The effect of apparent strain rate on primary water stress corrosion cracking (PWSCC) of alloy 600(UNS N06600) and 304 stainless steel(UNS S30400) was studied at 360°C at apparent strain rate ranging from 1×10-7 s-1 to 1×10-5 s-1 using the slow strain rate tensile technique (SSRT). In this study, it was revealed that the PWSCC fracture mode of nickel alloy is mainly intergranular, whereas the one for the austenitic stainless steel is transgranular. The total brittle fracture decreases with increasing strain rate; however, the fraction of transgranular mode in the brittle fracture increases with increasing strain rate, i.e., the fraction of intergranular fracture increases with decreasing strain rate for both materials. The mechanism for the fracture mode change is not clear yet, however, it might be possibly explained by a similar mechanism of PWSCC for both materials, because a trend of fracture mode change for the nickel alloy resembles that of austenitic stainless steel.
It is widely known that mill-annealed, nickel-base alloy 600 is susceptible to stress corrosion cracking in primary cooling water of pressurized water reactors (PWRs) since the studies conducted by Coriou et al. 1,2 . Whereas it has generally been believed that austenitic stainless steel does not have PWSCC susceptibility. However Totsuka et al.3reported that austenitic stainless steels have PWSCC susceptibility under cathodic potentials. Therefor, it is expected that the basic mechanism of PWSCC for austenitic stainless steel should be the same as that of alloy 600. More recently, Smialowska et al. 4,5 ,Andresen et al. 6,7 and Kaneshima et al. 8have also reported that 304 stainless steel has SCC susceptibility in deaerated and hydrogenated water. According to these studies, the PWSCC fracture mode of austenitic stainless steel is not always the same as that of alloy 600. The purpose of this study is to verify the influence of the apparent strain rate on PWSCC fracture mode of both alloy 600 and 304 stainless steel, and to discuss about common features and differences between alloy 600 and 304 stainless steel.
EXPERIMENTAL
Material The specimens used in these tests ware obtained from vacuum-melted material, which was hot-forged, hot-rolled and then thermal-treated. The chemical compositions, mill annealing temperature for alloy 600(UNS N06600), solution annealing temperature for 304 stainless steel(UNS S30400) and mechanical properties are shown in Table 1. SCC Test The stress corrosion cracking test was conducted by using the hump SSRT method9. The specimen configuration is shown in Figure1. The test was performed using an SSRT device, as shown in Figure 2. The water used for the test was simulated primary water typically used in PWRs (500 ppm B + 2 ppm Li )10. The test was conducted at 360°C hydrogen dissolved at 2.75ppm±10 %. The apparent strain rate in the SSRT test, assuming 20mm of the gauge length without hump, was varied from 1×10-7 s-1 to 1×10-5 s-1.
