PSI - Issue 28

Stepan Major et al. / Procedia Structural Integrity 28 (2020) 561–576 Stepan Major/ Structural Integrity Procedia 00 (2019) 000–000

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significantly automated (just write a script), the interpretation of sense of β angle is much more complicated. This angle can be used to map crack propagation.

Fig. 3. Meaning of measured angles: (a) deviation angel α between normal vector of elementary flat and sample axis; (b) deviation angel β between normal vector of elementary flat and primary crack growth direction.

3. Experimental procedure 3.1. Specimen material and surface treating

Steel specimens used for fatigue experiments in this article were made from steel EN 37CrA1Mo 6. Chemical composition of this material is following: C 0.357 %; Mn 0.468 %; Cr 1.49 %; Mo 0.194 %; V 0.01 %; Cu 0.072 %; Al 1.40 %; W 0.032 %; Si 0.292 %; P 0.006 %; S 0.005 % and Fe as remainder. Mechanical properties are: yield strength σ y = 840 MPa, ultimate tensile strength σ U = 954 MPa. When compared with commonly treated nitriding steels, the investigated steel has a slightly higher percentage of carbon. It offers higher values of strength but simultaneously introduces higher differences in microstructure resulting from variations in nitriding procedures. In this investigation two types of samples were used: virgin specimens and nitride specimens. Plasma nitriding, also called ion nitriding, is wide-spread surface hardening process utilized to improve the mechanical properties. Technology of plasma nitriding has been described in earlier reports such as Slámečka et al. (2010). This technology is characterized by adsorption of nitrogen atoms on the surface in the form of N+ ions or NH+, NH2+ and NH3+ ions. Owing to the low energy up to 10 eV, there is no implantation of N atoms and the nitrogen transport from the surface into the bulk is accomplished by the mechanism of an interstitial diffusion. Layers of different depth of nitrogen penetration and different phase compositions can be realized in this way. An important parameter determining the fatigue strength of nitrided parts is the thickness of the surface layer. The structure of the plasma nitrided layer consists basically of diffusion and compound layer (white layer). The latter one is composed of different iron nitride phases Fe4 N (γʹ) and Fe3 N(ε) nitrides. Mechanical properties of nitrided steel are σ y = 871 MPa and σ U = 1020 MPa, Vickers hardness on the surface is HV max = 1280, the depth of nitride layer h nitr = 200 μm. 3.2. Fatigue experiments Loading regimes and experimental fatigue life data are collected in Table 1 and Table 2, where σ a is the bending amplitude, τ a is the torsion amplitude, L R is the loading parameter and N f is the number of cycles to failure. The fatigue