Failure analysis of a steel slide ring of a tubular ball mill used in an iron ore mining plant

摘要

This work identified the root cause of an early failure occurring near welded joints that connect the slide ring with the head of a ball mill used in an iron mining plant. The ASTM-A516-G60 steel specified for use at this location had a lower carbon content than required. The microstructure presented by this steel consisted of a totally heterogeneous distribution of ferrite grains and pearlite colonies, with variations in grain size and volume fraction. Moreover, the union between the slide ring and the mill head created a significant geometric stress concentration and the welding process used, although meeting the specification, contributed to the microstructural heterogeneity, due to the thermal input required for this operation. These microstructural and geometrical characteristics led to the development and growth of fatigue cracks near the welded joints, that crossed the thickness of the slide ring. As a consequence, the iron ore pulp was contaminated with the lubricating oil, decreasing the adequate oil level and generating excessive wear on bearings and loss of pressure from the lubrication system. All these facts made impracticable the operation of the mill. Three different steels that meet the specification for application in pressure vessels were analyzed and compared with the steel of the failed mill by means of tension, Rockwell hardness, Charpy impact, fracture toughness (J-R curves) and fatigue tests (sigma(a) x N-f curves and da/dN x Delta K curves). It was concluded that the material selection for the mill component needs to consider a suitable chemical composition and microstructure of the steel for that application, to ensure adequate values of mechanical properties compatible with its use.