Mechanical strength and microstructural observations for remaining life assessment of service exposed 24Ni–24Cr–1.5Nb cast austenitic steel reformer tubes

Reformer furnace heater tubes made of cast austenitic alloy steels serve in the temperature range of 1073–1173 K and pressure around 3–4 MPa. Though the design life is about 100,000 h (∼11.4 years), premature failures within 3–8 years are more common, mainly caused by premature creep induced by overheating. The cast tube material undergoes microstructural changes with service exposure and as a consequence, degradation in strength. Microstructural changes, tensile strength and accelerated stress rupture strength of 96,000, 105,120 and 131,400 h service exposed tubes of 24%Ni24%Cr1.5%Nb alloy (equivalent to IN-519) served in a reformer furnace were analysed. Presence of coarsened carbides at the interdendritic boundaries, absence of significant amount of secondary carbides in the intra-dendritic regions together with sigma (σ) phase at both inter and intra-dendritic regions and isolated creep cavities indicated severity of service induced ageing and damage process accumulation. However, the stress rupture life obtained through Larson–Miller parametric method was good for further life extension. Attempts to correlate the microstructural changes to remaining creep life assessment (RLA) proved to be difficult as errors were mainly arising from the fact that temperature experienced by the individual tubes in a furnace differs and for a single tube through out its vertical length it varies. Non-destructive ultrasonic scan method employed too could not estimate the creep damage level accurately and the results vary with destructive test results. Lack of data on both microstructural changes and mechanical strength of the service exposed tube materials makes RLA more difficult.