On the issue of designing and creation of a composite grinding wheels of increased productivity for rails treatment

The article discusses the technological aspects of the development and design of composite modified wheels used in the mechanical treatment of railway rails. It allows to increase the productivity of treatment and tool durability. An increase in the physical and mechanical features of the shard of abrasive wheels can be achieved by modifying the binder component in the composition of the ceramic charge mixture with a nanomodifier such as carbon tubes. As a result of the modification of the abrasive composition of the charge mixture, it was possible to achieve an effective performance of railway rails treatment without a significant loss in the durability of the composite grinding wheel.


Introduction
As a result of the dynamic interaction between the rail and the wheel, various types of wearing are manifested, which affect the rolling surfaces of the rails and bands of rolling stock wheelsets to various grades [1].
The formation of vertical and lateral wear of rails occurs due to the loss of material, vertical wearing is formed in straight sections of the railway track, lateral wearing is formed in curved sections on the outer rail thread, or due to collapse (plastic deformation of the rail).
On damaged rail surfaces, contact stresses increase due to suboptimal contact in the wheel-rail zone.Therefore, to correct the transverse profile of the rail on railways, their profile and preventive grinding is used.
The modified repair profile of the rail head, obtained as a result of grinding the tread surface, ensures a uniform redistribution of forces at the points of contact between the wheel and the rail; as a result of such a redistribution, vertical irregularities on the tread surface of the rails are eliminated or reduced.To achieve such indicators, rail grinding complexes need to make several passes along the treated section, which leads to significant losses as a result of delays in the movement of trains along this section.Therefore, an urgent task is to increase the productivity of the process of rails grinding on the way [2].

Problem statement
The most effective areas for the design of abrasive tools (AT) with improved performance are the development and modification of recipe-technological methods aimed at optimizing the compositions of AT, the design and creation of new types of composite, highly porous tools used in high-speed grinding operations, etc. [3,4]; structural characteristics, with the optimal content of abrasive grains, binder and pores [5,6,16,17]; the use of various heat treatment modes and impregnation with new promising compositions [7,8]; recipe optimization [9,10].
Domestic and foreign scientists have carried out numerous theoretical and experimental studies on AT modernization.They reveal several directions in the creation of abrasive tools with improved technological performance.
Ways of AT modifying aimed at increasing efficiency by changing the structural and mechanical properties are the use of various complex multi-complex filler additives (structure modifiers) in the formulations, the effect of which on the structure of the tool shard can be classified according to their functional purpose: pore-forming, strengthening the structure, impregnating and multi-complex impact [11].

Research questions
The development of the direction associated with the optimization of sand formulations and granulometric compositions is quite widely reflected in domestic and foreign scientific works [12,13,18].
The cutting ability of the modified abrasive tools depends on complex formulation and technological criteria that are focused on certain points, such as the structural and mechanical properties of corundum abrasive particles, the physical and mechanical characteristics of the binder (ceramic, bakelite and other types of binder components).
The most vulnerable places in the structure of an abrasive tool are the areas between the grains of abrasive particles and the binder component (so-called "bridges").Cracks arising in the structure of the shard develop along easier trajectories, bypassing the abrasive grains of the main component and propagating to the "bridges", destroying the inner cavity of the tool shard [13].
It is possible to provide the necessary strength to an abrasive tool based on a ceramic bond through the use of a melting binder component of a vitreous mass, which includes refractory clay, feldspar, and quartz.In the process of firing the forming abrasive compositions, the vitreous mass forms a crock of an abrasive tool with the necessary parameters of rigidity, wear resistance and strength (including temperature), however, the binder has increased brittleness.

Materials and methods
The aim of this study is a development of an improved performance composite abrasive tool for rail grinding.An attempt is made to strengthen the weak points of the "bridges" in the shard of an abrasive composite grinding wheel through the use of innovative nanohardeners such as multi-wall carbon nanotube "Taunit" (hereinafter is referred to as MWCNT "Taunit") with the determination of the optimal geometric dimensions of conglomerates, taking into account the grain size, hardness and percentage ratio of abrasive particles (number of the structure of grinding wheels).

Results
To achieve this goal, a method was developed for filling the molding sands of an abrasive tool with a nanomodifier such as MWCNTs (multi-wall carbon nanotubes), which had previously passed through ultrasonic treatment, followed by mixing all the abrasive components of the composition.
The structure of the Taunit modifier is a tubular formation with an internal through channel, consisting of several carbon layers.
The geometric dimensions of the modifier particles have an outer radius of 10…35 nm and a length of 2 to 5 µm (figure 1).Conglomerates of carbon nanotube granules, like the multi-walled ones themselvesnanotubes are chemically inertwhich allows them to be used in various mixtures and compositions.Their use is limited by the fact that conglomerates are tangled balls of carbon tubes.As a result, a prerequisite for their effective use is their preliminary grinding.Ultrasonic treatment of the initial globules of Taunit MWCNTs was carried out in an ultrasonic disperser of the STEGLER 3DT model with the following characteristics: the ultrasound frequency is 40000 Hz, the total power of the ultrasonic waveguides is 0.4 kW.
At the stage of preliminary testing of the obtained materials with the introduction of a small amount of modifier in the form of nanotubes, a significant increase in tensile and bending strength has been noticed.Hence, it has been concluded that the indicators of the strength properties of the resulting shard of an abrasive tool increase significantly from the addition of a modifier.This is explained primarily by the high physical and mechanical characteristics of the nanotubes themselves and their high adhesion to base materials, which confirms the hypothesis of the reinforcing effect of this type of additive.The method of introducing the modifier is based on ultrasonic treatment of the starting material in isopropyl alcohol and its further mixing with sodium glass.Determining the size of the crushed globules of "Taunit" has been studied using a digital microscope with a built-in Altami 105 camera and the Altami application VideoKit for image treatment.

Findings
As a result of the analysis of the experimental results, the optimal treatment time was t=420 sec, at a frequency of f=40 kHz.At the same time, the molding sand of the grinding wheel will contain the modifier in the required amount and will be evenly distributed in the composition of the formed wheel.
After determining the effective modes and dispersion medium of nanomodifier globules, it was necessary to determine the proportion of particles (determined by size) that were processed in isopropyl alcohol for 7 minutes.
Thus, the suspension obtained by the proposed technology in its volume contains 15% of particles of nanometer geometric dimensions and 85% of particles of sizes from 1 to 10 microns, 5% of particles of sizes from 10 to 100 microns.
As a result of application of the technology of ultrasonic treatment nanomodifier of MWCNT "Taunit" in the medium of isopropyl alcohol, it is possible to ensure the grinding of the initial globules and optimize the fractional volume composition of the resulting suspension towards the nanoscale range.
The effect of this technological method is the possibility of nano-reinforcement of the weak points of the "bridges" in the shard of abrasive tools, the formulation of which is proposed to be supplemented with a premix in the form of nanostructured multilayer tubes.The model of the modified bridge of the nanostructured bond of the abrasive tool is shown in figure 2. The design and creation of a composite grinding wheel consisting of alternating cutting elements of two type (the first type of elements of which is made of abrasive grains on a bakelite bond, and the second type of abrasive grains on a nanostructured ceramic bond) is carried out using a combined technology for manufacturing of abrasive tools.
The composition of the mixture (base), weight in%: feldspar 53%, kaolin 31%, liquid sodium glass 7%, dextrin 9%, the abrasive material is white electrocorundum grade 25A.After preparing the modified mixture, standard samples were formed, which have been fired at a temperature of 1320°C, and have been subjected to standard physical and mechanical tests.
The test results of the modified ceramic segment of the grinding wheel are presented in table 1.As a result of the experiments, it has been found out that the optimal dosage of MWCNT "Taunit" in the basic composition of the charge mixture is 0.005 -0.001 wt.% of soda glass consumption.
If in the composition of the ceramic charge mixter there is a nanomodifier in the amount of 0.005%, the tensile strength increases by 1.62 (12.7 MPa) times compared to the base composition.The bending strength index during testing of control samples has reached 25.4 MPa, which is 25.6% more compared to the basic composition of the ceramic charge mixture.
As a result of the analysis of the microstructure of the modified ceramic segment of the 0.005% MWCNT "Taunit" nanomodifier, it has been revealed that the structure is homogeneous, the pores are evenly distributed throughout the ceramic matrix, the number of deep interconnected pores is insignificant.
Further studies have been aimed at determination of the structural characteristics of the developed composite grinding wheel with alternating bakelite and ceramic segments.The patent of the authors of the Russian Federation [14] was taken as a constructive basis for the grinding wheel.
Experimental samples of composite grinding wheels for a manually operated machine and for a rail grinding train are shown in figure 3a) and 3b).In further studies aimed at identifying effective modes of treatment the upper part of the rolling surface of the heads of railway rails, composite grinding wheels with a combined working surface were used (figure 3).
Further, in the work, a study was made of the relationship between the physical and mechanical parameters of the resulting tool and the operating modes of treatment the working surface of the rail head.
In the course of these studies, carried out in laboratories and on the railway tracks of the training ground of the Samara State Transport University, the following parameters have been controlled controlled: features of the abrasive tool (durability of composite grinding wheels); features of rails control samples (removable linear volume of rail steel, quality of the treated surface).At the same time, sets of composite grinding wheels based on a bakelite bond and modified ceramic segments with the following features have been tested: grain size (F36, F22 and F14 according to GOST R 52381-2005), hardness of grinding wheels as per GOST R 52587-2006 -Q, R, T. The content of particles of abrasive material per unit volume of the abrasive tool varied in the range of 40 -60%.
In process of experimental work, in order to effectively use the abrasive tool, the following cutting conditions have been changed: tool pressing force from 36 to 46 N, speed of a specialized mobile rail grinding laboratory installation from 4 to 8 km/h, peripheral speed of composite grinding wheels from 40 to 60 m/s.The geometric parameters of the grinding wheel and the machined surface of the heads of railway rails were determined by measuring instruments and devices that have passed verification.
In grinding operations, R65 type rails with a volume-hardened category of DT 350, with the grade of rail steel of E76F and with an operating time of 450 million gross tons have been used.
While planning experiments on grinding the working surface of the rail heads, the following has been planned: to measure the outlines of the transverse profile of the rail heads before grinding; record the geometric dimensions of the grinding wheels before and after the grinding operation; determine the linear stock removal rate after treatment the rolling surface of the rail heads; measure the height of the abrasive tool after the grinding operation; determine the overall scope of work.

Discussion
The processing of a large array of experimental data obtained in the course of full-scale experiments was based on analysis of variance, carried out using the specialized software package Statistica.
Optimization of the technological modes of the grinding process and of the structural features of the grinding wheel has been carried out by implementation of a three-factor experiment.
The response of the surface was the following parameters: the amount of stock removal tr.s(μm), durability (working capacity) of the composite grinding wheel T(h),roughness of the machined surface of the rails Rz r.s (µm).
Based on the obtained results, regression equations have been calculated.Homogeneity of variances, statistical significance of regression coefficients and adequacy of the obtained models have been checked.
The processing of experimental data on the physical and mechanical features of the composite grinding wheel has been carried out with a fixed value of the grain size of the abrasive material (accepted value is F14).
As a result of treatment the experimental data, the following dependence of the output parameters on the physical and mechanical features of the composite grinding wheel have been obtained: Tr.s = 106.011-0.928St -103.821Zh+ 0.023St 2 -0.804StZh + 50Zh 2  (1) Figure 4 shows a graphical interpretation of the results of mathematical model processing (1).From the resulting regression equation ( 1), it follows that with a hardness of the composite grinding wheel of 1.1 mm (Zh) and a dense structure St 3 (60% of abrasive), the maximum removal of the allowance tr.s = 28 μm is achieved with a grain size of F14.
The chips formed in the process of grinding at a high circumferential speed of the grinding wheel have a significantly larger width with an increased thickness (figure 5).process are of the flow type.In both cases the surface is smooth and curled, however, there are chips of considerable width removed by abrasive grains during the operation of the composite grinding wheel.
Considering the technological operation in details, using composite grinding wheels of various structures, the achieved result can be explained as follows: during the cutting process, chips of considerable width are formed, resulting from the work of abrasive particles on the rail tread surface with a general decrease in the number of formed chips [15,19,20].
Considering the grinding process, it has been found that the values of the rail stock removal rate increase with a decrease in the hardness of the composite grind wheel.It is due to the fact that during the operation of a composite grind wheel of lower hardness, the cutting ability of such a tool is higher, since the grains involved in the treatment process renew the cutting surface of the tool much faster, being released in a timely manner from the binder component of the abrasive tool shard, with the maximum use of the abrasive grain resource.
Also, the grinding coefficients (Kgr) for solid and composite grinding wheels with modified abrasive nanostructured ceramic binders (MANCB) have been determined for different technological treatment modes.A graphical interpretation of the results of the grinding coefficients is shown in figure 6.Thus the grinding coefficient while using the studied composite grinding wheel and effective technological modes was 4.2.While using a standard grinding wheel and treatment modes Kw =2.5.During treatment using studied composite grinding wheel in standard modes, the grinding coefficient was 3.3.
In process of experimental studies aimed at identification of factors that affect the performance and durability of composite grinding wheels used in grinding operations, it was possible to obtain correlations using the durability of the used grinding wheels can be adjusted the most productive process of treatment the rolling surface of the rail heads can be provided.
Tests conducted in "PKF" "SIMS" LLC, Samara (Russian Federation), on the grinding of railway rails using composite grinding wheels with alternating cutting elements of two types with different hardness and grain size of the abrasive, made it possible to reduce the treatment time of the working surface of the head of railway rails from 12 min 48 sec to 9 min 48 sec, as a result, it was possible to increase the productivity of operational process of grinding of the upper part of the working surface of the heads of railway rails by 24% or 1.35 times.In these circumstances, the most effective abrasive tool turned out to be a grinding composite wheel with the following features of segments on the working surface: grain size -F14, structure density (St 3), hardness -T.

Conclusion
Thus, in course of the study, a composite grinding wheel consisting of alternating cutting elements of two types has been developed.The first type of elements is made of abrasive grains on a bakelite bond, and the second type is made of abrasive grains on a nanostructured ceramic bond.The use of it can increase the productivity of grinding of the upper parts of the rolling surface of the heads of railway rails by rail-grinding trains of the RSHP type and mobile rail-grinding units by 1.35 times.

Figure 2 .
Figure 2. Model of the modified AT binding bridge.

Figure 3 .
Figure 3. Experimental samples of composite grinding wheels: a -for a manually operated machine; b -for rail grinding train.

Figure 4 .
Figure 4. Influence of physical and mechanical factors of the composite wheel on the amount of stock removal rate with a wheel grain size of F14.

Figure 5 .
Figure 5. Fragments of the microstructure of the E76F rail steel chips obtained after traditional treatment -(a) and composite grind wheel -(b) zoomed by x500.

Figure 6 .
Figure 6.Grinding coefficient for solid cast and composite (MANCB) grinding wheels for different treatment modes.

Table 1 .
Influence of the nanomodifier content on the physical and mechanical properties of the ceramic segment of the grinding wheel.