SCC growth behavior of SUS630 in high-temperature water. 2nd report. The effects of thermal aging, temperature and applied stress on SCC

SUS630 (17-4 PH stainless steel) is martensitic precipitation hardened stainless steel, and its properties of high-strength and high-hardness can be controlled by the precipitation-hardening heat treatment conditions. SUS630 has been widely used for valve stems in light water reactors, because the steel had high strength and good corrosion resistance. It is known from SCC failures experience of SUS630 in field, that higher hardness materials, exposure to high-temperature and long-term use affect SCC behavior of the materials. And age hardening of materials is known to occur for long-term thermal-aging at intermediate temperature, but the influence of thermal-aging on SCC is not clear. According to the author's previous report, the SCC growth rate of SUS630 (H1100 and H1150) was increased with aging at 400degC for 3,000h. In this study, SCC growth rate measurements were performed using lower and longer aging materials which were aged at 350degC for 10,000h to evaluate the influence of thermal aging on SCC for more details. In additions, the effects of test temperature and applied stress on SCC of the materials were investigated. In unaged H1100 and H1150 (better toughness and lower hardness materials), even at a high applied stress condition (K=30 MPa m^1/2), no SCC growth was observed in simulated PWR primary water at 290degC, which contained 500 ppm B, 2 ppm Li, and dissolved hydrogen (30cc STP H₂/kg-H₂O). On the other hand, after thermal aging at 350 and 400degC for up to 10,000h of H1100 and H1150, the SCC growth rate and the hardness increased with aging. (author)