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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 influence of load ratio on the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of shot-peened VDSiCr spring steel is investigated. S-N curves are measured with ultrasonic fatigue testing equipment at load ratio R = -1, R = 0.1 and R = 0.5 up to 10 10 cycles. Failures either occur below 5 9 10 6 or above 5 9 10 8 cycles. In the HCF regime cracks start exclusively at the surface. In the VHCF regime crack initiation occurs solely in the interior, at grain boundaries, inclusions or (rarely) in the matrix. The S-N curves continue to decrease beyond 10 9 cycles, most pronounced for R = -1 where the mean cyclic strength at 10 10 cycles is 5 % lower than at 10 9 cycles. VHCF strength presented in a Haigh diagram can be very well approximated with a straight line. The mean stress sensitivity factor is M = 0.47 for load ratios between R = -1 and R = 0.5.

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