OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. 5 No. 3 2023 without the appearance of hardening cracks. The main reason for the appearance of such microcracks is the internal stress-state of the material. In the case of surface hardening, special attention is paid to the depth of hardening, as this is the main parameter in the process. To achieve the desired level of hardness, it is necessary to choose the optimal steel grade. In this case, the effect on the magnitude and distribution of residual stresses is possible only by changing the size of the transition zone. Taking into account the fact that the location of the maximum tensile stresses is the fracture nucleus of the part during operation, it is advisable to move the dangerous zone deeper from the surface of the product. In this case, the greatest depth of occurrence is achieved if the thickness of the transition layer is maximum. However, a balance needs to be found, because as the depth increases, the level of compressive stresses on the surface also decreases. Studies have shown that the optimal size of the transition layer should be approximately 25–33 % of the depth of the hardened layer. If this requirement is met, a balance is achieved between the transfer of stresses into the deep layers of the material and the reduction of compressive stresses on the surface, not exceeding 6–10 %. It is especially important to provide a larger transition layer when hardening steels with a high carbon content. This makes it possible to control the mechanical properties and fracture resistance of parts effectively [7, 14, 17, 21, 47, 61, 71–73, 75, 82–83, 87, 102]. In the process of selecting modes of surface hardening of parts operating under cyclic loads, an additional criterion is used – the relative value of the transition zone, denoted as Ψ(qS,VS). This criterion is defined as the ratio of the size of the transition zone to the thickness of the hardened layer. By analyzing the experimental data, the corresponding functional dependence was established ΨU10(qS,VS) (fig. 8), valid for the material under study and the range of processing modes: 2 2 3 3 2 2 10 S S S S S S S S S S S S S S ( , ) U q V k lV mq nV oq pV q rV sq tV q uV q Ψ = + + + + + + + + + , (2) where 0.25 ≤ ΨU10(qS,VS) ≤ 0.33 The value of the coefficients of functional dependence for steel grade U10A: k = 0.55499986, l = 6.376, m = -3,0969982 ∙ 10-9, n = 2.1133193 ∙ 10-6, o = -6.697454 ∙ 10-24, p = -9.444857 ∙ 10-16, r = -1.1120113 ∙ 10-5, s = 8.2498316 ∙ 10-33, t = 1.5500134 ∙ 10-24, u = 1.3319075 ∙ 10-15. The determination of the specific power and the speed of the source movement during surface hardening is carried out by solving a system of equations U10 S S U10 S S ( , ); ( , ). h q V q V Ψ for given values of the hardening depth and the relative size of the transition zone. The graphical solution of this problem is shown in fig. 9. It should be noted that the resulting range of processing modes is much smaller compared to the range of modes to achieve only a given thickness of the hardened layer. To achieve the required thickness of the hardened layer h = 0.52 mm in the process of surface HEH HFC hardening, it is necessary to select the operation parameters in the range limited by points A and B on the curve (fig. 9). These parameters include the specific power qS, which will be in the range from 2.09 · 10 8 to 2.49 · 108 W/m2, and the source travel speed V S will be from 66 to 73 mm/s. These processing modes ensure that the required hardening depth is achieved and that the transition zone is optimally sized. Since HEH HFC hardening is performed in the one workpiece location, the auxiliary time is 0 seconds. Calculation of efficiency and energy consumption at the operation “Surface HEH HFC hardening” is performed using the following equations: , S sp V E L = S s S s sp s q bR q bR L EC P V = = , where L = 614 mm (fig. 3), b = 10 mm (fig. 2). Table 2 contains the results of the calculation of energy consumption and efficiency for all combinations of operating parameters during thermal hardening of the part.
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