specimen and a sliced specimen with 10 mm thickness taken from the dissimilar metal butt-weld were utilized. In the lattice constant evaluation using the standard specimen, average lattice constant derived from multiple hkl reflections was evaluated as the stress-free reference with cancelling out an intergranular strain. Comparing lattice constant distributions in each reflection with average lattice constant distribution in the standard specimen, ƒ¿Fe211 and ƒÁFe311 reflections were judged as a suitable reflection for neutron strain measurement to reduce intergranular strain effects. Residual stress distribution in the sliced specimen evaluated using a0 measured here exhibited higher accuracy than that measured using strain gauges. On the other hand, a0 distributions were evaluated using the sliced specimen under the plane-stress condition. Existence of slight longitudinal residual stresses near the weld center decreased accuracy of the a0 evaluations, which means that it is required to optimize the thickness of the sliced specimen for accurate a0 evaluation under plane strain condition. As a conclusion of this study, it was confirmed that procedures of accurate a0 evaluation, optimization of the measurement condition, and multiple evaluations on the results play an important role to improve accuracy of the residual stress measurement using neutron diffraction. Key Words:Neutron diffraction, Stress measurement, Butt weld, Intergranular strain, Strain-free reference