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

Part I of the summary/poster presentation session consisted of seven short poster introductions on Monday before the coffee break.

This session of brief (2 minute) summaries of posters started with a presentation by Elaine West (NNL, USA) entitled “Effect of grain size on the fatigue life of forged Alloy 600”. A fitting start for the meeting, this work dealt with identifying the cause for fatigue lives varying widely for a fixed strain amplitude. A historical archive of forged Alloy 600 fatigue life data was analyzed to evaluate the influence of: forging size/thickness, specimen orientation, test temperature, hardness and grain size. Grain size was, statistically, found to be the most influential. Following this finding, material characterization work was performed to determine the influence of material grain size on fatigue performance. Grain sizes of the Alloy 600 specimens in the archive ranged from <20 μm to >300 μm. It was noted that a larger grain size in Alloy 600 results in a shorter fatigue life, likely due to less homogeneity in deformation. Deformation in the coarse grained material is largely accommodated through the development of tangled dislocation clusters and dislocation sub-cell formation. Slip was more homogenous and linear in the fine grain material.

Xiaoqiang Liu (SNERDI, China) followed with a presentation of his poster entitled “Investigation on intergranular corrosion behavior of Alloy 690TT tubing materials”. This presentation addressed the question of finding the most suitable, of the many standard test methods applied to lower Cr alloys, for evaluating the susceptibility to IG attack of Alloy 690. The challenge is that Alloy 690 only sensitizes to a small degree (if at all). The authors noted that all the Alloy 690 TT tubes evaluated in their program have good IG attack resistance, as demonstrated by four evaluation methods. In terms of differences between the evaluation methods, it was noted that the sulfuric acid method  (ASTM 262 B) will induce some degree of uniform corrosion, making it insensitive to subtle grain boundary sensitization in this alloy. The nitric acid method (ASTM 262 C) is better but is not sufficiently discriminating; the optimum method was concluded to be nitric acid with 0.1% hydrofluoric.

Jiamei Wang (SJTU, China) presented a summary of his poster entitled “High temperature electrochemical corrosion behavior of Fe-Cr-Ni alloys in simulated PWR water”. The objective of the university research was to improve the understanding of electrochemical corrosion mechanisms of Fe-Cr-Ni alloys and its relationships with SCC by clarifying the electrochemical behavior of oxide films. The methodology and work scope was to obtain E-pH diagrams in 300ºC B/Li water with a pH range of 5 to 8 for 316 stainless steel and Alloy 690. Additional data and insights were gained via electrochemical impedance spectroscopy.

Juxing Bai (PSI, Switzerland) presented two related pieces of work. The first summary talk was dealing with “Electrochemical and spectroscopic characterization of oxide films formed on Alloy 182 in simulated BWR water: effects of dissolved hydrogen”. The surface oxide studies employed SEM, TEM, EDX, XPS, Raman spectroscopy, Mott-Schottky analysis and photoelectrochemical analysis, to determine oxide types as a function of dissolved hydrogen (below, on and above the Ni/NiO phase transition line). Oxides were formed in exposures at 274°C, lower than PWR focused studies and representative of BWR operating temperature. There is a clear transition in all oxide characteristics associated with the Ni/NiO transition.

Juxing’s following presentation dealt with the effect of grain boundary misorientation and local strains (determined by EBSD) on SCC initiation in an Alloy 182 fill, U-groove weld. It was found that SCC cracks typically initiated at favourably oriented high-angle grain boundaries, which separate grains with large misorientations. Local strain variations along crack propagation paths and at crack-tips were analyzed by EBSD allowing it to be concluded that a higher Schmid factor (mismatch) between adjacent grains was associated with the SCC propagation path as well as with initiation sites.

The two presentations summarized above, related to a full oral presentation made by Juxing Bai in the later weldments part II session “Stress corrosion cracking initiation and short crack growth behaviour in Alloy 182 weld metal under BWR/HWC conditions”.

Yusuke Sakai (TEPCO, Japan) presented a summary of a poster entitled “Effect of weld type on SCC growth rate of Alloy 82 in simulated BWR environment”. The underlying issue addressed by the work is the difference in yield strength between welds made of Alloy 82 compared with those made of Alloy 182. The weld metals also vary in yield strength based on weld process and heat input. Welds and deposits made with Alloy 182 using the submerged arc process have the lowest yield strength. The TEPCO work focused on how to make meaningful comparisons of SCC growth rates between high and low yield strength weld metals despite the restrictions (fracture mechanics) on sample size associated with low yield strength material (large specimens are required for validity). The ability to perform high K tests using smaller specimens than the validity criteria is the aim of the TEPCO project. See also a full presentation in the Weldments Part II session by Yohei Ono (Toshiba, Japan, “Crack growth properties of Ni-base alloy weld metal Alloy 82 in BWR normal water chemistry -2nd report-“).

Wen-Feng Lu (INER, Taiwan) summarized a poster entitled “Effect of dendrite axes on stress corrosion cracking of 308L/304L welds in a high-temperature water environment”. Crack growth rate testing was performed in oxygenated, high-purity water at 300°C (BWR normal water chemistry). The ferrite/austenite interface showed a higher SCC susceptibility than austenite/austenite interface in the tests performed. It was also observed that dendrite axis orientation affects the SCC growth rate. If the applied loading direction is nearly perpendicular to the dendrite axis, SCC growth rate increases.

Six short summary talks were given in part II of the summary/poster presentation session on Tuesday before the coffee break.

Radek Novotny (EC JRC, Netherlands) summarized a poster that focused on various pneumatic bellows driven loading devices to address EAC in nuclear power plants, including: 1) mitigating EAC through optimization of surface condition (EU MEACTOS project), 2) SCC initiation in 450°C supercritcal water (EU MEACTOS project), and 3) environmentally-assisted fatigue assessment (EU INCEFA+ project).

Fabio Scenini (University of Manchester, UK) introduced an experimental testing program on the effect of dissolved oxygen, hydrogen and hydrogen peroxide, and Zn on the oxide microstructure and chemistry. The outer layer was composed of hematite and the inner layer of Ni ferrite. Co was localized to the inner spinel layer. Zn injection produced a continuous Zn-enrichment in the inner oxide and reduced Co uptake. Zn injection also produced a thinner, but more protective oxide.

Mariia Zimina (CVR, Czech Republic) described an autoclave water loop system for simulation of primary circuit conditions up to 350°C and 16.6 MPa, the capability for mechanical testing of irradiated samples with activities of up to 300 TBq, and crack growth rate measurement capabilities for SCC assessments. These capabilities are enclosed in a 10-hot cell facility.

Taku Arai (CRIEPI, Japan) summarized a study involving microstructure analysis of thermally aged CF3M steels using APT, nanohardness indentation, fracture toughness (J-R) curve, and Charpy impact tests. Similar analyses were conducted on weld metal and on precipitation hardened martensitic stainless steel 17-4PH.

Takuyo Yamada (INSS, Japan) presented a study of the aging of precipitation hardened alloy 17-4 PH. Aging conditions included 320, 350 and 400°C for 3000 h followed by crack growth rate testing in PWR water at 290 and 320°C. Unaged H1100 and H1150 showed very slow SCC growth compared to unaged H900 and H1025 that showed significant SCC growth, in addition to H1100 and H1150 aged at 400°C for 3000 h.

Maria-Lynn Komar (Kinectrics, Canada) pointed out that several industry papers contain incorrect specifications for the chemistry of Alloy 800NG. The incorrect specification was N >0.03 wt.%, whereas the correct specification is N <0.03 wt.%.

There was also a poster presented without summary presentation:

Liberato Volpe (Manchester University, UK) presented a poster “In search of the true Ni/NiO transition”; the transition that underlies the susceptibility of Alloy 600 type materials to SCC in primary coolant. In the work, the Ni/NiO phase transition potential in a hydrogen-doped steam environment (oxygen added when potential was to be raised) was measured over a temperature range from 370 to 480°C using a solid state reference electrode. Preferential internal oxidation of Alloy 600 was also studied using STEM-EDX above and below the transition to understand SCC precursor events. The preferential internal oxidation to no oxidation transition was confirmed to coincide with the Ni/NiO transition. The intent of the work seems to be a preliminary step towards studying Alloy 690 under supercritical water conditions.