A remanufactured impact piston and method of making the same

Abstract

The application discloses a remanufactured impact piston and a preparation method thereof, and solves the problems of low residual value and use rate of the impact piston in the prior art. The remanufactured impact piston comprises, from front to back, an impact section, a front cavity section, a front cavity boss, a middle cavity section, a rear cavity boss and a rear cavity section, the front cavity section comprises a front cavity front section and a front cavity rear section, the front cavity front section and the impact section or the impact section form an impact end head, and the impact end head is connected to the front cavity rear section or the front cavity front section through a remanufacturing mode. Specifically, the remanufacturing mode is to cut off the old impact end head and then connect a new impact end head through a welding mode. The remanufacturing strategy of "local replacement and body reservation" is adopted, only the failed impact end head is cut off, and the new end head is connected through the welding mode, so that material waste caused by traditional overall replacement is avoided; the problem that the impact piston end head is failed and scrapped is solved, the service life of the impact piston is greatly prolonged, and the construction cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of rock drill technology, and in particular to an impact piston. Background Technology

[0002] As the power core of a rock drill, the impact piston operates at a frequency greater than 60Hz and has a single impact energy greater than 300J. This represents a typical high-frequency, high-impact condition, generally requiring high external hardness and a balanced internal strength and toughness. During prolonged use, impact pistons are prone to failure modes such as end-end spalling, roughening, cavitation, and fracture. Approximately 70% of these failures are due to end-end spalling, which reduces impact efficiency. Generally, an impact piston is considered unusable after 5-20mm of spalling.

[0003] Impact pistons are manufactured from high-purity raw materials, through complex heat treatment processes, and precision machining. The finished product can reach a length of 600mm. The scrapping of the end portion, rendering the entire impact piston unusable, results in significant resource waste and substantially increases construction costs. Existing technologies, such as the alloy steel disclosed in patent application CN107619992A and its manufacturing process for a hydraulic rock drill impact piston, focus on obtaining high-performance impact pistons. Meanwhile, the impact component and impact mechanism disclosed in patent application CN116220540A emphasize extending the lifespan during use. Therefore, improving the remaining value and utilization rate of impact pistons after end scraping, meeting the requirements of high strength, high toughness, and long lifespan, achieving green construction, and reducing construction costs have become urgent problems to be solved in this field. Summary of the Invention

[0004] To address the shortcomings in the aforementioned background technology, this invention proposes a remanufactured impact piston and its preparation method, which solves the problems of low residual value and low utilization rate of impact pistons in the prior art.

[0005] The technical solution of this invention is implemented as follows: A remanufactured impact piston comprises, from front to back, an impact section, a front cavity section, a front cavity boss, a middle cavity section, a rear cavity boss, and a rear cavity section. The front cavity section includes a front section and a rear section, and the front section and the impact section or impact section constitute an impact end. The impact end is connected to the rear section or front section of the front cavity through a remanufacturing method. Specifically, the remanufacturing method involves cutting off the old impact end and then connecting a new impact end by welding. The cutting boundary of the impact end can be flexibly selected according to the degree of failure: if only the end of the impact section peels off, it can be cut off at the front section of the front cavity; if the front section of the front cavity is also damaged, it can be cut off at the rear section of the front cavity. The welded joint is located at the rear section or front section of the front cavity, avoiding the end of the impact section where the stress is most concentrated, while utilizing the wall thickness advantage of the front cavity section to provide sufficient welding bearing area, ensuring the joint strength and fatigue performance.

[0006] The welding surfaces of the impact end and the welding surfaces of the rear or front section of the front cavity are rounded to eliminate stress concentration. The rounded corners are also shot-peened to introduce residual compressive stress.

[0007] A method for preparing a remanufactured impact piston, based on the aforementioned remanufactured impact piston; the steps are as follows:

[0008] S1. Screening of scrapped pistons: Visual inspection and internal flaw detection are carried out on scrapped impact pistons to screen out samples that only have end peeling, no internal cracks or other combined failure modes, and confirm their matrix material.

[0009] S2. Processing of the impact end of the scrapped piston: The impact end of the scrapped impact piston is cut off, the failed end is removed, and the remaining impact piston body to be welded is obtained. The surface to be welded is ground and cleaned to obtain the first welding surface.

[0010] S3. Selection of new impact end: Select a new impact end that is consistent with the original impact end in terms of material properties, structural form and heat treatment process, and reserve a machining allowance of 0.1~2cm for the axial length of the new impact end compared with the original end; define the welding surface of the new impact end as the second welding surface.

[0011] S4. Friction welding and post-weld shaping: The first welding surface and the second welding surface are welded together by friction welding process. After the welding is completed, a weld is formed between the two welding surfaces. The new impact end and the rear section of the front cavity or the front section of the front cavity of the remaining impact piston body form a welded joint. After the welded joint is cooled to below Ms point, it is slowly cooled. The welded joint after slow cooling is flash-machined to make the joint size meet the tolerance requirements.

[0012] S5. Local heat treatment: The welded joint is locally induction heated using a narrow-band induction heater whose width matches the weld width. The heating temperature is controlled at 800℃~950℃. Then, oil quenching is performed. When the temperature drops to 200℃, the joint is transferred to a slow cooling process. Finally, the welded joint is tempered to eliminate residual welding stress and restore the toughness of the joint.

[0013] S6. Welded joint treatment: Round the corners at the junction of the first and second welding surfaces of the welded joint, and then perform shot peening on the rounded corner area.

[0014] S7. Grinding: Precision grinding is performed on the welding position and surrounding area to ensure that the overall dimensional accuracy, form and position tolerance and surface roughness of the remanufactured impact piston meet the design standards, and finally form the finished remanufactured impact piston.

[0015] S8. Coating reinforcement: The front cavity boss, middle cavity section, and rear cavity boss section of the remanufactured impact piston are reinforced with coating.

[0016] Further optimization involves first visually inspecting the scrapped impact pistons using a magnifying glass to select samples that only have end peeling and no other failure modes. Then, ultrasonic testing is used to detect internal defects in the initially selected samples, further eliminating samples with internal cracks to ensure that scrapped impact pistons that meet both surface and internal requirements are obtained.

[0017] Further optimization, step S1 also includes using a spectrometer to perform full-element quantitative analysis on the selected scrapped impact pistons, accurately determining their chemical composition, and confirming that the base material is 50CrMo4 alloy structural steel; providing accurate material basis for the selection of new impact ends in step S3.

[0018] Further optimization involves using wire cutting to cut off the end of the scrapped impact piston in step S2; sanding the surface to be welded with sandpaper; and cleaning it with anhydrous ethanol after sanding.

[0019] Further optimization involves selecting a new impact end with a base material of 50CrMo4 alloy structural steel in step S3.

[0020] Further optimization involves the following steps: In step S4, during friction welding, the remaining impact piston body to be welded is clamped at the fixed end of the rotary friction welding machine, and the new impact end is clamped at the rotating end. The new impact end is driven to rotate at high speed, while the impact piston body is fed axially towards the rotating end. The two welding surfaces are continuously rubbed for 3 seconds under a friction pressure of 90 MPa, using frictional heat to bring the interface metal to a plastic state. Subsequently, an upsetting pressure of 150 MPa is applied and held for 2 seconds to allow the interface metal to fully diffuse and bond. The axial feed amount throughout the welding process is 0.5 cm.

[0021] In a further preferred embodiment, during the local heat treatment in step S5, a 15mm narrow-band induction heater with a width similar to that of the weld is used to heat the weld joint locally at a frequency of 20Hz to 880°C. Then, it is immediately oil quenched, and when the workpiece temperature drops to 200°C, it is transferred to slow cooling. Finally, the weld joint is tempered for 2 hours to eliminate residual stress and stabilize the microstructure.

[0022] Further optimization involves cleaning the area to be deposited on the remanufactured impact piston in step S8, and preparing the coating using the PVD method: the workpiece is rotated at a speed of 5~50 rpm, a 0.05~0.5µm Cr transition layer is deposited first, followed by a 1~4µm DLC surface coating, ultimately obtaining a high-performance composite coating with a hardness ≥1500HV and a friction coefficient of 0.05~0.1.

[0023] The beneficial effects of this invention are as follows: This invention adopts a remanufacturing strategy of "partial replacement and body retention", which only cuts off the failed impact end and connects the new end by welding, avoiding the material waste caused by traditional whole replacement; it solves the problem of the impact piston being scrapped when the end fails, greatly improves the service life of the impact piston and reduces construction costs.

[0024] The remanufacturing method of this invention achieves precise repair and performance regeneration of high-value alloy structural steel components by systematically screening, cutting off the ends, friction welding, local heat treatment and precision machining of scrapped impact pistons. Compared with the prior art, this invention adopts friction welding + local heat treatment + weld structure + local surface coating design to obtain high-strength and long-life impact pistons, solving the problems of low efficiency and insufficient life of impact pistons. Attached Figure Description

[0025] To more clearly illustrate the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the remanufactured impact piston structure of the present invention;

[0027] Figure 2 This is a schematic diagram of the remanufactured impact piston coating of the present invention;

[0028] Figure 3 This is a schematic diagram of the coating structure of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Example 1, such as Figure 1As shown, a remanufactured impact piston comprises, from front to back, an impact section 11, a front cavity section 12, a front cavity boss 13, a middle cavity section 14, a rear cavity boss 15, and a rear cavity section 16. Each of these sections can be hollow. The front cavity section 12 includes a front cavity front section 121 and a front cavity rear section 122. The front cavity front section 121 and the impact section 11, or the impact section 11, constitute the impact end; that is, the front cavity front section 121 + the impact section 11 constitute the impact end, with the cutting and replacement location at the front cavity section 12; or the impact section 11 is the impact end, with the cutting and replacement location at the impact section 11 or the front cavity front section 121. The impact end is connected to the front cavity rear section 122 or the front cavity front section 121 through a remanufacturing method. The remanufacturing method involves cutting off the old impact end and then connecting the new impact end by welding. The cutting boundary of the impact end can be flexibly selected according to the degree of failure: if only the impact segment end peels off, it can be cut off at the front section of the anterior cavity; if the front section of the anterior cavity is also damaged, it can be cut off at the rear section of the anterior cavity. The welded joint is set at the rear or front section of the anterior cavity, avoiding the impact segment end where the stress is most concentrated, while taking advantage of the wall thickness of the anterior cavity section to provide sufficient welding load-bearing area, ensuring the joint strength and fatigue performance.

[0031] Example 2, a remanufactured impact piston, is a further optimization based on Example 1. In this example, the new impact tip and the remaining body are made of the same material to ensure metallurgical compatibility at the welding interface and avoid electrochemical corrosion or thermal expansion mismatch problems caused by welding dissimilar steels. The welding surfaces of the impact tip and the welding surfaces of the rear section 122 or the front section 121 of the front cavity are rounded. The rounded corners are used to eliminate stress concentration, and the rounded corners are micro-shot peened to introduce residual compressive stress.

[0032] Example 3, a method for preparing a remanufactured impact piston, based on the remanufactured impact piston described in Example 2; the steps are as follows:

[0033] S1. Screening of Scrap Pistons: Scrap impact pistons undergo visual inspection and internal flaw detection to screen for samples exhibiting only end-end spalling without internal cracks or other combined failure modes, and to confirm their base material. First, a comprehensive failure analysis is performed on the scrap impact pistons. Through visual inspection combined with low-magnification observation, samples with a single failure mode, exhibiting only end-end spalling and lacking fatigue cracks, plastic deformation, corrosion damage, or excessive wear, are screened. Subsequently, ultrasonic flaw detection is used to perform non-destructive testing of internal defects on the samples that passed the initial visual screening, focusing on the piston head and stress concentration areas. Samples with hidden defects such as internal cracks, shrinkage cavities, or inclusions are further eliminated to ensure that the failure mode of the selected scrap impact pistons is purely dominated by end-end spalling. Finally, a spectrometer is used to perform chemical composition analysis on the samples that passed the double screening to confirm the base material, providing a basis for subsequent welding process parameter formulation.

[0034] S2. Processing of the Impact End of the Scrap Piston: The impact end of the scrapped impact piston is cut off to remove the failed end, obtaining the remaining impact piston body to be welded. The surface to be welded is then ground and cleaned to obtain the first welding surface. A mechanical cutting method is used to precisely cut along the transition area between the impact end and the body of the scrapped impact piston, completely removing the failed end portion to obtain the remaining impact piston body with sufficient wall thickness and structural integrity. The weldable surface formed by the cut is then subjected to rough grinding, fine grinding, and polishing to remove the oxide layer, heat-affected zone, and machining burrs, ensuring the surface roughness meets the requirements for friction welding. Subsequently, degreasing and cleaning are performed using an organic solvent (such as acetone or alcohol), followed by blowing with dry compressed air to obtain a clean, oil-free, and rust-free first welding surface, ensuring the metallurgical bonding quality of the weld interface.

[0035] S3. Selection of New Impact End: Select a new impact end that is identical to the original impact end in terms of material properties, structural form, and heat treatment process, with a machining allowance of 0.1~2cm for the axial length of the new impact end compared to the original end; define the welding surface of the new impact end as the second welding surface. Based on the material data confirmed in step S1, select a new impact end that is identical to the original impact end in terms of chemical composition, mechanical properties, metallographic structure, and heat treatment state to ensure the welding compatibility of the new and old materials.

[0036] S4. Friction welding and post-weld shaping: The first welding surface and the second welding surface are welded together by friction welding process. After the welding is completed, a weld is formed between the two welding surfaces. The new impact end and the rear section of the front cavity or the front section of the front cavity of the remaining impact piston body form a welded joint. After the welded joint is cooled to below Ms point, it is slowly cooled. The welded joint after slow cooling is flash-machined to make the joint size meet the tolerance requirements.

[0037] S5. Local heat treatment: The welded joint is locally induction heated using a narrow-band induction heater whose width matches the weld width. The heating temperature is controlled at 800℃~950℃. Then, oil quenching is performed. When the temperature drops to 200℃, the joint is transferred to a slow cooling process. Finally, the welded joint is tempered to eliminate residual welding stress and restore the toughness of the joint.

[0038] S6. Welded joint treatment: The corners at the junction of the first and second welding surfaces of the welded joint are rounded, and the rounded corner area is shot peened. A residual compressive stress layer is introduced on the surface by high-speed shot impact, with a depth of 0.1~0.3mm, which effectively inhibits the initiation and propagation of fatigue cracks and improves the fatigue life of the welded joint.

[0039] S7. Grinding: Precision grinding is performed on the welding area and surrounding regions to ensure that the overall dimensional accuracy, form and position tolerances, and surface roughness of the remanufactured impact piston meet the design standards, ultimately forming a finished remanufactured impact piston. After grinding, the remanufactured impact piston undergoes comprehensive dimensional inspection, ultrasonic flaw detection, surface hardness testing, and metallographic examination to ensure that all technical indicators meet or approach the standards of new parts, ultimately forming a qualified finished remanufactured impact piston.

[0040] S8, Coating reinforcement: such as Figure 2 , 3 As shown, the front cavity boss 13, middle cavity section 14, and rear cavity boss 15 of the remanufactured impact piston are coated for reinforcement. The areas to be coated are cleaned, and a PVD (Physical Vapor Deposition) method is used. The workpiece is rotated at 5-50 rpm to deposit a 0.05-0.5µm Cr transition layer, followed by a 1-4µm DLC surface coating, resulting in a coating with a friction coefficient of 0.05-0.1 and a hardness ≥1500HV. During the reciprocating motion of the impact piston, this area is prone to cavitation and high-temperature scratching. The Cr transition layer provides a bonding base and improves bonding strength, while the DLC surface layer provides a low-friction, high-wear-resistant functional surface. High wear resistance reduces the occurrence of cavitation and lowers the probability of scratching, while low friction significantly reduces heat generation during the reciprocating motion of the impact piston, further reducing the probability of scratching and extending the service life of the remanufactured piston.

[0041] According to the present invention, a reliable connection between the original impact piston and the new end can be achieved, thereby extending the service life of the impact piston and enabling the reuse of scrapped impact pistons.

[0042] Example 4, a method for preparing a remanufactured impact piston, such as... Figure 1As shown, based on the remanufactured impact piston described in Example 2; in this example, the impact end is composed of the front cavity section 121 + impact section 11, and the cutting and replacement position is in the front cavity section 12. The specific steps are as follows: Step S1: First, through visual inspection combined with magnification, samples with only end peeling and no other failure modes are selected from the scrap impact pistons; then, ultrasonic flaw detection technology is used to detect internal defects in the initially selected samples, further eliminating samples with internal cracks to ensure that the scrap impact pistons that meet the requirements in both surface and internal aspects are obtained. A spectrometer is used to perform full element quantitative analysis on the selected scrap impact pistons to accurately determine their chemical composition, confirming that the base material is 50CrMo4 alloy structural steel; providing accurate material basis for the selection of the new impact end in step S3.

[0043] Step S2: Cut off the end of the scrapped impact piston using wire cutting; the cutting position is the front cavity section. Remove the failed end to obtain the remaining impact piston body to be welded. Grind the surfaces to be welded with sandpaper, and then clean them with anhydrous ethanol. The diameter of the front cavity is larger than the diameter of the impact end, away from the high-stress area, reducing the stress level and significantly improving the connection reliability.

[0044] Step S3: Select a new impact end with a base material of 50CrMo4 alloy structural steel. The material properties, structural form, and heat treatment process of the new end should be consistent with those of the original end. To avoid interfacial stress caused by inconsistent thermal expansion and contraction during welding, the same material as the original impact piston should be selected. Since friction welding involves an upsetting process, which will lead to axial shrinkage, the length of the new end needs to be strictly controlled. The length should be determined to be 0.1-2 cm longer than the original end, based on the welding process feed rate, the total length tolerance of the impact piston, and the stress conditions at different positions. The preferred length is 0.5 cm longer than the original end.

[0045] Step S4: During friction welding, the remaining impact piston body to be welded is clamped on the fixed end of the rotary friction welding machine, and the new impact end is clamped on the rotating end. The new impact end is driven to rotate at high speed, while the impact piston body is fed axially towards the rotating end. The two welding surfaces are continuously rubbed under a friction pressure of 90MPa for 3 seconds, using frictional heat to bring the interface metal to a plastic state. Then, an upsetting pressure of 150MPa is applied and held for 2 seconds to allow the interface metal to fully diffuse and bond. The axial feed amount throughout the welding process is 0.5cm. After welding, the joint is cooled to below 200℃ and then slowly cooled with insulating cotton to reduce stress deformation. The flash is then precision machined to ensure that it meets the dimensional tolerances. Impact pistons are typical rotating parts, and friction welding is used for connection. Friction welding is a solid-state welding method, and the heat-affected zone is much smaller than that of fusion welding, reducing the impact on raw materials / base materials. In terms of welding parameters, short time, high upsetting, and precise compression control further reduce the welding heat input, while ensuring interface density and axial shortening, thus guaranteeing subsequent processing. The welding thermal cycle is equivalent to the weld undergoing a heat treatment, resulting in thermal stress and phase transformation stress. While obtaining martensite, stress deformation is reduced. Insulation cotton is used for slow cooling below the martensitic transformation end temperature, i.e., the Ms point.

[0046] In step S5, during local heat treatment, a narrow-band induction heater (15mm wide, similar to the weld width) is used for heating. The weld joint is locally induction heated to 880℃ at a frequency of 20Hz, followed immediately by oil quenching. Once the workpiece temperature drops to 200℃, slow cooling is initiated. Finally, the weld joint is tempered for 2 hours to eliminate residual stress and stabilize the microstructure. Although friction welded joints have a small heat-affected zone, they are not without one. The heat-affected zone can lead to a decrease in hardness, and hardness mismatch can cause early failure. Local heat treatment using induction heating followed by tempering allows the weld to regain the same microstructure and hardness as the base material.

[0047] Step S6: Round the corners at the weld joint, with a radius of R3~R10, preferably R5, to eliminate stress concentration. Then, perform micro-shot peening on the rounded corners to introduce residual compressive stress. Due to welding control issues, stress concentration is easily generated at the weld interface. Rounding the weld interface effectively disperses stress. Introducing residual compressive stress through micro-shot peening resists the tensile stress generated during piston operation, delaying crack initiation and propagation.

[0048] Step S7: Grind the welding area to achieve a surface roughness Ra≤0.3 and a tolerance ≤0.007mm. Grind the front boss, middle cavity section, and rear boss, reducing the grinding thickness by 3µm compared to the minimum value in the drawing, to remove cavitation pits and other imperceptible defects that may occur during use.

[0049] Step S8: Ultrasonic cleaning is performed on the front cavity boss 13, middle cavity section 14, and rear cavity boss 15 of the remanufactured impact piston. The PVD method is used, with the workpiece rotated at 20 rpm to deposit a 0.2 µm Cr transition layer, followed by a 2.8 µm DLC surface coating to obtain a coating with a friction coefficient of 0.1 and a coefficient of friction of 2500 HV.

[0050] Compared with existing technologies, this invention adopts a design approach of friction welding + local heat treatment + weld structure + local surface coating to obtain a high-strength, long-life impact piston, solving the problem of impact piston end failure and scrapping, significantly improving the service life of the impact piston and reducing construction costs.

[0051] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A remanufactured impact piston, characterized in that: It consists of an impact section (11), a front cavity section (12), a front cavity boss (13), a middle cavity section (14), a rear cavity boss (15), and a rear cavity section (16) from front to back. The front cavity section (12) includes a front cavity front section (121) and a front cavity rear section (122). The front cavity front section (121) and the impact section (11) or the impact section (11) constitute an impact end. The impact end is connected to the front cavity rear section (122) or the front cavity front section (121) by remanufacturing.

2. The remanufactured impact piston according to claim 1, characterized in that: The remanufacturing method involves cutting off the old impact end and then connecting the new impact end by welding.

3. The remanufactured impact piston according to claim 2, characterized in that: The welding surface of the impact end and the welding surface of the rear section (122) or the front section (121) of the front cavity are provided with rounded corners.

4. A method for preparing a remanufactured impact piston, characterized in that: Based on the remanufactured impact piston according to any one of claims 1 to 4; the steps are as follows: S1. Screening of scrapped pistons: Visual inspection and internal flaw detection are carried out on scrapped impact pistons to screen out samples that only have end peeling, no internal cracks or other combined failure modes, and to confirm their matrix material. S2. Processing of the impact end of the scrapped piston: Cut off the impact end of the scrapped impact piston, remove the failed end, and obtain the remaining impact piston body to be welded. Grind and clean the surface to be welded to obtain the first welding surface. S3. Selection of new impact end: Select a new impact end that is consistent with the original impact end in terms of material properties, structural form and heat treatment process, and reserve a machining allowance of 0.1~2cm for the axial length of the new impact end compared with the original end; define the welding surface of the new impact end as the second welding surface. S4. Friction welding and post-weld shaping: The first welding surface and the second welding surface are welded together by friction welding process. After the welding is completed, a weld is formed between the two welding surfaces. The new impact end and the rear section (122) or the front section (121) of the front cavity of the remaining impact piston body constitute a welded joint. After the welded joint is cooled to below Ms point, it is slowly cooled. The welded joint after slow cooling is flash-machined to make the joint size meet the tolerance requirements. S5. Local heat treatment: The welded joint is locally induction heated using a narrow-band induction heater whose width matches the weld width. The heating temperature is controlled at 800℃~950℃. Then, oil quenching is performed. When the temperature drops to 200℃, the joint is transferred to a slow cooling process. Finally, the welded joint is tempered to eliminate residual welding stress and restore the toughness of the joint. S6. Welded joint treatment: The corners at the junction of the first and second welding surfaces of the welded joint are rounded, and the rounded corner area is shot peened for strengthening. S7. Grinding: Precision grinding is performed on the welding position and surrounding area to ensure that the overall dimensional accuracy, form and position tolerance and surface roughness of the remanufactured impact piston meet the design standards, and finally form the finished remanufactured impact piston. S8. Coating reinforcement: The front cavity boss, middle cavity section, and rear cavity boss of the remanufactured impact piston are reinforced with coating.

5. The method for preparing a remanufactured impact piston according to claim 4, characterized in that: In step S1, visual inspection combined with magnifying glass observation is used to screen out samples from the scrapped impact pistons that only have end peeling and no other failure modes. Then, ultrasonic flaw detection technology is used to detect internal defects in the initially selected samples, and samples with internal cracks are further eliminated to ensure that scrapped impact pistons that meet the requirements in both surface and internal aspects are obtained.

6. The method for preparing a remanufactured impact piston according to claim 5, characterized in that: Step S1 also includes using a spectrometer to perform full-element quantitative analysis on the selected scrapped impact pistons, accurately determining their chemical composition, and confirming that the base material is 50CrMo4 alloy structural steel; providing accurate material basis for the selection of new impact ends in step S3.

7. The method for preparing a remanufactured impact piston according to claim 4 or 6, characterized in that: In step S2, the scrapped impact piston end is cut off by wire cutting; the surface to be welded is polished with sandpaper, and after polishing, it is cleaned with anhydrous ethanol.

8. The method for preparing a remanufactured impact piston according to claim 7, characterized in that: In step S3, a new impact end is selected with 50CrMo4 alloy structural steel as the base material.

9. The method for preparing a remanufactured impact piston according to claim 4 or 8, characterized in that: In step S4, during friction welding, the remaining impact piston body to be welded is clamped at the fixed end of the rotary friction welding machine, and the new impact end is clamped at the rotating end. The new impact end is driven to rotate at high speed, while the impact piston body is fed axially towards the rotating end. The two welding surfaces are continuously rubbed for 3 seconds under a friction pressure of 90 MPa, and the interface metal is brought to a plastic state by frictional heat. Then, an upsetting pressure of 150 MPa is applied and held for 2 seconds to allow the interface metal to fully diffuse and bond. The axial feed amount throughout the welding process is 0.5 cm.

10. The method for preparing a remanufactured impact piston according to claim 4, characterized in that: In step S5, local heat treatment, a 15mm narrow-band induction heater with a width similar to that of the weld is used to heat the weld joint locally at a frequency of 20Hz to 880℃. Then, it is immediately oil quenched and slow-cooled when the workpiece temperature drops to 200℃. Finally, the weld joint is tempered for 2 hours to eliminate residual stress and stabilize the microstructure.

11. The method for preparing a remanufactured impact piston according to claim 4 or 10, characterized in that: In step S8, the part to be deposited on the remanufactured impact piston is cleaned, and the coating is prepared by PVD method: the workpiece is rotated at a speed of 5~50 rpm, a 0.05~0.5µm Cr transition layer is deposited first, and then a 1~4µm DLC surface coating is deposited, finally obtaining a high-performance composite coating with a hardness ≥1500HV and a friction coefficient of 0.05~0.1.

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