An Investigation on the Cracking of Air Tubes of Rotary Kilns in a Sponge Iron Plant

Abstract

Cracking of an air tube used for supplying air in rotary kiln of a sponge iron plant is investigated in this paper. The material of the air tube is ASTM A297 HK 40, a member of the heat-resistant cast alloy family (H series) steels widely used for enhanced high-temperature properties. Microstructural degradation occurring at high temperature affects mechanical and corrosion-resistance properties of the component. The failure analysis consists of visual observation, chemical analysis, examination of microstructures using optical and scanning electron microscopes, and characterization with energy-dispersive spectroscopy (EDS). Microstructural characterization reveals (i) intergranular corrosion at the cross section of tube wall, (ii) formation of severe chromium carbide network at the grain boundary, (iii) formation of sigma phases along the grain boundary as well as within the matrix, and (iv) generation of crack at the grain boundary filled with oxides. Analyses of these results suggest that formation of carbide network and sigma phases at the elevated service temperature depletes the alloying elements from the surrounding matrix, enhancing intergranular corrosion, which leads to cracking of the air tube.