SUMMARY OF THE SESSIONS ON AUSTENITIC ALLOYS: Ni-BASE – Chaired by Peter Andresen and Hans-Peter Seifert

The first part of the Ni-base session was comprised of three talks, listed below. The talks covered a range of topics related to SCC of wrought vs. HIP Alloy 625, intergranular oxidation in PWR water, and the effect of Zn injection on Alloy 690 oxide films.

Charlotte Smith (Jocobs, UK), “Primary water stress corrosion crack growth rate testing of wrought and hot isostatically pressed Alloy 625 in a pressurised water reactor environment”. This talk evaluated the SCC behavior of four heats of HIP Alloy 625 and a wrought heat of Alloy 625. The microstructures of all of the as-received heats of HIP Alloy 625 are moderately inhomogeneous, with variability in grain size regions of prior (smaller and larger) Alloy 625. Al-rich (Alumina) and Mo/Nb-rich particles present. The SCC response was evaluated on materials with ~15% cold-work, with the HIP material exhibiting growth rates about 3-5 X higher than the wrought material. Overall, the growth rates are similar to the MRP-55 and MRP-420 curves for annealed Alloy 600.

Yoann Vidalenc (Framatome, France), “Intergranular oxidation of Alloys 600 & 182 in PWR primary water: Effect of cold-worked and industrial surface finishes”. This presentation addressed preferential intergranular oxidation (PIO) of Alloy 600 and Alloy 182 in PWR primary water. Alloy 600 specimens were cut from CRDM tubes, and Alloy 182 specimens were cut from bead-on-plate welds. 1500 h exposures were done in 325 °C primary water with 44 cc/kg H₂, with some specimens at 80% of the yield strength and some unloaded. The depth of penetration was evaluated by FIB sections and EBSD on polished sections. Embrittlement was evaluated using in-situ tensile tests with digital image correlation in an SEM. The as-received microstructures are less affected by PIO than bulk or surface cold-work material. 20% cold-work reduced the strain threshold and proportion of opened grain boundaries, but GB embrittlement was consistently deeper. Surface grinding produced an ultra-fine-grained structure with higher degree of PIO than as-received or 20% cold-work, suggesting an important change in the kinetics of oxidation between 500 and 1500 h. After recrystallisation or recovery, PIO is less pronounced.

Aidan Gunn (University of Bristol, UK), “Effect of Zn injection upon oxide passivation of Alloy 690 under simulated PWR conditions”. This talk addressed initial plans and studies on the effect of Zn and H₂ injection on the oxide formed on Alloy 690 and its adherence after exposure to high-temperature water. Characterisation of the oxide samples will use optical and scanning electron microscopy of FIB-SEM cross-sections.

Two further Ni-base talks were presented during the online session on Tuesday.

Yonghao Lu (University of Science and Technology Beijing, China), “Effect of surface scratch depth on high cycle fatigue behavior of Alloy 690TT steam generator tube”. The high-cycle fatigue behavior (due to flow-induced vibrations) of Alloy 690TT steam generator tubes with non-standard boat-shaped tube samples was evaluated at a load ratio of 0.1 at 30 Hz with a maximum stress in the range of 340 to 550 MPa at RT and supported by finite element modeling. For comparison reasons, bar specimens with the same microstructure, but slightly different strength level were also tested. In the boat samples, crack initiation started from the inner tube surface, whereas in the bar specimens, cracks initiated on the sub-surface. Different surface scratches with various scratch depths were introduced to the boat sample. The fatigue limit decreased with increasing scratch depths of 30 and 60 mm from 345 to 325 and 315 MPa, respectively. The effect of scratches increased with increasing stress amplitude. The crack initiation life was not only related to the stress concentration, but also to the cold-worked surface microstructure at scratch root. The notch sensitivity of the material (when considering the stress concentration) was small.

Eric Focht (US NRC, USA), “Overview and update on US NRC PWSCC initiation research”. In this talk, an overview on the EPRI-NRC cooperative research project “PWSCC crack initiation characterization of Alloys 600/182 and Alloys 690/52/152” was given. Result highlights from this project were presented by M. Toloczko in the Weldments Part I session. The goals of this program are to obtain PWSCC initiation data for Alloy 182 to support the xLPR Code development project on probabilistic leak before break (LBB) analysis for PWSCC, to better understand uncertainties and accuracy of PWSCC initiation models and to obtain PWSCC initiation data for Alloy 690/52/152 to develop inspection requirements (or releases by factor of improvements (FOI)) for components made from these alloys. The effect of stress, cold-work/yield stress and temperature or dissolved hydrogen content are under evaluation. In the last part of the project, also the effect of weld defects, dilution zones or HAZ or surface conditions shall be investigated. The current preliminary FOI based on mean initiations time of Alloy 690 and Alloy 152/52 with respect to Alloy 600 and 182 were 98 and 64 X, respectively.