Crack propagation and spalling of white cast iron balls subjected to repeated impacts

Abstract

In ongoing research by the Bureau of Mines directed toward the conservation of strategic and critical materials, the mechanisms of spalling were investigated in high chromium white cast irons. These irons are used extensively in mining and mineral processing equipment even though fracturing and spalling from severe contact pressures result in major wear problems. In the laboratory, specimens in the form of balls, 75 mm in diameter, were subjected to repetitive impacts, up to 300 000, to produce cracking and spalling. Three high chromium white cast iron alloys, each with 10 different heat treatments, were investigated. The interiors of the impacted specimens were examined by scanning electron microscopy. The crack patterns were related to residual and hertzian impact stresses and compared with the work of others on ball-bearing spalling. It was found that near the surface the eutectic carbides crack perpendicular to the surface owing to a tensile stress produced along the perimeter of contact. Farther from the surface, the eutectic carbides crack owing to a cyclic hertzian shear stress that initiates a crack front parallel to the surface of the specimen. It is primarily the merging of the perpendicular and parallel crack fronts that initiates spalling in white cast irons.