Radiation-Induced Segregation in 316 and 304 Stainless Steels Irradiated at Low Dose Rate
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MRS 2000 Fall Meeting Symposium R: Microstructural Processes in Irradiated Materials
RADIATION-INDUCED SEGREGATION IN 316 AND 304 STAINLESS STEELS IRRADIATED AT LOW DOSE RATE T. R. Allen and J. I. Cole, Argonne National Laboratory-West J. Ohta, K. Dohi, H. Kusanagi, Central Research Institute of Electric Power Industry E. A. Kenik, Oak Ridge National Laboratory ABSTRACT As part of the shutdown of the EBR-II reactor, structural materials were retrieved to analyze the effects of long-term irradiation on mechanical properties and microstructure. In this work, the effect of low dose rate irradiation (10-7 to 10-8 dpa/s) on grain boundary composition in 316 and 304 stainless steels was analyzed. Samples were taken from surveillance specimens and subassemblies irradiated in the reflector region of EBR-II at temperatures from 371-390°C to maximum doses of 30 dpa. The effects of dose, dose rate, and bulk composition on radiationinduced segregation are analyzed. In 316 stainless steel, changes in grain boundary chromium and nickel concentrations occur faster than changes in iron and molybdenum concentrations. In 304 stainless steel, decreasing the dose rate increases the amount of grain boundary segregation. For a dose of 20 dpa, chromium depletion and nickel enrichment are greater in 304 stainless steel than in 316 stainless steel, the difference most likely due to dose rate. In both 304 and 316 stainless steels, the presence of a grain boundary precipitate significantly changes the composition of the adjacent grain boundary. INTRODUCTION Radiation-induced grain boundary segregation (RIS) has been implicated as a contributor to failures cause by irradiation assisted stress corrosion cracking [1]. The majority of the available RIS measurements in austenitic stainless steels come from samples irradiated with high-energy protons [2] or Ni ions [3]. Fewer measurements of RIS in austenitic stainless steels are available from materials irradiated with neutrons. This study reports RIS measurements from 304 and 316 stainless steels irradiated in the EBR-II reactor at temperatures below 400°C. The effect of dose, dose rate, bulk composition, and the presence of a precipitate on segregation were investigated. Detailed RIS measurements at sinks in neutron-irradiated materials are rare and this paper represents one of the few published efforts to examine the dose dependence of RIS in materials irradiated with neutrons at temperatures below 400°C. EXPERIMENT Samples of 316 stainless steel were taken from a reflector subassembly irradiated in row 8 of EBR-II. Specimens were taken at various axial locations in the reflector subassembly so that irradiation conditions of 1, 20, and 30 dpa were analyzed. As all of the samples were taken R2.4.1
from the same subassembly, each had a different dose rate and temperature. The temperature differences were small (
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