Fatigue Fracture of Pie Channel Weld of Sinter Screen: A Metallurgical Insight

Sintering is an agglomeration process to convert iron ore fines into a larger mass suitable to be charged directly into the blast furnaces. A screen separates the incoming iron ore raw materials based on their size fraction. These screens are supported by pie channels. One of these channels failed during operation that led to unwarranted interruption in production. Owing to the criticality of the matter, detailed failure investigation was conducted. It involved visual observations, micro- and macro-fractography to determine the mode of fracture, chemical analysis and microstructure examination using optical and scanning electron microscopes aided with energy-dispersive x-ray spectroscopy and electron probe microanalyzer. The analysis revealed incompatible austenitic stainless-steel filler metal was used for welding mild steel material along with poor welding workmanship as suggested by underfilling and presence of thick layer of welding slag. There was formation of un-tempered martensite in dilution zone (the interface layer between heat-affected zone/fusion zone). The formation of undesirable martensite in the dilution zone is correlated with the time–temperature transformation diagram where the nose of the C-curve got significantly shifted toward the right due to enrichment with chromium and nickel as revealed by electron probe microanalysis. It was concluded that the failure occurred in fatigue mode due to deficiencies in welding and recommendations to prevent failures are also provided.