An Investigation on the Cracking of Air Tubes of Rotary Kilns in a Sponge Iron Plant
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CASE HISTORY—PEER-REVIEWED
An Investigation on the Cracking of Air Tubes of Rotary Kilns in a Sponge Iron Plant Goutam Mukhopadhyay • Sandip Bhattacharyya
Submitted: 30 June 2011 / Published online: 28 September 2011 ASM International 2011
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.
The sponge iron plant has a production capacity of 390,000 T/annum with two kilns of 300 T/day and one kiln of 500 T/day production capacities. Air is injected into the kiln through eight numbers of air tubes, which are mounted on the kiln shaft at alternate peripheral positions along the length. These air tubes provide the air for combustion of coal, which generates heat, and the main reducing agent CO gas, which reduces the iron ore to sponge iron. Swirl nozzles are fixed at the opening of the tube to direct the air into the kiln. Temperature of the air tubes vary from 850 to 1100 C depending on their respective position. During the operation of the rotary kiln furnace, transverse cracking of air tubes has been observed inside the kiln. Operation with the cracked air tube leads to false air ingress into the kiln, which causes localized heating of the charge mix or refractory and subsequent buildup of accretions. Cracking of the tubes requires either replacement with new tubes or ensuring no air inflow through the tube, otherwise conditions may deteriorate, which can lead to shutdown of the kiln. A small piece of sample as shown in Fig. 1 has been collected from a failed air tube for failure investigation. A detailed analysis of the sample has been carried out to find out the root cause of failure of the air tubes.
Keywords Air tube HK 40 steel Carbide network Sigma phase Intergranular corrosion Investigation Methods and Results Introduction The failure analysis of air tubes of rotary kilns in a coalbased sponge iron plant has been presented in this paper
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