Failure of a Secondary Superheater Tube in a 140-MW Thermal Power Plant

This investigation was conducted to determine the cause of failure of a welded joint of a secondary superheater tube from a 140-MW thermal power plant. Chemical analyses along with detailed optical microscopic examination of a secondary superheater tube were carried out to predict the probable cause of failure. Microstructure of the secondary superheater tube welded between austenitic stainless steel to low alloy steel revealed presence of a thin layer of metal carbide along the weld interface which eventually led to intergranular cracking at austenite grain boundaries. It was concluded that the formation of brittle carbide layer was due to migration of carbon at elevated temperature led to failure of the tube.