Experimental study demonstrates how varying inclusion types alter fatigue crack initiation in 18Ni maraging steel, highlighting composition tuning for higher durability.
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 aluminate-inclusions 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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Karr et al. (2017) studied this question.
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