The effect of microstructure on Low-Temperature Crack Propagation (LTCP) susceptibility of nickel-based Alloy 182, 152 and 52 weld metals in PWR primary water

The effect of microstructure on the Low Temperature Crack Propagation (LTCP) susceptibility of the nickel-based Alloy 182, 152 and 52 weld metals was studied comparing the obtained results from J-R tests, thermal desorption spectroscopy (TDS) and scanning electron microscope (SEM) fractography. The single edge bend (SE(B)) specimens were tested in hydrogenated (100, 30 and 5 cm3 H2/kg H2O) low temperature (55 ?C) water with and without exposure to high temperature hydrogenated PWR water (24 h or 30 days at 300?C) prior to loading at 55 ?C. The obtained J-R test results show that Alloy 182 is the most susceptible nickel-based weld metal to LTCP, whereas Alloy 52 retains its high fracture resistance in hydrogenated water with low hydrogen contents. The results obtained for all-weld metal Alloy 52 showed, however, a clear reduction of fracture resistance when tested at a high hydrogen content (100 cm3 H2/kg H2O), whereas narrow gap mock-up dissimilar metal weld (DMW) Alloy 52 showed to be considerably less susceptible to LTCP in the corresponding environment. Hydrogen content measurements show that bulk hydrogen concentrations of Alloy 182 and 152 weld metal samples decrease during the high temperature water exposure, even when exposed to water containing 30 cm3 H2/kg H2O. Also the fracture resistance values are improved after high temperature water exposure. Microstructural characterisation of fracture surfaces was performed using SEM and energy-dispersive X-ray spectroscopy (EDS). A clear relation between the low fracture resistance values and intergranular/interdendritic type of fracture was observed. The effect of grain boundary carbides and their trapping properties are discussed based on obtained SEM/EDS and TDS results.