Steel rail modifying and strengthening method based on friction stir processing

Abstract

The invention discloses a steel rail modification strengthening method based on friction stir processing, which comprises the following steps: removing oxides on the surface of a steel rail through polishing, and wiping the surface of the steel rail; a proper stirring head is selected, the rotating speed, the advancing speed, the downward pressure and the dip angle parameters of the stirring head during stirring friction machining are set in a controller, and a low-heat-input technological parameter combination is selected; rotating the stirring head at a high speed and moving downwards until the downward pressure between the stirring head and the surface of the steel rail reaches a preset value; after the stirring head rotating at a high speed and the surface of the steel rail are pressed tightly, in-situ rotating friction is conducted till the set time is reached; the stirring head horizontally moves in the set advancing direction while rotating, the surface of the key weak area of the steel rail is machined, and grain crushing and structure refining are achieved; after the machining process is completed, the stirring head is pulled back upwards and leaves the surface of the steel rail. According to the steel rail modifying and strengthening method based on stirring friction machining, the technological process is simplified, and operation is convenient and fast.

Description

Technical Field

[0001] The present invention relates to the technical field of friction stir processing, in particular to a friction stir processing modification technology, and more specifically, to a method for modifying and strengthening steel rails based on friction stir processing. Background Art

[0002] Friction stir processing is a low heat input solid-phase modification treatment technology that can break grains, refine the microstructure, and improve hardness and wear resistance. When applying it to the modification and strengthening of steel rails, it is necessary to pay attention to avoiding the formation of martensite structure while achieving modification and strengthening to meet the requirements of Chinese railway industry standards. Taking the U75V type steel rail as an example, according to the measured values, the temperature at which pearlite starts to austenitize (Ac1) and the temperature at which it completely transforms into austenite (Ac3) in the steel rail are 742 °C and 774 °C respectively; the starting temperature of martensite transformation (M S ) is 216 °C, and the critical cooling rate is about 3.5 °C / s. Generally, two methods can be used to inhibit the formation of martensite structure: one is to limit the peak temperature below 742 °C to avoid austenitization of the steel, so that martensite will not form during the cooling process; the other is to reduce the cooling rate so that when it is lower than 3.5 °C / s, the austenitized steel will not undergo martensite transformation. Therefore, during the friction stir processing, the peak temperature can be made lower than the austenitization temperature or the cooling rate can be made lower than the critical cooling rate of martensite transformation by reducing the rotational speed or the traveling speed respectively, thereby inhibiting the formation of martensite. This has been found to be achievable in the friction stir welding of steel rails, but there is no relevant report in the friction stir surface modification and strengthening of steel rails.

[0003] Railway steel rails are under the long-term impact and friction of vehicles, and it is extremely easy to form wear and low collapse on the surface of key weak areas (weld joint softening areas, turnouts, curves, etc.), inducing cracks and then leading to major accidents such as rail breakage, endangering the safety of train operation. Therefore, how to perform surface modification and strengthening treatment on the key weak areas of steel rails, improve the anti-impact wear performance of joints, and ensure the safe and smooth transportation is an urgent problem to be solved in the current railway field.

[0004] Currently, the main methods for modifying and strengthening steel rails are laser cladding strengthening, laser quenching strengthening, laminar plasma strengthening or ultrasonic vibration impact strengthening. The biggest problem with ultrasonic vibration impact is the unstable energy output, resulting in poor performance stability; laser cladding strengthening is prone to form weak bonding and pore defects between the cladding layer and the steel rail substrate, and the methods of laser and laminar plasma strengthening have too high temperature and fast cooling rate, which are prone to form martensite structure. Although the cooling rate can be reduced by means of auxiliary heat sources and other methods to inhibit the formation of martensite structure, the process flow is relatively complex and the operation is not convenient enough. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide a method for modifying and strengthening steel rails based on friction stir processing, so as to simplify the process flow and make the operation convenient.

[0006] Based on the above object, the present invention provides a method for modifying and strengthening steel rails based on friction stir processing, including the following steps: Step 1, remove the oxides on the surface of the steel rail by grinding, wipe the surface of the steel rail, and remove the oil stains and foreign matters on the surface of the steel rail; Step 2, the friction stir welding machine includes a main shaft, a controller and a stirring head installed on the main shaft. According to the required modification depth and range of the steel rail, select a suitable stirring head, and set the rotation speed, traveling speed, downward pressure, and inclination angle parameters of the stirring head during friction stir processing in the controller, and select a process parameter combination with low heat input; Step 3, rotate the stirring head at a high speed and move it downward until the downward pressure between the stirring head and the surface of the steel rail reaches a preset value; Step 4, after the high-speed rotating stirring head is pressed against the surface of the steel rail, rotate and rub in place for a set time to accumulate heat, so that the temperature of the steel rail rises, and the softening of the material in the area to be processed on the surface of the steel rail is realized; Step 5, the stirring head rotates while moving horizontally along the set traveling direction to process the surface of the key weak areas of the steel rail, realizing grain crushing and tissue refinement; Step 6, after the processing process is completed, the stirring head is retracted upward and leaves the surface of the steel rail; Step 7, perform grinding on the steel rail to restore the profile of the steel rail.

[0007] Optionally, the rotation speed is 300 r / min, the traveling speed is 30 mm / min, the inclination angle is 2.5°, and the downward pressure is 12 kN.

[0008] Optionally, in Step 7, mechanical processing and grinding are used to restore the profile of the steel rail.

[0009] The method for modifying and strengthening steel rails based on friction stir processing provided by the present invention, according to the required modification depth and range of the steel rail, by selecting a process parameter combination with low heat input, that is, setting the rotation speed, traveling speed, downward pressure, and inclination angle parameters of the stirring head during friction stir processing, so as to use the friction stir welding machine to process the surface of the key weak areas of the steel rail, realizing grain crushing and tissue refinement on the surface of the steel rail, improving the surface hardness and wear resistance of the steel rail, and can obtain low peak temperature and cooling rate under the conditions of low rotation speed and low traveling speed, which can inhibit the formation of martensite, simplify the process flow, and make the operation convenient. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following will describe in detail the preferred embodiments of the present invention with the help of the drawings, which will help to understand the object and advantages of the present invention, wherein:

[0011] Figure 1 is the process flow chart of the method for modifying and strengthening steel rails based on friction stir processing according to an embodiment of the present invention;

[0012] Figure 2 Schematic diagram when using a friction stir welding machine to modify and strengthen the surface of a steel rail in the friction stir processing-based steel rail modification and strengthening method according to an embodiment of the present invention;

[0013] Figure 3 Schematic diagram of Vickers hardness test in the heat affected zone during the steel rail modification and strengthening experiment of friction stir processing the surface of the steel rail by using high heat input process parameters, medium heat input process parameters, and low heat input process parameters respectively.

[0014] Description of the reference numerals:

[0015] 1: Friction stir welding machine; 2: Main shaft; 3: Stirring head; 4: Steel rail. Specific implementation mode

[0016] The present invention will be described in detail below in conjunction with the embodiments. The same components are denoted by the same reference numerals. It should be noted that the terms "front", "rear", "left", "right", "upper" and "lower" used in the following description refer to the directions in the drawings, and the terms "inner" and "outer" refer to the directions towards or away from the geometric center of a specific component respectively.

[0017] As Figure 1 and Figure 2 shown, the friction stir processing-based steel rail 4 modification and strengthening method provided by the present invention includes the following steps: Step 1, remove the oxides on the surface of the steel rail 4 by grinding, wipe the surface of the steel rail 4, and remove the oil stains and foreign matters on the surface of the steel rail 4; Step 2, the friction stir welding machine 1 includes a main shaft 2, a controller, and a stirring head 3 installed on the main shaft 2. Select a suitable stirring head 3 according to the required modification depth and range of the steel rail 4, set the rotation speed, traveling speed, downward pressure, and inclination angle parameters of the stirring head 3 during friction stir processing in the controller, and select a process parameter combination with low heat input; Step 3, rotate the stirring head 3 at a high speed and move it downward until the downward pressure between the stirring head 3 and the surface of the steel rail 4 reaches a preset value; Step 4, after the high-speed rotating stirring head 3 is pressed against the surface of the steel rail 4, rotate and rub in place for a set time to accumulate heat, so that the temperature of the steel rail 4 rises, and the softening of the material in the area to be processed on the surface of the steel rail 4 is realized; Step 5, while the stirring head 3 is rotating, move horizontally along the set traveling direction to process the surface of the key weak area of the steel rail 4 to realize grain crushing and tissue refinement; Step 6, after the processing is completed, the stirring head 3 is withdrawn upward and leaves the surface of the steel rail 4; Step 7, perform grinding on the steel rail 4 to restore the profile of the steel rail 4.

[0018] It should be noted that: in Step 4, the set time can be 45 seconds.

[0019] The rail 4 modification and strengthening method based on friction stir processing provided by the present invention, according to the required modification depth and range of the rail 4, by selecting a process parameter combination with low heat input, that is, setting the rotational speed, travel speed, downward pressure, and inclination angle parameters of the stirring head 3 during friction stir processing, so as to use the friction stir welding machine 1 to process the surface of the key weak areas of the rail 4, realize the grain crushing and microstructure refinement of the rail 4 surface, improve the surface hardness and wear resistance of the rail 4, and can obtain a low peak temperature and cooling rate under the conditions of low rotational speed and low travel speed, which can inhibit the formation of martensite, simplify the process flow, and is convenient to operate.

[0020] In one embodiment of the present invention, the rotational speed is 300 r / min, the travel speed is 30 mm / min, the inclination angle is 2.5°, and the downward pressure is 12 kN. The setting of the above parameters is the process parameter combination with low heat input, which can achieve the modification and strengthening of the rail 4 while avoiding the formation of martensite structure.

[0021] In one embodiment of the present invention, in step seven, mechanical processing and grinding are used to restore the profile of the rail 4, and the mechanical processing and grinding operation is convenient.

[0022] In one embodiment of the present invention, a plate with a length of 200 mm, a width of 70 mm, and a thickness of 10 mm is cut from a U75V rail, and a non-pin stirring head with a shoulder diameter of 12 mm is used to conduct rail modification and strengthening experiments on the rail surface by using high heat input process parameters, medium heat input process parameters, and low heat input process parameters respectively. Among them: the high heat input rotational speed is 400 r / min, the travel speed is 30 mm / min, the inclination angle is 2.5°, and the downward pressure is 12 kN; the medium heat input rotational speed is 350 r / min, the travel speed is 30 mm / min, the inclination angle is 2.5°, and the downward pressure is 12 kN; the low heat input rotational speed is 300 r / min, the travel speed is 30 mm / min, the inclination angle is 2.5°, and the downward pressure is 12 kN. Vickers hardness tests are carried out in the heat affected zone. As Figure 3 shown, the Vickers hardness of the base material is about 320 HV. After being processed by friction stir processing, when using high heat input process parameters, the hardness of the heat affected zone can reach 700 HV, when using medium heat input process parameters, the hardness of the heat affected zone can reach 550 HV, and when using low heat input process parameters, the hardness of the heat affected zone is about 350 HV. Generally speaking, after martensite structure appears in the rail, the hardness will increase significantly. As can be seen from Figure 3 it, when using low heat input process parameters, it can improve the surface hardness of the rail while avoiding the generation of martensite structure.

[0023] The rail modification and strengthening method based on friction stir processing provided by the present invention, according to the required modification depth and range of the rail, by selecting a process parameter combination with low heat input, that is, setting the rotational speed, traveling speed, downward pressure, and inclination angle parameters of the stirring head during friction stir processing, so as to use a friction stir welding machine to process the surface of the key weak areas of the rail, realize the grain crushing and microstructure refinement of the rail surface, improve the hardness and wear resistance of the rail surface, and can obtain a low peak temperature and cooling rate under the conditions of low rotational speed and low traveling speed, which can inhibit the formation of martensite, simplify the process flow, and is convenient to operate.

[0024] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for modifying and strengthening steel rails based on friction stir processing, characterized in that, It includes the following steps: Step 1: Remove the oxides on the surface of the rail by grinding, wipe the surface of the rail, and remove the oil stains and foreign matters on the surface of the rail; Step 2: The friction stir welding machine includes a main shaft, a controller, and a stirring head installed on the main shaft. Select a suitable stirring head according to the required modification depth and range of the rail. Set the rotation speed, travel speed, downward pressure, and inclination angle parameters of the stirring head during friction stir processing in the controller, and select a process parameter combination with low heat input; Step 3: Rotate the stirring head at a high speed and move it downward until the downward pressure between the stirring head and the surface of the rail reaches a preset value; Step 4: After the high-speed rotating stirring head is pressed against the surface of the rail, rotate and rub in place for a set time to accumulate heat, so that the temperature of the rail rises, and the material in the area to be processed on the surface of the rail is softened; Step 5: The stirring head rotates while moving horizontally along the set travel direction to process the surface of the key weak areas of the rail, realizing grain crushing and microstructure refinement; Step 6: After the processing is completed, the stirring head is retracted upward and leaves the surface of the rail; Step 7: Grind the rail to restore the rail profile.

2. The method for modifying and strengthening steel rails based on friction stir processing according to claim 1, characterized in that The rotation speed is 300 r / min, the travel speed is 30 mm / min, the inclination angle is 2.5°, and the downward pressure is 12 kN.

3. The method for modifying and strengthening steel rails based on friction stir processing according to claim 1, characterized in that, In Step 7, the rail profile is restored by mechanical machining and grinding.

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