Ion nitriding method for controlling micro-deformation of stainless steel thin-walled long cylinder

Abstract

The application discloses an ion nitriding method for controlling micro-deformation of a stainless steel thin-wall long-cylinder workpiece, which comprises a workpiece loading furnace process and an ion nitriding treatment process; the workpiece loading furnace process is specially designed for the special shape of the workpiece to ensure the temperature uniformity of the stainless steel thin-wall long-cylinder workpiece in the whole ion nitriding treatment process, and the specific parameters of the ion nitriding process of the application are determined, so that the micro-deformation of the long-cylinder workpiece after ion nitriding is ensured to be within the design range, and the yield of the stainless steel thin-wall long-cylinder workpiece after ion nitriding treatment is greatly improved.

Description

Technical Field

[0001] This invention relates to an ion nitriding method for stainless steel workpieces, and more particularly to an ion nitriding method for controlling micro-deformation of thin-walled long cylindrical stainless steel workpieces. Background Technology

[0002] Glow discharge ion nitriding is a novel surface modification technology. Compared with other nitriding methods, glow discharge ion nitriding utilizes the glow discharge phenomenon to ionize nitrogen-containing gas and generate nitrogen ions that bombard the surface of the part, heating it and performing nitriding. The workpiece forms three components from the surface to the core: a compound layer (also known as a bright white layer), a diffusion layer, and a substrate. Due to its characteristics such as fast nitriding rate, low brittleness, controllable phase composition, and micro-deformation, glow discharge ion nitriding has been regarded as a representative of green precision surface modification heat treatment processes.

[0003] For thin-walled long cylindrical workpieces (length exceeding 1500mm, wall thickness less than 10mm), there is no precedent for a good combination of material selection and processing to achieve excellent balance between hardness and strength. Currently, the commonly used combination is medium carbon steel combined with quenching and tempering pretreatment and ion nitriding or high-frequency induction hardening. However, during the quenching and tempering process, the workpiece is prone to deformation and cannot be straightened due to the influence of structural stress and thermal stress. Furthermore, there is a significant time difference between the upper and lower ends of the workpiece entering the quenching liquid, making it difficult to guarantee the overall uniformity of hardness. High-frequency induction hardening for thin-walled long cylindrical workpieces is constrained by multiple factors, including equipment limitations and induction heat treatment machine tool process parameters, which limits the induction hardening treatment of thin-walled long cylindrical workpieces.

[0004] Stainless steel is generally classified into martensitic stainless steel, austenitic stainless steel, and precipitation hardening stainless steel. Austenitic stainless steel is used to manufacture a variety of workpieces due to its excellent corrosion resistance. Common austenitic stainless steel parts (such as 304 and 316 stainless steel) are often used directly after solution treatment. Rust prevention is due to the dense passivation film Cr2O3 on the surface of austenitic stainless steel, but its surface hardness is relatively low (not exceeding 180 HB), and its wear resistance is relatively poor. If austenitic stainless steel workpieces are treated with ion nitriding, although some of the steel's inherent corrosion resistance will be sacrificed, the wear resistance of the workpiece can be significantly enhanced.

[0005] For stainless steel thin-walled cylindrical workpieces, machining is extremely difficult due to their length and thin walls, making it challenging to fully guarantee coaxiality and straightness. Conventional ion nitriding processes also present several problems: First, deformation occurs due to the thin walls and length, leading to deviations in the outer and inner diameters, straightness, and coaxiality, potentially rendering the workpiece unusable. Second, temperature control during ion nitriding is problematic. The thin walls (less than 10mm) result in low heat storage in the substrate (thinner walls mean lower heat storage), easily causing the workpiece to exceed the predetermined nitriding temperature. Overheating disrupts the coherent relationship between the nitride and the substrate, reducing surface microhardness. Third, the length of the workpiece results in uneven nitriding temperatures across the upper, middle, and lower parts, leading to inconsistent effective nitriding depths.

[0006] How to perform ion nitriding treatment on thin-walled long cylindrical stainless steel workpieces, while ensuring that the surface hardness meets the requirements and controlling the micro-deformation to meet the design requirements, so as to improve the yield of thin-walled long cylindrical stainless steel workpieces, is a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0007] The purpose of this invention is to address the existing technical deficiencies by providing an ion nitriding method for controlling the micro-deformation of thin-walled long cylindrical stainless steel components.

[0008] To achieve the above objectives, the present invention can adopt the following technical solutions:

[0009] The ion nitriding method for controlling micro-deformation of stainless steel thin-walled long cylindrical parts according to the present invention includes a workpiece loading process and an ion nitriding treatment process.

[0010] The workpiece loading process includes:

[0011] S11. First, place positioning plugs at the upper and lower ends of the long cylindrical workpiece, and seal the boss of the positioning plugs into the long cylindrical workpiece. Then, pass the lifting screw through the central through hole of the upper and lower positioning plugs. The lower end of the lifting screw extending out of the lower positioning plug is fastened with a nut, and the upper end of the lifting screw extends out of the positioning plug. To avoid abnormal arcing caused by the gap between the positioning plug and the workpiece, a nut can also be used to fix the upper end of the lifting screw extending out of the positioning plug.

[0012] S12. Connect and fix the upper end of the lifting screw of the long cylindrical workpiece to the auxiliary baffle of the special tooling;

[0013] S13. Three support columns are symmetrically installed on the auxiliary baffle around the long cylindrical workpiece, and the length of the support columns is slightly longer than that of the long cylindrical workpiece.

[0014] S14. Suspend multiple auxiliary round bars, shorter than the length of the support columns, side by side between two adjacent support columns to complete the clamping of the long cylindrical workpiece.

[0015] S15. Use a lifting device to hoist the special fixture with the clamped long cylindrical workpiece into the nitriding furnace as a whole, so that the three support columns are evenly placed on the cathode plate of the nitriding furnace, and the furnace loading process is completed.

[0016] The ion nitriding treatment process includes:

[0017] S21. When the workpiece is placed in the furnace, turn on the two vacuum pumps to draw the air pressure inside the furnace hood to below 60Pa, and adjust the voltage to 600-700V and the current to 10-15A. Then gradually increase the duty cycle value so that the workpiece inside the furnace starts to arc, in order to remove surface impurities and gradually increase the temperature.

[0018] Adjusting the duty cycle is a routine operation during the heating and homogenization process of the ion nitriding furnace. On-site, the duty cycle is gradually increased: starting from 0, as arcing occurs inside the furnace and gradually stabilizes, the duty cycle is gradually increased until it reaches the maximum value of the equipment, around 80-83. During the homogenization stage, the duty cycle is reduced by 30-35. Then, during the process of heating from 400℃ to the nitriding temperature of 540℃-560℃, the duty cycle is increased again. When the nitriding temperature range is reached, the duty cycle is controlled within the range of 45-50.

[0019] S22. After the workpiece is heated to 200℃, the temperature is uniformly maintained for 2 hours.

[0020] S23. Continue heating until the workpiece reaches 260℃±10℃. Introduce hot ammonia into the furnace and maintain the ammonia flow rate at 0.2-0.4L / min. Turn off one vacuum pump and allow the workpiece to reach 400℃ for 2 hours to homogenize. To reduce ammonia consumption while ensuring airflow in the furnace, the valve of the other vacuum pump can be closed halfway.

[0021] S24. When the temperature continues to rise to 540-560℃, increase the voltage to 700-750V, the current to 40-50A, and the ammonia flow rate to 1.0-1.2L / min, and keep it at that temperature for 40h.

[0022] S25. After the heat preservation is completed, stop the ammonia gas supply when the temperature is reduced to 300℃, and turn off the voltage and current. After the furnace cools to 160℃, remove the furnace and air cool.

[0023] This invention controls the micro-deformation of stainless steel thin-walled long cylindrical workpieces during ion nitriding from two aspects: clamping fixtures and process control. It ensures that the effective nitriding layer depth on the surface of the stainless steel thin-walled long cylindrical workpiece after ion nitriding meets the design requirements, and the deformation of the entire workpiece is controlled within the machinable range, which greatly improves the yield of stainless steel thin-walled long cylindrical workpieces after ion nitriding.

[0024] The hot ammonia gas introduced into the furnace is ammonia gas that has been decomposed by an ammonia decomposition furnace (it is a mixture of ammonia, nitrogen, and hydrogen). Since ammonia can decompose into nitrogen and hydrogen at high temperatures (the reaction formula is 2NH3→N2+3H2), under the vacuum and high pressure of the nitriding furnace, nitrogen decomposes into nitrogen ions, which bombard the surface of the workpiece for nitriding. Hydrogen ions will react with oxides or other substances on the metal surface to increase the activity of the nitriding reaction, thereby improving the quality and efficiency of nitriding.

[0025] In steps S23-S24, the heating rate of the workpiece during the entire heating process, from 260℃ to 400℃ and from 400℃ to 540-560℃, is ≤30℃ / h.

[0026] In step S25, after the heat preservation is completed, the temperature is first reduced to 480℃ and then uniformly cooled for 2 hours. This can prevent the workpiece from deforming due to excessive temperature drop during furnace cooling. Then, the temperature is reduced further.

[0027] To ensure temperature uniformity of the stainless steel thin-walled long cylindrical workpiece during nitriding, the present invention provides a specially designed fixture for clamping the long cylindrical workpiece, comprising: an auxiliary baffle with a lifting hole at the center; three support columns evenly fixed around the auxiliary baffle; the length of the three support columns being slightly greater than the length of the long cylindrical workpiece; multiple auxiliary round bars arranged between adjacent support columns; and workpiece fixing holes and several ventilation holes on the auxiliary baffle located inside the support columns.

[0028] The auxiliary baffle serves two purposes: firstly, to fix the long cylindrical workpiece, and secondly, to assist in heat concentration, preventing the upper part of the workpiece from being too cold and failing to reach the nitriding temperature. Several vent holes on the auxiliary baffle facilitate the smooth flow of the nitriding agent (thermally decomposed ammonia) from top to bottom. The length of the three supporting columns is slightly longer than the length of the long cylindrical workpiece, ensuring that the lifting screw will not hit the nitriding furnace chassis when straightened during the nitriding process. Multiple auxiliary round bars arranged between adjacent supporting columns surround the long cylindrical workpiece from three directions for protection, achieving uniform temperature of the long cylindrical workpiece.

[0029] A horizontal support rod is fixed between two adjacent support columns near the top and middle positions. The auxiliary round bar is suspended on the horizontal support rod in two layers, upper and lower. Since the auxiliary round bar is generally made of φ20mm steel bar, it will be very heavy when the length is long. If the long cylindrical workpiece being processed is long, the auxiliary round bar can be arranged in layers for easier layout.

[0030] The three horizontal support rods located in the middle are respectively fixed with auxiliary round rod support columns at both ends. The auxiliary round rod support columns extend downward and their bottoms are on the same horizontal plane as the bottoms of the three support columns. The support columns can provide strong support for the horizontal support rods that suspend the auxiliary round rods.

[0031] The advantage of this invention lies in starting with the tooling for suspending long cylindrical workpieces and determining the specific parameters of the ion nitriding process, thereby ensuring that the micro-deformation of the long cylindrical workpieces after ion nitriding is within the design range, which greatly improves the yield of ion nitriding treatment of thin-walled stainless steel long cylindrical workpieces. Attached Figure Description

[0032] Figure 1 This is an enlarged view of the plug used to clamp the workpiece.

[0033] Figure 2 This is a structural diagram of a long cylindrical workpiece after the plug has been clamped in place.

[0034] Figure 3 This is a schematic diagram of the special tooling structure designed for this invention.

[0035] Figure 4 This is a bar chart showing the microhardness of samples at different distances from the surface after ion nitriding treatment.

[0036] Figure 5 , Figure 6 The images show the microstructure of the samples after ion nitriding treatment. Detailed Implementation

[0037] The following detailed explanation, using specific examples of long cylindrical workpieces processed, is provided to facilitate understanding by those skilled in the art.

[0038] The dimensions of the long cylindrical workpiece treated with ion nitriding according to the present invention are: length 1958mm, outer diameter 65mm, inner diameter 56mm, wall thickness 4.5mm, and material is 0Cr18Ni10Ti austenitic stainless steel.

[0039] Technical requirements: After ion nitriding treatment, the surface hardness shall meet ≥900HV0.1; the effective nitriding layer depth shall be 0.08~0.15mm.

[0040] According to the alloy element content specifications in GB / T 1220-2007 "Stainless Steel Bars", the alloy element content (mass percentage) of the raw material bar used in this workpiece was determined using an ARL3460 direct-reading spectrometer. Specifically, the content is as follows: C≤0.08 (0.06), Si≤1.0 (0.34); Mn≤2.00 (1.26); P≤0.045 (0.02); S≤0.03 (0.01); Ni 9.00-12.00 (10.02); Cr 17.00-19.00 (18.32); Ti C 5 (0.35). The values ​​in parentheses are the actual measured components, all of which meet the requirements of the national standard.

[0041] The workpiece is processed according to the following procedure: solution treatment -- machining -- stabilization treatment -- grinding -- cleaning -- ion nitriding -- ion nitriding result detection.

[0042] According to the solution treatment temperature range recommended in the heat treatment manual, the solution treatment temperature used for this workpiece is 1100℃-1120℃.

[0043] I. Solution treatment

[0044] The raw material for making the workpiece (φ72mm stainless steel bar) was placed in a high-temperature box furnace, heated to 650℃ and held for 60 minutes, then heated to 900℃ and held for 60 minutes, then heated to 1100℃ and held for 90 minutes, and then oil-cooled, with the quenching oil temperature controlled at 45-50℃; after solution treatment, the end face hardness was tested using a portable hardness tester, and the end face hardness was 172HBS.

[0045] II. Machining Processing

[0046] The outer diameter of the bar is rough-turned using a CNC machine tool, with a 3mm machining allowance removed on one side, leaving a 0.50mm allowance on one side of the outer diameter. The inner hole is formed by deep drilling, and the inner hole size after machining is 55.3mm, with a 0.35mm allowance on one side.

[0047] III. Stabilization Treatment

[0048] The thin-walled cylindrical parts were stabilized using an RJ3-380kw trolley tempering furnace with a holding temperature of 260℃ and a holding time of 300min. The parts were then furnace cooled to a temperature of 80℃~100℃ and then air-cooled to room temperature.

[0049] IV. Grinding Process

[0050] Both ends are fixed with tooling with boss structure, and the center point hole is used as the center hole to ensure that the center holes at both ends are on the same horizontal line. The surface roughness of the finished workpiece after grinding is Ra0.8. The workpiece machining dimensions are: length 1958mm, outer diameter 65mm, inner diameter 56mm, and wall thickness 4.5mm.

[0051] V. Cleaning Treatment

[0052] To ensure the appearance quality of this workpiece, a cleaning solution containing metal cleaning agent can be used first during cleaning. The temperature of the cleaning solution should be 80-90℃ to ensure that the cutting fluid, rust-preventive oil, iron filings and other impurities adhering to the surface are cleaned. Then, during the furnace loading process, the surface of the workpiece should be wiped again with a clean white cloth soaked in alcohol.

[0053] VI. Ion Nitriding Treatment

[0054] 1. Loading the furnace

[0055] Ion nitriding heating relies on ion bombardment for heating, with each workpiece serving as a heat source. Heating is limited by the surface area / weight ratio. Considering the characteristics of bell-type ion nitriding furnaces—lower temperatures at the top and lower parts, higher temperatures in the middle and lower temperatures at the outer layer—the first consideration is the furnace loading fixture:

[0056] First, a positioning plug is designed, the structure of which is as follows: Figure 1 As shown, a through hole D101 (φ10mm) is provided in the middle, with a boss D102 (φ54mm). During installation, plugs (upper plug D1, lower plug D2) are installed at both ends of the long cylindrical workpiece A. The boss D102 of the plug is embedded into the long cylindrical workpiece A. The through hole in the middle is used to pass through the lifting screw DL. The lower plug D2 is fixed with a nut. The upper part of the lifting screw DL passes through the upper plug D1 and is connected to the special tooling for clamping the long cylindrical workpiece. The installation of the upper and lower plugs is completed. In actual installation, in order to avoid abnormal arcing caused by the gap between the through hole of the upper plug and the lifting screw DL, a nut can also be placed above the upper plug D1, such as... Figure 2 As shown.

[0057] Because the lifting screw DL is slender and lacks rigidity in the early stage of nitriding in the furnace, it is not in a straight hanging state. During the nitriding process, the screw will straighten. Since the bosses on both ends of the long cylindrical workpiece A are embedded inside the long cylindrical workpiece A, it can be ensured that the upper and lower bosses will not be displaced during the nitriding process.

[0058] To ensure temperature uniformity during the ion nitriding process of thin-walled elongated workpieces, it is necessary to shield and protect the upper part and surrounding area of ​​the workpiece. Therefore, this invention designs a special tooling, the structure of which is as follows: Figure 3 As shown:

[0059] It includes an auxiliary baffle 2 with a lifting hole 1 at the center, three supporting columns 3 evenly fixed around the auxiliary baffle 2, and workpiece fixing holes 4 and several ventilation holes 5 on the auxiliary baffle 2 located inside the supporting columns 3. Figure 3The image shows eight workpiece fixing holes, two of which are used to fix long cylindrical workpieces A. The eight workpiece fixing holes are evenly spaced around the hoisting hole 1, and can be used to perform ion nitriding treatment on eight long cylindrical workpieces simultaneously when needed.

[0060] The auxiliary baffle 2 has the following functions: First, it is used to fix the long cylindrical workpiece A, that is, to connect and fix the upper end of the lifting screw DL (extending the upper plug D1) of the long cylindrical workpiece to the auxiliary baffle 2; Second, it can play an auxiliary heat-gathering role to prevent the upper part of the workpiece from being too low and unable to reach the nitriding temperature; Third, the several vent holes 5 (irregular small holes) opened on the auxiliary baffle facilitate the smooth flow of the nitriding agent (thermal decomposition ammonia) from top to bottom.

[0061] It is important to note that the length of the three support columns 3 must be greater than the length of the long cylindrical workpiece A. When the support columns 3 are placed on the nitriding furnace base DP, it can be ensured that the bottom of the long cylindrical workpiece A (the lifting screw at the bottom of the lower plug D2) will not touch the nitriding furnace base DP when the lifting screw DL is straightened during the nitriding process. Multiple auxiliary round bars 6 are arranged between two adjacent support columns. Since the long cylindrical workpiece A being processed is quite long (nearly 2 meters), the auxiliary round bars 6 can be arranged in two layers. Horizontal support rods 7 are fixed between two adjacent support columns 3 near the top and middle positions. Multiple auxiliary round bars 6 (φ20 steel bars can be used) are suspended from the horizontal support rods 5 in two layers (the figure shows 3 auxiliary round bars suspended at intervals on each horizontal support rod; the number of auxiliary round bars suspended on each horizontal support rod can be increased if necessary). This forms a three-sided closed protection for the long cylindrical workpiece A, achieving temperature uniformity during the ion nitriding process of the long cylindrical workpiece.

[0062] Since the auxiliary round rods suspended on the horizontal support rod 7 are relatively heavy, auxiliary round rod support columns 8 can be fixed at both ends of the horizontal support rod 7. The auxiliary round rod support columns 8 extend downwards, and their bottoms are on the same horizontal plane as the bottoms of the three support columns. During the ion nitriding process, they can provide strong support for the horizontal support rod 7 that suspends the auxiliary round rods.

[0063] After fixing the lifting screw DL of the long cylindrical workpiece to be processed on the workpiece fixing hole 4, the special tooling with the long cylindrical workpiece clamped is lifted into the nitriding furnace by the lifting tool, so that the three support columns are evenly placed on the cathode plate DP of the nitriding furnace, and the auxiliary round bar support column 8 is also placed in a suitable position. The auxiliary round bars on each horizontal support rod 7 are suspended in sequence to complete the furnace loading process.

[0064] Because the wall thickness of the austenitic stainless steel thin-walled long cylindrical part is too thin and the effective nitriding layer depth is too shallow, the portable Leeb hardness tester cannot accurately and effectively detect the surface hardness of the workpiece. Therefore, the furnace-loaded sample is used as the basis for detecting various data of the nitriding layer of the workpiece. In order to facilitate the inspection of the surface hardness, compound layer and effective hardness layer depth of the furnace-loaded sample after ion nitriding treatment, the applicant placed rectangular strip samples (15mm×15mm×40mm) of the same material as the long cylindrical workpiece in three positions in the nitriding furnace (near the long cylindrical workpiece): the lower sample was placed on a cylindrical pad with a diameter of φ20mm and a height of 50mm, and the placement height was close to the lower position of the long cylindrical part; the upper sample was placed at the upper end D1 position; and the middle sample was positioned using a short screw fixed to the auxiliary baffle and an adjusting fixing plate screwed on the screw (placed in the middle position of the stainless steel thin-walled cylinder).

[0065] 2. Ion nitriding treatment

[0066] 1) The ion nitriding equipment uses an LDM-150KW bell-type ion nitriding furnace. After placing the workpiece in the furnace, turn on two vacuum pumps to evacuate the gas pressure inside the furnace hood to below 60Pa. Then, rotate and increase the voltage to 600-700V and the current to 10-15A. Gradually increase the duty cycle value to make the workpiece inside the furnace start to arc, so as to remove surface impurities and gradually increase the temperature.

[0067] 2) After the workpiece has been heated to 200℃, allow it to cool evenly for 2 hours.

[0068] 3) Continue heating until the workpiece reaches approximately 260°C. Then, introduce thermally decomposed ammonia (a mixture of ammonia, nitrogen, and hydrogen) into the furnace. Maintain the ammonia flow rate at 0.2-0.4 L / min, and turn off one vacuum pump. Allow the workpiece to reach 400°C and then homogenize it for 2 hours. Throughout the entire heating process, the heating rate must be controlled to not exceed 30°C / h.

[0069] 4) Continue heating to the nitriding temperature (540-560℃), increase the voltage to 700-750V, the current to 40-50A, and the ammonia flow rate to 1.0-1.2L / min, and hold for 40h; similarly, the heating rate should be controlled not to exceed 30℃ / h throughout the entire heating process.

[0070] 5) After the heat preservation is completed, first lower the temperature to 480℃ and then uniformly heat for 2 hours to prevent the workpiece from deforming due to excessive temperature drop during furnace cooling; then continue to lower the temperature to below 300℃, stop the ammonia gas supply, and turn off the voltage and current. After the furnace cools to 160℃, remove the workpiece from the furnace and air cool.

[0071] VII. Verification of Ion Nitriding Results

[0072] The surface hardness of ion nitrided surfaces was tested according to GB / T 34883-2017 "Ion Nitriding" standard. The Vickers hardness method was used to test the surface hardness of the samples. Due to the shallow depth of the nitrided layer, a testing force of 0.1 kgf was applied. The rectangular strip samples were 15*15*40 mm in size, and the surface needed to be polished before testing the hardness. The nitrided layer was tested according to GB / T 11354-2005 "Determination of Nitrided Layer Depth and Metallographic Structure of Steel Parts" standard, including the testing of the compound layer and the effective hardness layer depth of the nitrided layer. The test location was the cross-section of the sample. The sample needed to be polished, and the effective depth of the nitrided layer was determined by the hardness method.

[0073] 1. Microstructure

[0074] The surface hardness was tested using an HV-10B low-load Vickers hardness tester with a testing force of 0.1 kgf. Five locations were tested for each sample. The surface hardness results are shown in Table 1 below.

[0075] Table 1 Surface hardness test values ​​at various locations

[0076]

[0077] Analysis of the surface hardness test results above shows that there is no significant difference in the test results of the samples at different locations. After removing the highest and lowest values ​​in the test data, the average surface hardness values ​​at different locations are 1088.5HV0.1, 1092.2HV0.1 and 1096.4HV0.1, respectively.

[0078] Conclusion: The surface hardness at all locations of the sample meets the technical requirement of ≥900HV0.1.

[0079] The effective microhardness gradient of the nitrided layer was detected using an HXD-1000TMC / LCD microhardness tester with a testing force of 0.3 kgf. The test results are as follows: Figure 4 As shown.

[0080] Figure 4This is a columnar schematic diagram of the microhardness of samples at different distances from the surface. According to GB / T 11354-2005 "Determination of Nitrided Layer Depth and Metallographic Structure of Steel Parts", the hardness measured at a depth greater than three times the nitrided layer is the core hardness. After five measurements, the average value was 218.9 HV0.3. The core hardness + 50 HV is used as the boundary for judging the nitrided layer depth. The hardness at 0.1 mm from the surface is greater than 268.9 HV0.3, while the surface hardness at 0.15 mm is lower than this value. Considering that the microhardness tester calculates the hardness value based on the principle of measuring the diagonal length, superimposing test points within the 0.1-0.15 mm range on the same horizontal line can easily cause overlapping indentations and data distortion. Therefore, the minimum range of effective nitrided layer depth at each location is 0.10-0.12 mm, which meets the design requirements.

[0081] The ion-nitrided sample was etched using FeCl3 solution. Figure 5 and Figure 6 The images show the microstructure of the sample, from the surface inwards: the compound layer (bright white layer), the transition layer, and the substrate. The bright white layer is approximately 5 μm thick, accounting for about 5% of the total hardened layer depth. Surface strengthening is mainly due to two factors: firstly, the dispersion strengthening effect resulting from the combination of Cr, Ti, and N elements to form a second-phase compound; secondly, the ionization of thermally decomposed NH3 under a vacuum high-voltage electric field, generating ions that bombard the metal surface and combine with sputtered Fe ions to form Fe²⁺. x N compounds.

[0082] Conclusion: From Figure 5 It can be observed that after the ion nitriding process of this invention, the thickness of the bright white layer is slightly shallower than that of conventional medium carbon alloy steel (6-8 μm). However, the strengthening effect of nitrides formed by the combination of Ti and Cr elements with N elements in austenitic stainless steel far exceeds that of medium carbon alloy steel, significantly enhancing its surface wear resistance. Furthermore, due to the long-term nitriding treatment at 540℃ for thin-walled long cylindrical parts of austenitic stainless steel, this temperature falls within the sensitization temperature range of austenitic stainless steel (450-850℃), resulting in… Figure 6 The precipitation of high-chromium carbides at grain boundaries and within grains, as shown, leads to a decrease in the corrosion resistance of austenitic stainless steel. However, the dense outermost compound layer provides a certain degree of corrosion resistance. Therefore, glow discharge ion nitriding can effectively modify the surface of austenitic stainless steel, resulting in a microstructure that is both ultra-hard and wear-resistant while also providing corrosion resistance.

[0083] 2. Deformation data

[0084] Since the workpieces being processed were thin-walled long cylindrical parts, on-site inspection was conducted using vernier calipers and an inside micrometer to measure the data of the inner hole and outer diameter. The inner hole could only be measured at both ends, while the inside micrometer was used to measure the data of the upper, middle, and lower sections of the outer diameter. The deformation data of the three thin-walled cylinders before and after nitriding were measured, and the results are shown in Table 2 below.

[0085] Table 2 Data on thin-walled long cylindrical austenitic stainless steel components before and after ion nitriding

[0086]

[0087] The deformation of the workpiece mainly comes from two aspects: one is the nitride formed by the combination of N ions with Fe or alloying elements during nitriding, which produces lattice distortion and deformation that is usually 70% of the thickness of the white layer; the second, more important factor is the effect of thermal stress during nitriding, which causes the workpiece to deform due to the release of residual machining stress during heating and furnace cooling.

[0088] Conclusion: The test data above shows that the inner diameter increased by 0.01 mm, and the outer diameter deformation ranged from 0.008 to 0.014 mm, which fully meets the design requirements.

Claims

1. An ion nitriding method for controlling micro-deformation of thin-walled long cylindrical stainless steel components, characterized in that: This includes the workpiece loading process and the ion nitriding treatment process; The workpiece loading process includes: S11. First, place positioning plugs at the upper and lower ends of the long cylindrical workpiece, and seal the boss of the positioning plug into the long cylindrical workpiece. Then, the lifting screw passes through the central through hole of the upper and lower positioning plugs, and the lower end of the lifting screw extending out of the lower positioning plug is fastened with a nut. The upper end of the lifting screw extends out of the positioning plug. S12. Connect and fix the upper end of the lifting screw of the long cylindrical workpiece to the auxiliary baffle of the special tooling; S13. Three support columns are symmetrically installed on the auxiliary baffle around the long cylindrical workpiece. The length of the support columns should be longer than that of the long cylindrical workpiece. S14. Suspend multiple auxiliary round bars, shorter than the length of the support columns, side by side between two adjacent support columns to complete the clamping of the long cylindrical workpiece. S15. Use a lifting device to hoist the special fixture with the clamped long cylindrical workpiece into the nitriding furnace as a whole, so that the three support columns are evenly placed on the cathode plate of the nitriding furnace, and the furnace loading process is completed. The ion nitriding treatment process includes: S21. When the workpiece is placed in the furnace, turn on the two vacuum pumps to draw the air pressure inside the furnace hood to below 60Pa, and adjust the voltage to 600-700V and the current to 10-15A. Then gradually increase the duty cycle value so that the workpiece inside the furnace starts to arc, in order to remove surface impurities and gradually increase the temperature. S22. After the workpiece is heated to 200℃, the temperature is uniformly maintained for 2 hours. S23. Continue heating until the workpiece reaches 260℃±10℃. Introduce hot ammonia gas into the furnace and maintain the ammonia flow rate at 0.2-0.4L / min. Turn off one vacuum pump and allow the workpiece to reach 400℃ for 2 hours to homogenize. S24. When the temperature continues to rise to 540-560℃, increase the voltage to 700-750V, the current to 40-50A, and the ammonia flow rate to 1.0-1.2L / min, and keep it at that temperature for 40h. S25. After the heat preservation is completed, stop the ammonia gas supply when the temperature is reduced to 300℃, and turn off the voltage and current. After the furnace cools to 160℃, remove the furnace and air cool.

2. The ion nitriding method for controlling micro-deformation of thin-walled stainless steel long cylindrical parts according to claim 1, characterized in that: The hot ammonia gas refers to the ammonia gas produced by transporting ammonia gas from an ammonia gas tank into an ammonia decomposition furnace and then heating and decomposing it.

3. The ion nitriding method for controlling micro-deformation of thin-walled stainless steel long cylindrical parts according to claim 1, characterized in that: In steps S23-S24, the heating rate of the workpiece during the entire heating process, from 260℃ to 400℃ and from 400℃ to 540-560℃, is ≤30℃ / h.

4. The ion nitriding method for controlling micro-deformation of thin-walled stainless steel long cylindrical parts according to claim 1, characterized in that: In step S25, after the heat preservation is completed, the temperature is first reduced to 480℃ and then uniformly cooled for 2 hours to prevent the workpiece from deforming due to excessive temperature drop during furnace cooling. Then the temperature is reduced further.

5. The ion nitriding method for controlling micro-deformation of thin-walled stainless steel long cylindrical parts according to claim 1, characterized in that: The special tooling for clamping long cylindrical workpieces includes an auxiliary baffle with a lifting hole at the center, three support columns evenly fixed around the auxiliary baffle, the length of the three support columns being greater than the length of the long cylindrical workpiece, and multiple auxiliary round bars arranged between two adjacent support columns. The auxiliary baffle located inside the support columns has workpiece fixing holes and several ventilation holes.

6. The ion nitriding method for controlling micro-deformation of thin-walled stainless steel long cylindrical parts according to claim 5, characterized in that: A horizontal support rod is fixed between two adjacent support columns near the top and middle positions, and the auxiliary round rod is suspended on the horizontal support rod in two layers, upper and lower.

7. The ion nitriding method for controlling micro-deformation of thin-walled stainless steel long cylindrical parts according to claim 6, characterized in that: The horizontal support rod is fixed with auxiliary round bar support columns at both ends. The auxiliary round bar support columns extend downward and their bottoms are on the same horizontal plane as the bottoms of the three support columns.

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