Elaboration of methodological base of quality evaluation of nitrogenization of steels of tribotechnical application

Abstract

Nitrogenization is one of most spread methods of surface hardening of critical parts of machines. To elaborate methodological base of structural evaluation of quality parameters of nitrogenized surface layers of sliding friction couple by methods of X-ray structural analysis and tribotechnical tests, structure and performance properties of nitrogenized structural steels and alloys were studied. Namely pearlitic and martensitic steels (40Х, 38Х2МЮА, ВКС-7), model iron-based alloys with a BCC lattice, steels alloyed by Cr, Mo, Al in amount of up to 4 % (at.), iron-based alloys with FCC lattice, alloyed by Ni (29 % (at.)), Cr, Al, Ti in the amount of up to 4 % (at.), as well as austenitic high-chromium steels 12Х18Н10Т, 08Х16Г15Н5МАФ, steel 16Х3НВФМБ-Ш and high manganese steel 40Г14Н8Х3Б1. The surface saturation of the samples was made by gas, ion, gas-baric nitrogenizing and by ion implantation of nitrogen. The sum of macroscopic (tribotechnical) and microscopic (structural) indicators was evaluated, which characterize the physics and mechanics of friction process at various hierarchical levels. This sum of indicators also provides information on properties of antifriction coatings, modified layers and performance ability of the friction couple under conditions of contact deformation. It was established, that increasing of the nitrogenizing temperature from 540 up to 700 °C for alloys with a ferrite matrix results in a decrease of the hardness of the surface layer and physical broadening of X-ray lines. In this case, the relative wear resistance reaches maximum at 620 °C. For alloys with an austenitic matrix, an increase of hardness, broadening of X-ray lines and relative wear resistance occurs with increasing of temperature. These parameters grow in the sequence Fe−Ni, Fe−Ni−Al, Fe−Ni−Cr, Fe−Ni−Ti. Gas-baric nitrogenizing enables to obtain higher performance characteristics of parts (wear resistance and contact strength). Based on the generalization of the results of experimental studies of nitrogenized steels and alloys with various compositions and various crystal lattices, the most significant characteristics of the structural state and properties of near-surface microvolumes were established, that affect the level of surface destruction during friction, as follows: the size of the alloying element nitrides particles, the distance between them, the micro-deformation of the crystal lattice of the solid solution, the values of the physical broadening of the X-ray lines of the solid solution, its hardness and the change in hardness during friction, the plasticity reserve. Generalized criteria for the properties of the surface layer, taking into account the characteristics of the structure and properties of the nitrogenized layer and the zone of surface contact plastic deformation elaborated. Also methodological foundations for the structural assessment of the quality parameters of the surface layers of sliding friction couples elaborated. Bench tests have confirmed the effectiveness of the application of the generalized structure parameter and properties of surface layers for the selection of materials for the friction unit. The formulated conditions are recommended for optimizing the nitrogenizing technological process from the standpoint of tribology and choice of materials for friction couple.