Low-temperature carburized snap ring and ferrule type joint and preparation method thereof

Abstract

The invention discloses a low-temperature carburizing snap ring and ferrule type connector and a preparation method thereof, and belongs to the technical field of low-temperature carburizing methods. The preparation method comprises the steps that the austenitic stainless steel snap ring is pretreated, so that the surface roughness Ra of the austenitic stainless steel snap ring reaches 1.6-3.5 microns; pTFE aqueous dispersion liquid with the solid content being 10-15 wt% is sprayed to the surface of the pretreated snap ring, and the fluorine-plated snap ring is obtained; the method comprises the following steps: carrying out gradient heating heat treatment on the fluorine-plated snap ring at 420-550 DEG C in a protective atmosphere, carrying out carburizing treatment at 500-565 DEG C in a carburizing gas atmosphere, and cooling in the protective atmosphere to obtain the low-temperature carburized snap ring. The carburizing efficiency is high, the carburizing layer depth is large, the production period is short, and the surface strengthening and matrix corrosion resistance of the product can be well considered.

Description

Technical Field

[0001] This invention relates to the technical field of low-temperature carburizing processes, and particularly to a low-temperature carburizing retaining ring and ferrule type joint and its preparation method. Background Technology

[0002] Compression fittings are critical connection components widely used in piping systems, achieving reliable sealing and connection between pipes and valves / equipment through mechanical means. This structure typically consists of three parts: the fitting body, a retaining ring (or ferrule), and a compression nut. Its working principle is as follows: when the compression nut is tightened, the retaining ring undergoes plastic deformation under axial pressure, and its cutting edge is pressed into the outer wall of the pipe, forming a sealing ring. Simultaneously, the central part of the retaining ring arches, generating elastic deformation, which acts as a buffer and provides continuous compensation, thus achieving a reliable seal that prevents leakage, resists vibration, and is resistant to pull-out. As the most crucial load-bearing and deformation component, the performance of the retaining ring directly determines the overall reliability of the fitting. It typically needs to possess extremely high surface hardness and wear resistance, good toughness and elasticity, and corrosion resistance. To meet these performance requirements, retaining rings are usually made of austenitic stainless steel or duplex stainless steel. These materials themselves have good toughness and corrosion resistance, but their surface hardness is insufficient. Therefore, the surface must be strengthened during the preparation process. The key treatment method is carburizing, which involves infiltrating carbon atoms into the surface layer of the workpiece at high temperature to form a hardened layer with high carbon content, thereby significantly improving its surface hardness and wear resistance.

[0003] When conventional carburizing processes are used in the preparation of snap rings, they usually need to be carried out at high temperatures above 800°C and have the following drawbacks: (1) At high temperatures, chromium, which plays a key role in corrosion resistance in stainless steel, will preferentially combine with the diffused carbon to form chromium carbides (such as Cr). 23 C6) precipitates in large quantities at grain boundaries and on the surface. This leads to "chromium deficiency" in the surface and grain boundary areas of the material, forming a chromium-poor zone, which causes the core corrosion resistance of stainless steel (especially intergranular corrosion resistance) to drop sharply, making it unable to meet the requirements of harsh working conditions; (2) High-temperature long-term treatment easily leads to coarse austenite grains, resulting in uneven or loose carburized layer structure, affecting the hardness and toughness of the material. At the same time, high temperature will cause increased deformation, increasing subsequent processing costs and processing difficulty; (3) High-temperature process consumes a lot of energy and has a long production cycle.

[0004] To address the aforementioned contradictions, low-temperature carburizing technology has been introduced into the field of stainless steel surface strengthening. This technology typically involves infiltrating carbon atoms into the surface of austenitic stainless steel at temperatures lower than those of traditional carburizing, using a special process. Because the processing temperature is significantly lower than the sensitive temperature range for the large-scale precipitation of chromium carbides, this technology effectively inhibits the formation of chromium carbides, avoids chromium-depleted areas, and thus achieves high hardness while essentially maintaining the original excellent corrosion resistance of stainless steel. Furthermore, the low-temperature treatment greatly reduces thermal deformation of the workpiece, which is beneficial for maintaining the geometric accuracy of precision components such as retaining rings. However, existing low-temperature carburizing processes still face the following technical challenges when applied to precision stainless steel parts such as snap rings: the dense Cr2O3 passivation film naturally formed on the stainless steel surface severely hinders the carburizing process, resulting in shallow carburized layers, low efficiency, and long processing times; at low temperatures, the diffusion rate of carbon atoms is slow, making it difficult to obtain a sufficient effective hardened layer depth (e.g., >20μm), and it is extremely sensitive to process parameters such as temperature, atmosphere, and time, making it difficult to control uniformity and consistency; to activate the surface and break through the passivation film, complex pretreatment (such as sandblasting, pre-oxidation and reduction, plasma bombardment, etc.) is often required, or specific expensive equipment (such as plasma or salt bath) is needed, resulting in complex process flows, long overall processing cycles, and high costs, which limits its large-scale application in high-volume, high-requirement industrial parts such as snap rings. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a low-temperature carburizing clamping ring and ferrule joint and its preparation method. This preparation method overcomes the problems of low carburizing efficiency, insufficient carburizing depth, long process time, and difficulty in uniformity control caused by the obstruction of the stainless steel passivation film in existing low-temperature carburizing technologies. This preparation method is efficient, controllable, and can balance the deep strengthening of the material with the corrosion resistance of the substrate.

[0006] The technical solution of the present invention is as follows: A method for preparing a low-temperature carburized retaining ring, comprising the following steps: (1) The austenitic stainless steel retaining ring is pretreated to obtain a pretreated retaining ring; the pretreatment includes cleaning and surface roughening treatment to reduce its surface roughness R. a Reaching 1.6-3.5μm; (2) By electrostatic spraying, a PTFE aqueous dispersion with a PTFE solid content of 10-15wt% is uniformly sprayed onto the surface of the pretreated snap ring to form a coating film with a thickness of 3.0-4.5μm, which is then dried to obtain a PTFE-plated snap ring. (3) Under a protective atmosphere, the fluorine-plated clasp is subjected to a gradient heating heat treatment at 420-550℃ to obtain an activated fluorine-plated clasp. (4) The activated plated retaining ring is carburized at 500-565°C in a carburizing gas atmosphere, and then cooled in a protective atmosphere to obtain a low-temperature carburized retaining ring.

[0007] According to some preferred embodiments of the present invention, the cleaning includes alkaline ultrasonic cleaning before the surface roughening treatment and water ultrasonic cleaning after the surface roughening treatment.

[0008] According to some preferred embodiments of the present invention, the drying conditions in step (2) are: drying at 80-90°C for 10-15 minutes.

[0009] According to some preferred embodiments of the present invention, the gradient heating heat treatment includes: heating to 420-480°C at a heating rate of 3-8°C / min, holding at that temperature for 30-60 min, and then heating to 525-550°C at a heating rate of 3-8°C / min, holding at that temperature for 20-40 min.

[0010] According to some preferred embodiments of the present invention, the carburizing gas atmosphere comprises nitrogen, hydrogen and methane in a volume ratio of (50-75):(24-45):(1-5).

[0011] According to some preferred embodiments of the present invention, the holding time for the carburizing treatment is 300-420 min.

[0012] According to some preferred embodiments of the present invention, in step (1), the surface roughness Ra is 1.6-2.0 μm; in step (2), the thickness of the coating film is 3.5-4.0 μm; in step (3), the gradient heating heat treatment includes: heating to 420-450℃ at 3-5℃ / min and holding for 60 min, and then heating to 535-550℃ at 3-5℃ / min and holding for 20-30 min; in step (4), the carburizing temperature is 535-550℃ and the holding time is 300-420 min.

[0013] The inventors unexpectedly discovered that the product obtained by this preferred embodiment has superior hardened layer thickness, hardness, and corrosion resistance, and achieves a good balance between product performance, production process control, and production cost.

[0014] According to some preferred embodiments of the present invention, in step (1), the surface roughness Ra is 2.5-3.0 μm; in step (2), the thickness of the coating film is 4.0-4.5 μm; in step (3), the gradient heating heat treatment includes: heating to 450-480℃ at 5-8℃ / min, holding for 30-50 min, and then heating to 535-550℃ at 5-8℃ / min, holding for 20-30 min; in step (4), the carburizing temperature is 550-565℃, and the holding time is 300-360 min.

[0015] The inventors unexpectedly discovered that this preferred embodiment can achieve a better hardened layer thickness.

[0016] The present invention further provides a low-temperature carburized retaining ring prepared according to the above preparation method and a retaining fitting containing the low-temperature carburized retaining ring.

[0017] The present invention has the following beneficial effects: This invention can modify the passivation film on the stainless steel surface in situ by fluorination through PTFE spraying and subsequent mild gradient pyrolysis activation, thereby disrupting the continuity and density of the passivation film and significantly improving the diffusion efficiency and depth of carbon atoms in the carburizing process. This invention can achieve deep carburizing effect at a lower carburizing temperature and a shorter holding time. Compared with traditional high-temperature carburizing or long-term holding processes, it significantly shortens the production cycle, improves production efficiency, and reduces energy consumption. The low-temperature carburized retaining ring obtained by this invention has high surface hardness and hardened layer thickness, and the hardened layer has a uniform and dense structure, which significantly improves the wear resistance, fatigue resistance and service life of the product, and avoids early failure caused by loose structure. The preparation method of the present invention only modifies the surface passivation film without compromising the overall corrosion resistance of the base stainless steel. While achieving deep surface hardening, it successfully preserves the inherent good corrosion resistance of the austenitic stainless steel base, solving the problem that corrosion resistance is often sacrificed during strengthening treatment in the prior art. The preparation method of the present invention has strong process controllability and is suitable for large-scale production. At the same time, by adjusting the process parameters, it can produce clasp products with different performance focuses, such as products with high hardness and excellent corrosion resistance or products with a thicker hardened layer, thus exhibiting high process flexibility. Attached Figure Description

[0018] Figure 1 These are sampled microscopic images of the low-temperature carburized retaining rings obtained in Example 4, where (a) is image of sample 1, (b) is image of sample 2, and (c) is image of sample 3. Figure 2The sampling test results are for the low-temperature carburized retaining rings obtained in Example 4; Figure 3 Example of a cross-sectional microscopic image of a compression fitting with a low-temperature carburized retaining ring (as described in Example 4) assembled with a steel pipe. Detailed Implementation

[0019] The technical solutions of the present invention will be further described below with reference to embodiments thereof. The embodiments described below are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort should fall within the scope of protection of the present invention.

[0020] The PTFE dispersion with a solid content of 10wt% used in the following examples was obtained by adjusting the solid content after purchasing commercial products (such as Teflon® PTFE DISPersion) (by directly adding deionized water or by further adding functional additives such as stabilizers, leveling agents, defoamers, etc.).

[0021] Example 1 The low-temperature carburized retaining ring is prepared by the following process: (1) The 304 stainless steel retaining ring is ultrasonically cleaned in an alkaline cleaning solution to remove surface oil stains, and then sandblasted to reduce its surface roughness R. a After reaching 1.6μm, it is then subjected to ultrasonic cleaning with deionized water and hot air drying to obtain a pretreated retaining ring; (2) Using an electrostatic spraying device, a PTFE dispersion with a solid content of 10wt% is uniformly sprayed onto the surface of the pretreated snap ring to form a coating film with a thickness of 3.5μm. Then, it is dried in an oven at 80℃ for 10min to obtain a fluorine-plated snap ring. (3) The PTFE-plated snap ring is placed in a tube furnace and subjected to gradient heating under nitrogen protection. The process includes: heating to 420°C at a heating rate of 5°C / min and holding for 60 min to achieve preliminary pyrolysis of PTFE and mild fluorination of the surface passivation film Cr2O3; then heating to 550°C at a heating rate of 5°C / min and holding for 20 min to achieve rapid conversion and cleaning of the passivation film; and obtaining an activated PTFE-plated snap ring. (4) The activated fluorine-plated snap ring is quickly transferred to the carburizing furnace and carburized in the carburizing gas. The carburizing gas includes N2, H2 and CH4 in a volume ratio of 50:45:5. The carburizing temperature is 550℃ and the holding time is 300min. Then the carburizing gas is stopped and the snap ring is cooled and taken out of the furnace under nitrogen protection to obtain a low-temperature carburized snap ring.

[0022] The thickness and hardness of the obtained low-temperature carburized retaining ring were tested. The results showed that it contained a hardened layer with an average thickness of 28.7 μm and a surface hardness of HV920. Compared with the hardened layer structure obtained by conventional high-temperature carburizing, it can be found that the hardened layer structure of Example 1 is uniform and dense, while the hardened layer structure obtained by high-temperature carbonization has a loose layer.

[0023] Furthermore, the obtained low-temperature carburized retaining ring was subjected to a salt spray test (neutral salt spray). The test showed that after 168 hours of testing, the obtained low-temperature carburized retaining ring still showed no rusting, with only a small number of yellow spots in some areas.

[0024] Example 2 The low-temperature carburized retaining ring is prepared by the following process: (1) The 316L stainless steel retaining ring is ultrasonically cleaned in an alkaline cleaning solution to remove surface oil stains, and then sandblasted to reduce its surface roughness R. a After reaching 2.0μm, it undergoes ultrasonic cleaning with deionized water and hot air drying to obtain a pretreated retaining ring; (2) Using an electrostatic spraying device, a PTFE dispersion with a solid content of 10wt% is uniformly sprayed onto the surface of the pretreated snap ring to form a coating film with a thickness of 4.0μm. Then, it is dried in an oven at 90℃ for 15min to obtain a fluorinated snap ring. (3) The PTFE-plated snap ring is placed in a tube furnace and subjected to gradient heating under nitrogen protection. The process includes: heating to 450°C at a heating rate of 3°C / min and holding for 60 min to achieve preliminary pyrolysis of PTFE and mild fluorination of the surface passivation film Cr2O3; then heating to 535°C at a heating rate of 3°C / min and holding for 30 min to achieve rapid conversion and cleaning of the passivation film; and obtaining an activated PTFE-plated snap ring. (4) The activated fluorine-plated snap ring is quickly transferred to the carburizing furnace and carburized in the carburizing gas. The carburizing gas includes N2, H2 and CH4 in a volume ratio of 50:45:5. The carburizing temperature is 535℃ and the holding time is 420min. Then the carburizing gas is stopped and the snap ring is cooled and taken out of the furnace under nitrogen protection to obtain a low-temperature carburized snap ring.

[0025] The thickness and hardness of the obtained low-temperature carburized retaining ring were tested. The results showed that it contained a hardened layer with an average thickness of 29.5 μm and a surface hardness of HV980, and the hardened layer had a uniform and dense structure.

[0026] Furthermore, the obtained low-temperature carburized retaining ring was subjected to a salt spray test (neutral salt spray). The test showed that after 190 hours of testing, the obtained low-temperature carburized retaining ring still showed no rusting, with only a small number of yellow spots in some areas.

[0027] Example 3 The low-temperature carburized retaining ring is prepared by the following process: (1) The 304 stainless steel retaining ring is ultrasonically cleaned in an alkaline cleaning solution to remove surface oil stains, and then sandblasted to reduce its surface roughness R. a After reaching 3.0μm, it is then subjected to ultrasonic cleaning with deionized water and hot air drying to obtain the pretreated retaining ring; (2) Using an electrostatic spraying device, a PTFE dispersion with a solid content of 10wt% is uniformly sprayed onto the surface of the pretreated snap ring to form a coating film with a thickness of 4.5μm. Then, it is dried in an oven at 80℃ for 15min to obtain a fluorinated snap ring. (3) The PTFE-plated snap ring is placed in a tube furnace and subjected to gradient heating under nitrogen protection. The process includes: heating to 480°C at a heating rate of 8°C / min and holding for 40 min to achieve preliminary pyrolysis of PTFE and mild fluorination of the surface passivation film Cr2O3; then heating to 540°C at a heating rate of 8°C / min and holding for 35 min to achieve rapid conversion and cleaning of the passivation film; and obtaining an activated PTFE-plated snap ring. (4) The activated fluorine-plated snap ring is quickly transferred to the carburizing furnace and carburized in the carburizing gas. The carburizing gas includes N2, H2 and CH4 in a volume ratio of 75:24:1. The carburizing temperature is 565℃ and the holding time is 360min. After that, the carburizing gas is stopped and the snap ring is cooled and taken out of the furnace under nitrogen protection to obtain a low-temperature carburized snap ring.

[0028] The thickness and hardness of the obtained low-temperature carburized retaining ring were tested. The results showed that it contained a hardened layer with an average thickness of 35.0 μm and a surface hardness of HV850, and the hardened layer had a uniform and dense structure.

[0029] Furthermore, the obtained low-temperature carburized retaining ring was subjected to a salt spray test (neutral salt spray). The test showed that after 130 hours of testing, the obtained low-temperature carburized retaining ring still showed no rusting, with only a yellow spot in a local area.

[0030] Example 4 Low-temperature carburized retaining rings are prepared in batches using the following process: (1) The 304 stainless steel retaining ring is ultrasonically cleaned in an alkaline cleaning solution to remove the oil stains on its surface. Then it is sandblasted to make its surface roughness Ra reach 3.0-3.5μm. Then it is ultrasonically cleaned with deionized water and dried with hot air to obtain the pretreated retaining ring. (2) Using an electrostatic spraying device, a PTFE dispersion with a solid content of 10wt% is uniformly sprayed onto the surface of the pretreated snap ring to form a coating film with a thickness of 3.0-3.5μm. Then, it is dried in an oven at 80℃ for 10-15min to obtain a fluorinated snap ring. (3) Place the PTFE-plated snap ring in a tube furnace and perform gradient heating treatment under nitrogen protection. The treatment process includes: heating to 420-430℃ at a heating rate of 5-8℃ / min and holding for 30-50min to achieve preliminary pyrolysis of PTFE and mild fluorination of the surface passivation film Cr2O3; then heating to 525℃ at a heating rate of 5-8℃ / min and holding for 30-40min to achieve rapid conversion and cleaning of the passivation film; and obtaining an activated PTFE-plated snap ring. (4) The activated fluorine-plated snap ring is quickly transferred to a carburizing furnace and carburized in a carburizing gas. The carburizing gas includes N2, H2 and CH4 in a volume ratio of 50:45:5. The carburizing temperature is 500-510℃ and the holding time is 360-390min. After that, the carburizing gas is stopped and the snap ring is cooled and taken out of the furnace under nitrogen protection to obtain a low-temperature carburized snap ring.

[0031] The obtained low-temperature carburized retaining rings were sampled and tested. Three samples were selected for microstructure observation, and the results are attached. Figure 1 As shown, the hardened layer structure of the sampled specimens is dense and uniform. Salt spray testing (neutral salt spray) revealed that after 150 hours, the resulting low-temperature carburized retaining rings showed no rusting, with only a few localized yellow spots. Further sampling and testing of the performance parameters of the obtained low-temperature carburized specimens were conducted, and the results are attached. Figure 2 As shown, through the attached Figure 2 As can be seen, the hardened layer (i.e., the carburized layer shown in the figure) of the tested sample has a thickness of 28.35-29.43 μm, a surface hardness of HV876.8, and a core hardness of HV157.7.

[0032] The low-temperature carburized retaining ring obtained in Example 4 was assembled into a compression fitting and then assembled with a steel pipe. Microscopic examination of the cross-section of the assembled structure was performed. Examples of microscopic images obtained under different tightening degrees (Figure (a) 1 / 2 turn, Figure (b) 3 / 4 turn, Figure (c) 1 turn, Figure (d) 1 1 / 4 turns) are attached. Figure 3 As shown, by attaching Figure 3 The tests showed that when the tightening degree was 1 / 2 turn, 3 / 4 turn, 1 turn, and 1 1 / 4 turns, the cutting depth of the cutting edge of the low-temperature carburized retaining ring was 0.635 mm, 0.953 mm, 1.27 mm, and 1.59 mm, respectively. The cutting edge shape was intact and without wear, and it could cut completely into the steel pipe to form a good seal, and produce obvious indentations, cracks or metal chips on the steel pipe.

[0033] It should be noted that the above descriptions are merely preferred embodiments of the present invention and should not limit the scope of protection of the technical solutions of the present invention. Any modifications made to the technical solutions described in the foregoing embodiments, or equivalent substitutions of technical features, by those skilled in the art within the spirit and principles of the present invention, should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a low-temperature carburized retaining ring, characterized in that, It includes the following steps: (1) The austenitic stainless steel retaining ring is pretreated to obtain a pretreated retaining ring; the pretreatment includes cleaning and surface roughening treatment to reduce its surface roughness R. a Reaching 1.6-3.5μm; (2) By electrostatic spraying, a PTFE aqueous dispersion with a PTFE solid content of 10-15wt% is uniformly sprayed onto the surface of the pretreated snap ring to form a coating film with a thickness of 3.0-4.5μm, which is then dried to obtain a PTFE-plated snap ring. (3) Under a protective atmosphere, the fluorine-plated clasp is subjected to a gradient heating heat treatment at 420-550℃ to obtain an activated fluorine-plated clasp. (4) The activated plated retaining ring is carburized at 500-565°C in a carburizing gas atmosphere, and then cooled in a protective atmosphere to obtain a low-temperature carburized retaining ring.

2. The preparation method according to claim 1, characterized in that, The cleaning process includes alkaline ultrasonic cleaning before the surface roughening treatment and water ultrasonic cleaning after the surface roughening treatment.

3. The preparation method according to claim 1, characterized in that, The drying conditions described in step (2) are: drying at 80-90℃ for 10-15 minutes.

4. The preparation method according to claim 1, characterized in that, The gradient heating heat treatment includes: heating to 420-480℃ at a heating rate of 3-8℃ / min, holding at that temperature for 30-60min, and then heating to 525-550℃ at a heating rate of 3-8℃ / min, holding at that temperature for 20-40min.

5. The preparation method according to claim 1, characterized in that, The carburizing gas atmosphere contains nitrogen, hydrogen, and methane in a volume ratio of (50-75):(24-45):(1-5).

6. The preparation method according to claim 1, characterized in that, The holding time for the carburizing treatment is 300-420 minutes.

7. The preparation method according to claim 1, characterized in that, In step (1), the surface roughness Ra is 1.6-2.0 μm; in step (2), the thickness of the coating film is 3.5-4.0 μm; In step (3), the gradient heating heat treatment includes: heating to 420-450℃ at 3-5℃ / min and holding for 60min, and then heating to 535-550℃ at 3-5℃ / min and holding for 20-30min; in step (4), the carburizing temperature is 535-550℃ and the holding time is 300-420min.

8. The preparation method according to claim 1, characterized in that, In step (1), the surface roughness Ra is 2.5-3.0 μm; in step (2), the thickness of the coating film is 4.0-4.5 μm. In step (3), the gradient heating heat treatment includes: heating to 450-480℃ at 5-8℃ / min and holding for 30-50min, and then heating to 535-550℃ at 5-8℃ / min and holding for 20-30min; in step (4), the carburizing temperature is 550-565℃ and the holding time is 300-360min.

9. The low-temperature carburized retaining ring prepared by the preparation method according to any one of claims 1-8.

10. A compression fitting containing the low-temperature carburized retaining ring as described in claim 9.

Images