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High cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of two 18Ni maraging steels with different cobalt and titanium content and similar static strength are investigated. Ultrasonic fatigue tests are performed with thin sheets with nitrided surfaces at load ratio R = 0.1. The specimens are mounted on a carrier and are forced to joint vibrations at approximately 20 kHz. The increase of Co content and the elimination of Ti improved the HCF and VHCF strength of 18Ni maraging steel. TiN inclusions if Ti is present and Al2O3 inclusions in the Ti free material with sizes (areaINC)1/2 smaller than 10 µm were preferential crack initiation locations. Considering inclusions as initial cracks, the minimum stress intensity range for VHCF failure is 1.2 MPam1/2 for TiN inclusions and 1.8 MPam1/2 for Al2O3 inclusions. Data scatter may be slightly reduced if lifetimes are presented versus stress amplitudes multiplied by (areaINC)1/12 rather than in an S–N diagram.

The very high cycle fatigue (VHCF) properties of four 18Ni maraging steels were investigated. Ultrasonic fatigue tests were performed on thin sheets with nitrided surfaces at load ratio R = 0.1. Traditional maraging steel containing Ti (material A) showed crack initiation at TiN-inclusions. The elimination of Ti and the increase in Co content (material B) lead to preferential crack initiation at aluminate-inclusions. Aluminate-inclusions are less damaging than TiN-inclusions, and material B shows higher VHCF strength than material A. A further developed maraging steel (material C) with reduced Co content that is compensated for by alloying with Al showed crack initiation at aluminate-as well as at Zr(N,C)-inclusions. Zr(N,C)-inclusions are more damaging than aluminateinclusions and less damaging than TiN-inclusions. The highest VHCF strength was found for a recently developed alloy with further increased Al content (material D). In addition to inclusion-initiated fracture, this material showed shear mode crack initiation in the nitrided zone at the surface.

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