Improving the efficiency of surface-thermal hardening of machine parts in conditions of combination of processing technologies, integrated on a single machine tool base

OBRABOTKAMETALLOV Vol. 23 No. 3 2021 technology Introduction In the era of globalization and high competitiveness, it is extremely important for modern mechanical engineering to work on reducing production costs and at the same time ensuring the production of high- quality products with maximum productivity [1-20]. In this regard, the manufacturing industry clearly shows a particular interest in the development of a new type of technological equipment that allows imple- menting methods for modifying the surface layers of parts by processing it with concentrated energy sourc- es [21-25]. Local and ultra-high-speed thermal effects make it possible to obtain higher values of hardness, strength, and toughness due to the formation of a highly dispersed metastable structure on the surface of the parts with a much higher dislocation density compared to bulk processing and traditional methods of surface hardening [17, 21–23, 26–31]. The development of high-frequency heating technology associated with the use of radio frequencies, with the work on the miniaturization of inductors and equipping it with ferrite magnetic cores, led to the emergence of a new method – high-energy heating with high-frequency currents (HEH HFC), which is currently of particular interest from the point of view of strengthening structural steels [26-28, 32-36]. This method makes it possible to implement the technological process of quenching with specific heating capacities of about 400 MW/m 2 , which successfully allows it to compete with other concentrated sources (laser, electron beam) when hardening the material without melting (Fig. 1). It should be noted that the provision of the required constant gap ( d = 0.1...0.2 mm) between the inductor and the workpiece, which is a necessary condition for the implementation of the HEH HFC, becomes possible only when combining two processing technologies – mechanical and surface-thermal operations – on a single machine base [20, 27, 28, 37]. Since the development of new machine tools requires a large amount of financial and labor resources, we propose the modernization of standard metal-cutting machines by retrofitting it with an ad - ditional concentrated energy source, which can be used as a HFC generator. Taking into account the current level of development of microprocessor technology in the field of high-frequency industrial thyristor-type installations, as well as the criteria for digestible integration into a standard machine system, our area of in- terest will include high-frequency generators of the microwave type. At the same time, it becomes an urgent task to develop new methods for assigning processing modes that consider the combined operations of the technological process not in isolation, but in interrelation, and allow obtaining parts with a predetermined accuracy and physical and mechanical properties of its working surfaces [27, 28]. The phase transformations, and, consequently, the resulting structure and quenching depth, as well as the grain size of austenite depend on the integral temperature-time effect of heating on the steel structure and, therefore, are also determined by the shape of the thermal curve. The use of the average rate of heat- ing and maximum heating temperature as the main parameters for the purpose of surface quenching modes is rather approximately characterized by the temperature-time conditions of phase transformations during austenitization of steel [28, 38-40]. In addition to the above parameters, it is necessary to take into account both the average cooling rate and residence time of the material in the temperature range of phase trans- Fig. 1. The scheme of HEH HFC processing