SUMMARY OF THE SUMMARY/POSTER PRESENTATION SESSIONS – Chaired by Gary Was and Mike Wright

Part I of the summary/poster presentation session consisted of four short poster introductions on Monday morning followed by the poster presentation during the coffee break.

Jiamei Wang (SJTU, China) presented a poster on the “Effect of residual δ-ferrite on SCC behavior of 321 stainless steel”. This research focused on the effect of lower δ-ferrite content (<2%) and the water chemistry (DO/DH + Cl^–) on the SCC propagation behavior of 321 stainless steel. Their investigations showed that (1) the CGR obtained in pure water with O₂ was about 3.4·10^-8 mm/s, and changing to H₂ decreased the CGR significantly with or without Cl^– addition, (2) the δ-ferrite seems to impede crack propagation even with Cl^– addition, and promotes the crack propagation along the γ/δ phase boundary, and (3) the δ-ferrite appears to promote the stable Cr-rich oxide formation near the δ-ferrite and retards crack growth.

Yoshiyuki Kaji (JAEA, Japan) summarized investigations on the “SCC initiation susceptibility in L-grade austenitic stainless steels after long-term thermal aging treatment”. Experiments were conducted on creviced bent beams in 288°C water containing 8 ppm DO and conductivity <0.2 µS/cm at the outlet for 1000 h. Crack number and crack morphology were recorded. Samples were tested in both the CW (20% reduction) and long-term thermal aged for 1400 h at 288°C (LTA) conditions. Results showed that for CW 304L, LTA caused only a slight increase in SCC susceptibility. For CW 316L, the number of IGSCC cracks significantly increased following LTA.

The short presentation of Yung-Hsiang Chang (National Tsing Hua University, Taiwan) advertised the poster titled “Electrochemical characteristics of platinum-treated type 304 stainless steels of different surface oxide structures in high-temperature water”. Samples of 304 stainless steel that were sensitized, pre-oxidized and coated with Pt were exposed in deaerated or 300 ppb DO water and then subjected to Electrochemical and Raman analysis. Results showed that Pt treatment enhanced the general corrosion rate of type 304 stainless steel in highly oxidizing environments. The efficiency of Pt deposition on pre-oxidized specimens exposed to H₂ is better than pre-oxidized specimens exposed to H₂O₂.

The objective of Katsuhiko Fujii’s (INSS, Japan) work “APT analysis of solute clustering in neutron-irradiated stainless steels” is to obtain a deeper understanding of solute clustering and precipitation of stainless steels under LWR irradiation condition. Flux thimble tube (FTT) materials irradiated to 3, 11 and 74 dpa were analyzed by APT. Results show that clusters fell into two groups; small (3-4 nm) and large (6-10 nm). The total cluster number density increased due to an increase in small clusters and a decrease in large clusters. Ni-Si is likely a precursor of γ’ phase (Ni₃Si) rather than γ’ phase. The formation of Mn-rich Ni-Si clusters, which possibly relate to γ phases (M₆Ni₁₆Si₇, M = Mn, Mo) as precursors, is likely to be a high-dose effect.

Four further summary presentations were given in part II of the summary/poster presentation session on Tuesday before the coffee break. This session focused on austenitic alloys and specifically metallurgical factors, often subtle ones, affecting material performance, with three presentations addressing long-term aging conditions or the higher operating temperatures of advanced reactors.

The session started with a presentation by Kai Chen (SJTU, China) entitled “Comparison of the stress corrosion cracking behavior of 310S stainless steel and Alloy 690 in supercritical water”. He reported crack growth rate tests showing that both annealed 310 stainless steel and CW Alloy 690 cracked in a supercritical water environment (argon deaerated), showing the same temperature dependence from 400 to 500-550°C. The 310 stainless steel cracked at higher rates however, approx. 100 times higher than the 30% CW Alloy 690. Alloy 690 with no CW cracked at a very low rate.  Both the 310 stainless and the Alloy 690 materials showed evidence of grain boundary Cr depletion occurring ahead of the crack-tip (in-situ sensitization). The greater susceptibility of the 310 stainless steel was attributed to depletion of Cr to levels as low as 7%. For both materials the author noted indications of grain boundary creep damage ahead of the crack-tip. Note that this work is linked with work reported separately by Xianglong Guo, also of SJTU, “Effect of temperature and cold-work on the SCC behavior of Alloy 690 in high-temperature water”. Guo reported direct evidence of creep crack growth for CW Alloy 690 at 450 to 550°C but a very limited role of creep in sub-critical water tests (<360°C).

Jiunn-Yuan Huang (INER, Taiwan) presented a summary entitled “Effects of dendrite orientation and fusion boundary on stress corrosion cracking of 308L/304L weldments in a high-temperature water environment”, the test environment being simulated BWR water. Jiunn-Yuan reported on two (compact tension) crack growth tests, one performed with the crack growing within the 308L weld metal, sampling differing dendrite orientations plus another test where a crack was initiated beside the weld in the HAZ of the 304L and then propagated so as to approach the fusion boundary at a shallow angle. The 304L HAZ was found to be susceptible to SCC and the high susceptibility was attributed to the residual strain caused by weld shrinkage. But as the crack encountered the fusion boundary, the crack arrested. The boundary acted as a barrier for the SCC propagation. For the crack initiated within the weld metal, the ferrite/austenite interfaces showed higher SCC susceptibility than the austenite phase. It was also noted that the dendrite axis affects the SCC growth rate: if the applied loading direction is nearly perpendicular to the dendrite axis, the SCC growth rate increases.

Yasufumi Miura (CRIEPI, Japan) summarized his poster with the title “Effect of thermal aging on fracture property of type 316L stainless steel welds”. He noted that type 316L welds with 8%, or more, ferrite are widely used in Japanese BWR plants and some thermal aging leading to degradation in fracture toughness is expected. To date, studies of thermal aging of type 316L weld metal with relatively high ferrite is limited, compared to the extensive studies focused on cast austenitics, and there is no prediction model specifically for thermal aging of stainless steel welds.  The work reported included different welds, gas tungsten arc welds (GTAW) and shielded metal arc welds (SMAW) aged at 350 and at 400°C. Tests performed were tensile and fracture toughness tests at 288°C, and microscopic aging effects were investigated employing APT (see also the numerous presentations investigation irradiation effects at this meeting). It was reported that the fracture toughness of both, aged and unaged GTAW was higher than that of SMAW, although the ferrite content of the GTAW weld was larger than that of the SMAW weld. The welding process is therefore a very important starting variable. Relatively good correlation was observed between normalized fracture toughness (J_1.5) and flow stress in both types of weld regardless of aging temperature. Microstructural evolution in ferrite phase of the welds were similar to that of a cast austenitic tested at the same aging temperature (350°C). This work is a first step in the development of a thermal aging prediction model for stainless steel welds.

The last summary presentation, “Oxidation of 316L stainless steel in primary water environment” was made of behalf of Ryan Matthews (ESKOM, South Africa) by Thierry Couvant (EdF, France). Ryan is working on his PhD with Thierry and the work presented is connected with a latter full presentation by Thierry, entitled “Improvement of SCC predictions considering the surface finish”.  Ryan’s work addresses a previously noted peak in IGSCC growth rates observed in 316 stainless steel with respect to temperature, with the peak at approx. 320°C. The drop in CGR above 320°C has been accounted for by a mechanical response. However, in this study a similar trend was observed in 316 stainless steel oxidation growth kinetics, in the absence of applied stress. FIB cross-sections were prepared through the surface to physically measure the oxide penetration thickness. This method allowed the significant influence of metal orientation on growth rates to be observed.