Effect of strength mismatch on fracture mechanical behavior of NG-DMW

In modern pressurized water reactor (PWR) designs, dissimilar metal joints, e.g. reactor pressure vessel (RPV) safe-ends, are manufactured using a new weld design which takes advantage of narrow-gap (NG) welding technique. In addition to the new weld design, the filler metals have been changed from Alloys 82 and 182 to higher Cr containing Alloys 52 and 152 to ensure the structural integrity of the welds. In dissimilar metal welds (DMW), the mismatch in material properties between the two joined materials and their narrow local variation in different zones of the weld are of importance because the local strength mismatch state plays an important role in the fracture behavior of the weld. For the experimental determination of the local strength variations in a narrow-gap dissimilar metal weld (NG-DMW), a weld mock-up was manufactured using narrow-gap gas-tungsten arc welding (GTAW) method. The weld consisted of SA 508 pressure vessel steel with AISI 309L/308L cladding, AISI 304 piping steel, and Alloy 52 weld metal. The weld was characterized in two different heat treatment conditions, in as-welded condition and in post-weld heat treated (PWHT) condition. The microstructure of the weld mock-up was characterized using FEGSEM. The fusion zone (FZ) between the SA 508 pressure vessel steel and Alloy 52 weld metal was characterized using micro- and nano-hardness testing and the strength mismatch state of the FZ was determined with tensile testing using miniature-sized tensile testing specimens allowing the determination of the local tensile properties of the narrow weld zones near the fusion line (FL). The fracture mechanical testing was performed at room temperature to examine the effect of local strength mismatch on the fracture behavior and crack propagation. The results of the tensile tests revealed that Alloy 52 weld metal had close to equivalent strength with SA 508 base material and the highest strength mismatch existed between the SA 508 heat-affected zone (HAZ) and the Alloy 52 weld metal. According to the hardness measurements, the mismatch at the FL was increased as a result of the post-weld heat treatment (PWHT). In the fracture mechanical tests, high fracture toughness values were observed at the different zones of the weld and the fracture surfaces consisted of ductile fracture. In the specimens with the pre-fatigue cracks located near the FL as well as in the specimens with the pre-fatigue cracks located in the HAZ of SA 508, the cracks seemed to propagate towards the FL. In the PWHT specimens, once the cracks reached the FL, they propagated along the SA 508 side of the FL. In the case of the as-welded specimens, the cracks occasionally jumped across the FL to Alloy 52 weld metal.