The influence of thermal ageing on the tensile and fatigue properties of advanced 316 steel for FBR structures

Results of tensile and fatigue tests at 550oC are reported for advanced 316 steel (316FR; FBR grade), which was thermally pre-aged at three temperatures (550oC, 600oC, and 650oC) and for periods up to 10000 h. These data show that the influence of thermal aging on the tensile and fatigue properties can be estimated based on the equivalent aging time at 550oC (teq) which is an Arrhenius-type function of temperature and time. Aging has no detrimental effect on the tensile strength. Long-term aging decreases the tensile ductility. The changes in fatigue life caused by aging are dependent on strain range. Aging improves the fatigue life at high strain range (Δ ε t''>='' 0.6%), but decreases the fatigue life at low strain range (Δ ε t< 0.6%). Microstructural changes caused by thermal aging were examined. Film-like precipitates observed at the grain boundary were composed of very fine particles which were identified as mainly Laves (Fe2Mo) phase and a small amount of M23C6 type carbides. The changes in fatigue strength caused by aging can be explained in terms of crack initiation and propagation properties related to the microstructural changes. (author)