Mechanismen der Rissentstehung und -ausbreitung im Vergütungsstahl 50CrMo4 bei sehr hohen Lastspielzahlen

This study deals with the fatigue mechanisms of a 50CrMo4 low-alloy tempered martensitic steel at very high load cycles. Low-alloy tempered steels are widely used in general mechanical engineering and in vehicle construction. Among other things, this material group stands out due to its moderate procurement costs and the possibility of individually adapting its strength and toughness to its intended application. For safe component design, knowledge of fatigue mechanisms at high-load cycles is of crucial importance. In this study fatigue tests were carried out, taking into account the influence of material strength and load cycle frequency. Experimental in situ methods, such as thermography or scanning electron microscopy, allow early detection of fatigue damage. These methods have shown that damage mechanisms change significantly, depending on material strength. Lower material strength showed a pronounced localization of damage in chromium-depleted areas. Fatigue damage occurred in this case in the form of local plastic deformation, which subsequently leads to crack initiation within the martensitic microstructure and then to fractures. Through systematic investigations, microcracks were detected in the test samples. Additionally, the material showed the pronounced influence of load-cycle frequency. In fatigue tests, higher strength materials showed a mechanism change in internal crack initiation at non-metallic inclusions. No influence of the test frequency could be observed in materials with these properties. The results of the investigations show that damage mechanisms in a material are highly dependent on the heat treatment process, the distribution of the alloying elements, the test frequency and the critical sample volume