Evaluation of Material Deterioration based on Microstructural Evolution during Long-Term Creep of Austenitic Stainless Steel

Abstract

In order to assess material deterioration at long times, the microstructural evolution has been investigated for SUS316 austenitic stainless steel after creep rupture tests for up to 150,000 h at 550 to 750℃. A variety of second phases, such as M_<23>C_6 carbides, G phase, Laves phase, σ phase and χ phase, precipitate at grain boundaries and within grain during creep. The σ phase forms earlier as temperature increases and continues to grow in size over 100,000 h. The length of σ phase can exceed 50μm during long-term creep tests. The total amount of precipitates reaches an equilibrium by 2,000 h at high temperatures of 700 and 750℃, while it needs long times of 20,000 and 50,000 h at 650 and 600℃, respectively. The acceleration effect of stress on the precipitation during creep decreases with decreasing stress and increasing test duration. The distributions of precipitates are substantially the same between the specimen gauge portion under stress and the specimen head portion under no stress at long time conditions over 100,000 h. The amount of total precipitates or σ phase particles can provide a useful measure for the estimation of operating temperature and time of high-temperature components.