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 Thus, the simultaneous solution of the system of equations 45 U8 45 U8 ( ); ( , ) s p S h S q V      will allow to determine the modes of HEH HFC according to the required quenching depth. However, as the results of mathematical and field experiments have shown, the obtained ranges of hardening modes do not guarantee the formation of a hardened layer without the presence of quenching cracks, the main cause of which is the internal stress state of the material. Due to the fact that the main technological characteristic of surface hardening is the depth of hardening (the required level of hardness is provided by the selection of the appropriate steel grade), it is possible to influence the magnitude and nature of the distribution of residual stresses only by changing the size of the transition zone. Taking into account the fact that the seat of destruction of the part in use is the location of the maximum tensile stresses, it is necessary to move the danger zone as far as possible from the surface of the product. Naturally, the depth of occurrence of σ t max will be the greatest if the value of the transition layer turns out to be the maximum. But, in this case, there is a significant decrease in compressive stresses on the surface. The analysis of the results of experimental and theoretical studies has shown that the value of the transition layer should be 25 ... 33 % of the depth of the hardened layer. It is when this requirement is met that there is a certain balance between the fact that the values are shifted to deeper layers of the material and at the same time the value of compressive stresses on the surface decreases on average by no more than 6...10 %. At the same time, large values of the transition zone must be provided when quenching steels with a high carbon content [27, 28, 39, 45]. In this case, when choosing the modes of surface hardening of parts operating under cyclic loads, an- other criterion is introduced – the relative value of the transition zone Ψ ( q s , V p ), that is, the ratio of the value of the transition zone to the depth of the hardened layer. As a result of processing the results of experimental studies, the corresponding functional dependencies were obtained for the studied materials and the ranges of processing modes ( q s [W/m 2 ], V p [m/s]): 2 2 3 3 2 2 , ) , ( s p p s p s p s p s p s p s q V k lV mq nV oq pV q eV sq tV q uV q Ψ = + + + + + + + + + (6) where 0.25 ≤ Ψ( q s , V p ) ≤ 0.33. The value of the coefficients of functional dependence: for steel 45: k = 0.087564, l = –7.429933, m = 1.062284 · 10 –8 , n = 235.19293, o = –3.424286 · 10 –17 , p = –8.850919 · 10 -8 , r = –1309.3045, s = 2.9423 · 10 –26 , t = 1.403793 · 10 –16 , u = 1.010925 · 10 –7 . for steel U8A: k = 0.013232, l = 7.354214, m = 5.814168 · 10 –9 , n = 31.678703, o = –1.724837 · 10 –17 , p = –8,746601 · 10 –8 ,