Mechanical characterization of alloy 709 stainless steel after high temperature aging for application in advanced reactors

Alloy 709, NF709, austenitic stainless steel, tensile testing, mechanical properties

Riso, Victoria M. ; Heuser, Brent J. ; Stubbins ; James F.

关键词: Alloy 709, NF709, austenitic stainless steel, tensile testing, mechanical properties;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101063/RISO-THESIS-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Sodium fast reactors are among the leading generation IV nuclear reactor designs being studied for future development. They will operate at higher temperatures and have higher neutron fluxes and thus require the use of novel materials. These materials will need to withstand an extreme environment, including corrosion from liquid sodium. Alloy 709 is a derivative of NF709, a high strength austenitic stainless steel originally developed for boiler tube applications. It is being developed by Oak Ridge National Laboratory for sodium fast reactor applications. The first heat of Alloy 709 was aged at 550, 650, and 750\textcelsius\ for 10, 100, 300, 1000, and 3000 hour durations. These temperatures were chosen due to their relevance to advanced nuclear reactor designs. Alloy 709 was then machined into tensile specimens with gauge lengths of 5.0 mm and tested using an Instron 1331 load frame. Tensile tests were conducted with a constant strain rate of 0.001 s\textsuperscript{-1} and the results were compared. It was found that overall, the total percent elongation decreases with increasing aging duration and increasing aging temperature.At 550\textcelsius\ there was little, if any, change in the tensile test results across the aging durations. At 650\textcelsius\ the total elongation only started decreasing after 100 hours of aging. At 750\textcelsius\ the total elongation immediately decreased until 300 hours of aging, after which it remained relatively constant. Ultimate tensile strength (UTS) measurements were found in the range of 725-670 MPa, total elongation in the range of 31-41\%, Young's modulus in the range of 142-190 GPa, toughness in the range of 197-280 MPa, and strain hardening exponent values in the range of 0.17-0.31 across all conditions. Scanning electron microscopy, transmission electron microscopy, and energy dispersive spectroscopy were utilized to examine precipitation behavior, how this affected the mechanical properties, and investigate the oxide growth behavior. Grain boundary precipitates were found to first occur in the 650\textcelsius\ sample aged for 100 hours and were not in the sample aged for 10 hours. Grain boundary precipitates were also found in the 10 hour sample aged at 750\textcelsius. Precipitation along grain boundaries was confirmed to continue in the remaining 750\textcelsius\ samples and is presumed to also continue in the 650\textcelsius\ samples aged beyond 100 hours. At 750\textcelsius\ the 300, 1000, and 3000 hour samples all contained matrix precipitation in addition to the grain boundary precipitates. When compared to the tensile test results, it was concluded that the grain boundary precipitates had a larger impact on the mechanical properties.