Damage Analysis of Catalyst Tube of a Reformer Furnace Used in Hydrogen Production

A heat-resistant HP-Nb steel casting tube of a reformer furnace used in hydrogen production was designed on the basis of 100,000-h service life. However, after about 400 h of operation at temperatures ranging from about 950 to 750 °C, a problem was encountered involving corrosion at the outer tube surface and formation of internal cracks. A section of the tube was analyzed to determine the cause of damage using various techniques including light microscopy, scanning electron microscopy combined with energy dispersive x-ray spectroscopy, as well as transmission electron microscopy and diffraction. It was concluded that the tube was subjected to two separate types of damage: (i) heating the outer tube surface by burning a low grade fuel contaminated with highly corrosive species such as chlorine, which precluded the material from developing and maintaining a surface protective oxide, and (ii) using a heat of the alloy with Si content on the higher side of the specifications stabilizing the detrimental Ni₃Nb₂Si Laves phase precipitation of which at grain boundaries could lead to intergranular cracking; however, the cracking could also result from localized plastic deformation in relatively "soft" zones of γ-phase (solid-solution) alongside grain boundaries.