Many components used for long term applications in the regime of very high cycle fatigue (VHCF) show failure even above the typical fatigue limit of high strength steel (2 x 106 cycles). Instead of typical surface induced crack initiation three-dimensional inhomogeneities like pores, non-metallic inclusions and shrinkage holes lead to crack initiation, even from internal positions. Especially non-metallic inclusions have a detrimental influence on the fatigue life depending on their size and position. However, larger non-metallic inclusions located in the volume of specimens are less detrimental than smaller inclusions located directly at the specimen surface. In order to study the influence of inclusions in the volume of specimens the specimen surface has to be modified.
The present paper focuses on the effect of plasma nitriding on the fatigue behavior. Thus, fatigue life and crack initiation were investigated on 42CrMo4 cast steel after plasma with different thickness of surface nitride layer at various conditions leading. Before the plasma nitriding, the cast steel was hot isostatically pressed in order to remove pores and shrinkage holes.
Fatigue tests up to 109 cycles were performed on quenched and tempered specimens using an ultrasonic fatigue testing machine. The fatigue tests were complemented by detection of in situ parameters like resonant frequency, non-linearity parameter and temperature evolution. The fracture surfaces of all specimen were studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Thereby the crack initiating non-metallic inclusions were characterized in terms of size, location and chemical composition.
The results of the present study show that plasma nitriding has a significantly influence on the fatigue lives. It leads to (i) an increase of fatigue strength, (ii) a prevention of surface induced failure and consequently (iii) exclusively internal crack initiation. The variation of the thickness of the nitride layer between 0.2 mm and 0.4 mm has a significant influence on the appearance of the fatigue fracture surface. Thus, the size of the fish-eye, the formation of the fatigue fracture area as well as the formation of the fine granular area (FGA) around the non-metallic inclusions are influenced.
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