SUMMARY OF THE SESSIONS ON WELDMENTS – Chaired by Pål Efsing and Ian de Curières

The weldments session started on Thursday morning with four talks.

Ting Xiao (NPIC, China) discussed the effect of thermal ageing on SCC behavior in 316L welds. The conclusion from the study is that TA does not severely alter the sensitivity of the structure to SCC and that the precipitation of a’ and G-phase is limited at the same tie as the residual stresses are relaxed with time. The tests were performed on samples aged at 400 °C for times comparative to up to 50 years of operation. The weld contained some 5-6% ferrite and the relatively low content may be a reason to the limited effect of ageing in the experiments.

Massimo Cocco (Forsmark NPP, Sweden) discussed a degradation issue that they have been observing and following for relatively long time period regarding material loss of alloy 82-type welds in the bottom dome to the most recently built Swedish BWR’s Oskarshamn 3 and Forsmark 3. Suggestions from the audience includes considering the Fe-content and couple this to the local ECP as well as studying the outline of the dendrites after welding to understand the degradation.

Annesha Das (PSI, Switzerland) presented some findings from a program on effects of cavitation peening on the microstructure and stress distribution of Alloy 182. Peening is commonly used in the industry as a mitigation method to prevent initiation of SCC and as such, relaxation of the local stresses at the surface may be detrimental for the degradation behvior since there are microstructural changes also.

Finally, Mychailo Toloczko (PNNL, USA) discussed the on-going initiation tests that US-NRC sponsors at PNNL. Mychailo showed that there is a connection to the time for initiation and the temperature such that the specimens that show defects are significantly more compressed and aligned at lower temperatures. In fact, it seems as if the testing at 360 °C should be abandoned in favor of testing at 270 °C. The apparent difference in estimated initiation time is very low and a more homogeneous behavior gives a better basis for assessment.

The second weldments session on Thursday after the lunch break included four talks.

The first one, by Zaiqing Que (VTT, Finland) showed a case of cracking of old Alloy 52 CT specimens, which had been tested in high-temperature water for 13 years, then stayed for 3 years at lower temperature, due to a reactor shutdown. The specimens exhibited 3 to 4 mm propagated cracks. They are intergranular, brittle in appearance and their oxidation low, which suggests LTCP. The features (transgranular cracking with a rather low stress (K = 20) or possibly less) reveal a new case that is surprising and would require more investigation.

Shuichi Yoshida (Toshiba, Japan) presented results on the SCC susceptibility of various welds in Alloy 52M (and modified) in BWR water. The crack growth rates they obtained are similar to the ones obtained by other laboratories and one order of magnitude lower than those in Alloy 82.

Mychailo Toloczko (PNNL, USA) gave an update on the status of their initiation testing of various Ni-base alloys, to implement in an initiation module in the xLPR Code. Their results on Alloy 182 are consistent with previous data and enable to expand the statistical relevance for this alloy data. On higher chromium containing alloys, tests are still going on and indicate clearly a stronger initiation resistance.

Jiamei Wang (SJTU, China) performed SCC tests, considering the thermal ageing of stainless steel welds with low and high ferrite content, as well as carbon content. They observed that thermal treatment was detrimental in aerated water, especially in the high ferrite-high carbon welds, due to sensitization, formation of gamma phase and strong segregation of minor elements (sulfur particularly) at the ferrite-austenite interfaces.

After the tea break the session continued with four more presentations.

The session was kick-off by Tetsuo Shoji (Honorary Member, Japan) who made a connection to the ICG-meeting in 2013 in Karuizawa when he discussed an issue where a saltwater ingress occurred in one of the Japanese BWR plants following the Fukushima earthquake. The presentation was a discussion of a novel test method to study corrosion/stress corrosion for stainless steel cladding on pressure vessel material. The test is a disc test which is stressed by a pressure difference between the upper and lower surfaces using a 10 cm diameter disc.

Zhao-Wei Fu (Tohoku University, Japan) presented his PhD work at Tohoku, which is a study of the influence of d-ferrite on SCC. The study so far shows that the d-ferrite at grain boundaries tend to act as crack stoppers at least as the second area.

Yasufumi Miura (CRIEPI, Japan) presented a study on effects of thermal ageing of stainless steels on SCC in BWR environments. It seems as if materials aged at 350 °C do not show increased CGR, even though the cracks propagated in the d/g-interface. where-as for materials aged at 400 °C there appears to be a slight increase in CGR with time, but totally un-related to the d-ferrite distribution. No apparent effect was observed on fracture mechanical properties as measured in the study even though the aged material in general had lower initiation toughness than the as delivered condition.

The final paper was presented by Seunghyun Kim (KIMS, Korea) on chloride-induced SCC on stainless steels for use in spent fuel storage. It seems quite clear that weldments are susceptible to CISCC where-as the base metal is less so. It also seems as if lean duplex can be a viable option for use here. The chloride is present in the atmosphere and can enter the concrete container through the ventilation system.

As a conclusion it can be noted that much interest now is at effects on stainless steel weldments, which is connected to the knowledge gap associated with the long-term structural integrity effects due to the ageing. Issues such as delta-ferrite effects, spinodal decomposition and break down of the meta stable gamma-austenite still are open for discussion and as such much work remains. An observation from the initiations tests on nickel-based alloys is that it appears as if the lower temperature results are more consistent, possibly due to stress relaxation effects at high T, and as such, more emphasis could be placed to these tests. Finally, it is obvious that there are still failure cases in the weldment area and as such, more research is needed. One such area could be the long-term effect from some mitigative efforts such as peening, where the results presented this year implies a relatively quick relaxation of the needed/wanted compressive stresses.

Please note that one more weldments-related presentation was given during one of the summary/poster presentation sessions and that one more presentation file is included in the meeting minutes (without being presented during the meeting):

Xinhe Xu (Shanghai University, China), “Nano-micrometer scale characterization of PWSCC crack-tips in the transition zone of Alloy 52M overlay and the implication to intergranular cracking”

Hans-Peter Seifert (PSI, Switzerland), “K-dependence of SCC crack growth in Alloy 182/82 dissimilar metal welds in oxygenated high-temperature water”