Cracking of an austenitic stainless steel lance pipe in a lime calcining plant

Kiln of a lime plant, a part of an integrated steel plant, converts limestone (CaCO3) into lime (CaO) by heating at a temperature of around 900 °C using coke oven gas as a fuel. Coke oven gas is a by-product of the coke plant, which contains mainly hydro-carbons. The coke oven gas is lanced into the kiln through a number of pipes attached to the kiln. The lance pipes are cracking prematurely within 6 months of their service against an expected service life of 3 years. The analysis of cracking of the lance pipe has been presented. The investigation includes visual observation, chemical analysis, characterization of microstructures using optical and scanning electron microscopes, EDS analysis, and measurement of hardness. The lances are found to be made of AISI 302 grade of austenitic stainless steel. Visual observation shows longitudinal cracks on the pipe wall associated with irregular yellowish surface having multiple pits. Analysis of the deposit on the pipe wall shows the presence of sulphur. Microstructural examination shows intergranular corrosion with thick porous layer of scale on the surface. Microstructural and EDS analysis indicate sulphidation and chromium carbide networks along the grain boundary as well as sigma phases within the grains. Sulphidation, sensitization due to grain boundary carbides and sigma phases led to intergranular corrosion causing cracking of the pipe wall.