Cracking of AISI 309S Stainless Steel Lance Tube Used for Coal Injection in Blast Furnace

Abstract

The cracking of coal injection lance tubes in a blast furnace has been analyzed in this paper. Most modern blast furnaces have facilities for injecting pulverized coal as an auxiliary fuel. The coal is injected in front of the tuyeres through lance tubes using nitrogen as a carrier gas, and the injected coal is burnt in contact with the hot blast (blast temperature ≈ 1225 °C) enriched with oxygen and humidity. The injection lances were failing prematurely within 2 months of installation. The material of the lance tube was AISI 309S grade austenitic stainless steel. Visual observation showed multiple branched cracks on the tube wall. Fractography of the crack surface under scanning electron microscope showed cleavages indicating brittle fracture. Microstructural examination revealed branched transgranular cracks initiating from the outer surface which was in contact with the hot blast. Energy dispersive spectroscopy on the cracks near the outer surface showed presence of corrosive elements like chlorine and sulfur in the corrosion product, which may have been introduced by the raw materials such as coal and coke used in the blast furnace. Measurement of stress indicated significant amount of internal stress within the tube. Sulfidation was also observed at the inner surface of lance tube. Analyses of the failure, particularly, multi-branched transgranular cracks of austenitic stainless steel suggested stress corrosion cracking induced by exposure to a chlorine-containing environment; hot blast conditions (temperature, humidity, and oxygen enrichment) aggravated the cracking. A suitable material (Inconel 600) was recommended for the application which increased the service life from 2 to 4 months. Microstructural characterization exhibited oxidation associated with voids at the grain boundary suggesting creep of the Inconel 600 lance tube after 4 months.