Effects of High Damage Dose on Laser Welded, Irradiated AISI 304SS
The objective of this project is to evaluate the high irradiation dose evolution of laser weld repairs of AISI 304 stainless steel (SS). We will select regions from EBR-irradiated AISI 304SS "hex blocks" with LWR-representative He contents, irradiation doses, and irradiation temperature histories. These shall be the regions on which we perform welds and eventually conduct microstructure analysis. The selected regions should also enable us to determine scientific dependencies of weld success/performance on He content, dose, and temperature. We will make bead-on-plate, single-pass laser welds in hot cells, on the selected sections of the hex blocks. Weld parameters (e.g. traverse speed, power) will have already been optimized as much as possible, through prior work carried out by our Advisory Panel. Following completion of welding, the welds will be cross-sectioned and polished. A transmission electron microscopic (TEM) lamella will be fabricated by focused ion beam (FIB) lift-out, to characterize the as-welded condition. The weld cross-sections will then undergo an ion irradiation campaign on the cross-sectioned weldments at Texas A&M. Irradiations will be carried out with 5 MeV Fe2+ ions in a chamber capable of handling radioactive materials. Ion irradiation conditions that will be studied range 400-460°C, and doses of 50 and 100 displacements per atom (dpa). Note these doses are in addition to the previously neutron-irradiated dose. Characterization of the irradiated weldments involves mechanical and microstructural understanding. We will gain an understanding of the residual stresses in and around the weldments using a combination of x-ray diffraction (XRD), nanoindentation, and scanning electron microscopic (SEM) in situ compression pillar and microcantilever bend tests. We will conduct a comprehensive microstructure characterization of weldments before and after ion irradiations, using TEM techniques. Microstructure characterization shall include observation of dislocation loops, voids, and precipitates. This project has achieved four major accomplishments as summarized here: Accomplishment 1 – We have welded on three pieces of the hex blocks (5D2, 3A2, 3E3) spanning doses ~1-28 dpa and He contents ~0.1-8 appm He. Four slices (each ~3 mm x 3 mm x 250 micrometer thickness) have been made through the cross-section of all three weldments, totaling twelve slices. The slices have all been polished to metallographic standards, etched, then examined by light optical microscopy. Four “sets” of slices have been assembled, each containing one slice from each of the three weldments – the four “sets” are designated as one for as-welded observation, and three for ion irradiation. Two of these three ion irradiations have been carried out in this fiscal year: 50 dpa at 400°C and 50 dpa at 460°C. The ion irradiation conditions enable us to compare the effect of a temperature shift for ion irradiation emulation of neutron irradiations. Accomplishment 2 – The as-welded set of specimens has been completely characterized for a baseline microstructure using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Grain morphologies have been observed, and a dendritic grain structure is found in the heat affected zone (HAZ) of the weldments. Cavities and dislocation loops are observed and quantified in the base metal. These irradiation-induced microstructural features have relatively consistent sizes across all three weldments, but their number densities increase with irradiation dose. In the HAZ, cavities and grain boundary radiation-induced segregation (RIS) appear to be annealed due to the heat input from welding; loops are only somewhat annealed in the HAZ compared to base metal. Accomplishment 3 – Mechanical properties of the as-welded materials are determined by nanoindentation. Hardness and modulus of elasticity are isotropic with respect to crystallographic orientation, enabling us to consider the average of several nanoindentation measurements on random grain orientations to be representative of the mechanical properties of that material. Hardness and modulus are measured across the base metal, HAZ, and weld metal in all three weldments. The HAZ is notably softer than the base metal. Accomplishment 4 – We have completed TEM and scanning TEM (STEM) analysis to characterize the ion irradiated microstructure following both 50 dpa ion irradiations (400°C and 460°C). Dislocation loops and cavities are observed in the base metal and HAZ in all three weldments. We have also completed mechanical property characterization using nanoindentation across the weldments in all ion irradiated specimens. Relatively little ion irradiation induced hardening is observed in the base metal, consistent with the relatively unchanged dislocation loop morphology between the as-welded and ion irradiated base metals. More significant ion irradiation induced hardening is observed in the HAZ and weld metal, likely because they start out with a less damaged microstructure before ion irradiation.
- Research Organization:
- Boise State Univ., ID (United States)
- Sponsoring Organization:
- USDOE Office of Nuclear Energy (NE)
- Contributing Organization:
- Electric Power Research Institute (EPRI); L.A. Giannuzzi & Associates; Idaho National Laboratory (INL)
- DOE Contract Number:
- NE0008525
- OSTI ID:
- 1581113
- Report Number(s):
- DOE-BSU-11111
- Country of Publication:
- United States
- Language:
- English
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