In Situ Failure Investigation of Weld Cracking in a Linz–Donawitz Vessel

Abstract

Linz–Donawitz (LD) vessel is an integral part of the blast furnace–basic oxygen furnace route of steelmaking. Failures of the LD vessel can result in safety issues as well as production loss. In the event of any such failure, the thrust should be to carry out the investigation at the minimum lead time and suggest for corrective actions to prevent any recurrence in the future. For this purpose, an on-site or in situ nondestructive method of investigation can be extremely useful. Present work is a case study of analysis of cracking of the shell of a vessel. Visual observation revealed that a large crack of around 6.5 m length developed at the bottom of the top cone connected to the vertical portion of the vessel shell by means of welding. Crack had propagated along the weld. The investigation consisted of chemical analysis using portable x-ray florescence spectroscopy, hardness testing across the weld and replica technique for microstructural analysis. These were aided by detailed analysis of refractory wall thickness measurements and thermography data for over a year. The study revealed that the failure occurred in thermo-mechanical fatigue mode from the weld joint due to use of inferior welding electrode. In contrast to the micro-alloyed base metal, no precipitation strengthening was present at the weld zone that resulted in the onset of softening mechanism therein. Recommendations are provided to prevent similar failures in the future.