Reduced stress welding process for nuclear plant piping

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This paper discusses the development and application of FineLine™ Welding (FLW). FLW is a modified Gas-Tungsten-Arc Welding mechanized process developed by GE Nuclear Energy. The FLW process offers significant improvements over standard and narrow-gap welding. For piping, FLW reduces the weld groove width and volume of weld metal compared to standard ‘V’ or J-Bevel welds by approximately 30% for smaller pipe and 70% or more for larger sizes. Also, extremely low heat inputs are achieved, and typically result in: (1) significant reductions in welding elapsed time, particularly for thicker walls, (2) compressive/very low tensile residual stresses on the pipe inside surface, and (3) improved heat-affected-zone microstructure, with reduced shrinkage/distortion. The improved residual stress and metallurgical states provide increased resistance to stress corrosion cracking in high temperature, oxidizing water. The FLW process is successful in eliminating the need for secondary stress improvement process such as induction-heating stress improvement. Verifications of the stress improvement were performed on standard V-groove and narrow-gap welds, which were compared to thin and thick-wall stainless steel pipe FLW welds. Testing included strain-gage, X-ray diffraction, magnesium chloride and metallographic evaluations. The results show significant improvement in the residual stress level and uniformity of the FLW welds.