Hannu Hänninen

Effect of Thermal Aging on SCC, Material Properties and Fracture Toughness of Stainless Steel Weld Metals

An experimental program has been conducted in order to understand how the spinodal decomposition may affect material properties changes in Type 316L BWR pipe weld metals. The program includeed Charpy-V, tensile, SCC crack growth and in-situ fracture toughness testing as a function of aging time and temperature. In this paper we report results of fracture toughness, SCC crack growth rate and fracture morphology studies of Type 316L stainless steel weld metals under simulated BWR conditions, consisting of 288°C, high purity water containing 300 ppb dissolved oxygen (defined for purposes of this paper as “In-Situ”). SCC crack growth results show an approximately 2X increase in crack growth rate over that of the unaged material. In-situ fracture toughness measurements indicate that environmental exposure can result in a reduction of toughness by up to 40% over the corresponding at-temperature air values. Detailed analysis of the results strongly suggest that spinodal decomposition is responsible for the degradation in properties measured ex-environment. Analysis of the results also strongly suggests that the in-situ properties degradation is the result of hydrogen absorbed by the material during exposure to the high temperature aqueous environment.

Stress Corrosion Cracking and Crack Tip Characterization of Alloy X-750 in Boiling Water Reactor Environments

The stress corrosion cracking susceptibility of Inconel alloy X-750 in the HTH heat-treated condition has been evaluated in high-purity water @ 93 and 288°C under Normal Water Chemistry (NWC) and Hydrogen Water Chemistry (HWC) conditions. SCC crack growth rates of approximately 1.2x10-7 mm/s (K=28 MPa√m) under NWC conditions and 1.4x10-8 mm/s (K=28 MPa√m) under HWC conditions in 288°C high-purity water were observed. The alloy was also tested in HWC at 93 °C. No SCC crack growth was observed at K= 35 MPa√m for the 525 h of testing at 93°C. At 288°C the fracture mode transitioned from predominantly transgranular cracking under fatigue conditions to intergranular cracking under constant stress intensity for both environments. Atom Probe Tomography (APT) was used to characterize the crack tip and ahead of the crack tip. The results suggest that oxygen diffuses ahead of the crack tip along specific crystallographic directions.