Vortrag (20 Min., 5 Min. Diskussion, 5 Min. Raumwechsel)
The development of new techniques in manufacturing and mobility for the reduction of CO2-emission as well as the protection of the environment has led to a new material and component quality. Better surfaces result in more harmful effects of internal defects such as shrinkages holes, porosity and, especially, non-metallic inclusions. In combination with long term use of above 107 cycles, the very high cycle fatigue (VHCF) describes a regime, which is, especially for high-strength steels, dominated by the internal crack initiation resulting in the well-known fisheye fracture. In order to improve the fatigue life, surface-induced crack initiation should be inhibited. Thus, different surface treatments in combination with adjusted heat treatments can be used for the protection of the surface against crack initiation due to residual compressive stresses In the present paper a 42CrMo4 cast steel was investigated in the VHCF regime at four different material conditions: (i) classical quenching and tempering (QT) procedure, (ii) modified quenching and tempering procedure for enhanced material hardness, (iii) classical QT with subsequent plasma nitriding, and (iv) modified QT in combination with shot peening. The fatigue investigations were performed with an ultrasonic fatigue testing equipment at a frequency of about 19.5 kHz. The temperature was kept constant (± 5 K) by a pulse-pause-mode and continuous cooling with a spot cooling system. The resonant frequency fres and damage parameter βrel were analyzed in order to evaluate both the stress amplitude and the number of cycles to crack initiation. Facture surfaces were intensively studied by scanning electron microscopy in terms of location, type and chemical composition of the inclusions as well as features such as size of fisheye, non-metallic inclusion and fine granular area, respectively. The results show that the increased volume hardness leads to higher fatigue strength and enhanced lifetimes, as expected. Failure was caused by mainly internal crack initiation through a higher notch effect of the non-metallic inclusions. Through shot peening, the initiation and propagation of surface cracks was impeded only in combination with the modified quenching and tempering treatment resulting in a higher volume hardness. Most effective was the plasma nitriding resulting in a nitriding depth of 0.2 mm. Thus, a procedure was developed to study the largest non-metallic inclusion in a certain material´s volume.