Creep behavior of Alloy 709 at 700 °C

Abstract

Alloy 709 is an austenitic stainless steel with potential for structural applications in advanced nuclear reactors. In this study, creep tests on Alloy 709 specimens were performed at a temperature of 700 °C under applied stresses in the range of 125–250 MPa. The alloy exhibited a brief primary, insignificant secondary and prolonged tertiary creep stages. A Norton stress exponent (n) of 5.9±0.7 was obtained from the double-logarithmic plot of minimum creep rate versus stress. Microstructural analyses were carried out for both as-received and crept specimens using optical microscopy, electron backscattered diffraction, and transmission electron microscopy. The microstructures of the as-received and crept specimens were compared. The mode of fracture was transgranular as revealed by scanning electron microscopy of the creep-ruptured specimens. Thermodynamic and precipitation modeling was performed in order to obtain information on the phase types and the kinetics of precipitation for the sigma−phase and the M23C6 carbide. Based on the creep data, microstructural observations and relevant modeling, creep deformation and failure mechanisms at the considered temperature are elucidated.