In nuclear power plants, the automated narrow gap welding (NGW) technique has been widely used in joining pipes in primary coolant system. Meanwhile, to apply the leak-before-break (LBB) design, mechanical properties of the structural materials of piping systems should be evaluated, especially at various welded joints. In this study, the fatigue crack growth rate (FCGR) and fracture resistance of stainless steel weld fusion zone and nearby heat affected zone (HAZ) were evaluated to support the LBB application. Tests were performed at plant operating temperature (315°C) and room temperature. FCGR test results showed higher crack growth rate in HAZ and the weld fusion zone compare to the base metal. Fracture resistance tests showed higher fracture toughness in HAZ compared to the weld fusion zone. By analyzing the microstructures in the weld fusion zone and HAZ, their effects on crack growth rate were discussed. Also, the crack growth behavior in circumferential direction was compared with that in radial direction which was previously reported.

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