Carburisation of Fe-Ni-Cr alloys at high temperatures

A large number of radiant tubes belonging to an ethylene furnace of a petrochemical plant failed during service. All tubes exhibited severe carburisation, while some of them lost their structural integrity and sagged. The tube material was based on a Fe-Ni-Cr alloy system with three varying compositions. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the microstructure and elemental composition of the tube material. Microhardness was tested to determine their mechanical strength. Experimental results indicated that the sagged tubes exhibited a higher degree of carburisation as compared to other tubes. The microstructure of these tubes also revealed coarser Cr-carbide precipitation and a continuous carbide lattice at austenite grain boundaries. It was concluded that exposure to excessive temperature during service was responsible for the degradation of all tube materials. Based on the above results, it is recommended that better control of furnace temperature should be employed in order to avoid overheating during service.