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

Title
G-phase formation in twenty-years aged heat-resistant cast austenitic steel reformer tube
Authors
심재혁Majid Abbasi박이호노윤조지영수Raghavan Ayer
Keywords
Reformer tube; Heat-resistant steel; G-phase; HP50M; Precipitate; MC carbide; M23C6 carbide; Scanning electron microscopy; Transmission electron microscopy
Issue Date
2019-02
Publisher
Materials characterization
Citation
VOL 148, NO 1-306
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.
URI
http://pubs.kist.re.kr/handle/201004/69151
ISSN
1044-5803
Appears in Collections:
KIST Publication > Article
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