Piston pin batch ion nitriding process and its special tooling

Abstract

A piston pin batch ion nitriding process and its special tool, the process steps are as follows: deburring, cleaning; the piston pin is fixedly installed on the special tool and placed in the glow ion nitriding furnace; a preset temperature value is set and greater than the current actual temperature value of the nitriding furnace system; the pressure of the nitriding furnace is set to 50 Pa, the vacuum pump is started to vacuum the nitriding furnace; the voltage is set to 650 V, and the pulse duty cycle is given to the rising key intermittently until the pulse duty cycle gradually increases to 70%; the pulse duty cycle is reduced to 10%, ammonia gas is added to the nitriding furnace, and then the pulse duty cycle is slowly increased; the resistance auxiliary heating system is started to heat the nitriding furnace; the parameters of the nitriding furnace system are set to quickly heat the workpiece to 520-540 DEG C; enter the holding stage, then stop the furnace and cool down, and when the temperature is below 150 DEG C, the furnace can be taken out. The process can effectively reduce the thermal deformation of the piston pin, refine the grains, and improve the size accuracy and matrix toughness.

Description

Technical Field

[0001] This invention relates to a workpiece surface treatment process, specifically a batch ion nitriding process for piston pins and its dedicated tooling. Background Technology

[0002] Piston pins are key force transmission components between engine pistons and connecting rods. During operation, they must withstand high-frequency alternating loads and sliding friction, requiring extremely high surface hardness, wear resistance, and fatigue strength.

[0003] Currently, most ordinary piston pins are surface-strengthened using the traditional gas carburizing and quenching process. This process requires carburizing in the austenitic region at 900-950℃, followed by quenching and tempering to achieve surface hardening. This process has some drawbacks: ① The dual thermal effects of carburizing and quenching cause significant thermal deformation and coarse grains in the workpiece, which not only affects the dimensional accuracy and form and position tolerances of the piston pin but also reduces its matrix toughness and fatigue strength, making it difficult to meet the high precision and high reliability requirements of high-end piston pins; ② The carburized layer is prone to oxidation and grain boundary oxidation, reducing the overall mechanical properties of the workpiece.

[0004] Ion nitriding technology involves filling a furnace with ammonia gas in a vacuum environment, with the furnace body as the anode and the workpiece as the cathode. After being energized, the ammonia gas is ionized into nitrogen ions and hydrogen ions, etc. N+ and H+ bombard the workpiece at high speed to heat it. The ion bombardment of the workpiece surface produces atomic sputtering. Through adsorption and diffusion, nitrogen ions penetrate into the surface of the workpiece to form a nitrided layer, thereby achieving surface hardening. Ion nitriding technology has the advantages of fast nitriding speed and good nitrided layer quality.

[0005] 31CrMoV9 steel, a medium-carbon chromium-molybdenum-vanadium alloy steel widely used in Europe for nitriding, possesses excellent hardenability, high-temperature strength, and nitriding performance. However, existing technologies lack ion nitriding processes specifically for 31CrMoV9 piston pins, making it difficult to fully utilize the material's nitriding potential. This results in insufficient adhesion of the nitrided layer, increased brittleness, and limitations on the service life and reliability of the piston pin.

[0006] Furthermore, existing ion nitriding fixtures often use simple supports or clamps for positioning, which cannot accurately locate the piston pins requiring nitriding. They lack targeted shielding structures, making it difficult to ensure that only the outer cylindrical surface and end faces are nitrided. This can easily lead to the misnitriding of non-nitrided surfaces (such as inner holes), affecting the piston pin assembly accuracy and performance. Clamping stability is poor; during batch processing, differences in the furnace temperature field can affect the uniformity of nitriding. High-efficiency batch clamping cannot be achieved, resulting in low production efficiency. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a batch ion nitriding process for piston pins and its special tooling that can effectively reduce the thermal deformation of piston pins, refine grains, and improve dimensional accuracy and matrix toughness.

[0008] The technical solution of the present invention is as follows: A batch ion nitriding process for piston pins includes the following steps: 1. Deburr and clean the piston pins to be processed, and air dry or bake them immediately after cleaning. 2. Fix multiple dried piston pins onto a special tooling, with the spacing between adjacent piston pins after installation being 30-50mm; then place the special tooling inside the glow discharge ion nitriding furnace; 3. A preset temperature value is set on the nitriding furnace, which is greater than the current actual temperature value displayed by the nitriding furnace system; 4. Set the pressure of the nitriding furnace to 50Pa, and then turn on the vacuum pump to evacuate the nitriding furnace; when the vacuum degree of the nitriding furnace is ≤50Pa, set the voltage to 650V, and intermittently click the pulse duty cycle given increase key to increase the pulse energizing time. When the pulse duty cycle increases to a certain value, the workpiece starts to arc and produce glow, until the pulse duty cycle is gradually increased to 70% to remove oil and impurities from the surface of the workpiece; 5. Reduce the pulse duty cycle to 10%, and start adding ammonia gas into the nitriding furnace. Set the ammonia gas flow rate to 50-100 ml / min and the gas pressure to 30-50 Pa. Then slowly increase the pulse duty cycle. The added ammonia gas will cause arcing to occur again in the nitriding furnace, and the temperature of the workpiece in the furnace will gradually increase. 6. After the arc light basically disappears and the glow becomes stable, start the resistance-assisted heating system set on the furnace wall of the nitriding furnace to heat the nitriding furnace, and control the heating temperature to always be slightly lower than the workpiece temperature in order to reduce the temperature difference between the workpiece surface and the furnace wall. 7. Set the given voltage of the nitriding furnace system to 750V, the pulse duty cycle to 60%-80%, the gas pressure to 300Pa, and the ammonia flow rate to 1300ml / min, so that the workpiece can be rapidly heated to 520-540℃; 8. Enter the heat preservation stage and start calculating the heat preservation time. After the heat preservation time is reached, the furnace can be stopped for cooling. When the actual temperature of the workpiece drops below 150℃, it can be taken out of the furnace.

[0009] A further improvement to the technical solution of this invention lies in the following steps for cleaning the piston pin to be treated: Soak the workpieces in the first cleaning agent tank for 2-3 minutes, then clean and dry them one by one with a brush. During the cleaning process, oil and iron filings in the small holes and narrow crevices of the workpieces should be cleaned. Water-based cleaning agent is used. After the first cleaning and drying, the workpiece is placed in the second cleaning agent tank for a second cleaning. Then it is rinsed with clean water and immediately air-dried or oven-dried to prevent rust.

[0010] A further improvement of the technical solution of the present invention is that the preset temperature value is 400℃.

[0011] A further improvement of the technical solution of the present invention is that: before evacuating the nitriding furnace, the "ammonia flow rate set value" is checked to see if it is zero. If it is not zero, the set value is set to zero.

[0012] A further improvement of the technical solution of the present invention is that the heat preservation time is 48-58 hours and the furnace shutdown and cooling time is 13-17 hours.

[0013] A specialized tooling for batch ion nitriding of piston pins includes a base plate with multiple support columns evenly distributed around its circumference below the base plate. A top plate is connected to the base plate via multiple connecting columns evenly distributed around its circumference. A support beam is connected at the center between the base plate and the top plate. The support beam has a square cross-section. Multiple layers of mounting rods are evenly distributed and fixed on the support beam from top to bottom. Each layer of mounting rods consists of four rods evenly distributed around its circumference and connected to the four sides of the support beam. The outer diameter of each mounting rod is clearance-fitted with the inner diameter of the piston pin. A tapered boss is provided on one end of each mounting rod near the support beam. A positioning screw is threaded to the outer end of each mounting rod for positioning the piston pin to be processed.

[0014] A further improvement of the technical solution of the present invention is that a number of through holes are provided on the base plate to allow nitrogen ions to bombard the piston pin through the through holes.

[0015] A further improvement of the technical solution of the present invention is that a number of through holes are provided on the top plate to allow nitrogen ions to bombard the piston pin through the through holes.

[0016] A further improvement to the technical solution of the present invention is that the center distance between two adjacent mounting rods is 55-75mm, so as to avoid the abnormal arcing in the furnace caused by the spacing being too small, and the workpiece being underheated due to the spacing being too large.

[0017] A further improvement of the technical solution of the present invention is that: there are four connecting columns, and each layer of mounting rods passes through the space between two adjacent connecting columns, so as to balance the temperature field and reduce the temperature difference between workpieces.

[0018] The technological advancements achieved by this invention due to the adoption of the above technical solutions are as follows: 1. This invention provides a batch ion nitriding process for piston pins, which enables a core upgrade from traditional gas carburizing and quenching to ion nitriding for ordinary piston pins. It avoids the disadvantages of traditional gas carburizing and quenching processes, such as large workpiece thermal deformation, coarse grains, high energy consumption, and high scrap rate caused by the high temperature of 900-950℃. This process uses a low temperature treatment of 520-540℃, which can effectively control piston pin deformation and improve the piston pin's dimensional and positional tolerance accuracy by 60%. After nitriding, no subsequent fine grinding process is required, significantly reducing processing costs and scrap rate. It also refines the matrix grains, taking into account both surface hardness and matrix toughness, while reducing energy consumption, shortening the production cycle, and improving production economy.

[0019] 2. After evacuating the nitriding furnace by turning on the vacuum pump, the voltage is set and the pulse duty cycle is intermittently increased by clicking the pulse duty cycle setting up key to increase the pulse energizing time until the glow is ignited. This can play a role in cleaning the workpiece surface, removing oil and impurities from the workpiece surface, thereby improving the final nitriding effect of the workpiece surface.

[0020] 3. This process can fully utilize the nitriding potential of 31CrMoV9 material, significantly improve the surface hardness, wear resistance and fatigue strength of piston pins, and extend their service life; compared with the traditional gas carburizing and quenching process, it greatly reduces workpiece deformation and scrap rate, and improves dimensional accuracy; it enables stable industrial-scale mass production, meets the needs of high-end piston pins, and has outstanding practicality and economy, and can completely replace the traditional gas carburizing and quenching process.

[0021] 4. The present invention provides a special tooling for the batch ion nitriding process of piston pins, which has precise positioning and can accurately control the nitriding surface, avoiding accidental nitriding of non-nitriding surfaces and improving the assembly accuracy of piston pins; by fixing multiple layers of mounting rods evenly distributed from top to bottom on the support beam, a batch clamping design can be realized, resulting in high production efficiency; the clamping is firm and easy to disassemble and assemble, which can prevent piston pin displacement during nitriding and ensure batch processing consistency; by having multiple support columns evenly distributed around the circumference below the base plate and connecting the top plate to the base plate through multiple connecting columns evenly distributed around the circumference, the temperature field inside the nitriding furnace can be balanced, significantly improving the uniformity of the nitriding layer. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the present invention.

[0023] Figure 2 This is a schematic diagram of the special tooling of the present invention.

[0024] Figure 3 yes Figure 2 A three-dimensional structural diagram.

[0025] Figure 4 This is a cross-sectional depth image of the piston pin permeation layer after processing using the process of this invention, obtained by examining the layer using a 100x metallographic microscope.

[0026] Figure 5 This is a microstructure of the nitrided layer on the surface of the piston pin after processing using the process of this invention, as examined using a 500x metallographic microscope.

[0027] In the diagram: support beam 1, base plate 2, connecting column 3, mounting rod 4, tapered boss 5, positioning screw 6, top plate 7, support column 8, through hole 201, through hole 701. Detailed Implementation

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

[0029] like Figure 1 As shown, the present invention relates to a batch ion nitriding process for piston pins, comprising the following steps: The piston pins made of 31CrMoV9 material are deburred and cleaned, and then immediately air-dried or oven-dried. The specific steps are as follows: 1.1. Inspect each workpiece for burrs and rust spots. Remove burrs from workpieces with burrs and remove rust spots with fine sandpaper before cleaning. 1.2. Immerse the cleaned workpieces in the first cleaning agent bath for 2-3 minutes, then clean each piece individually with a brush. During the cleaning process, remove oil stains and iron filings from small holes and narrow crevices in the workpieces; a water-based cleaning agent should be used. 1.3. Place the workpiece that has been cleaned and dried in the second cleaning agent tank for a second cleaning, then rinse it with clean water and air dry or bake it immediately to prevent rust.

[0030] 2. Fix several dried piston pins onto a special fixture using positioning screws, with a spacing of 30-50mm between adjacent piston pins after installation; then place the special fixtures with the piston pins installed evenly inside the glow discharge ion nitriding furnace.

[0031] 3. A preset temperature value is set on the nitriding furnace, which is greater than the current actual temperature value displayed by the nitriding furnace system, so that the nitriding furnace system is in working condition; the preset temperature value is preferably 400℃.

[0032] 4. Check if the "ammonia flow rate setting value" is zero. If it is not zero, set the setting value to zero. Then set the pressure of the nitriding furnace to 50Pa and turn on the vacuum pump to evacuate the nitriding furnace. When the vacuum degree of the nitriding furnace is ≤50Pa, set the voltage to 650V and intermittently click the pulse duty cycle setting increase key to increase the pulse energizing time. When the pulse duty cycle increases to a certain value, the workpiece will start to arc and produce glow. Continue until the pulse duty cycle is gradually increased to 70% to clean the workpiece surface, remove oil and impurities from the workpiece surface, and thus improve the final nitriding effect of the workpiece surface.

[0033] 5. Reduce the pulse duty cycle to 10% and start adding ammonia gas into the nitriding furnace. Set the ammonia flow rate to 50-100 ml / min and the gas pressure to 30-50 Pa. The added ammonia gas will cause arcing to occur again in the nitriding furnace. As the arcing frequency decreases, slowly increase the pulse duty cycle to continue arcing. As the arcing progresses, the ammeter reading will gradually increase, causing the temperature of the workpiece in the furnace to gradually rise.

[0034] 6. After the arc light basically disappears and the glow becomes stable, start the resistance-assisted heating system installed on the furnace wall of the nitriding furnace to heat the nitriding furnace, and control the heating temperature to always be slightly lower than the workpiece temperature in order to reduce the temperature difference between the workpiece surface and the furnace wall.

[0035] 7. Set the given voltage of the nitriding furnace system to 750V, the pulse duty cycle to 60%-80%, the gas pressure to 300Pa, and the ammonia flow rate to 1300ml / min, so that the workpiece can be rapidly heated to 520-540℃.

[0036] 8. Enter the heat preservation stage and begin calculating the heat preservation time, which is 48-58 hours. After the heat preservation time is reached, the furnace can be stopped for cooling, which takes 13-17 hours. The workpiece can be removed from the furnace when its actual temperature drops below 150°C.

[0037] like Figures 4-5 As shown, when piston pins processed using this technology undergo furnace inspection, the inspection layer depth is 0.5-0.7mm, and the nitrided layer hardness is ≥730HV10. Under high-temperature conditions, the hardness retention is far superior to that of carburized martensitic structures, increasing wear life by more than 50%. The nitrided bright white layer depth is less than 0.015mm, and the nitride level is no greater than level 2, avoiding the risk of brittle spalling. It balances surface wear resistance and carburized toughness, increasing bending fatigue strength by 40%, making it more suitable for heavy-duty alternating working conditions.

[0038] This process enables a core upgrade of ordinary piston pins from traditional gas carburizing and quenching to ion nitriding. It adopts a low-temperature treatment of 520-540℃, which can effectively control piston pin deformation and improve the piston pin's dimensional and positional tolerance accuracy by 60%. After nitriding, no subsequent fine grinding process is required, and the processing cost and scrap rate are significantly reduced. It also refines the matrix grains, taking into account both surface hardness and matrix toughness, while reducing energy consumption, shortening the production cycle, and improving production economy.

[0039] like Figure 2 and Figure 3 As shown, the present invention relates to a special tooling for a batch ion nitriding process of piston pins, comprising a circular base plate 2, with multiple support columns 8 evenly distributed and fixed on the circumference of the base plate 2, and a top plate 7 fixedly connected to the base plate 2 by multiple connecting columns 3 evenly distributed on the circumference, and a support beam 1 fixedly connected at the center between the base plate 2 and the top plate 7, wherein the two ends of the support beam 1 and the support columns 8 are welded to the base plate 2 and the top plate 7 respectively or fixed by screws.

[0040] The cross-section of the support beam 1 is square. Multiple layers of mounting rods 4 are evenly distributed and fixed on the support beam 1 from top to bottom. The center distance between two adjacent layers of mounting rods 4 is 55-75mm to avoid the abnormal arcing in the furnace caused by too small a spacing, and the insufficient heating of the workpiece caused by too large a spacing.

[0041] Each layer of mounting rods 4 consists of four rods evenly distributed around the circumference, and each mounting rod 4 is connected to the threaded holes around the support beam 1 by threads. The outer diameter of each mounting rod 4 is clearance-fitted with the inner diameter of the piston pin. A tapered boss 5 is provided on one end of each mounting rod 4 near the support beam 1. The tapered boss 5 is an integral structure with the mounting rod 4. A positioning screw 6 is threaded to the outer end of each mounting rod 4 for positioning the piston pin to be processed.

[0042] Several through holes 201 are provided on the bottom plate 2, and several through holes 701 are also provided on the top plate 7, so that nitrogen ions can bombard the piston pin through the through holes.

[0043] There are four connecting columns 3. The mounting rods 4 of each layer pass through the middle position between two adjacent connecting columns 3 to balance the temperature field and reduce the temperature difference between workpieces.

[0044] In use, unscrew the positioning screw 6, put the piston pin to be processed onto each mounting rod 4, with the inner end of the piston pin resting against the tapered boss, and then install the positioning screw 6 to fix the piston pin to be processed onto the mounting rod 4.

[0045] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A batch ion nitriding process for piston pins, characterized in that, The steps are as follows: S1. Deburr and clean the piston pin to be processed, and air dry or bake it immediately after cleaning. S2. Fix multiple dried piston pins onto a special fixture, with the spacing between adjacent piston pins after installation being 30-50mm; then place the special fixture inside the glow discharge ion nitriding furnace. S3. A preset temperature value is set on the nitriding furnace, which is greater than the current actual temperature value displayed by the nitriding furnace system; S4. Set the pressure of the nitriding furnace to 50Pa, and then turn on the vacuum pump to evacuate the nitriding furnace. When the vacuum degree of the nitriding furnace is ≤50Pa, set the voltage to 650V, and intermittently click the pulse duty cycle given increase key to increase the pulse energizing time. When the pulse duty cycle increases to a certain value, the workpiece starts to arc and produce glow, until the pulse duty cycle is gradually increased to 70% to remove oil and impurities from the surface of the workpiece. S5. Reduce the pulse duty cycle to 10%, start adding ammonia gas into the nitriding furnace, set the ammonia gas flow rate to 50-100 ml / min, and the gas pressure to 30-50 Pa, and then slowly increase the pulse duty cycle; through the added ammonia gas, the arc is struck again in the nitriding furnace, and the temperature of the workpiece in the furnace gradually increases. S6. When the arc light basically disappears and the glow light tends to stabilize, start the resistance-assisted heating system set on the furnace wall of the nitriding furnace to heat the nitriding furnace, and control the heating temperature to always be slightly lower than the workpiece temperature in order to reduce the temperature difference between the workpiece surface and the furnace wall. S7. Set the given voltage of the nitriding furnace system to 750V, the pulse duty cycle to 60%-80%, the gas pressure to 300Pa, and the ammonia flow rate to 1300ml / min, so that the workpiece can be heated to 520-540℃ quickly. S8. Enter the heat preservation stage and start calculating the heat preservation time. After the heat preservation time is reached, the furnace can be stopped for cooling. When the actual temperature of the workpiece drops below 150℃, it can be taken out of the furnace.

2. The batch ion nitriding process for piston pins according to claim 1, characterized in that, The specific steps for cleaning the piston pins to be treated are as follows: Soak the workpieces in the first cleaning agent tank for 2-3 minutes, then clean and dry them one by one with a brush. During the cleaning process, oil and iron filings in the small holes and narrow crevices of the workpieces should be cleaned. Water-based cleaning agent is used. After the first cleaning and drying, the workpiece is placed in the second cleaning agent tank for a second cleaning. Then it is rinsed with clean water and immediately air-dried or oven-dried to prevent rust.

3. The batch ion nitriding process for piston pins according to claim 1, characterized in that: The preset temperature value is 400℃.

4. The batch ion nitriding process for piston pins according to claim 1, characterized in that: Before evacuating the nitriding furnace, check if the "ammonia flow rate setpoint" is zero. If it is not zero, set the setpoint to zero.

5. The batch ion nitriding process for piston pins according to claim 1, characterized in that: The heat preservation time is 48-58 hours, and the furnace shutdown and cooling time is 13-17 hours.

6. A special tooling for the batch ion nitriding process of piston pins as described in any one of claims 1-5, characterized in that: The system includes a base plate, with multiple support columns evenly distributed around its circumference below the base plate. A top plate is connected to the base plate via multiple connecting columns evenly distributed around its circumference. A support beam is connected at the center between the base plate and the top plate. The support beam has a square cross-section. Multiple layers of mounting rods are evenly distributed and fixed on the support beam from top to bottom. Each layer of mounting rods consists of four rods evenly distributed around its circumference and connected to the four sides of the support beam. The outer diameter of each mounting rod is clearance-fitted with the inner diameter of the piston pin. A tapered boss is provided on one end of each mounting rod near the support beam. A positioning screw is threaded to the outer end of each mounting rod to position the piston pin to be processed.

7. The special tooling according to claim 6, characterized in that: Several through holes are provided on the base plate to allow nitrogen ions to bombard the piston pin through these holes.

8. The special tooling according to claim 6, characterized in that: Several through holes are provided on the top plate to allow nitrogen ions to bombard the piston pin through these holes.

9. The special tooling according to claim 6, characterized in that: The center distance between two adjacent mounting rods is 55-75mm to avoid abnormal arcing in the furnace if the spacing is too small, and insufficient heating of the workpiece if the spacing is too large.

10. The special tooling according to claim 6, characterized in that: There are four connecting columns, and each layer of mounting rods passes between two adjacent connecting columns to balance the temperature field and reduce the temperature difference between workpieces.

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