Mirror surface machining method for outer circle of piston made of M42 quenched steel material

Abstract

The invention relates to a part surface machining method, in particular to a piston outer circle mirror surface machining method of an M42 quenched steel material, and aims to solve the technical problem that according to an existing mirror surface machining method, the surface roughness of a piston workpiece of the M42 quenched steel material cannot meet the lowest mirror surface effect requirement easily. The method comprises the steps of piston machining through a turning technology, heat treatment, precise turning of an inner hole and an outer cylindrical surface, precise grinding of the outer cylindrical surface, grinding and polishing of the outer cylindrical surface and vibration grinding and polishing of the outer cylindrical surface, and finally a piston finished product with the outer cylindrical surface achieving the mirror surface effect is obtained. According to the piston machined through the machining method, the surface roughness Ra can reach 0.01 mu m-Ra 0.02 mu m, the surface is flat and smooth, natural light can present the machining state of clear mirror reflection, the surface is free of microscopic protrusions and machining lines, visual flaws are avoided, the bare hand touch feeling is not blocked, and the mirror effect is completely achieved.

Description

Technical Field

[0001] This invention relates to a method for machining the surface of a part, specifically to a method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel. Background Technology

[0002] In the field of mechanical engineering, the piston is a core moving and pressure-bearing component, widely used in various engines, compressors, hydraulic cylinders, and precision instruments. Its basic function is to perform reciprocating linear motion within the cylinder or chamber, thereby completing energy conversion, fluid compression, or power transmission. A schematic diagram of a commonly used piston is shown below. Figure 2 It includes a hollow cylindrical piston body 1, a bottom end face 102 located at one end of the piston body 1, and two side through holes 101 symmetrically arranged on the side wall of the piston body 1 for cooling oil flow. On the outer cylindrical surface 104 of the piston body 1, located on both sides of the two side through holes 101, two V-shaped annular grooves 105 for installing oil rings are also provided. For ease of description, in this document, the inner cylindrical surface of the piston body 1 and the inner surface of the bottom end face 102 constitute the inner hole 103 of the piston, and the term "piston outer circle" in this document refers to the outer cylindrical surface of the piston.

[0003] Pistons typically operate under extreme conditions of high temperature, high pressure, and high-speed reciprocating motion, thus placing extremely high demands on their materials and manufacturing processes: 1) High strength and high hardness: to withstand enormous mechanical loads and wear; 2) Excellent thermal stability (red hardness): to maintain dimensional and performance stability at high temperatures; 3) Low coefficient of friction and high wear resistance: to reduce power loss and extend service life; 4) High dimensional and geometric accuracy: to ensure a precise fit with the cylinder block, prevent media leakage, and guarantee working efficiency. To meet these extreme operating requirements, users have placed extremely high demands on the piston material and the surface roughness of the outer cylindrical surface 104.

[0004] To meet the material requirements of pistons, one method is to use M42 hardened steel to manufacture them. M42 hardened steel is a high-speed tool steel with ultra-high hardness, high wear resistance, and high red hardness. It is obtained by subjecting M42 steel to specific heat treatment (quenching and tempering), achieving a hardness value of 68-70 on the HRC (Rockwell hardness scale). This high hardness makes it excellent in resisting wear and deformation. Simultaneously, M42 hardened steel also has high compressive strength, capable of withstanding significant pressure without plastic deformation. Furthermore, it exhibits good bending strength, meeting the requirements of applications demanding high bending resistance. Finally, the hot hardness of M42 hardened steel is one of its key characteristics; it maintains high hardness and strength even at high temperatures, such as 550℃-600℃, giving it unique advantages in high-speed cutting and high-temperature machining.

[0005] To ensure the surface roughness (Ra) of the piston's outer cylindrical surface 104 meets the required standard and achieves a precise fit with the cylinder's inner cylindrical surface during application, a mirror finish is typically required. A mirror finish refers to an extremely low surface roughness, usually defined as Ra (Arithmetical Mean Deviation of the Profile) ≤ 0.02 μm, and sometimes Ra ≤ 0.01 μm. At this level, the piston's outer cylindrical surface 104 is smooth and exhibits a clear mirror reflection. To achieve this mirror finish, it is usually necessary to remove microscopic protrusions and machining lines from the piston's outer cylindrical surface 104, reducing its roughness to a visually flawless and tactilely smooth state. Polishing processes can improve the surface roughness of the piston's outer cylindrical surface 104. Polishing is a precision machining process that uses physical / chemical actions to remove minute excess material (usually at the micrometer level) from the surface of a workpiece, reducing surface roughness and improving smoothness (up to mirror finish). It does not significantly alter the dimensional accuracy or shape of the workpiece. Existing polishing methods mainly include the following four types: 1) Mechanical polishing: using tools such as grinding wheels and cloth wheels with abrasives (such as diamond powder, alumina, etc.) to achieve a smooth surface through mechanical cutting / grinding; this is the most widely used method. 2) Chemical polishing: immersing the workpiece in a chemical solution to achieve a smooth surface through microscopic dissolution; suitable for workpieces with complex shapes and has high efficiency. 3) Electrolytic polishing: using electrolytic chemical reactions to selectively dissolve the metal on the workpiece surface, achieving a uniform and bright surface effect; commonly used for materials such as stainless steel. 4) Magnetorheological polishing: utilizing the controllable rheological properties of magnetorheological fluids to achieve high-precision, low-damage polishing; suitable for precision parts. There are also other new polishing methods, such as finishing processes and vibratory grinding machine polishing.

[0006] In existing technologies, the machining method for the outer diameter of pistons made of M42 hardened steel typically includes the following steps: Step a: Machining the M42 steel piston workpiece using turning process to obtain a rough-machined piston workpiece, wherein each surface of the rough-machined piston workpiece is reserved with a machining allowance of 0.1mm to 0.3mm on each side; Step b: Heat treat the rough-machined piston workpiece to obtain the heat-treated piston workpiece. The heat treatment includes quenching and tempering. During the heat treatment process, a deformation and oxide layer of 0.05 mm to 0.1 mm are generated in the machining allowance areas on various surfaces of the workpiece. Step c: Use a grinding wheel made of white fused alumina or brown fused alumina to perform precision grinding on the outer cylindrical surface of the heat-treated piston workpiece to remove all deformation, oxide layer and machining allowance on each surface, and obtain the precision-ground piston workpiece. Step d: The outer cylindrical surface of the piston workpiece after precision grinding is mechanically polished using the cloth wheel polishing method to obtain the finished piston and complete the outer diameter machining of the piston made of M42 hardened steel.

[0007] However, this method has the following drawbacks: the machining marks generated in step a and the grinding marks generated in step c may be too large. Mechanical polishing in step d alone cannot completely remove the microscopic protrusions and / or machining marks on the outer cylindrical surface of the piston workpiece. This not only easily results in visual defects, but also the final surface roughness Ra value measured by a surface roughness meter is typically 0.03μm to Ra0.05μm, which is close to a mirror finish but not quite. To achieve a mirror finish, the mechanical polishing time in step d needs to be extended to remove the microscopic protrusions and / or machining marks on the outer cylindrical surface of the piston workpiece as thoroughly as possible. However, such an operation is highly likely to damage the piston workpiece itself, affecting its size and shape, leading to the final piston product failing to meet specifications.

[0008] Furthermore, prior art document 1, with application publication number CN119952411A, discloses a mirror finishing method and the workpiece formed therefrom, which includes: selecting a workpiece of the required size; clamping the workpiece on a machining table; selecting a first milling cutter to semi-finish the shape and size of the workpiece to the required standard size, and then using a second milling cutter and a toolpath to directly cut the machining surface of the workpiece through a mirror finishing process; cooling the workpiece while it is being milled to prevent the temperature from being too high during the milling process, which would cause tool deformation and chips to stick to the tool; and after the workpiece is finished, removing it from the machine table, thus completing the machining of the required workpiece.

[0009] Prior art document 2, with application publication number CN115890432A, discloses a precision machining method for the surface of a thrust end face friction pair, which belongs to the field of friction pair machining. It includes: processing the surface of the thrust end face friction pair by successively using a combination of rough grinding, fine grinding and polishing processes. The rough grinding and fine grinding of the surface use white corundum grinding wheels, and the polishing of the surface uses cubic boron nitride grinding wheels.

[0010] However, both Prior Artwork 1 and Prior Artwork 2 have certain limitations. Prior Artwork 1 targets "soft" stainless steel with a hardness lower than HRC 52, and its mirror milling method is completely unworkable for the HRC 68-70 material in this application (the tool cannot cut it). Although Prior Artwork 2 uses a CBN grinding wheel, the hardness of the object (HRC 56-60) and the final surface roughness requirement (Rz1.6, far from mirror finish) are both lower than those of this application.

[0011] In summary, existing mirror finishing methods are insufficient to achieve the minimum mirror finish required for the surface roughness of piston outer cylindrical surfaces made of M42 hardened steel. Summary of the Invention

[0012] The purpose of this invention is to solve the technical problem that existing mirror finishing methods are insufficient to make the surface roughness of piston workpieces made of M42 hardened steel meet the minimum mirror effect requirements, and to provide a method for mirror finishing the outer circle of pistons made of M42 hardened steel.

[0013] To achieve the above objectives, the technical solution provided by this invention is: A method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel, characterized by the following steps: Step 1: Using M42 steel as the material, the piston workpiece is machined by turning according to the preset required dimensions to obtain a rough-machined piston workpiece. A machining allowance of 0.1mm to 0.3mm is reserved on each side of each surface of the rough-machined piston workpiece. Step 2: Heat treat the rough-machined piston workpiece obtained in Step 1 to obtain a heat-treated piston workpiece. The material of the heat-treated piston workpiece is changed to M42 hardened steel, and its hardness value is expressed as HRC 68~70. During the heat treatment process, a deformation and oxide layer of 0.05mm~0.1mm are generated in the machining allowance areas on various surfaces of the workpiece. Step 3: The inner hole and outer cylindrical surface of the piston workpiece after heat treatment obtained in Step 2 are precision machined by turning process to correct deformation and remove all oxide layer and part of the machining allowance on each surface. The turning process uses CBN as the material of the effective working area of ​​the tool to obtain the precision turned piston workpiece. Then, the inner hole of the precision turned piston workpiece is used as the reference and fixed on the mandrel of the lathe. Step 4: Use a CBN grinding wheel to perform precision grinding on the outer cylindrical surface of the precision-turned piston workpiece to remove all remaining machining allowances on each surface, and obtain the precision-ground piston workpiece. Step 5: Use a grinding tool containing abrasive to perform preliminary polishing on the outer cylindrical surface of the precision-ground piston workpiece to remove microscopic protrusions and / or machining marks on the surface, and obtain the piston workpiece after preliminary polishing. Step 6: Use a vibratory grinding and polishing machine to perform mirror polishing on the outer cylindrical surface of the piston workpiece after preliminary polishing, so that the surface roughness of the outer cylindrical surface reaches the mirror effect, thus obtaining the finished piston of M42 hardened steel material, and completing the mirror surface machining of the outer cylinder of the piston of M42 hardened steel material.

[0014] Furthermore, step 2 specifically involves: Step 2.1: Perform segmented preheating on the rough-machined piston workpiece. The rough-machined piston workpiece obtained in step 1 is preheated to 500℃~600℃ and held at that temperature for a first time; then preheated to 800℃~850℃ and held at that temperature for a second time to obtain the preheated rough-machined piston workpiece. Step 2.2: Heating the preheated rough-machined piston workpiece. The preheated rough-machined piston workpiece is transferred to a high-temperature furnace at 1100℃~1200℃ to transform the crystal structure of the steel inside the rough-machined piston workpiece into uniform austenite; and then held at that temperature for a third time to allow the alloying elements inside the steel of the rough-machined piston workpiece to be fully dissolved in the austenite, thus obtaining the heated rough-machined piston workpiece. Step 2.3: Quench the roughly machined piston workpiece after heating. The heated rough-machined piston workpiece is removed from the high-temperature furnace and immediately immersed in a cooling medium to rapidly cool it to room temperature, thus obtaining the quenched rough-machined piston workpiece. Step 2.4: Temper the rough-machined piston workpiece after quenching. Step 2.4a: Remove the rough-machined piston workpiece after quenching; Step 2.4b: Immediately place the removed piston workpiece into a tempering furnace at 550℃~600℃ and hold for 1h~2h. Then remove the piston workpiece and cool it to room temperature in the air. Step 2.4c: Using the method in step 2.4b, the piston workpiece is tempered, held at a high temperature, and cooled 1-2 times to obtain the heat-treated piston workpiece. The material of the heat-treated piston workpiece is changed to M42 hardened steel, and its hardness value is expressed as HRC 68-70. The heat treatment process produces a deformation and oxide layer of 0.05mm-0.1mm on each surface of the workpiece.

[0015] Furthermore, in step 4, the mesh count of the grinding wheel is greater than 1600.

[0016] Further, in step 5, the grinding tool is a disc made of ductile iron, with a central through hole in its center for placing the outer cylindrical surface of the piston. The sidewall of the central through hole has two expansion grooves and one mounting groove evenly distributed on the circumference along the radial direction of the disc. The mounting groove communicates with the outer ring of the disc. Both expansion grooves are closed at their outer ends to disperse stress during the grinding process. A right-angle notch is provided on the outer ring of the disc on one side of the mounting groove. Two mounting holes are provided on corresponding positions on both sides of the mounting groove for screws to pass through to adjust the clamping degree of the piston. The sidewall of the central through hole has a spiral groove for coating and storing abrasive.

[0017] Furthermore, in step 5, the abrasive is white corundum micro powder.

[0018] Furthermore, in step 6, the vibration frequency of the vibratory grinding and polishing machine is 20kHz to 50kHz, and the amplitude is 5μm to 20μm.

[0019] Furthermore, in step 2.1, the first duration is 40 minutes, and the second duration is 30 minutes; In step 2.2, the third duration is 15 minutes.

[0020] Furthermore, each of the expansion slots has a circular hole-shaped groove at its closed end, and the diameter of the circular hole is greater than the width of the outer closed slot.

[0021] Furthermore, in step 5, the abrasive white corundum micro powder.

[0022] Compared with the prior art, the beneficial effects of the present invention are: 1. The piston outer cylindrical mirror surface machining method of M42 hardened steel material provided by the present invention adds a precision turning step after the heat treatment step of the prior art, which improves the positioning datum accuracy of the inner hole, so that it can be precisely matched with the mandrel, which is beneficial to the subsequent steps; and the tool material used for precision turning is CBN. CBN tools have good thermal stability and can maintain their cutting performance at high temperature. They are not prone to tool softening and accelerated wear, and are conducive to increasing cutting speed and reducing the generation of built-up edge.

[0023] 2. The piston outer cylindrical mirror surface machining method for M42 hardened steel material provided by the present invention replaces the grinding wheel material in the precision grinding step of the prior art with CBN, avoiding the problems of low hardness, high grinding force and excessive wear of existing white corundum or brown corundum grinding wheels; the high-hardness CBN grinding wheel has low grinding force and low grinding temperature, which can effectively avoid defects such as burns and cracks on the piston workpiece surface; at the same time, the CBN grinding wheel has high machining accuracy and can obtain better surface quality.

[0024] 3. The piston outer cylindrical mirror surface machining method of M42 hardened steel material provided by the present invention uses a specific abrasive tool in the initial polishing, and the material is ductile iron with relatively soft hardness. It is combined with white corundum micro powder as abrasive to effectively remove micro protrusions and / or machining lines on the outer cylindrical surface and improve the flatness of the outer cylindrical surface.

[0025] 4. The piston outer cylindrical mirror surface machining method of M42 hardened steel material provided by the present invention adds a polishing step using a vibratory grinding and polishing machine after the initial polishing, which further reduces the surface roughness of the piston workpiece outer cylindrical surface, and finally Ra can reach 0.01μm~Ra0.02μm, which meets the actual requirements for extremely low surface roughness.

[0026] 5. The piston outer circle mirror surface machining method for M42 hardened steel material provided by the present invention has each step closely linked and arranged in a reasonable and scientific manner, and will not damage the shape and size of the piston workpiece itself. Attached Figure Description

[0027] Figure 1 This is a flowchart of the piston outer cylindrical mirror surface machining method of M42 hardened steel material according to the present invention; Figure 2 This is a schematic diagram of the piston structure; Figure 3 This is a schematic diagram of the lathe structure in step 5 of the embodiment of the piston outer cylindrical mirror surface machining method for M42 hardened steel material of the present invention; Figure 4 This is a schematic diagram of the CBN material tool in step 3 of the embodiment of the piston outer circle mirror surface machining method of M42 hardened steel material of the present invention, wherein (a) is the front view of the tool and (b) is the side sectional view of the tool. The attached figures are labeled as follows: 1-Piston cylinder, 101-Side through hole, 102-Bottom end face, 103-Inner hole, 104-Outer cylindrical surface, 105-V-shaped ring groove; 2-Grinding tool, 201-Central through hole, 202-Expansion groove, 203-Mounting groove, 2031-Mounting hole. Detailed Implementation

[0028] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0029] A method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel, see flowchart. Figure 1 This includes the following steps: Step 1: Using M42 steel as the material, the piston workpiece is machined by turning according to the preset required dimensions to obtain a rough-machined piston workpiece. A machining allowance of 0.1mm to 0.3mm is reserved on each side of each surface of the rough-machined piston workpiece. Step 2.1: Perform segmented preheating on the rough-machined piston workpiece. The rough-machined piston workpiece obtained in step 1 is preheated to 500℃~600℃ and held at that temperature for a first time (40 min); then preheated to 800℃~850℃ and held at that temperature for a second time (30 min) to obtain the preheated rough-machined piston workpiece. Step 2.2: Heating the preheated rough-machined piston workpiece. The preheated rough-machined piston workpiece is transferred to a high-temperature furnace at 1100℃~1200℃ to transform the crystal structure of the steel inside the rough-machined piston workpiece into uniform austenite; and then held at this temperature for a third time (15min) to allow the alloying elements inside the steel of the rough-machined piston workpiece to be fully dissolved in the austenite, thus obtaining the heated rough-machined piston workpiece. Step 2.3: Quench the roughly machined piston workpiece after heating. The heated rough-machined piston workpiece is removed from the high-temperature furnace and immediately immersed in a cooling medium to rapidly cool it to room temperature, thus obtaining the quenched rough-machined piston workpiece. Step 2.4: Temper the rough-machined piston workpiece after quenching. Step 2.4a: Remove the rough-machined piston workpiece after quenching; Step 2.4b: Immediately place the removed piston workpiece into a tempering furnace at 580℃ and hold for 1-2 hours. Then remove the piston workpiece and cool it to room temperature in the air. Step 2.4c: Using the method in step 2.4b, the piston workpiece is tempered, held at a high temperature, and cooled 1-2 times to obtain the heat-treated piston workpiece. The material of the heat-treated piston workpiece is changed to M42 hardened steel, and its hardness value is expressed as HRC 68-70. The heat treatment process produces a deformation and oxide layer of 0.05mm-0.1mm on each surface of the workpiece. Step 3: The inner hole 103 and outer cylindrical surface 104 of the piston workpiece obtained in Step 2 are precision machined by turning to correct deformation and remove all oxide layers and some machining allowances on each surface. The turning process uses CBN (Cubic Boron Nitride) as the material of the effective working area of ​​the tool to obtain the precision-turned piston workpiece. Then, using the inner hole 103 of the precision-turned piston workpiece as a reference, it is fixed on the mandrel of the lathe. See the schematic diagram of the CBN material tool. Figure 4 Where d is the nominal side length of the insert; l is the overall length of the insert; s is the thickness of the insert, which determines the tool's installation stability and impact resistance; α is the clearance angle of the insert, used to reduce friction between the tool and the workpiece during cutting; r is the tip radius, affecting the surface finish and the tool's resistance to chipping; l e The effective length of the cutting edge is the cutting edge length. In reality, the CBN material only covers the effective working area of ​​the cutting edge, which is the core cutting part of the tool. Step 3, for the precision turning of the inner hole 103, improves the positioning datum accuracy of the inner hole 103, enabling it to precisely fit with the mandrel, which is beneficial for subsequent steps. The reason for choosing CBN material for the tool is that CBN tools have good thermal stability, maintaining their cutting performance at high temperatures, and are less prone to tool softening and accelerated wear. It also helps to increase cutting speed and reduce the formation of built-up edge (built-up edge forms unstable deposits on the tool surface, affecting the surface roughness and dimensional accuracy of the machined surface. When the cutting speed increases to a certain level, the cutting temperature rises, and the built-up edge softens and falls off, easily causing machining errors).

[0030] Step 4: Precision grinding is performed on the outer cylindrical surface 104 of the precision-turned piston workpiece using a CBN grinding wheel with a mesh size greater than 1600. This removes all remaining machining allowances from each surface, resulting in a precision-ground piston workpiece. Existing white or brown fused alumina grinding wheels have relatively low hardness, leading to high grinding force and excessive wear during processing. The replaced high-hardness CBN grinding wheel has lower grinding force and temperature, effectively preventing defects such as burns and cracks on the piston workpiece surface. Furthermore, the high machining accuracy of the CBN grinding wheel ensures a better surface quality. After Step 4 is completed, the piston workpiece basically meets the dimensional requirements, except that the surface roughness of the outer cylindrical surface 104 does not reach a mirror finish.

[0031] Step 5: Using a grinding tool 2 containing abrasive, the outer cylindrical surface 104 of the precision-ground piston workpiece is preliminarily polished to remove microscopic protrusions and / or machining marks on the surface, resulting in a preliminarily polished piston workpiece; see [link to relevant documentation]. Figure 3 The grinding tool 2 is a disc made of soft ductile iron (material grade: QT450). It has a central through hole 201 in the middle for placing the outer cylindrical surface 104 of the piston (the size of which can be adjusted according to the size of the workpiece). The side wall of the central through hole 201 has two expansion grooves 202 and one mounting groove 203 evenly distributed on the circumference along the radial direction of the disc. The mounting groove 203 is connected to the outer ring of the disc. Both expansion grooves 202 are closed grooves at the outer end to disperse the stress during the grinding process. Each expansion groove 202 has a circular hole-shaped groove at the closed end, and the diameter of the circular hole is larger than the width of the outer closed groove, which can further disperse the stress during the grinding process. The outer ring of the disc has a right-angled notch on one side of the mounting groove 203; two mounting holes 2301 are respectively provided on both sides of the mounting groove 203 for screws to pass through to adjust the clamping degree of the piston; the side wall of the central through hole 201 is provided with a spiral groove for coating and storing abrasive; the mechanical polishing abrasive is usually selected according to the surface roughness requirements of the workpiece, commonly used abrasives include diamond micro powder and alumina, and after improvement, white fused alumina micro powder is selected. White fused alumina micro powder is divided into coarse grinding micro powder and fine grinding micro powder. For coarse grinding, W10 specification white fused alumina micro powder is usually selected to remove larger protrusions and textures on the surface, while for fine grinding, W1 / W0.5 specification white fused alumina micro powder is usually selected to remove small protrusions and textures, reduce surface roughness, and improve surface smoothness.

[0032] Step 6: The outer cylindrical surface 104 of the piston workpiece after preliminary polishing is mirror-polished using a vibratory grinding and polishing machine. This achieves a mirror-like surface roughness, resulting in the finished piston made of M42 hardened steel, thus completing the mirror-finish machining of the outer cylindrical surface of the M42 hardened steel piston. Vibratory grinding and polishing utilizes the microscopic forces between the abrasive and the workpiece surface to achieve precision machining. This machining method effectively removes microscopic defects from the workpiece surface, reduces surface roughness, and improves surface smoothness and finish. High-frequency vibration generates minute and continuous impacts and friction between the abrasive and the workpiece surface. During the machining process, the abrasive continuously contacts the workpiece surface under vibration, removing microscopic protrusions through grinding and simultaneously causing plastic deformation of the surface material, filling the microscopic pits and gradually smoothing the surface. This microscopic machining method avoids macroscopic scratches and damage that may occur in traditional machining methods, facilitating the achievement of a mirror-like finish. Vibratory grinding and polishing involves using a machine to impact the workpiece surface with abrasive particles at high frequencies, typically between 20 and 50 kHz, with amplitudes controllable within the range of 5 to 20 μm. The choice of abrasive is crucial; abrasives can be categorized as metal-based, non-metal-based, or special abrasives. This process utilizes a non-metallic composite abrasive, made by mixing hard abrasive grains with elastic materials. Macroscopically, this abrasive is soft and elastic, preventing mechanical damage to the workpiece. Microscopically, the embedded hard abrasive grains provide high hardness, enabling effective cutting and grinding. The abrasive slides and compresses the workpiece surface, reducing surface roughness and achieving a mirror-like polishing effect.

[0033] Each step in this embodiment is interconnected and arranged scientifically. The finishing process after heat treatment essentially eliminates microscopic protrusions and machining marks on the outer cylindrical surface of the piston, enabling the outer cylindrical surface of the piston to achieve a mirror finish. This embodiment improves upon the manual mechanical polishing method using a cloth wheel, using a grinding tool 2 to grind the outer cylindrical surface. Simultaneously, the abrasive is replaced with softer white corundum, and the abrasive grade is improved, further eliminating microscopic protrusions and machining marks on the outer surface, laying the foundation for achieving a mirror finish. This embodiment improves the precision grinding wheel type to a CBN wheel and increases the grit to over 1600 grit, greatly reducing microscopic protrusions and grinding marks generated during grinding. This embodiment introduces a vibratory grinding and polishing method, allowing the abrasive to continuously contact the workpiece surface under vibration. The grinding action of the grinding tool removes the microscopic protrusions on the workpiece surface, and a matching non-metallic composite abrasive is selected, thereby improving the surface roughness of the workpiece and achieving a mirror polishing effect.

[0034] The piston processed according to the processing method of this embodiment has an outer cylindrical surface with a surface roughness tester. The surface roughness Ra can reach 0.01μm to Ra0.02μm. The surface is flat and smooth, and it can show a clear mirror reflection under natural light. The surface has no microscopic protrusions or processing lines, no visual defects, and no resistance when touched by bare hands. It completely achieves a mirror effect and meets the actual application requirements.

[0035] It should be noted that the above description outlines the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the present invention. Various changes and modifications can be made to the present invention without departing from its spirit and scope (such as applying the method to the surface mirror processing of other workpieces with similar structures). All such changes and modifications fall within the scope of the present invention as claimed, and the scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel, characterized in that, Includes the following steps: Step 1: Using M42 steel as the material, the piston workpiece is machined by turning according to the preset required dimensions to obtain a rough-machined piston workpiece. A machining allowance of 0.1mm to 0.3mm is reserved on each side of each surface of the rough-machined piston workpiece. Step 2: Heat treat the rough-machined piston workpiece obtained in Step 1 to obtain a heat-treated piston workpiece. The material of the heat-treated piston workpiece is changed to M42 hardened steel, and its hardness value is expressed as HRC 68~70. During the heat treatment process, a deformation and oxide layer of 0.05mm~0.1mm are generated in the machining allowance areas on various surfaces of the workpiece. Step 3: The inner hole (103) and outer cylindrical surface (104) of the piston workpiece after heat treatment obtained in Step 2 are precision machined by turning process to correct the deformation and remove all oxide layers and some machining allowances on each surface. The turning process uses CBN as the material of the effective working area of ​​the tool to obtain the precision turned piston workpiece. Then, the inner hole (103) of the precision turned piston workpiece is used as the reference and fixed on the mandrel of the lathe. Step 4: Use a CBN grinding wheel to perform precision grinding on the outer cylindrical surface (104) of the precision-turned piston workpiece to remove all remaining machining allowances on each surface, and obtain the precision-ground piston workpiece. Step 5: Use a grinding tool (2) containing abrasive to perform preliminary polishing on the outer cylindrical surface (104) of the precision-ground piston workpiece to remove micro-protrusions and / or machining lines on the surface, and obtain the piston workpiece after preliminary polishing. Step 6: Use a vibratory grinding and polishing machine to perform mirror polishing on the outer cylindrical surface (104) of the piston workpiece after preliminary polishing, so that the surface roughness of the outer cylindrical surface (104) reaches the mirror effect, thus obtaining the finished piston of M42 hardened steel material, and completing the mirror surface processing of the outer cylindrical surface of the piston of M42 hardened steel material.

2. The method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel according to claim 1, characterized in that, Step 2 is as follows: Step 2.1: Perform segmented preheating on the rough-machined piston workpiece. The rough-machined piston workpiece obtained in step 1 is preheated to 500℃~600℃ and held at that temperature for a first time; then preheated to 800℃~850℃ and held at that temperature for a second time to obtain the preheated rough-machined piston workpiece. Step 2.2: Heating the preheated rough-machined piston workpiece. The preheated rough-machined piston workpiece is transferred to a high-temperature furnace at 1100℃~1200℃ to transform the crystal structure of the steel inside the rough-machined piston workpiece into uniform austenite. The temperature is maintained for a third time to allow the alloying elements inside the steel of the rough-machined piston workpiece to fully dissolve in the austenite, resulting in the heated rough-machined piston workpiece. Step 2.3: Quench the roughly machined piston workpiece after heating. The heated rough-machined piston workpiece is removed from the high-temperature furnace and immediately immersed in a cooling medium to rapidly cool it to room temperature, thus obtaining the quenched rough-machined piston workpiece. Step 2.4: Temper the rough-machined piston workpiece after quenching. Step 2.4a: Remove the rough-machined piston workpiece after quenching; Step 2.4b: Immediately place the removed piston workpiece into a tempering furnace at 550℃~600℃ and hold for 1h~2h. Then remove the piston workpiece and cool it to room temperature in the air. Step 2.4c: Using the method in step 2.4b, the piston workpiece is tempered, held at a certain temperature, and cooled 1-2 times to obtain the heat-treated piston workpiece. The material of the heat-treated piston workpiece is changed to M42 hardened steel, and its hardness value is expressed as HRC 68-70. The heat treatment process produces a deformation and oxide layer of 0.05mm-0.1mm on each surface of the workpiece.

3. The method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel according to claim 1 or 2, characterized in that: In step 4, the mesh count of the grinding wheel is greater than 1600.

4. The method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel according to claim 1, characterized in that: In step 5, the grinding tool (2) is a disc made of ductile iron. It has a central through hole (201) in its middle for placing the outer cylindrical surface (104) of the piston. The side wall of the central through hole (201) has two expansion grooves (202) and one mounting groove (203) evenly distributed on the circumference along the radial direction of the disc. The mounting groove (203) is connected to the outer ring of the disc. Both expansion grooves (202) are closed grooves at the outer end to disperse the stress during the grinding process. A right-angle notch is provided on the outer ring of the disc on one side of the mounting groove (203). Two mounting holes (2301) are provided on the corresponding positions on both sides of the mounting groove (203) for screws to pass through to adjust the clamping degree of the piston. The side wall of the central through hole (201) is provided with a spiral groove for coating and storing abrasive.

5. The method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel according to claim 4, characterized in that: In step 5, the abrasive is white corundum micro powder.

6. The method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel according to claim 1, characterized in that: In step 6, the vibration frequency of the vibratory grinding and polishing machine is 20kHz to 50kHz, and the amplitude is 5μm to 20μm.

7. The method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel according to claim 2, characterized in that: In step 2.1, the first duration is 40 minutes, and the second duration is 30 minutes; In step 2.2, the third duration is 15 minutes.

8. The method for machining the outer cylindrical mirror surface of a piston made of M42 hardened steel according to claim 4, characterized in that: Each of the expansion slots (202) has a circular hole-shaped groove at its closed end, and the diameter of the circular hole is greater than the width of the outer closed groove.

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