Comparative Study of Microcrack Initiation and Fatigue Failure in C75 Steel Quenched in Oil and Salt Bath

Mechanical properties and fatigue performance are highly dependent on the microstructure formed during heat treatment. C75 steel is widely used in mechanically loaded components subjected to cyclic stresses, where fatigue resistance plays a critical role in ensuring operational reliability and service life. In this context, the present study investigates the influence of bainitic and martensitic microstructures, obtained through salt-bath quenching and oil quenching, respectively, on the fatigue behaviour of C75 steel. To isolate the effect of microstructure, both heat treatment processes were designed to achieve an identical final hardness of 52 HRC, enabling a direct comparison of the resulting microstructures and their influence on the evolution of dynamic stiffness during cyclic loading. Salt-bath quenching resulted in a fine and homogeneous bainitic microstructure, whereas oil quenching produced a predominantly martensitic structure. The treated samples were characterised using hardness measurements, optical microscopy and scanning electron microscopy (SEM). To evaluate the influence of microstructure on fatigue resistance, Wöhler (S-N) curves were established using high-frequency four-point bending fatigue tests performed in accordance with ISO 22407:2007: Metallic Materials—Fatigue Testing—Axial Plane Bending Method. The progression of fatigue damage was assessed through frequency monitoring during cyclic loading, enabling the identification of the onset of frequency decrease and the subsequent evolution of dynamic stiffness during cyclic loading. The results revealed noticeable differences in fatigue behaviour, expressed as descriptive trends, despite similar hardness levels. The two microstructures exhibited different crack-initiation and propagation characteristics, while the overall fatigue performance remained comparable. Furthermore, the results demonstrated that fatigue life is governed not only by hardness but also by microstructural morphology and phase distribution. These findings highlight the critical role of the quenching medium in controlling fatigue damage mechanisms and provide valuable insights for optimising the fatigue performance of C75 steel components subjected to cyclic loading.