Investigation of Premature Failure of a Coal-Fired Boiler Superheater Tube

Abstract

A metallurgical investigation has been carried out to ascertain the cause of premature failure of boiler superheater tube. The tube material was SA213-T22-grade steel. Visual observation of fractured region of failed component revealed thick-lip fracture, parallel cracks along the longitudinal direction, bulging, and non-uniform wall thickness. Microstructural investigation showed complete spheroidization of second phase, formation of thick scale to the tune of ~ 1 mm, the presence of creep voids, and significant decrease in hardness of the tube material. These signatures are indicative of overheating and creep of the alloy during service exploitation. Fracture surface predominantly contained the signature of intergranular failure. The microstructure near-fracture region exhibited a large number of creep voids and their coalescence along grain boundaries. Under the hoop’s stress, the coalescence of these creep voids triggered intergranular fracture leading to rupture of the tube. Operating temperature was estimated from steam side oxide-scale thickness using kinetic model equations. It indicated that for the formation of ~ 1-mm scale over a service span of 9 months required operating temperature of ~ 677 °C. However, the material is designed to withstand a maximum temperature of 540 °C. It was thereby concluded that overheating during the operation led to accelerated creep and subsequent failure within a short period.