Very high cycle fatigue of nitrided 18Ni maraging steel sheet

Thin sheets of nitrided 18Ni maraging steel are tested under cyclic tension (load ratio R = 0.1) in the very high cycle fatigue (VHCF) regime. The ultrasonic fatigue testing method with a cycling frequency of about 20 kHz has been further developed for these experiments. Sheet specimens with 0.35 mm thickness are mounted on a carrier specimen, they are pre-stressed and are forced to vibrate jointly. Between 107 and 109 cycles, fatigue cracks are initiated exclusively at internal TiN inclusions. The areas of the crack initiating inclusions projected perpendicular to the applied tensile stress are evaluated. The square root of inclusion areas, (areaINC)1/2 lies between 2.5 μm and 5.3 μm. Considering inclusions as cracks, their stress intensity range is between ΔKINC = 1.3 MPa m1/2 and 2.4 MPa m1/2. The sizes of crack initiating inclusions influence fatigue lifetimes. This is considered in a crack propagation model and by presenting lifetimes versus the stress amplitudes multiplied by (areaINC)1/12. A mean lifetime of 109 cycles is found at a stress amplitude of 22% of the tensile strength, which is comparable to other high strength steels tested under cyclic tension.