Microstructure and Post-Irradiation Annealing Behavior of 20% Cold-Worked 316 Stainless Steel
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Microstructure and Post-Irradiation Annealing Behavior of 20% Cold-Worked 316 Stainless Steel J. I. Cole, T. R. Allen, H. Kusanagi1, K. Dohi1 and J. Ohta1 Argonne National Laboratory-West, PO Box 2528, Idaho Falls, ID 83403 1 Central Research Institute of Electric Power Industry, 2-11-1, Iwado Kita, Komae-shi, Tokyo 201-8511, Japan ABSTRACT Microstructural examination and in situ post-irradiation annealing studies were carried out on 20% cold-worked 316 stainless steel (SS) hexagonal duct material following irradiation in the reflector region of the EBR-II reactor. Stainless steel hexagonal ducts were used to house reactor subassemblies and provide a valuable source of information on irradiation behavior of reactor structural materials at lower dose-rates (on the order of 10-8 dpa/sec) than previously examined. The microstructural development of samples irradiated to doses of 1, 20 and 30 dpa is examined, while the post-irradiation annealing behavior of a sample irradiated to 20 dpa is described. Annealing studies were performed at 370 and 500°C to examine the kinetics of radiation damage recovery as a function of annealing temperature. The initial (pre-annealed) microstructures consists of a substantial density of irradiation induced chromium-rich M23C6 and M6C carbides which form both on the grain boundaries and within the grain interiors. Recovery of the coldwork is evident in the 1 dpa sample while samples irradiated to 20 and 30 dpa possess dense populations of voids and dislocation structures consisting of networks of line dislocations and faulted dislocation loops. Results indicate that post-irradiation annealing of the samples at 370°C for 1 hour has little effect on the microstructure, while further annealing at 500°C for 1 hour results in void shrinkage, the formation of small cavities, and a reduction in the dislocation loop and network density. INTRODUCTION With the shutdown of the EBR II fast reactor, a variety of irradiated structural hardware has become available for examination covering a range of dose, dose-rate and temperature regimes. These materials provide the opportunity to better understand the effects of long term irradiation on reactor structural materials, and specifically address swelling and microstructural evolution in materials irradiated at a lower flux, further from the core center, than previously studied. For the current study, samples were taken from a single 20% cold-worked hexagonal duct used to house a reflector subassembly located in row 8 of the EBR-II reactor. Because the flux varied axially as well as radially from the center, the hex duct was exposed to a range of dose and dose rates along its length. For the present study, samples were selected for microstructural examination which had received doses of 1, 20 and 30 dpa. EXPERIMENTAL The alloy composition for typical 316 SS hexagonal ducts is shown in table I, while a list of the irradiation parameters for the 3 samples is provided in table II. TEM discs were punched from the hexagonal duct flats and then prepared both as
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