In the last decades, significant improvements regarding the design, materials and technology of rock drills have been made. Likewise, in sheet metal forming, forming tools experience very high contact pressures when processing high strength steel sheets. In both applications components operate under extremely tough contact conditions that result in an accelerated component failure. Enhancements on mechanical properties of components material subjected to extreme contact conditions are highly required in order to withstand the application loads and prevent severe wear.

The present thesis was focused on understanding of machinery component damage mechanisms under severe contact conditions. A case study of worn components used in rock drilling and sheet metal cold work was carried out. Thread joints from rock drilling and punches from sheet metal pressing were selected for the investigation. For these components, sliding contact under high contact pressure is a common load condition under the components usage. Then, to understand and quantify the influence of contact parameters, load and surface quality on material performance, laboratory simulations were performed. The results were used for a comparative analysis of the typical damage mechanisms observed in the tests and the case study of the components.

The case study results showed that the threaded surfaces underwent severe plastic deformation due to the high-pressure sliding contact. The microstructure beneath the worn surface was altered and surface cracks and delamination were frequently observed at the worn surface. The dominant damage mechanism found on the investigated punches was adhesive wear. Material transfer adds friction stresses at the punch surface and ultimately, with repeated punch strokes, it leads to initiation and propagation of fatigue cracks.

Wear process in thread joint and punch wear was simulated using the SOFS. The worn specimens tested experimentally showed similar wear mechanisms obtained in the case study. The thread joint wear simulation showed that the total damage at the worn surface was a result of adhesive wear, plastic deformation, and initiation and propagation of fatigue cracks. In addition, the results showed that the type of motion had a significant influence on the worn volume and crack initiation, and more severe wear was observed at reciprocal motion. The punch wear simulation showed that the friction quickly increased as work material from metal sheets transferred to the disc surface. The rate of the material transfer was strongly dependent on the combination of sheet material and tool steel. Further, the present experimental simulations were applicable to characterize and predict wear of components in the application.

Components used in rock drilling and sheet metal forming operate under harsh contact conditions that result in an early-life component failure. Wear and fatigue are considered as the most common damage mechanism for these components. Commonly, the service life of a component is designed based on its fatigue life. However, wear might have a significant effect on the components life too. Wear results in a surface damage that in turn may cause a fatigue crack initiation. Therefore, knowledge about wear of materials and components is a key factor in design and prediction of the lifetime of the components. In order to predict wear of a certain component, a thorough understanding of the component with regards to its material properties, application loads and working environment, and damage mechanisms is required. The overall aim of the present work was to define the typical wear mechanisms occurred on machinery components used in rock drilling and sheet metal forming. A comparative analysis of the case studies and results from performed laboratory tests simulated wear mechanisms in the applications highlighted wear mechanisms and factors influencing severity of wear in the applications. Obtained information is crucial for ranking and selection of the best material in the applications.

Machinery components used in demanding applications are required to transmit or carry mechanical loads under severe loading conditions. They are subjected to cyclic loading and repeated sliding contact that in many cases result in a premature failure of components. Cyclic loading cause mechanical fatigue failure, while repeated sliding contact cause wear damage at the surface of components that can initiate crack nucleation and propagation under cyclic load. Wear and fatigue are the most common failure modes occurring in machinery components, and a synergetic effect of these two mechanisms accelerates component failure and reduces its service life. Understanding failure mechanisms and understanding the synergetic effect of wear and fatigue in relation to the components are therefore of high importance. In the present study, a detailed failure analysis was conducted on rock drilling components used under severe working conditions. Rock drill thread joints failed in the field application and cold-work punches working against advanced high-strength steels were investigated. Repeated laboratory sliding wear tests under high contact stresses have been performed on a number of high-strength metal alloys frequently used in demanding applications. A slider on flat surface SOFS tribo-tester and a three-point bending fatigue tester were used to simulate the wear and fatigue found in demanding applications. In particular, the influence of wear on the fatigue life of a high-strength steel was investigated. Surface analysis techniques were employed using instruments as 3D profile optical interferometer, scanning electron microscope, scanning transmission electron microscope, light optical microscope and X-ray diffractometer, to investigate the wear damage on the worn specimens, and to study fracture mechanisms of the failed specimens. The study describes the dominant failure modes of the present components when subjected to severe loading conditions. Further, the results explained dominant wear mechanisms encountered under high-pressure sliding contact. In addition, it described the influence of wear damage on fatigue life when a high-strength steel was exposed to cyclic stresses.

Machinery components used in demanding applications where severe contact conditions results in premature component failure. Wear and fatigue are considered as the most common failure mechanisms for such components. In general, the service life of a component is estimated based on its fatigue strength. However, wear might also have a significant effect on the component’s life too. Sliding wear results in surface damages that can be critical for metal alloys when subjected to cyclic stresses. Therefore, knowledge about sliding wear of metal alloys is a key factor in component designing. In addition, understanding the influence of sliding wear on fatigue life of metals will help to predict the component’s service life. 

The present study was focused on wear mechanisms of high-strength steels and ductile cast irons occurred under repeated sliding contact. Further, this study investigated the influence of wear on fatigue performance of a high-strength steel when subjected to cyclic bending.