RESIDUAL STRESS BY X-RAY DIFFRACTION AND MICROSTRUCTURE FOR MULTI-PASS GIRTH WELDED PIPE JOINT IN AUSTENITIC STAINLESS STEEL TYPE 316L

摘要

Residual stress due to welding can result in brittle fracture, fatigue failure, and stress corrosion cracking in welded structures. Measuring residual stresses are of great importance, if crack propagation needs to be evaluated. However, it is especially known that the X-ray diffraction method makes remarkable different for austenitic stainless steel, because the microstructures in welds change from the original microstructures during welding thermal cycle. That is, there are the preferred orientation due to the unidirectional solidification and the grain growth in the heat-affected zone. In order to average the sin 2~Ψ plots to exclude them, Ψ oscillation of ±3 deg was performed and the incident beam size was broadened to 4 by 4 mm. Consequently, typical residual stress distributions due to welding were obtained to various conditions. The residual stress distribution measured by X-ray diffraction agrees very well with that the estimated by thermal-elastic-plastic analysis, if the spatial resolution is correlated. It is attributed that the 5-ferrite grows as the primary phase and the austenite precipitates or crystallizes as the secondary phase. When the secondary austenite nucleates with the Kurdjiumov-Sachs relationship which satisfy δ{110}//γ{111} and δ<111>//γ<110>, plate-like austenite grows randomly into the ferrite and austenite grains are braked up. That is, Specific systems in austenitic stainless steels should be classified, as a material that residual stress can be measured accurately by X-ray diffraction.