Abstract
Pressure rollers are used for gripping pellet cars as they travel through the track. Of 776 pressure rollers, four were found broken after three years of service in a pellet plant of an integrated steel plant. Fatigue cracks had initiated from heavy dent marks at outer surface in all the failed roller. Microstructural analysis revealed dents with some foreign particles embedded on them. EDS analysis of particles showed the presence of iron, calcium and oxide and sulfide elements, indicating presence of FeO with CaS, which are components of sinter particles. No grain deformation was observed around the dents as these were created by sinter particles which are softer than rollers (i.e., a hardness of ~ 450–600 HV against 770 HV for the rollers). After creating these “soft” dents, the sinter particles got embedded in them. Subsequent crevice corrosion caused pits to form, a result of the difference in oxygen potential between the sinter particles and the roller. Sinter fines were probably deposited on the sprocket teeth at either the discharge or the feed ends. The surface roughness (Ra) of used and failed rollers were measured, post which it has been recommended that rollers that develop a surface roughness, Ra higher than 4.5 should be replaced to avoid a failure. This maintenance practice has been implemented in plant with an elimination of such failures.
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