THE EFFECTS OF METALLURGICAL FACTORS ON STRESS CORROSION CRACK GROWTH RATE IN SIMULATED PWR PRIMARY WATER FOR COLD WORKED TT ALLOY 690

摘要

It is well known that the stress corrosion crack growth rate (SCCGR) in simulated PWR primary water for cold worked TT Alloy 690 shows significant scatter from heat-to-heat. Authors have pointed out that the metallurgical factors due to the fabrication process are very important for understanding the heat-to-heat SCCGR differences in simulated PWR primary water. In this paper, the SCCGRs were measured for various laboratory heats and commercial heats for control rod drive mechanism (CRDM) tube materials. The cracks and voids at grain boundary carbides were carefully observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to estimate the SCCGR of various hot finished thermally treated (TT) Alloy 690 materials that were subsequently cold rolled based on their metallurgical characteristics. Furthermore, the behavior of primary M_(23)C_6 carbides in simulated PWR primary water was studied to clarify the mechanism of PWSCC for hot finished and highly cold worked TT Alloy 690. From this study, it became clear that the PWSCCGR depends not only on cold work and grain size but also on the number of micro cracks and voids in or near primary carbides, etc. In addition, the cracked large primary M_(23)C_6 carbides should be dissolved by simulated PWR primary water, and M_(23)C_6 might be changed to oxides such as (Cr, Ni)Fe_2O_4. The SCCGR for highly cold worked after hot worked TT Alloy 690 including primary carbides may be affected by this dissolving and changing process.