Failure of the roller mill caused by rolling contact, shrinkage stress, and intentionally introduced defects

The article concerns a failure of a two-roller mill used for grinding pigments in a chemical plant. During operation, two large spalls detached from the surface of the shaft ring, leading to the mill being shut down and removed from the production process. The shaft ring is a forged component. The forging process eliminates internal defects, minimizes porosity, and ensures a continuous, directional metal grain flow. As a result, the forged part exhibits excellent directional, structural, impact, and fatigue strength. Initial visual inspection revealed that the localized damage to the cylindrical surface of the ring occurred directly adjacent to threaded holes drilled in one of the ring’s frontal faces. To determine the root cause of the failure, tensile tests, chemical composition analysis, and microstructural examinations of the damaged ring material were carried out. Additionally, fractographic analysis of the fracture surfaces was performed. The stress distribution in the ring, resulting from assembly on the shaft and subsequent mill operation, was determined using the finite element method. The results indicated that the stresses around the hole surfaces were concentrated to a critical level, ultimately leading to fatigue crack initiation and propagation under cyclic rolling contact between cooperating rings. The detrimental effect of machining operations on the mechanical properties of forged machine components is well known. Nevertheless, the failure described in this article demonstrates that this fact is sometimes overlooked, resulting in substantial operational and financial losses.