In this paper, it is studied the relationship between cyclic durability of parts and their finishturning modes. An analysis of the influence of various factors on the fatigue resistance of parts was carried out. It is shown that it is advisable to use turning as the final stage for surface machining, which allows to obtain high-quality parts with smooth surface finish, and therefore, with good operational properties. It is provided experimental results of fatigue tests for the specimens made of 40Х steel (DSTU 7806:2015). The specimens were machined by turning with a cutting speed range 80 and 180 m/min; a federate range of 0.08 and 0.12 mm/rev and a cutting depth of 0.3 mm. Based on the experimental result, a mathematical model of the cyclic durability of parts made of 40X steel was obtained by the method of multifactorial regression analysis considering turning modes and stress amplitude.
surface. In this regard, during the manufacturing process of the part, irregularities are formed on its surface, the structure, phase and chemical composition changes, and residual stresses arise.
The production of many important parts usually requires grinding, polishing, or even some specific machining after roughing, semi-finishing, and finishing transitions in turning or milling.
Thus, the production of such parts is quite expensive. One of the possible ways to reduce the cost
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Finishing of parts made of hardened steels (for example, AISI 52100 steel for bearings and 16MnCr5
steel for automotive gears and shafts) by turning with tools made of ultra-hard cutting materials (PCNB,
ceramics, etc.) has increased industry application [
It is possible to obtain slander and residual compressive or tensile stresses of different intensities
and depths during turning and grinding [
The results of the use of super hard tool material hexane-R [
It was found that micro-uniformities with a depth of up to 5 μm after turning can be compensated
due to the formation of residual compressive stresses for stainless steel machining [
Considering the above, it can be stated that turning allows to obtain parts with high-quality surface, and therefore, with good operational properties.
Therefore, to ensure the necessary cyclic durability of parts, it is required to control technological
heredity which corresponds to surface roughness during turning [
The purpose of the work is to determine the influence of turning finishing modes and stress amplitude on the cyclic durability of parts.
The processes of gradual accumulation of damage in the material, which arise because of applying cyclic loading to the specimens obtained under different modes of turning finishing processing, are considered.
To study the cyclic durability of parts during turning finishing made of structural material, the fatigue tests of specimens were carried out under different turning modes.
Structural alloyed chromium steel 40X DSTU 7806:2015 was chosen for this study. Parts subjected to vibration and dynamic loads are usually made from this material, and requirements for increased strength, viscosity and durability are imposed on them.
To study the influence of technological conditions of turning processing on the fatigue properties, smooth specimens of round cross-section type I were used (Fig. 1).
The specimens were tested for fatigue failure on a machine MUI-6000 on the basis of N = 2·107 cycles with 20°С temperature and rotation frequency of 2000 rev/min.
Turning of specimens was performed on a HAAS ST20 CNC machining center with a PVVNN 2525M-16Q cutter with a VBGW 160404T00815SE cutting plate without cooling. The cutting modes of turning are given in Table 1.
The machining of the gage region of the specimens was carried out in several stages. First, turning processing of the surface of the specimens was performed with an allowance for the final finishing turning transition.
At the next stage, in order to remove scratches on the surface of the specimens from previous turning, the grinding and polishing operation were used. Finally, the specimens were subjected to heat treatment.
The heat treatment of the specimens was carried out in a shielding gas environment to prevent oxidation of the surfaces under the following regime: heating to a temperature of 450 ºС, exposure for 2 hours and cooling in the oven. The results of experimental studies are given in the Table 2-4. The fatigue curves are shown in Figure 3, Figure 3, and Figure 4 respectively.
The analysis of high-cycle fatigue curves demonstrates the influence of machining modes on the fatigue life. Thus, the lowest fatigue life was observed for the regime of V=80 m/min; S=0.08 mm/rev; t=0.3 mm, and the maximum one for the regime of V=120 m/min; S=0.12 mm/rev; t=0.3 mm. At the same time, the scatter factor is getting higher for decreased stresses.
At a stress of 500 MPa, the number of cycles to failure for specimens machined under the regime V=80 m/min, S=0.08 mm/rev, t=0.3 mm is 11300 cycles (Figure 3); for specimens machined under the regime V=180 m/min, S=0.08 mm/rev, t=0.3 mm is 15000 cycles (Figure 4); for specimens machined under the regime V=120 m/min; S=0.12 mm/rev; t=0.3 mm is 20500 cycles (Figure 2).
At a stress of 400 MPa, the number of cycles to failure for specimens machined under the regime V=80 m/min, S=0.08 mm/rev, t=0.3 mm is 40600 cycles (Figure 3); for specimens machined under the regime V=180 m/min, S=0.08 mm/rev, t=0.3 mm is 59700 cycles (Figure 4); for specimens machined under the regime V=120 m/min, S=0.12 mm/rev, t=0.3 mm is 95000 cycles (Figure 2).
At a stress of 300 MPa, the number of cycles to failure for specimens machined under the regime V=80 m/min, S=0.08 mm/rev, t=0.3 mm is 150000 cycles (Figure 3); for specimens machined under the regime V=180 m/min, S=0.08 mm/rev, t=0.3 mm is 290000 cycles (Figure 4); for specimens machined under the regime V=120 m/min, S=0.12 mm/rev, t=0.3 mm is 430000 cycles (Figure 2).
Thus, the maximum difference in the number of cycles to failure at a stress of 300 MPa for specimens machined under V=80 m/min; S=0.08 mm/rev; t=0.3 mm and specimens machined under V=120 m/min; S=0,12 mm/rev; t=0.3 mm was 2.9 times.
Therefore, it is possible to make a conclusion about the significant influence of turning modes on the cyclic durability of parts which justifies the necessity to find a mathematical relationship between the cyclic durability of parts and turning mode, which helps to predict the value of relevant operational characteristic at the stage of technological preparation of production.
In order to predict the fatigue life of specimens machined with different cutting modes multiple
regression was used. This approach makes it possible to quantitatively assess the relationships between
the initial parameters and variables (factors) [
Based on the results of experimental studies (Tables 2-4) and its correlation by the method of multiple regression data analysis, a mathematical model of the cyclic durability of steel 40X related to the modes of turning processing and stress amplitude was found:
(, , ) = , (1) where = 14.437 + 0.0048 + 13.006 − 13.19 + 0.002 − 0.002 − 5.941 + +0.0000004 + 2.929 + 3.013 .
Given equation (1) is true according to Fisher's F-test at a confidence probability of 0.95: V=80-180 m/min, S=0.08-0.12 mm/rev, σ=225-670 МPа. The integral influence of turning modes and stress amplitude on the cyclic durability of 40X steel specimens is shown in Figure 5 and Figure 6.
Thus, according to the results of the experimental studies, it was found that the cyclic durability during turning of specimens made of 40X steel with a cutting depth of 0.3 mm for the feed range from 0.08 to 0.12 mm/rev, and cutting speeds from 80 to 180 m /min increases as both feed and cutting speed increase. At the same time, the influence of the feed rate is more important.
It was found that fine turning increase quality parameters of the surface layer in comparison with grinding. It leads to increased durability of machined parts subjected to cyclic loading. The mathematical model for fatigue life prediction was obtained which allows to determine the number of cycles for failure in correlation with finish-cutting modes. It was also found that the durability of 40X steel specimens increases with increasing both feed rate and cutting speed.