Microstructure Evaluation During Short Term Creep of Cr35Ni45Nb Cast Alloy Reformer Tube
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Microstructure Evaluation During Short Term Creep of Cr35Ni45Nb Cast Alloy Reformer Tube Young Wha Ma1 · Gimo Yang2 · Kee Bong Yoon3 · Thi Giang Le3,4 Received: 28 June 2020 / Accepted: 17 August 2020 © The Korean Institute of Metals and Materials 2020
Abstract In this work, a Cr35Ni45Nb alloy tube for use in a reformer furnace is subjected to high temperature (871 °C and 927 °C) creep regimes, and the microstructural changes (e.g. carbide precipitation and phase transformation) and consequent creep damage are investigated. The results of creep tests under closely similar applied stresses give different values of the Larson–Miller parameter (LMP) at different temperatures. As a result, the calculated creep rupture time at 871 °C based on the LMP value measured at 927 °C is approximately twice that which was actually measured at 871 °C. The microstructure of the as-cast tube is found to consist of an austenitic matrix with networks of MC (NbC) and eutectic (M23C6) carbides located both at grain boundaries and between dendrites. It is noted that the M 23C6 carbide is not a primary eutectic carbide that can be observed in the as-cast Cr35Ni45Nb alloys. It can be argued that the primary eutectic M7C3 carbides were transformed into M23C6 due to the heat of spiral welding during the tube manufacturing process. After creep, the NbC carbides at both locations were mostly transformed to the G-phase (Nb3Ni2Si) and all the precipitates formed inside the austenitic matrix were composed of the M 23C6 and G-phase. Creep cavities were initiated around the G-phase and grew cracks along the grain boundaries due to the formation of a Cr-depleted zone and the G-phase. Keywords Cr35Ni45Nb · Creep · Larson–Miller parameter · Phase transformation · Precipitation · Reformer tube
1 Introduction Reformer furnace tubes are critical components in the petrochemical industries and are exposed for long periods to severe conditions of high temperatures and pressures during service. Although these are generally designed for nominal lifetimes of around 100,000 h, failures often occur before this projected service lifetime has been reached [1–3]. Consequently, the actual service lifetime is found to vary from 30,000 to 180,000 h depending on the quality of the * Thi Giang Le [email protected] 1
Materials Technology Development Team, Doosan Heavy Industries and Construction, Changwon, Gyeongnam 51711, Republic of Korea
2
Subsidiary Research Institute, Metal Lab Inc., Daejon 34016, Republic of Korea
3
Department of Mechanical Engineering, Chung-Ang University, Seoul 06974, Republic of Korea
4
Faculty of Mechanical Engineering, Thuyloi University, 175 Tay Son, Dong Da, Hanoi, Vietnam
reformer tube materials and the service conditions [1, 2]. Previously, reformer tubes were manufactured using highchromium and high-nickel HK and HP type steels in order to withstand oxidation and creep deformation at operating temperatures above 650 °C [4–6]. However, as the operating temperature of reformer tubes increased to approximately
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