Influence of the Modes of Ion Nitriding on the Fretting-Fatigue of 40Kh Steel
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INFLUENCE OF THE MODES OF ION NITRIDING ON THE FRETTING-FATIGUE OF 40KH STEEL P. V. Kaplun1, 2 and V. A. Honchar1
UDC 821.894+620.17
We present the results of experimental investigation of the fretting-fatigue with bending for 40Kh steel after ion nitriding in hydrogen-free and hydrogen-containing media. The optimal mode of ion nitriding in hydrogen-free media is determined according to the criterion of maximum service life of 40Kh steel under the conditions of fretting-fatigue. We also establish the effect of hydrogen on steel after nitriding in hydrogen-containing medium. Keywords: fretting-fatigue with bending, ion nitriding, service life, hydrogen-free medium, hydrogencontaining medium.
Introduction In the engineering, a great number of components, units, and mechanisms operate under the conditions of fretting, fretting-fatigue, and corrosion fretting-fatigue. Numerous investigations are devoted to the evaluation of their service lives under the indicated conditions [1–16]. These processes run at the sites of contact of the components, where the surfaces perform relative oscillatory motions with small amplitudes caused by the influence of cyclic loads in various media. The surface defects and microcracks appearing under the conditions of fretting play the role of nuclei of fatigue cracks whose growth causes fatigue fracture under loads that do not lead to destruction in the absence of fretting [5–8, 15]. Fretting-fatigue is a very dangerous and insidious kind of fracture because fretting takes place mainly at the sites of joints inaccessible for inspections and leads to premature or even unexpected (sudden) catastrophic fatigue failures [13, 14]. Corrosive media accelerate this process [1, 5–11, 14, 15]. Hence, prolongation of the service life of the components operating under conditions of this kind is very important for the national economy. The service life of structural elements subjected to fretting fatigue depends on numerous factors determined by the structure of elements, the properties of materials, loading, media, etc. [5–9, 15]. For the description of the mechanisms of fracture, the researchers of fretting fatigue advance various hypotheses and give preference to different factors, which often are contradictory and do not give adequate explanation to the observed phenomena. Thus, there is no clear understanding of the relative importance of two main groups of factors: chemical and mechanical. According to [6, 9, 12], the mechanical factors are predominant in air. However, other researchers [7, 14, 15, 17–21] emphasize the effect of chemical phenomena (influence of the medium). As follows from [9], the influence of fretting on the fatigue strength most clearly manifests itself via the formation of surface cracks directly under the action of cyclic contact friction but not via cavities, wear, corrosion, or erosion. 1 2
Khmel’nyts’kyi National University, Khmel’nyts’kyi, Ukraine. Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 54, No. 6
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