Factors affecting low temperature crack propagation (LTCP)

The effect of hydrogenated PWR primary water on the Low Temperature Crack Propagation (LTCP) susceptibility of the nickel-based Alloy 182, 82, 152 and 52 weld metals was studied performing J-R tests for all-weld metal and dissimilar metal weld (DMW) mock-up specimens at a slow displacement rate in simulated low temperature PWR primary water. Single edge bend (SE(B)) specimens were tested in hydrogenated environments (100, 30 and 5 cm3 H2/kg H2O) with and without exposure to high-temperature hydrogenated PWR water prior to loading (24 h or 30 days at 300??C). Additionally, the effect of post-weld heat treatment on LTCP behaviour of Alloy 182 and 52 weld metals was studied. It was observed that Alloy 182 was the most susceptible material to LTCP, but also some specimens of Alloy 52 exhibited rather low JQ values, similar to those of Alloy 182, when tested with highest hydrogen content. Specimens of DMW Alloy 52 behaved in varying manner when tested with high hydrogen content, in both AW and PWHT conditions. PWHT seems to increase the tearing resistance of Alloy 182. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive Xray spectroscopy (EDS) were applied in order to examine the cracking path and to characterize the microstructural features affecting hydrogen trapping in Alloy 182 and 52 weld metals. A clear relation between high hydrogen content of the water, low fracture resistance values and intergranular/interdendritic type of fracture was observed, although also transgranular brittle-like fracture was observed in DMW Alloy 182 and all-weld metal Alloy 82 specimens tested in low temperature hydrogenated water. Hydrogen thermal desorption spectrometry (TDS) measurements were performed for Alloys 182 and 52 by using varying heating rates in order to study the effect of high temperature pre-exposure, different trapping sites and trapping activation energies. The high temperature pre-exposure was found to decrease the hydrogen content of Alloy 182 and 152 weld metals and consistently the fracture resistance values were increased. Different factors affecting the LTCP behaviour of nickel-based weld metals are discussed.