Properties of 20 % Cold-Worked 316 Stainless Steel Irradiated at Low Dose Rate

Abstract

Abstract:To assess the effects of long-term, low-dose-rate neutron exposure, tensile, hardness, and fracture properties were measured and microstructural characterization performed on irradiated 20% cold-worked Type 316 stainless steel. Samples were prepared from reactor core components retrieved from the EBR-II reactor following final shutdown. Sample locations were chosen to cover a dose range of 1-56 dpa at temperatures from 371-390°C and dose rates from 0.8-3.3 x10 -7 dpa/s. Irradiation caused hardening, with the ultimate tensile strength (UTS) reaching about 800 MPa near 20 dpa and appearing to saturate at higher doses. The yield strength (YS) follows approximately the same trend as the ultimate tensile strength. At higher dose, the difference between the UTS and YS decreases, suggesting the work-hardening capability of the material is decreasing with increasing dose. The hardness and yield strength increases occur roughly over the same range of dose. While the material retained respectable ductility at 20 dpa, the uniform and total elongation decreased to <1 and <3%, respectively, at 47 dpa. Fracture in the 30 dpa specimen is mainly ductile but with local regions of mixed-mode failure, consisting mainly of dimples and microvoids. The fracture surface of the higher-exposure 47 dpa specimen displays more brittle features. Changes in yield strength predicted from the microstructural components are roughly consistent with the measured changes in yield strength.

Integrated modeling of second phase precipitation in cold-worked 316 stainless steels under irradiation

“…Despite the high volume fraction of carbide in modeling, the bulk of experimental data show a steady state volume fraction in the order of 0.1%. The exception is the data of Allen et al [45], particularly the value at ~30 dpa, which shows a volume fraction of about 1.3% (all precipitate formation percentages are given as volume fractions in this work unless otherwise labeled). While we cannot presently be sure of the explanation of the differences in this data vs. other experiments, it is observed that the highest value is comparable to the values predicted by our model.…”

“…The phase fraction study ( Figure 7) shows that FCC_A1 (austenite), Sigma, Laves, and M 23 C 6 are stable phases at temperatures around 300 °C. However, reported experimental data on precipitates in 316 SS under irradiation in the LWR temperature range (~ 300 °C) do not show Sigma and Laves phases [9,11,12,[43][44][45]. This fact suggests that Sigma and Laves are kinetically inhibited phases in 316 SS under LWR conditions.…”

“…The mechanism of radiation enhanced dissolution may help us to understand the Allen et al [45] data points. The dose rate in Allen's experiment is 1×10 -7 dpa/s while the Renault's experiments were conducted at 5.3×10 -7 , and 8.7×10 -7 dpa/s.…”

“…By incorporating enhanced diffusion of M atoms and dislocation evolution (which alters heterogeneous nucleation sites) in our thermo-kinetic model, we are able to model the carbides precipitation under irradiation. Since we have better experimental data on carbides precipitation at temperature around 390 °C [11,12,43,45] we focus on this temperature range. We get CD input data for this condition form Sec.…”

“…In addition to the reported data in Figure 8, Tan et al [13] and Edwards et al [9] also reported very small nanoscale carbide precipitates under irradiation at BOR-60 at 320 °C, which further supports the result that there is very little carbide precipitation under irradiation. [11,12,45] (modeling dose rate = 8.7 × 10 -7 dpa/s). We note that the nature of precipitates in Renault (2009) were uncertain and could be carbide or G-phase.…”