SUMMARY OF THE LOW-ALLOY STEELS SESSION – Chaired by Armin Roth
The low-alloy steel (LAS) session comprised a total of four oral presentations and was heldt on Thursday morning.
Timo Saario (VTT, Finland) presented the “Effect of potential on stress corrosion cracking of low-alloyed steel in conditions of a RPV cladding flaw bottom in boiling water reactor coolant with chloride transient”. Based on earlier work, the question arose whether the water chemistry at the crack tip of a 1” C(T)-specimen is the same as the one forming at the bottom of a “real” cladding flaw. Cladding flaws are semi-elliptical and migration of chlorides into the crack might be different in contrast to C(T) specimens, where the side faces of the crack are open to the environment. According to modelling, in BWR bulk water with 50 ppb chloride, a much more aggressive crevice environment, that is clearly not representative of the cladding flaw case, is established in the LAS 1ʹʹ C(T) specimen. Specifically, corrosion potential at the LAS/solution interface is predicted to be much higher for the 1ʹʹ C(T) LAS specimen (approx. -0.2 VSHE) than in the realistic cladding flaw bottom case (approx. -0.38 … -0.47 VSHE). Slow strain rate tensile (SSRT) test experiments have shown that SCC in LAS in cladding flaw bottom environment occurs at potentials E > -0.35 VSHE. Thus, chloride-induced SCC/SICC would not be expected to occur in the realistic cladding flaw case. These results would support the hypothesis that the very high crack growth rates measured in laboratory with 1” C(T) LAS specimens in BWR water with chloride might not be representative of the realistic cladding flaw case.
Zaiqing Que´s (PSI, Switzerland) talk showed latest results of a project investigating “High-temperature water and hydrogen effects on the fracture behaviour of low-alloy RPV steels”. The fracture toughness of reactor pressure vessels could be reduced by the absorption of hydrogen from the reactor coolant and corrosion reactions. Exposure to high-temperature water of a commercial pressure vessel steel (Biblis C base metal) to simulated conditions for PWR, BWR/NWC, BWR/HWC at 150 and 288°C did not reduce the initiation toughness and tearing resistance. Occurrences of sudden unstable ductile crack extensions were observed in a simulated high strength coarse-grain HAZ material. There is evidence for possible reduction of the initiation toughness in PWR environment, but further tests are needed for confirmation. Synergy between dynamic strain aging (DSA) and hydrogen was observed in tensile tests in air in a 277-steel with high DSA susceptibility. There was first evidence for reduction of initiation toughness in high-temperature water. More severe toughness values were measured at slower loading rates, partially due to SICC crack growth.
Yasufimi Miura (CRIEPI, Japan) continued the session presenting a “Test method for evaluation of elastic-plastic fracture resistance in high temperature water”. In general, fracture toughness of materials which compose the components in light water reactor (LWR) is obtained by fracture toughness tests in air although some components are subjected to coolant water because of the difficulty of the test in high temperature water. The objective was to establish a test method for evaluation of elastic-plastic fracture resistance in high temperature water based on direct measurement of load line displacement LLD of the specimen. A test method for evaluation of elastic-plastic fracture resistance in high temperature water using the clip gage and normalization data reduction technique (NDR) was established. In this study, the obtained value using NDR technique tended to be higher than the value using elastic unload compliance. Obvious reductions of fracture resistance of the LAS in high temperature water were not observed in this study.
Colin Judge (CNL, Candada) talked about CANDU feeder cracking. Carbon steel feeder cracking has occurred in only one CANDU reactor. As of 2007, feeder cracking is no longer an active degradation mechanism. A retrospective examination in ex-service carbon steel feeder cracks was performed by applying similar techniques utilized in characterizing heavily irradiated Inconel X-750 CANDU spacers. Note that the mechanism for carbon steel feeder cracking has never been determined. No explanation to why this has only affected one plant has been given. CANDU feeder cracking has been linked with regions of high residual stress, high hardness, high temperatures (relatively), and locations of FAC. High-resolution microscopy has not been performed to investigate feeder cracking prior to this work. Inside surface cracks and outside surface cracks show the same general microstructural features (i.e., carburization of the grain boundaries). Preliminary observations conclude the breakdown of Fe3C to amorphous carbon at the Ferrite-Cementite intergranular fractures. Potential explanation is hydrogen attack from FAC. Nano-cavities observed ahead of the ferrite-ferrite intergranular crack tip. Potential explanation is hydrogen enhanced creep from FAC. This appears to be the first time that high-resolution STEM mapping has been performed on carbon steel cracking in a hydrogen environment. Therefore, there are no benchmarks for comparison. It is possible that if other experiments in a high hydrogen environment may have caused the breakdown of Fe3C similar to that observed in ex-service feeder tubes. Recall the amorphous carbon layer is only a few nm thick (easily missed).
Please note that two additional LAS papers were presented during a summary/poster presentation session:
Qian Xiao (Shanghai University, China): “The effects of environmental factors on corrosion and fracture behaviors of A508III low alloy steel”
Xiaouyuan Lou (GE-GRC, USA): “SCC memory effect on pressure vessel steel after chloride transient in high temperature water”
General conclusions from the session chairman:
In contrast to the preceding years, when predominantly EAC aspects were addressed, the sessions of last year and this year indicate a trend towards contributions looking at environmental and hydrogen effects on characteristic properties related to fracture mechanics.