Microstructural evolution of Super304H stainless steel during high-temperature creep rupture

Abstract

This study is aimed to explore the microstructural evolution of Super304H stainless steel during long-term creep. Creep-rupture specimens of this steel are analyzed via microhardness measurement, metallographic structure observation, X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results show that the matrix structure of the steel was stable during high-temperature stress-rupture test without grain growth. Twin boundaries in austenite gradually disappeared with the prolongation of time, which process was accelerated at higher temperatures. The steel hardness sharply increased in the initial stage of high-temperature stress-rupture test due to the second-phase precipitation, and then gradually dropped with time, due to the coarsening of precipitates. The Cu-rich phase and secondary Nb(C,N) with the size less than 50 nm, and M23C6 were the main precipitated phases in the high-temperature stress-rupture test. This allowed the steel under study to retain a high stress-rupture limit due to the precipitation-strengthening effect. The microstructure aging resulted in the formation of M23C6 chain at the grain boundary and coarsening of M23C6 and primary Nb(C,N) in grains. With the coarsening of precipitated phases, hardness and stress-rupture limit of Super304H stainless steel decreased.