G-phase formation in twenty-years aged heat-resistant cast austenitic steel reformer tube

Abstract

HP50M heat-resistant cast austenitic steel tubes are exposed to high temperatures (i.e. 900 degrees C) for long periods (10 years or longer) in petrochemical reforming plants. Twenty-years aged tubes exhibit microstructural changes such as primary carbide transformation and growth, MC carbide transition to G-phase, and fine secondary M23C6 precipitation. Analytical electron microscopy reveals that silicon tends to segregate at austenite inter-dendritic boundaries in fresh as-cast tubes. Thermo-kinetic simulation of long-term precipitate evolution of the steel is carried out together with Si segregation compositions. The experimentally observed general trend that G-phase forms at the expense of MC carbide is successfully predicted for the Si segregation compositions after a few thousand hours of aging in a temperature range between 700 and 900 degrees C. This study demonstrates that the Si segregation plays a crucial role in the formation of G-phase in this type of heat-resistant cast steels. Additional heat-treatment experiment confirms that G-phase becomes unstable above 900 degrees C and the reverse transition to MC occurs.