Bulging of inner cover of a batch annealing furnace in a cold rolling mill

Bulging of an inner cover made up of AISI 309S austenitic stainless steel of a batch annealing furnace (BAF) in a cold rolling mill has been investigated. Inner cover envelopes the cold rolled (CR) coils to be annealed and shields the coils from direct heating by the burners on the heating hood surrounding the inner cover. The investigation of the bulged out inner cover consists of visual inspection, chemical analysis, characterization of microstructures by optical and scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and measurement of micro-hardness profile. Visual inspection reveals deposition of soot at the inner surface, microstructural examination and EDS analysis indicate the formation of carbide network and sigma phase in the matrix, and micro-hardness profile shows gradual increase in hardness values from the outer surface to the inner surface. Analyses of the results suggest that carburization attack of the hydrocarbon gas generated at the annealing temperature from the rolling oil adherent to the coils favors the formation of carbide network at the inner cover matrix. Formation of carbides associated with volume expansion and depletion of alloying elements from the matrix deteriorates the mechanical properties of the inner cover material making it prone to bulging.