Failure analysis of oscillating hammer mill used in the sugarcane industry

Abstract

Sugar-alcohol industries are affected by unexpected and costly interventions caused by loss of efficiency of key components, stoping the production for maintenance. The main station suffering severe degradation is the shredder, where rotating hammers are subjected to abrasive wear from mineral contaminants and corrosion from the sugarcane juice. This work presents a failure analysis of hammers removed from an on-site shredder of a sugar-alcohol industry in Brazil. First, the materials were chemically and microstructurally characterized. Second, the resistance against abrasive wear and corrosion in sugarcane juice was assessed, respectively, by dry sand/rubber wheel test, and cyclic potentiodynamic polarization. Finally, the results were discussed in conjunction to propose the cause of the failure of the rotating hammer. The base material was a carbon steel (∼0.3 wt% C), protected by a ferrous coating with high content of Cr (22.2 wt%) and C (4.4 wt%). Ferritic stainless steel (410 stainless steel) was included in the study due to its increased use in sugar-alcohol industries. The coating showed greater resistance to abrasive wear compared to the carbon steel and 410 stainless steel. The superior wear resistance of the coating was ascribed to the large fraction of hard carbides, which acted as effective reinforcement to prevent excessive removal of the soft matrix. 410 stainless steel performed better against the corrosive effects of sugarcane juice, due to the Cr being distributed in solid solution in a single ferritic structure, which ensured the formation of a protective passive film. Nevertheless, the coating was susceptible to corrode, since carbides in excess cause interfaces depleted in Cr, representing preferential points for corrosion. The cracks initially present in the coating allowed the penetration of the sugarcane juice, and subsequent detachment of the coating by impact, exposing the base material to severe abrasion and corrosion with large volume loss. Greater control during the coating preparation process and the use of base material with greater corrosion resistance such as 410 stainless steel could significantly extend the life of the hammers.