SUMMARY OF THE SESSIONS ON WELDMENTS – Chaired by Pål Efsing and Ian de Curières
The first half of the weldment session on Thursday morning consisted of three papers that, from a distance, seem a fair bit apart from each other, but when looking closely, indeed have much in common.
The first paper by Martin Bjurman (Studsik, Sweden) discussed thermal and irradiation induced aging of a type 308 stainless steel weld metal harvested from an actual plant (Zorita NPP, Spain). High-resolution APT studies showed a number of features, including a clear aging effect of the material such that increasing dose level (and time at temperature) resulted in an increased g-phase formation as well as increased spinodal decomposition. Martin also reported on an observation that at low dose (0.15 dpa) P segregated to the phase boundary between ferrite and austenite, whereas this segregation appears to disappear at higher doses. In addition to this, no Cr-depletion was noted at the phase boundary.
Qian Xiao (Shanghai University, China) studied the microstructure and corrosion behavior of stainless steel weld metal typical of cladding material in LWRs. Studying the oxide formation on cross sections of the two layers of cladding material, 309 and 308 weld metal, it was noted that the protective inner oxide had significantly less Cr on the 309 weld metal compared to the 308. This was proposed to be the reason for the apparent sensitivity to SCC that was found during slow strain rate tests of the cladding material.
In the final paper Jianqui Wang (IMR, China) presented a comprehensive study on weld defects formed during welding of a full thickness Alloy 52 weld between a simulated nozzle of A508 and a 316 LN simulated piping. It was noted that there is a strong correlation between the grain boundary morphology and size, orientation and distribution of carbides on the susceptibility to form ductility dip cracks, a solid state crack formation mode that occurs during cooling of the weld. Fairly large defects (up to 3 mm in length) were observed in the area representative of inner, media touched portion of the weld. However, throughout the weld defects were observed. Most of them were well below 500 µm, and as such not an issue from a structural integrity point of view.
Concluding remarks from the session chairman:
All three presentations consisted of high-resolution microstructural characterization of the studied materials and presented some key observations that can be important in understanding of the aging and degradation of weld materials and also to direct future work. It also was shown very clearly that it is necessary to understand the defects that are present in welded components to lay a strong foundation for long-term operation of the power plants and to avoid issues with future developments in NDE. It is an industry necessity to be able to understand and discriminate manufacturing defects from service-induced, and to be able to understand the effect of aging on mechanical properties and degradation of structures and components. As such, the studies presented here have a strong correlation to plant operation.
After the coffee break the weldments session continued with three papers.
The first paper was presented by Mychailo Toloczko (PNNL, USA) and dealt with SCC initiation in Alloy 182 in PWR primary water. He tested 36 specimens from four different welds, with a slight amount of CW applied to the specimens (15%). Although some tests had not been performed yet, he observed that the SCC initiation behavior revealed a significant scatter for each weld. Nonetheless, in many coupons cracks initiated in a relatively short amount of time (<150 h at 360 °C), which proved to be below the PNNL curve for Alloy 600. In the low initiation times, the cracks were not correlated with pre-existing weld defects. His results are comparable to the ones obtained by other labs.
The second talk given by Yohei Sakakibara (IHI Corp., Japan) dealt with PWSCC initiation in Ni-based alloy welds in primary water and hydrogenated steam. The selected materials were Alloy 82 and 52 (with some susceptibility to ductility dip cracking DDC) as reverse U-bend specimens. The tests indicate a clear susceptibility of Alloy 82 and SCC cracking was observed in some of the 52 specimens with 7 or 15% of pre-strain, implying a factor of improvement of 4.6 with respect to Alloy 82. DDC seems to be an aggravating factor for SCC initiation in Alloy 52 in hydrogenated steam. The question of the effect of pre-existing weld defects of the initial surface condition of the RUB specimen are required to understand the SCC response.
The last presentation of the weldments session was held by Masao Itatani (Toshiba, Japan). It dealt with the fatigue behavior of Ni-base alloys under long-term fatigue cycling in BWR environments. His objective is to assess the behavior of Ni-base alloys in domains out of the scope of the JSME code (notably slow load rising times). He performed fatigue tests in BWR environments, both NWC and HWC, on weld metals (Alloy 82 and 182) and base metal Alloy 600. He observed that increasing the load rise time increases the fatigue crack growth rate at constant DK, which could be ascribed to some SCC contribution during the rising load. He proposed corresponding fatigue crack growth rate curves.
Concluding remarks from the session chairman:
Overall, the papers in this session indicate that the corrosion behavior of welds remains a complex topic, even more than the one of base metals. The scatter in the results concerning even the seemingly simplest alloys indicate that some work remains necessary to assess the behavior of these materials important for the plant component’s reliability and safety.
Please note that two further papers were presented during one of the summary/poster presentation sessions:
Jiunn-Yuan Huang (INER, Taiwan): “Effects of dendrite axes and fusion boundary on stress corrosion cracking of 308L/304L weldments in a high-temperature water environment”
Yasufumi Miura (CRIEPI, Japan): “Effect of thermal aging on fracture property of type 316L stainless steel welds”