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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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This study investigates the effect of laser volume energy density (VED) on the properties of AISI 420 stainless steel and TiN/AISI 420 composite manufactured by selective laser melting (SLM). The composite contained 1 wt.% TiN and the average diameters of AISI 420 and TiN powders were 45 µm and 1 µm, respectively. The powder for SLMing the TiN/AISI 420 composite was prepared using a novel two-stage mixing scheme. The morphology, mechanical, and corrosion properties of the specimens were analyzed, and their correlations with microstructures were investigated. The results showed that the surface roughness of both SLM samples decreases with increasing VED, while relative densities greater than 99% were achieved at VEDs higher than 160 J/mm3. The SLM AISI 420 specimen fabricated at a VED of 205 J/mm3 exhibited the highest density of 7.7 g/cm3, tensile strength (UTS) of 1270 MPa, and elongation of 3.86%. The SLM TiN/AISI 420 specimen at a VED of 285 J/mm3 had a density of 7.67 g/cm3, UTS of 1482 MPa, and elongation of 2.72%. The microstructure of the SLM TiN/AISI 420 composite displayed a ring-like micro-grain structure consisting of retained austenite on the grain boundary and martensite in the grain. The TiN particles strengthened the mechanical properties of the composite by accumulating along the grain boundary. The mean hardnesses of the SLM AISI 420 and TiN/AISI 420 specimens were 635 and 735 HV, respectively, which exceeded previously reported results. The SLM TiN/AISI 420 composite exhibited excellent corrosion resistance in both 3.5 wt.% NaCl and 6 wt.% FeCl3 solutions, with a resulting corrosion rate as low as 11 µm/year.