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 formations. Therefore, the processing modes should be assigned in such a way as to provide the necessary thermal cycles with the specified parameters in the surface layers of the material [27, 28]. At the same time, as it was shown in studies [27, 41], it is not possible to establish an unambiguous relationship of the numeri- cal values of these parameters with the processing modes and quality characteristics of the hardened layer. However, it is obvious that the numerical values of the parameters of thermal cycles are determined by the magnitude of the transmitted energy and the nature of its distribution in the material. In this regard, and on the basis of the works [41, 42], it is proposed to use the integral temperature-time characteristic S , which combines all the listed parameters of thermal cycles [28, 43–45], as the main param- eter of the purpose of surface hardening modes. The process of austenite formation will occur in the time period t t = t 3 – t 1 (Fig. 2) regardless of whether the thermal curve has an upward or downward character in a given time period. This means that the total time t t and the temperatures at which the austenitization process occurs can be characterized by the area value ( S ABC ) limited at the top by the heating curve, and at the bottom – by a straight line corresponding to the temperature A C 1 . 3 1 1 3 1 ( ) ( ), C S T d A t t = t t − t − t ∫ (1) The physical meaning of this characteristic becomes clear from the dependence S = Q · R T , where Q is the energy, J; R T is the thermal resistance of the material, o C s/J. Fig. 2. Kinetic curve of steel heating and cooling during quenching The thermal resistance of a metal is the ability of a material to resist heat transfer. In this case, we consider a metal heated above the temperatureA C1 . In this case, the numerical value of the thermal resistance of the metal will depend not only on the thermal conductivity coefficient, but also on the structural-phase transitions that mainly occur in this temperature range endothermically with heat absorption. In other words, this characteristic indirectly determines the amount of energy transferred to the material and spent on structural-phase transformations. It can be easily calculated from the dependence (1) in the process of simulation temperature fields in the material [28]. Based on the above, in order to develop a methodology for assigning surface hardening modes in hybrid processing conditions, it is necessary to establish the relationship of the numerical values of the integrated temperature-time characteristic with the processing modes, on the one hand, and with the hardening depth, on the other hand. The solution of this problem is carried out by combined simulation of temperature fields and structural-phase transformations in the material [27, 28, 39, 44, 45].