Physicochemical Properties of the Surface Layers of 40Kh Steel After Hydrogen-Free Nitriding in Glow Discharge
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PHYSICOCHEMICAL PROPERTIES OF THE SURFACE LAYERS OF 40KH STEEL AFTER HYDROGEN-FREE NITRIDING IN GLOW DISCHARGE M. S. Stechyshyn,1, 2 M. E. Skyba,1 N. M. Stechyshyna,1 A. V. Martynyuk,1 and R. S. Mardarevych1 By the methods of mathematical statistics, we deduce equations specifying the relationship between the technological parameters of hydrogen-free nitriding in glow discharge (temperature, composition of gas mixture, and its pressure) and the physicochemical properties of the nitrided surface layers of 40Kh structural steel. Keywords: nitriding, high-cycle and low-cycle fatigue, thickness, surface microhardness, residual stresses, electrode potential, corrosion rate.
Introduction In [1–4], the authors studied the influence of the conditions of hydrogen-free nitriding in glow discharge (HNGD) on the structure and phase composition, corrosion and electrochemical characteristics, level and character of distribution of internal stresses, and the characteristics of low- and high-cycle fatigue of the nitrided layers of structural steels. In the present work, we analyze the possibility of application of the methods of mathematical statistics (the method of active planning of experiments and the Box–Wilson method) for the construction of statistical models aimed at determination of the properties of surface nitrided layers (temperature, pressure of gas atmosphere and its composition). The duration of nitriding τ = 4 h and remained constant in all experiments. Procedure of Investigations The procedure of nitriding of steel in glow discharge was carried out in an installation designed and constructed in the Podillya Scientific Physical-Technological Center at the Khmelnytskyi National University [5]. We used the procedures of metallographic, electron-microscopic, and X-ray diffraction analyses. The structure of diffusion-nitrided layer in the cross section and the distribution of microhardness over the thickness of the layer were studied on metallographic specimens of 40Kh steel etched in a 3% alcohol solution of nitric acid with the help of a MIM-10 metallographic microscope. The submicroscopic structure of the nitrided zone was analyzed in a REM-200 scanning microscope. The X-ray phase diffraction analysis of nitrided surface layers was carried out with the help of a DRON-200 diffractometer in the K α -radiation [1]. The corrosion and electrochemical 1 2
Khmelnytskyi National University, Khmelnytskyi, Ukraine. Corresponding author; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 55, No. 6, pp. 104–109, November–December, 2019. Original article submitted March 7, 2019. 892
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PHYSICOCHEMICAL PROPERTIES OF THE SURFACE LAYERS OF 40KH STEEL AFTER HYDROGEN-FREE NITRIDING
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characteristics of the nitrided layers on 40Kh steel in a model acid medium [buffer solution of citric acid (5 g/liter) and double-substituted sodium phosphate (10 g/liter)] with pH = 6.5 were investigated by using the gravimetric and potentiostati
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