Wear of high strength steels and ductile iron and its effect on fatigue performance

Sammanfattning: 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.