Movable hood tube in LD convertor: Failure analysis and coating solution

Abstract

An assembly of movable and fixed hood tubes provide safe passage to LD gas produced during primary steel making process. These water-cooled tubes, made up of ASTM A106: Grade B steel are exposed to most extreme environment involving severe thermal cycling, corrosion from hot LD gas as well as cooling water, wear and erosion from lime and slag splashing, etc. Current work analyzed the failure of movable tubes that led to generation of multiple cracks on the outer surface of the tube facing the LD gas.Analysis revealed cracks parallel to each other in circumferential direction of the tubes. Most of the cracks originated from outer wall of the tube and propagated inwards, however, some of the cracks emanated from inner wall which was in contact with cooling water as well. Elemental analysis revealed the presence of calcium, potassium and sulphur in the oxide scale along the cracks. The nature of scale was confirmed using Energy Dispersive Spectroscopy and Raman Spectroscopy. Presence of these elements are known to reduce the thermal conductivity of the scale and thus can make the cooling inefficient and promote greater thermal stresses. A number of circumferential and longitudinal cracks were observed in tube-to-tube weld zone as well and these were related to microstructural abnormality therein. Overall failure analysis suggested that “thermo-mechanical corrosion fatigue” resulted in premature failure of the hood tubes. Based on detailed laboratory study and field trial, Inconel 625 coating using twin wire thermal spray method has been proposed as a viable techno-economic solution for improved reliability of these tubes.