Microstructure Evaluation During Short Term Creep of Cr35Ni45Nb Cast Alloy Reformer Tube

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 M23C6 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 M23C6 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.