Microstructural evolution of Alloy 709 during aging

The creep-resistant austenitic stainless steel Alloy 709 (Fe-20Cr-25Ni (wt%) based steel) is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500–550 °C. However, the study of microstructural evolution of Alloy 709 during aging is lacking. In this study, thus, the microstructure of Alloy 709 has been investigated using electron microscopy in the as-received state and after static aging at 550, 650 and 750 °C. The results show that the prominent precipitate in the as-received Alloy 709 is Nb(CN), with the rod-like Z phase (CrNbN) observed very occasionally. After aging at 550 °C even up to 2000 h, no significant microstructure change was observed, which means that Alloy 709 is fairly stable at 550 °C. Aging at 650 °C produced globular M₂₃C₆ phase on grain boundaries, plate-like M₂₃C₆ carbides at twin boundaries and in the grain interior, and blocky M₂₃C₆ carbide nucleated on Nb(CN). Fine dispersoid Z phases were found on dislocations after aging at 650 °C for 500 h; their amount increases with aging time and temperature. (Cr,Mo)₃(Ni,Fe)₂SiN θ phase forms at grain boundaries after aging at 650 °C for 1000 h. After aging at 750 °C, θ phase nucleated on M₂₃C₆ carbide and a transformation of M₂₃C₆ to θ phase was found, which suggests that θ phase is the more stable. Aging at 550 °C promotes the segregation of Cr and Mo to grain boundaries whereas clear Cr depletion was observed at 650 °C due to the precipitation of Cr-rich M₂₃C₆ carbides at grain boundaries. Such depletion nearly disappears at 750 °C. The implications of aging for the subsequent mechanical behaviour of Alloy 709 are discussed briefly.