RU2430166C1 - Procedure for strenghening railroad wheels and railroad wheel with strengthened working surface - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: metal surface strengthening is carried out by means of low temperature plasma arc of direct action. As plasma generating gas there is used argon, mixture of argon with helium, and mixture of argon with carbon containing gases. Also, heating is performed with magnetic oscillation of arc with triangle shape of voltage pulse, scanning amplitude 20-45 mm and distance of treatment 10-30 mm. A strengthened surface layer consists of several sub-layers: at depth of 0.2-0.8 mm - structure of martensite with inclusions of upper bainite, - from 0.8 to 1.8 mm - trosto-martensite structure, - from 1.8 to 2.5 mm - sorbitic-martensite structure, - from 2.5 to 3 mm - sorbite, - over 3 mm - ferrite-pearlite structure of basic metal.

EFFECT: raised efficiency of strengthening process, 3 times increased wear resistance of strengthened working zone of wheel, 3 times reduced probability of development of hydrogen metal brittleness and tendency to brittle fracture.

2 cl, 1 tbl

Description

The invention relates to the field of mechanical engineering and heat treatment of metals, can be used in the manufacture of new railway wheels, as well as in the process of their operation on railway and other rolling stock.

A known method of improving the volumetric heat treatment of railway wheels (RU 2124056 C1, 12.27.98, IPC C21D 9/34) and bandages using volumetric furnace heating, which consists in regulating the heating and cooling rate during quenching and subsequent tempering. The disadvantages of this technical solution include low indicators of wear resistance and cracking due to the fact that there is a heat treatment of the entire surface of a large product.

There is also known a method of plasma heat treatment of products, which consists in heating the surface of the products with a plasma arc created by the anode and cathode, directed at an angle to the processing plane, placing the anode spot on the anode and exposing the electric arc to a constant magnetic field (RU 2121514 C1, 10.11.1998, IPC C21D 1/09). The disadvantages of this technical solution include the fact that the low-temperature plasma is defocused due to reflection from the rotating anode, indirectly heats the surface layer with the high-temperature gas, the lowest temperature region of the plasma, and is not a concentrated source of heating in physical essence, and therefore, the heating and cooling rates are very low and comparable to flame quenching.

Wheel sets are known that are obtained as a result of processing by the hardening method described in patent RU 2183223 C2, 10.06.2002. The disadvantages of this wheel include the shallow depth of the hardened layer, which affects the optimal combination of hardness and ductility of the hardened layer and the formation of sliders and white spots on the surface during braking.

This, in turn, does not allow obtaining a finely dispersed martensite structure in the surface layer, as a result of low indicators of increasing wear resistance.

The closest analogue according to claim 1 is a method of hardening a railway wheel, including heating the working surface of the wheel with a plasma arc of a plasma torch located perpendicularly and remotely to the working surface of the rotating wheel, and subsequent surface hardening (RU 2064511 C1, dated July 27, 1996, IPC C21D 9 / 34, C21D 1/09). The plasma jet (indirect arc) is placed perpendicular to the surface of the wheel and with electric torch values of 35-45 kW, linear wheel rotation speed of 4-10 mm / min, and hardening distance of 40-55 mm, the surface layer of the wheel is quenched. In this case, 13-18% sorbitol is considered the optimal structure of the surface layer, the rest is martensite. The process is conducted without melting the surface layer. This analogue describes a railway wheel having a hardened layer on the working surface. The studies conducted by the authors showed that when hardening the working surface of the wheels by quenching with nitrogen plasma, several types of structures are possible: from a structure consisting of martensite to structures containing a different percentage of quenching sorbitol in combination with martensite. According to the test results, in which the actual operating conditions of the wheels were modeled, the optimal structure was selected, which consists of 130-18 percent of quenching sorbitol, the rest is martensite. However, such a microstructure of the wheel does not have the optimal combination of strength and ductility, which leads to its reduced service life.

As a plasma-forming gas, nitrogen and air are used. This method is closest to the proposed method of hardening and adopted as a prototype of the invention.

Significant disadvantages of the method adopted for the prototype are:

- low efficiency of the hardening process: 0.55-0.70 (most of the heat generated in the indirect plasma torch is used to heat the anode, not the product);

- high power of the plasma torch 35-45 kW, which increases the cost of hardening the wheelset;

- high power of the plasma torch, which leads to an increase in the dimensions of the equipment and the complexity of their maintenance;

- the large dimensions of the equipment used, which do not allow it to be compactly placed under an electric locomotive;

- low speeds of rotation of the wheel (4-10 mm / s), which leads to overheating when the wheelset is heated and, as a result, to obtain a large size of austenitic grain, which during the next cooling operation leads to average sizes of martensite crystals and the formation of a ferrite-pearlite mesh over the entire cross section of the hardened layer;

- a large processing distance, which causes the suction of the atmosphere into the plasma jet and contributes to the oxidation of the surface layer and hydrogen adsorption. Hydrogen in the hardened layer contributes to the formation of cracks and hydrogen embrittlement, and therefore, to reduce wear resistance (Balanovsky A.E., Nesterenko N.A. The problem of hydrogen in plasma surface hardening. "Welding", 1992, No. 11, p.13-15) ;

- the minimum and maximum width of the hardened layer are not indicated;

- the minimum and maximum hardness of the surface layer are not indicated.

The task to which the claimed technical solution is directed is to reduce the energy intensity of the process, simplify the processing process, increase the wear resistance of railway wheels by obtaining the optimal thickness and structure of the surface layer.

To solve the problem, the method of hardening railway wheels includes heating the working surface of the wheel with a plasma jet of a plasma torch located perpendicularly and remotely to the working surface of the rotating wheel, and subsequent surface hardening, while the working surface is heated with a direct-acting argon plasma arc with a linear wheel speed of 6-32 mm / s and the processing distance of 10-30 mm, and during heating, magnetic oscillation of the plasma arc with a triangular pulse shape on maskers and scanning amplitude of 20-45 mm.

A railway wheel having a hardened layer on the working surface according to the invention is characterized in that the hardened surface layer is obtained by the method according to claim 1, while it has a martensite structure interspersed with upper bainite at a depth of 0.2-0.8 mm, at a depth of 0.8 to 1.8 mm, a reed-martensitic structure, at a depth of 1.8 to 2.5 mm, a sorbitol-martensitic structure, at a depth of 2.5 to 3.0 mm, a sorbitol structure, and more than 3 mm deep - ferrite-pearlite structure.

The use of a plasma arc of direct action allows you to reduce the power of the plasma torch by 2–3 times. The use of argon, argon mixed with helium, argon mixed with carbon-containing gases as a plasma-forming gas allows to reduce the open circuit voltage of the power source from 200 V to 60-90 V, which reduces the energy consumption of the hardening process, reduces the dimensions of the equipment, protects the treated surface from oxygen and hydrogen.

The use of magnetic scanning of the arc allows to increase the area of the processed surface, and the use of a sawtooth (triangular) shape of the voltage pulse on the magnetic system ensures uniform heating over the entire scanning amplitude without the formation of stagnant zones (where surface melting is possible), as in the case of a sinusoidal, rectangular, trapezoidal pulse shape voltage. Heating by a scanning plasma arc allows increasing the depth of the hardened layer due to the longer residence time of the metal at high temperatures, which favorably affects the course and completeness of phase and structural processes in the surface layer of the metal.

These advantages, in comparison with the prototype, are achieved by the fact that the heating by a scanning plasma arc is carried out under the mandatory conditions of the heating process by a direct-acting plasma arc with a power to melt the surface of the products, with a scanning amplitude of 20-45 mm, a linear speed of the product of 6-32 mm / s, with a processing distance of 10-30 mm.

Tests of the proposed method of plasma-arc hardening showed that, subject to the conditions of hardening within the specified limits, the highest positive hardening results are achieved.

The simultaneous intensive cooling of the hardened surface of the product allows for the intensive removal of heat accumulated during hardening in the volume of the metal, and to ensure uniform distribution of hardness along the width and depth of the hardened layer.

The inventive method is illustrated by the following example. Since the inventive method is mainly focused on a specific application - railway wheels, it is necessary to comply with the requirements of VNIIZhT existing on railway transport in the field of road safety of rolling stock.

Implementation of the technological solution in accordance with the above requirements was carried out on a serial installation UPNS-304 (factory "Electric", RF) with a modernized plasma torch with a magnetic oscillation system. As samples used segments cut from railway wheels (wheeled steel grade 2, GOST 10791-89). As a movement mechanism, a table with smooth adjustment of linear movement from 0 to 60 mm / s was used.

At the beginning, the hardening process was carried out on a model specimen, on which, by varying the power of the plasma torch, linear displacement velocity, amplitude and frequency of magnetic oscillations, hardening modes were determined for their compliance with the requirements of the technological instruction. Each technological parameter was investigated for minimum and maximum, with the other parameters unchanged, but so that the hardening process was carried out, i.e. quenching processes occurred in the surface layer. In this case, a metallographic express assessment of the samples for microhardness, width and depth of the hardened layer was made.

The initial power (1 kW) of the plasma arc does not lead to structural and phase transformations in the surface layer of the wheel steel, however, steam measurements recorded a temperature of 490 degrees Celsius on the surface, which is clearly not enough to begin structural rearrangements in the metal. The first signs of quenching were recorded with increasing power of the plasma arc to 2.5 kW, with other parameters unchanged. A further increase in power leads to an increase in the width and depth of the surface layer of the metal, in which structural and phase transformations proceed more fully, which is also confirmed by the linear nature of the increase in hardness. At this stage, in order to establish the optimal parameters of the hardening regimes, we used the method of measuring only surface hardness on the Rockwell scale, since the method of measuring microhardness requires the preparation of microsections, which is very cumbersome and laborious at the stage of determining and finding the optimal hardening parameters. However, in the future, all the parameters found were checked using metallographic studies with microhardness measurements (as required according to the technical conditions (VNIIZhT)).

Thus, on the basis of the conducted studies, the optimal parameters of the surface hardening modes of railway wheels were established, namely, when the scanning plasma arc is heated under the required conditions of the heating process by a direct-acting plasma arc with a power equal to 0.75-0.95% of power (13 kW ) for melting the surface of products, with a scanning amplitude of 20-45 mm, a linear speed of the product 6-32 mm / s, a processing distance of 10-30 mm.

Subsequently, full-scale tests of the proposed method were carried out in the conditions of the Irkutsk-Sorting depot, more than 500 wheel pairs of electric locomotives were processed in the claimed method. The implementation of the invention was carried out on the basis of the locomotive depot Irkutsk-Sortirovochny, while using the standard rotator of the wheelset of locomotives (pumped out from under the electric locomotive). This rotator is used to test wheelsets during ultrasonic inspection, and it provides a linear rotation speed in the range of 4-5 mm / s. The equipment used was the UPN S-304 installation (Electric factory, RF) with a modernized plasma torch. The hardening modes were as follows: the plasma arc power was 12 kW, which corresponds to 92% of the power (13 kW) required for surface melting at constant other parameters, linear rotation speed 25 mm / s, scanning amplitude 45 mm, hardening distance 20 mm, shape pulse is triangular.

The wheel hardened by the method described above contains a surface layer consisting of several sublayers: the surface and at a depth of 0.2-0.8 mm is a martensite structure interspersed with upper bainite, then 0.8-1.8 mm is reed-martensite.

Data on the hydrogen content in the hardened layer are given in table. It is seen that under optimal conditions of surface hardening, there is a significant decrease in hydrogen in the hardened layer, which reduces the likelihood of hydrogen brittleness of the metal, in addition, the table shows a decrease in the wear rate in comparison with the prototype.

The proposed method of surface plasma-arc hardening of railway wheels, in comparison with the prototype, allows to increase the width of the hardened surface by more than 2 times, the hardening depth by 1.5 times, reduce the likelihood of the development of hydrogen brittleness of the metal and the tendency to brittle fracture by at least 3 times, wear resistance increases by 3.5 times.

Physico-chemical and technical characteristics of the hardened surface Hardness, MPa Depth mm The hydrogen content in cm ^3/100 g Wear after run of 5000 km, mm Original wheelset 300 - 0.5-1.5 7-8 Prototype 550 1,5-2 1.8-5.8 4.2-5.1 The proposed method 700-850 1,5-3 0.8-1.0 1.0-1.9

Claims (2)

1. A method of hardening railway wheels, including heating the working surface of the wheel with a plasma jet of a plasma torch located perpendicularly and remotely to the working surface of the rotating wheel, characterized in that the heating of the working surface of the wheel is carried out by a direct-acting argon plasma arc with a linear wheel speed of 6-32 mm / s and the processing distance of 10-30 mm, and during heating, magnetic oscillation of the plasma arc with a triangular voltage pulse and a scanning amplitude of 20 -45 mm. 2. A railway wheel having a hardened layer on the working surface, characterized in that the hardened surface layer is obtained by the method according to claim 1, wherein at a depth of 0.2-0.8 mm, the hardened layer has a martensite structure interspersed with upper bainite, at a depth from 0.8 to 1.8 mm — the reed-martensitic structure, at a depth of 1.8 to 2.5 mm — the sorbitol-martensitic structure, at a depth of 2.5 to 3.0 mm — the sorbitol structure, and at a depth more than 3 mm - ferrite-pearlite structure.