The wear resistance research of the rail contact surface depending on the grinding process

Background. Continuous mechanical loads on the rails during its contact with the wheel lead to an accumulation of residual stresses in the surface layers of the rails, resulting in fast-growing fatigue cracks. In addition, the interaction of the wheel and the rail leads to micro-and macro-slip during their contact, abrasive wear, as well as plastic deformation of the rail. Rail grinding is the repair method by which defective material layers removes from the rail surface, provides the necessary accuracy of size and shape as well as surface quality. Objective. The aim of the work is to determine the effect of grinding on the tribological properties of the rail surface and establish the optimal parameters of the grinding process to ensure the best wear resistance of the rail surface. Methods. The research on wear and contact damage of samples of surfaces cut from grinding rails conducted on a friction machine M-22M. The studies were carried out by dry friction of a sample (cut from a rail) with a counter-sample from the material used in the manufacture of railway wheels, for 1 hour, the friction path was 3.60 km. Samples were weighed on a VLR-200 balance before and after the study was performed on the friction machine. As a result, the mass wear value was determined for each sample. Results. Based on the results of tribological studies, we obtained graphical dependencies of the wear intensity on the hardness of surfaces of samples and histograms which showing the effect of grinding process parameters on the amount of the wear intensity of samples. In the work was investigated influence the next main parameters of the grinding process on wear resistance there are the temperature of the rail, depth of cut, grinding wheel speed. The results of the work can find practical application in railway transport when repairing rails by grinding. Conclusions. Based on the analysis of experimental data, the empirical relationship revealed between the depth of cut, the surface hardness of the sample and the intensity of its wear. The nature of the influence of grinding process parameters (rail temperature, depth of cut, grinding wheel speed) on the wear resistance of the rail surface is established. The most optimal values of the process parameters that provide greater wear resistance of the rail surface are depth of cut – 0.007 mm, grinding wheel speed – 30 m/s, rail temperature – 20°C (it is better to conduct the processing of rails in the warm season). The results of the work can find practical application in railway transport when repairing rails by grinding.


Introduction
Currently, there is a constant increase in the speed of rail transport, the relative movements of friction surfaces as well as an increase in specific loads and the presence of abrasive (sand). The most important factor in the rail travel cost optimization is the use of resource-saving technologies that extend inter-repair period and reduce the complexity of current track retention. Therefore, there is a need for new treating methods, working surfaces hardening and technologies to restore the geometrical parameters of the worn out parts. In addition, it is necessary to develop technologies for extending the service life of railway vehicles rails and wheels, including necessary equipment and machinery.
The problem is also complicated the large size of the structural elements, for example, worn out rail surfaces reach tens of meters. This complicates quality assurance and control, and possible manual processing does not provide sufficient productivity and quality.
Continuous mechanical loads on the rails during operation lead to an accumulation of residual stresses in the surface layers of the rails, resulting in fast-growing fatigue cracks. In addition, the interaction of the wheel and the rail leads to micro-and macro-slip, abrasive wear as well as plastic deformation of the rail. The repair of rail defects can do by the grinding. You can positioned the grinding tool at the site of the necessary repair, on the rail surface or edge, thanks to flexible kinematics, where needs removed completely material damaged layers. The tool kinematics and designs selected depending on the requirements for repair. The grinding rails surface roughness should be achieved Rz < 25 µm. Accuracy requirements of the rails shape and dimensions are defined by the technical requirements in Ukraine (DSTU4344: 2004) [1], that are being used in manufacturing of rails P50, P65 and P74. In Europe, rails are being manufactured in accordance to the technical requirements EN 13674-1:2011 [2].
Currently, rail grinding is a repair method that allows the removal of defective material layers from the rail surface as well as to provide the necessary accuracy of size and shape. There are a number of papers investigating the technological process of grinding rails, namely: to explore the effects of grinding passes, grinding direction, rotational speed of grinding stone on the material removal behavior of grinding rails during the grinding process [3][4][5], the grinding effect on surface roughness and hardness of the rail [6], the effects of contact pressure on the abrasive belt grinding performance of rail material [7]. There are few fundamental [3,[8][9][10] works on the impact of the grinding process and tools on the surface layers of rails, but there is very little fundamental knowledge about the tribological properties and the nature of the use of ground rails. In the work [10] investigated the influence of rails surface roughness on their durability, but the increase of hardness didn't taken into account in the process of grinding of rails. In some articles have researched the optimization of rails grinding in terms of the economy [11] and noise [12]. In the paper [11], based on the management theory of life-cycle cost, the model of analyzing rail grinding parameters is established, the calculation method is determined, grinding factor is optimized from the point of view of minimizing the rail life-cycle cost.
Thus, the aim of the work is to determine the effect of grinding on the tribological properties of the rail surface and establish the optimal parameters of the grinding process to ensure the best wear resistance of the rail surface.
The main tasks set forth in the presented work are:  Investigation of the effect of various surface hardness achieved by grinding on the wear resistance of rails;  Optimization of rail grinding parameters based on the results of the effect of rail surface hardness on its wear resistance;  Creation of a tribological model for determining the nature of the use of grinding rails in real conditions of operation.

Results
The research was carried out on wear and contact damage of the surfaces of ground rail samples, which were carried out on the friction machine M-22M under the scheme of a roll -square in conditions of dry friction (Fig.1).
As the sample material ( Fig. 2), German rail steel was used (HRC 35-38), which was grinding using varying process parameters. The counter-sample ( Fig. 3) consist of the steel material, which used in railway wheels and has the hardness HRC 38-40. The tribological tests were carried out with the contact force -P = 100 H, the rotational speedn = 490 min -1 resulting in the sliding speed -1 m/s (Fig. 1). The samples were examining on the friction machine M-22 M with dry friction for 1 hour accounting for a friction path -3.60 km. The rail samples for the tribological tests excised from grinding rails as shown in Fig. 4. The rails were prepared with a rib of the same height prior to grinding to ensure homogeneous grinding conditions. The laboratory balance of class 2 VLR-200 with a maximum weighing limit of 200 g and a weighing error of not more than 0.0001 g (in accordance with GOST 24104-80) used to determine the mass wear of the samples.
In addition to the above shown grinding parameters, different abrasive grain sizes within the grinding wheel (16, 20, 24 Mesh) as well as varying lubrication conditions were used while grinding. Furthermore, the rail temperature was varied while grinding (-60, -40, -20, 0, 20, 40, 60, 80С) to simulate climatic influences. Next, by plotting the graphical dependencies, an analysis was made of the effect of the indicated parameters on the value of their wear intensity.  The wear intensity can find using the analytical dependency. Known the work of V. Tsybulsky [13], who obtained the analytical dependence to determine the wear intensity of crane wheels based on the work of G. Yampolsky, I. Kragelsky [14], which has the following form: where C is a some constant value; ε is the concentration of abrasive particles (sand); R is the volumetric radius of the particle; σ c.s. is the conditional stress of the compression resistance of an abrasive particle; σ c is the contact stress; Δ is the complex parameter that depends on the size of the tolerance on the wheel size, shape, geometry of the work surface and the accuracy of the wheels; δ w is the elongation at break of material; HB w and HB r are Brinell hardness, respectively, of wheel and rail materials; R w is the wheel radius; n w is the wheel rotation frequency.
Dependence (1)  -influence of physical and mechanical properties of materials. In the article [13] dependency obtained and shows in Fig. 6 in accordance to the experimental results, which relates the magnitude of the wear intensity of the crane wheels and the complex parameter HK 120 3/2 ·δ 5B -2 of the deformationstrength properties for the three grades of steels. This is the same as the parameter . This parameter has HK 120 is the Ludwig hardness and δ 5B is the relative extension. You can calculate them using experimental method assessment of plasticity surfaces of machine parts [15]. Then we decided to compare our results and results from paper [13]. It show on fig.7. For the sake of comparison, all values resulted in a Brinell hardness. At work [13], wheel samples tested using a friction machine with the contact force -P = 736 H, that is higher than we used (P = 100 H). That is why values of the rail samples wear intensity in our tribological tests lower than in results of work [13]. In addition, the rail material and wheel materials are different. The very nature of the dependence of the wear intensity on hardness is similar in both cases (look fig. 7). This confirms the correctness of the results of the experimental studies.
From the above graphs, we can see that the lowest wear intensity occurs for a surface hardness of the samples in the range of 550 ... 650 HV. The wear intensity for varying grinding depths of cut are shown in Fig. 8 to analyze the influence this parameter on the rail surface hardness. As can be seen from the graphs, a large value of the depth of cut (0.021 mm) leads to an increase in the average wear intensity by 1.5 times compared with the minimum depth of cut (0.007 mm). This also confirmed in the work [8].
Since the magnitude of the wear intensity fluctuate with the same grinding surface parameters (depth of cut, rail temperature, grinding wheel speed), it was decided to analyze the effect of the depth of cut on the probable value of the wear intensity. For such the analysis, the dependence of the probability of the normal distribution on the magnitude of the wear intensity was constructed using normal Laplace-Gauss distribution. It described by the next formula [16]: where σ is the standard deviation and ̅ is the mean or expectation of the distribution. Using the formula (2), we plotted the graphical dependencies in fig. 9. Describes the probability of finding the magnitude of the wear intensity at certain values. As can be seen from fig. 9 the probability density function is narrower and shifts towards lower values of the wear intensity at lower depth of cut values. Therefore, a lower depth of cut value is preferable.
It obtains the empirical dependence of the wear intensity on the surface hardness at different depth of cut values based on the graphs in Fig. 5 where a e is the depth of cut. For the depth of cut of a e = 0.007 mm, a feed speed of v f = 22 m/min and a grinding speed of v s = 30 m/s, the effect of varying rail temperature during processing on the amount of wear is analyzed on the basis of the graph shows in Fig. 10.   Fig. 10. The dependence of wear intensity on the temperature while grinding with a depth of cut of ae = 0.007 mm An increase in temperature leads to a decrease in rail the wear intensity and vice versa. A similar pattern can observe at higher depths of cut (Fig. 11). а b Fig. 11. Dependence of wear intensity on the processing temperature of the rails for different depths of cut: a -0.014 mm, b -0.021 mm From fig. 12, it can be seen that for large depths of cut (a e = 0.021 mm), there is an influence of the grinding wheel speed on the wear intensity. At lower depths of cut, the effect of grinding speed is minimal.  fig. 12, you may see that the more grinding wheel speed leads the more wear intensity.

Conclusion
1) The influence of the basic technological parameters of the grinding process on the wear resistance of rail samples established through tribological research. These parameters include the grinding temperature, the depth of cut and the grinding wheel speed. The empirical dependence of the wear intensity on the hardness of the rail surface obtained for various depths of cut values.
2) The lowest wear intensity occur for a surface hardness of the samples is in the range of 550 ... 650 HV.
3) A different character of the dependence of the wear intensity on hardness is noted. For a sample hardness of 300 ... 500 HV, the wear intensity decreases, with a hardness of 500 ... 700 HV it stabilizes and with a hardness of more than 700 HV increases.
4) The dependence of the wear intensity on the hardness for different depths of cut (0.007 mm; 0.014 mm; 0.021 mm) has a similar character: The lowest wear rate occurs for the smallest depth of cut of a e = 0.007 mm. Increasing the grinding depth of cut leads to an increase in the average wear intensity by 1.5 times compared to the minimum depth of cut (0.007 mm). 5) A higher value of the temperature of the rails during grinding process provides a smaller amount of the wear intensity of samples cut from these rails. The intensity of wear at a grinding temperature of -60 is approximately 3 times higher than at temperatures of 40-80 . A similar pattern is typical for all values of depth of cut.
6) The dependence of the grinding wheel speed during rails grinding on the wear intensity at a treatment grinding depth of cut of 0.007 mm was established. The lowest wear intensity is at the minimum (30 m/s) and the maximum (40 m/s) grinding wheel speed. The wear intensity is the maximum (2.5-3 times higher) at grinding wheel speeds of 35-40 m/min.