**2. Effect of carbide grade on machining parameters**

The study investigated the inserts made of tool carbides of (WC-TiC-Co) and (WC-TiC-TaC-Co) groups (without wear-resistant coatings). The machining was conducted at standard cutting modes typical for locomotive depots: the milling cutter rotational speed was 93 rpm (at the cutting speed of 60 m/min), the working feed rate was 120–160 mm/min, and the allowance was taken in 23 passes (at the depth of cut of 23 mm per pass). Complete failure (blunting) of more permissible value observed on the tested carbide inserts due to macro- and microchipping along the periphery of the rake face was taken as a criterion for blunting of the tested carbide inserts. The tool life indicators for the tested carbide inserts were defined through the ratio between the number of wheel sets machined and a complex set of inserts made of a certain grade of carbide (considering normally worn, broken, inverted, and replaced inserts). The test results are presented in **Figure 3**.

The analysis of the obtained results shows that the modern carbides of the (WC-TiC-TaC-Co) group have wear resistance 1.5–2.0 times higher than the carbides of the (WC-TiC-Co) group. Meanwhile, the change in manufacturing conditions produces less effect on wear resistance of the carbides of the (WC-TiC-TaC-Co) group.

Following the analysis of the external wear pattern and the structure of failures of the cutting edges on the standard RNGX 121200-shaped carbide inserts, it can be noted that in milling tire steel, for the carbide inserts of the (WC-TiC-Co)

*Main Ways to Improve Cutting Tools for Machine Wheel Tread Profile DOI: http://dx.doi.org/10.5772/intechopen.80302* 

**Figure 3.** 

*Results for the tests of RNGX 121200-type carbide inserts made of various carbide grades: 1—WC-15%TiC-6%Co, 2—WC-14%TiC-8%Co, 3—T5 (WC-TiC-TaC-Co), and 4—T1 (WC-TiC-TaC-Co).* 

#### **Figure 4.**  *Failure patterns on (WC-TiC-Co) (a) and (WC-TiC-TaC-Co) carbide inserts (b).*

 (78.0%WC, 14.0%TiC, 8.0%Co and 79.0%WC, 15.0%TiC, 6.0%Co) grades, currently most widely used for wheel milling, the most typical mechanism of failure is brittle fracture of the cutting edges as macrochipping on the rake and flank faces of the carbide inserts with depth over 3 mm (see **Figure 4a**). Meanwhile, for cutting inserts of the (WC-TiC-TaC-Co) carbides (T1 grade 79%WC, 4.4%TiC, 3.6%TaC, 5.8%Co and T5 grade 78.2%WC, 4.0%TiC, 5.0%TaC, 6.0%Co), failure occurs as blunting of the cutting edges due to wear and microchipping along the periphery of the rake face, due to contact-fatigue chipping with an area of 1.5–2.0 mm2 and a depth of 0.3–0.5 mm (see **Figure 4b**). The obtained results are in good agreement with the data of production tests of various carbide grades in wheel milling of wheel sets of locomotives, electric, and subway trains on machines of the KZH-20 type [1].

The investigation of the wear rate of the carbide inserts of various carbide grades along the wheel tread profile is shown in **Figure 5**. The analysis of **Figure 4** shows that for the (WC-TiC-Co) and (WC-TiC-TaC-Co) carbides groups, the highest wear rate of the carbide inserts was registered on the area of the wheel tread until the circular cutting of ridge (with the maximum in a plane from the wheel rolling circle), with the presence of thermomechanical defects on the wheel tread (slides, white spots, chipping, etc.).

 A slight increase in the wear rate was also registered at the ridge top where pointed rolling appeared. Meanwhile, it should be noted that the wear rate of carbide inserts of the (WC-TiC-TaC-Co) carbide group is more uniform in the wheel tread profile, which makes it possible to significantly reduce the tool costs by installing carbide inserts of various carbide grades in the milling cutters. Thus, the fifth-tenth cutter knife seats on the wheel rolling circle face should

**Figure 5.**  *Wear rates for the carbide inserts of various carbide grades depending on their placement in cutter knife seats.* 

bear more stable expensive carbides of the (WC-TiC-TaC-Co) group, while cheaper carbides of the (WC-TiC-Co) group can be installed to less critical sections of the profile. This technology provides a decrease in the cutting tool costs by 30% at maximum.

According to the basic principles of the theory of metal cutting, in turning and milling with carbide inserts, the changes in the pattern of the chip formation process and the temperature-stressed state on the tool cutting edges are determined by the geometric parameters of the sharpening of the cutting edges on the carbide inserts. At present, for the modern carbide inserts used to machine wheel sets of locomotives (LNMX 191940, SNMM 190616, RPUX 2709MO, and RNGX 121200 shapes), the geometry of the cutting edge sharpening is determined by two main parameters: the inclination angle of the negative reinforcing wear land (γF) on the rake face and the width of the negative reinforcing wear land (f).

 In [17, 18], it was found that under conditions of intermittent cutting, the creation of a negative wear land on the rake face of the carbide insert increases the mechanical and heat resistance of the cutting edge. If there is a negative wear land on the cutting edge, the center of chip pressure shifts from the top of the cutting wedge and thereby increases the cutting edge strength. In this case, the angle of sharpening exceeds 90°, and the cutting wedge starts working under the conditions of compression deformation (in contrast to the bending conditions when a sharp tool is used). Moreover, an increase in the angle of sharpening improves the conditions for the heat transfer from the cutting edge to the tool body.

For carbide inserts used for wheel turning of locomotive tires (LNMX 191940, SNMM 190616, and RPUX 2709MO shapes), the maximum wear land width is

#### *Main Ways to Improve Cutting Tools for Machine Wheel Tread Profile DOI: http://dx.doi.org/10.5772/intechopen.80302*

 limited by the start of the chip-breaking groove and is f = 0.4–0.6 mm on average. The carbide inserts of the RNGX 121200 shape (used for wheel milling) have the reinforcing wear land of f = 0.1–0.2 mm on average. The special study to determine the effect of the width of the reinforcing wear land f on the tool life of cutting tools in machining the tire steel showed (see **Figure 6**) that regardless of the tool material properties, a reduction in the wear land width led to a significant decline in the tool life indicator. For example, for inserts made of the (WC-TiC-TaC-Co) carbides with high hardness of carbide matrix and the (WC-TiC-Co) carbides with high brittleness of carbide matrix, a reduction of the reinforcing wear land by two times led to a decrease in the tool life indicators for the inserts on average by 45–55% and 80–90%, respectively. The analysis of the wear patterns on the inserts showed the presence of considerable plastic deformation of the cutting edge, while for the (WC-TiC-Co) carbides, in the area of maximum wear of the carbide insert cutting edge, there were formation centers of microcracks, chipping, and brittle fracture.

 According to the data of [18], finishing-reinforcing machining by the method of cutting edge rounding makes it possible to achieve an increase in the depth of penetration of residual compressive stresses with simultaneous reduction of their gradient in a thin surface layer; i.e., it leads to the creation of a favorable profile of the residual compressive stresses in the near-surface layer of the insert. Meanwhile, during cutting, a tip of the cutting wedge will be under the action of compressive rather than tensile stresses, like in cutting with a sharp cutting edge, and smooth rounding of the edges ensures no voltage concentrators [17]. The production experimental tests conducted in wheel milling of wheel sets of locomotives and subway trains on machines of the KZH-20 type showed that the use of the inserts of the RNGX 121200 shape from the (WC-TiC-Co) and (WC-TiC-TaC-Co) carbide groups with a radius of rounding r = 0.06–0.08 mm increases the tool life indicators by 30 and 25%, respectively, with a general reduction in the number of large chipping and chipping of the cutting edges, as compared to the inserts with the width of the sharpened negative wear land of 0.1–0.2 mm.

#### **Figure 6.**

*Relation between the width of the reinforcing wear land f and the tool resistance of the cutting tools made of various carbide grades in machining the tire steel, where Kwr is tool resistance coefficient, at f = 0.15 mm, Kwr = 1.0.* 
