Relation Between the Mechanical Behaviour of a High Strength Steel and the Microstructure in Gigacycle Fatigue

Many components can reach or exceed 109 cycles in their service time. When fatigue life is beyond 106, the Wohler S-N curve was always considered to be asymptotic in horizontal axis, but the fatigue behaviour over 106 cycles can not be neglected. It is not usual to carry out a fatigue test beyond 109 cycles due to the conventional fatigue test's constraints, time consuming and expensive. High strength steel is widely applied in automobile, railway industry after surface treatment in order to improve performance of material in practice. Carburizing process hardens surface to increase wear and fatigue resistance and shot peening has a beneficial effect on the material fatigue strength from the surface residual compressive stresses. A piezoelectric gigacycle fatigue machine is used to do the tests in gigacycle regime on specimens with different surface treatments. The effect of different surface treatments is investigated in gigacycle regime at a frequency of 20KHz with a fixed stress ratio R=0.1 at room temperature. Moreover, Scanning Electron Microscopy (S EM) observations of fracture surfaces are analyzed to evaluate the mechanism of damage related to surface treatments, microstructure scored inclusion size. The role of inclusions and microstructure is emphasized at 10(9) cycles.