Failure Analysis of Inconel 601 Radiant Tubes in Continuous Annealing Furnace of Hot Dip Galvanizing Line

Abstract

Metallurgical investigation was conducted to establish the genesis of crater-like defects and perforations in Inconel 601 radiant tubes used in Radiant Tube Heating/ Radiant Tube Soaking (RTH/RTS) section of the Continuous Annealing Furnace in Hot Dip Galvanizing Line (HDGL) of an integrated steel plant. The radiant tubes, several of them, had undergone catastrophic damage after only about 70 days of service under mixed gas firing inside the furnace. Visual examination, chemical analysis and secondary electron imaging (SEI) coupled with energy-dispersive x-ray spectrometry (EDS) were concomitantly utilized for the investigation of failed radiant tube samples. Based on the findings, the unprecedented failures were attributed to the high temperature sulphidation attack experienced by the tubes on their inner walls, owing to sulphur dioxide (SO2) resulting from combustion of hydrogen sulphide (H2S) in the mixed gas (2H2S + 3O2 → 2H2O + 2SO2). The investigation also pointed toward the apparent formation of low-melting nickel-nickel sulphide (Ni-Ni3S2) eutectic because of sulphidation, which led to unique spherical manifestations of the defects and served to highlight the deleterious influence of sulphur-bearing compounds on nickel-base alloys at high temperatures. Suitable alternative materials of construction are proposed for the radiant tubes operating under such environments.