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

The first weldments session included four papers with a broad mixture of areas of interest.

The first paper was presented by Mychailo Toloczko (PNNL, USA). Mychailo presented an extended assessment of two different specimens from the initiation studies on Alloy 182. During the test, which included samples from four different manufacturers of the welded material, it was noticed a significant skew towards short initiation times which has been noted before by Amzallag et al. in a study for EDF. During the study some 80% of the specimens initiated and failed. Mychailo also pointed to the fact that there is a large variability within the sampling with respect to crack growth, this can partially be connected to “fingered growth” and/or portion of the crack front showing a lagging effect, where the crack dwells instead of propagates.

Ada Mackiewicz’s (PSI, Switzerland) work is aiming to fill some of the gaps when it comes to Zn injection to BWR reactor water. In the study three different specimen types have been used: coupons, flat tapered tensile and CT specimens. Initiation of propagation occurred at higher K-levels, and the obtained crack growth rates were lower for both, 15 and 40 ppb Zn injection. In the accelerated SCC initiation tests with tapered specimens, no beneficial/mitigative effect on SCC was noted at 15 ppb of Zn content. A general observation made is that the inner oxide layer is significant thinner and more compact for specimens subjected to Zn injection than without. This appears to be true for both, 15 and 40 ppb injection.

Yutaka Watanabe (Tohoku University, Japan) presented a study on creep deformation at BWR temperatures, i.e., 288 °C. The preliminary results showed a significant influence of hydrogen on the creep rates, specifically for LAS base metal but also for Alloy 182 weld metal. There will be more data coming out of the program and the Japanese organization EAC-J is looking for collaboration in this area from the international community.

The US NRC has together with EPRI developed a computer code for evaluation of risk for degradation/failure probability, xLPR to supersede previous efforts on leak before break and similar concepts. Both, NRC and EPRI have given a number of presentations on the tool and interested readers can for example look at [1] and [2]. The point here being that most material are susceptible to some sort of degradation mode, we thus need to cover and quantify this. The basis for the Code is the understanding of how material behave regarding both, initiation stages and growth stages and as such much effort has been put on these areas with testing being performed at PNNL in Richland as well as at ANL in Chicago. A key to success is to quantify the factor of improvement when selecting the less susceptible alloys on the market now replacing Alloy 600 and its weld metals. Eric Focht (NRC, USA) presented the current scope and on-going activities and highlighted the interest to test materials harvested from various reactors that have been shut-down in the near history. Eric pointed to the fact that hardening of aged Alloy 690 materials have been observed by several researchers, which may imply some ordering effects occurring in the material, but at the same time he also noted the absence of validated crack growth data for these materials (service or furnace aged). The point here is to achieve validated and consistent data to include in the xLPR Code to enable good decision making for both, utilities and regulatory bodies to relate to. [1] https://www.nrc.gov/docs/ML2214/ML22143A836.pdf; [2] https://www.epri.com/xlpr.

On Thursday afternoon there were four more presentations, which were of high scientific and technical level, providing interesting inputs for both engineers and regulators.

Peter Andresen (Honorary Member, USA), “Final Expert Panel Alloy 82 SCC dependencies and parameters with effect in BWR water”. The speaker presented the frame and analysis of Alloy 82 SCC data in BWR water and the screening process used to assess their quality and rank them, considering reproducibility, confidence in data and relevance of materials and test conditions. Based on this selection, the expert panel selected many data which enable to propose a disposition curve for the crack growth rates in Alloy 82 in BWR water. This disposition curve will likely be of interest for BWR utilities who operate with a flaw-tolerance approach to manage the SCC of such weld materials, in the frame of an LBB process and in determining inspection strategies.

Katsuhiko Kumagai (TEPCO, Japan), “Effect of PWHT on the material properties and SCC growth behavior of Ni-base weld metals”. Results of SCC crack growth tests on CT specimens of Ni-base alloy weld metals were shown, both in the as-welded and PWHT conditions. The laboratory performed residual stress measurements on the CT specimens to properly correct the loading during the test. They observed that the residual stress fields were quite different in as-welded and PWHT specimens. The use of the residual stress fields results in that the accurate CGRs for as-welded and PWHT specimens are not the same, suggesting that the classical disposition curves, which do not consider the residual stress fields and provide distinct curves for the two cases may not be relevant; one single curve is accurate. This phenomenon is unlikely to be an issue for base materials but it gives some important information for utilities, engineers and regulators to chose accurate data in the engineering or safety evaluations.

Yun Wang (Hitachi Ltd., Japan), “Comparison between Alloy 82 and Alloy 52i based on stress corrosion cracking initiation mechanism in BWR environment”. The speaker performed some SSRT SCC tests with Alloys 82 and 52i in BWR environments. He obtained microcracks of about 2 µm in both alloys. Extensive analysis of the coupons and the cracks revealed that SCC likely occurred by the coalescence of micro-voids present in oxides. The number of these voids is lower in 52i than in 82, thus the SCC initiation susceptibility of Alloy 52i is expected to be much better than that of Alloy 82, especially in plant conditions where the loads are constant and not slowly rising. His work is on-going to asses a more precise SCC mechanism based on internal oxide penetrations of grain boundaries.

Mychailo Toloczko (PNNL, USA), “Alloy 152(M)/52(M) GB microstructures of SCC initiation specimens after 5.7 years of exposure in simulated PWR primary water”. PNNL performed SCC tests on coupons of Alloy 152M and52M coupons (taken from various positions in the welds) for 4 and 5.7 years in PWR simulated primary water under constant load at 360 °C. The welds were 15 % cold-forged before the tests. After the SCC tests, they observe a few very small defects on the surface, of about 100 µm of length and 10 µm of depth. Most of them are correlated with weld defects, but two of them require more attention. The first one, in Alloy 52M after 5.7 years seem to be due to the coalescence of microcavities. The second one, in Alloy 152M, also after 5.7 years, showed a depth of nearly 800 µm. Some specimens will be tested for an additional time to verify whether they are active cracks or pre-existing defects.

The third weldments session included three papers of three different topics. It connected nicely to the special session on SCC in stainless steels with two papers related to this, whereas the third paper related to degradation of modified Alloy 82 weld metal.

Takahiro Hayashi (Toshiba, Japan) gave a presentation on the sensitivity of Ni-base weld materials with Cr-contents, i.e., modified Alloy 82 with up to 36% Cr. The objective of the study is to investigate possible replacement materials for BWRs. All weld metals showed similar microstructure. Both, general corrosion tests and bending tests to study SCC were included in the study. Whereas conventional Alloy 182 and 82 showed evidence of cracking, none of the modified materials exhibited any defects, even after 2000 h of accelerated testing. The final step of the study was to perform crack growth tests under both, NWC and HWC conditions. The comprehensive study shows that there is a possibility to implement the high-Cr alloys (regardless of if it is the standard Alloy 52, modified Alloy 82 or something else) from an ageing and degradation perspective. In an accompanying study Toshiba has studied the weldability of the high-Cr materials, but this will be a presentation of its own.

Takuyo Yamada (INSS, Japan) discussed SCC in the HAZ of stainless steels, connecting to the special session’s objectives. INSS has studied crack growth response on materials deemed reasonably representative to actual pipe welds. Very little influence was detected from the distance to the weld fusion line (1 to 4 mm tested) and the data falls reasonably well into the current database of available data within Japan. It was also noted that there in essence is a threshold temperature above which no crack growth was detected, which well emulates data available from cold-worked 316 stainless steel. One possible explanation for the increased susceptibility to crack growth in cold-worked materials has been the introduction of strain-induced martensite. No such transformation was noted in the current study.

The final paper of the session was given by Zdeněk Fulín (CVR, Czech Republic). They studied the influence of laser shock peening on the relative degradation risk of degradation, or its mitigative effect on the risk. As expected, the areas outside the actual peening zone, exhibits higher tensile stresses than before the treatment, whereas it was clearly shown with deep hole drilling to evaluate the residual stresses that the area around the treated weld showed compressive stresses. The testing was done on bolt-loaded C-ring specimens in accordance with ASTM G36 (“Evaluating the sensitivity to SCC by subjecting the specimens to boiling MgCl solution”) and SSR tests in hydrogenated steam. One of the main conclusions is that longer test time is essential to draw the right conclusions, but the LSP treatment appears to improve the SCC resistance of the materials. Also to note was that much of the observed cracking is on the outer, non-treated (and non-media touched under normal conditions) surface, which may obscure the actual test outcome.

In total, the sessions mentioned here provided a good variation of tests and studies which shows the widths of the issue to be covered and also the complexity of understanding the issues related to EAC of weldments.

Please note that one more weldments-related presentation was given during one of the summary/poster presentation sessions:

Adrianna Mackiewicz (PSI, Switzerland), “Effect of Zn on the oxide film properties of Alloy 182 exposed to BWR environment”