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

Highlights: • Precipitation behavior in HP50M alloy tubes used for 20 years was investigated. • Si segregation at interdendritic boundaries plays a key role in formation of G-phase. • Kinetic simulation successfully predicts the MC to G-phase transition during aging. -- Abstract: HP50M heat-resistant cast austenitic steel tubes are exposed to high temperatures (i.e. 900 °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 °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 °C and the reverse transition to MC occurs.