2015

Ahonen, Matias

Various nickel-based materials are susceptible to low temperature crack propagation (LTCP) in simulated PWR (pressurized water reactor) water at a temperature range of about 50 to 150 °C. Experimental evidence from various sources shows that the hydrogen content of the water has a decreasing effect on the fracture resistance, thus LTCP is widely regarded as a hydrogen-induced phenomenon. This thesis concentrates on the LTCP phenomenon of Alloy 182, 82, 152 and 52 weld metals. The studied materials were both all-weld metals and dissimilar metal weld (DMW) mock-ups. The material conditions studied in this work were as-welded (AW), post-weld heat treated (PWHT) and high temperature water pre-exposed. The experimental work was divided in fracture mechanical testing in an environment, microstructural examination of fracture surfaces and hydrogen thermal desorption measurements. 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 moderate hydrogen content. 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 Alloy 52 DMW appeared to be less susceptible to LTCP in the corresponding environment. 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, and the fracture resistance values of Alloys 182 and 152 are improved. 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 hydrogen trapping properties are discussed based on the obtained SEM/EDS and thermal desorption spectroscopy results and a model was applied in order to determine the activation energies for hydrogen desorption of Alloys 182 and 52. The different LTCP behaviour of Alloy 182 and 52 weld metals is believed to be caused mainly by different types of carbides dominating the hydrogen-induced fracture. The carbides may have an effect on hydrogen distribution at the grain/dendrite boundaries and the availability of hydrogen close to the crack tip, by acting as trapping sites for hydrogen and by affecting the strain distribution at the grain boundary area.