A heat treatment strengthening process and apparatus for carbon steel rivet nuts

Abstract

This invention discloses a heat treatment strengthening process and apparatus for carbon steel rivet nuts, relating to the field of ferrous metal heat treatment. The strengthening process includes the following steps: S1, quenching the cold-headed carbon steel rivet nut in a heating furnace; S2, tempering the quenched carbon steel rivet nut at 396~410℃ and cooling; S3, tempering the tempered carbon steel rivet nut at 272~295℃ and cooling to obtain a heat-treated and strengthened carbon steel rivet nut. This invention, through precise control of the phase transformation behavior of medium carbon steel combined with two-stage tempering, enables the deformed portion of the rivet nut to obtain a tempered troostite structure with uniform carbide strengthening, resulting in a significant simultaneous increase in tensile strength and yield strength. Compared with traditional heat treatment processes, this invention can improve the ability of the deformed portion of thin-walled parts to resist instantaneous overload during high-speed riveting, thereby significantly reducing the risk of installation failure during riveting.

Description

Technical Field

[0001] This invention relates to the field of ferrous metal heat treatment technology, and in particular to a heat treatment strengthening process and apparatus for carbon steel rivet nuts. Background Technology

[0002] Rivet nuts, as efficient fasteners, are widely used in the assembly of thin-plate structures in aerospace, automotive, and other fields. Their installation relies on automated high-speed riveting equipment, which applies axial tension to a thin-walled mandrel at the nut's tail using external tools, forcing the mandrel to expand and deform, forming a permanent locking structure. With increasing demands for lightweighting, thin-walled, M3-M6 small-size carbon steel rivet nuts with a wall thickness ≤1.0 mm are increasingly widely used. These products need to withstand instantaneous high-speed riveting forces within confined installation spaces, placing stringent requirements on the mechanical properties of the deformed portion of the rivet nut. As the core load-bearing component of the rivet nut, the deformed portion must achieve locking through tail expansion deformation during the riveting process, and the tensile stress it withstands can reach 1.5-2 times the material's yield strength.

[0003] Currently, the industry generally adopts a conventional heat treatment process of quenching followed by tempering, that is, heating to 800-900 ℃ for quenching, and then tempering at 200-300 ℃ to relieve stress. However, for thin-walled small-sized rivet nuts, although the mandrel obtains a martensitic structure after quenching, it is difficult to accurately control the carbide precipitation morphology during tempering, and the stress relief is insufficient. This results in limited improvement in the tensile strength and yield strength of the mandrel. Under the extreme load of high-speed riveting or under slight material fluctuations, the deformed part of the rivet nut is prone to instantaneous tensile failure, with an installation failure rate often exceeding 4.3%, which seriously restricts production efficiency and connection reliability.

[0004] Therefore, it is necessary to improve upon the shortcomings of existing technologies in order to solve the above problems. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides a heat treatment strengthening process and apparatus for carbon steel rivet nuts.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: Firstly, the present invention provides a heat treatment strengthening process for carbon steel rivet nuts, applicable to carbon steel rivet nuts with a carbon content of 0.4~0.6 wt% and a manganese content of 0.6~0.9 wt%, comprising the following steps:

[0007] S1. Quenching treatment: The cold-forged carbon steel rivet nuts are quenched in a heating furnace.

[0008] S2, First tempering: Temper the quenched carbon steel rivet nuts at 396~410 ℃ and cool them to room temperature;

[0009] S3. Secondary tempering: The carbon steel rivet nut after primary tempering is tempered at 272~295 ℃ and cooled to 60 ℃ to obtain the heat-treated and strengthened carbon steel rivet nut.

[0010] In a preferred embodiment of the present invention, in step S1, the quenching temperature is 840~860 ℃ and the holding time is 5~10 min.

[0011] In a preferred embodiment of the present invention, in step S1, the quenching medium is rapid quenching oil, and the oil temperature is 50~70 ℃.

[0012] In a preferred embodiment of the present invention, in step S2, the holding time for the first tempering is 13-25 minutes.

[0013] In a preferred embodiment of the present invention, in step S3, the holding time for the secondary tempering is 8-15 minutes.

[0014] In a preferred embodiment of the present invention, the wall thickness of the deformed part of the carbon steel rivet nut is ≤1.0 mm, and the thread specification is M3~M6.

[0015] Secondly, the present invention provides an apparatus for heat treatment strengthening process of carbon steel rivet nuts, comprising: a base, a plurality of threaded mandrels fixed on the top of the base, and a plurality of alloy sleeves disposed on the top of the base.

[0016] The bottom of the alloy sleeve is open, and the top is provided with a deformation hole with a taper of 1:45~52;

[0017] The threaded mandrel, from top to bottom, includes: an upper section for clearance fitting with the deformation hole, a middle section for connecting the threaded portion of the carbon steel rivet nut, and a lower section for positioning the carbon steel rivet nut.

[0018] The deformation hole is fitted onto the side of the upper shaft and has the same cross-sectional shape; the room temperature gap between the upper shaft and the inner wall of the deformation hole is 0.05-0.08 mm.

[0019] In a preferred embodiment of the present invention, the alloy sleeve is made of a material with a thermal expansion coefficient of 12.7~13.4×10⁻⁶. -6 The material of the threaded mandrel is a nickel-based alloy at / ℃; the material of the threaded mandrel is martensitic stainless steel.

[0020] In a preferred embodiment of the present invention, the side of the middle section shaft is threadedly matched with the threaded portion of the carbon steel rivet nut; the diameter of the lower section shaft is larger than the diameter of the middle section shaft.

[0021] In a preferred embodiment of the present invention, a plurality of positioning pins are fixed on the top of the base, and a plurality of positioning holes are provided on the bottom of the alloy sleeve, wherein the positioning pins are inserted into the positioning holes.

[0022] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0023] (1) This invention provides a heat treatment strengthening process for carbon steel rivet nuts. By precisely controlling the phase transformation behavior of medium carbon steel, during the main tempering stage, manganese element inhibits the coarsening of Fe3C carbides and promotes the uniform precipitation of ε-carbides at the nanoscale. Its crystal plane forms a coherent strain field with the crystal plane of the α-Fe matrix, generating a dislocation pinning effect. During the secondary tempering, residual carbon atoms are driven to diffuse towards the dislocation lines to form Kotler atmospheres, neutralizing the lattice distortion energy. This allows the deformed part of the rivet nut to obtain a tempered troostite structure with uniform carbide strengthening. The tensile strength and yield strength are significantly improved simultaneously. Compared with traditional heat treatment processes, this can improve the ability of the deformed part of thin-walled parts to resist instantaneous overload during high-speed riveting, thereby significantly reducing the installation failure risk during the riveting process.

[0024] (2) The present invention provides a heat treatment strengthening device for carbon steel rivet nuts. By assembling the rivet nut on the threaded mandrel during the main tempering stage and combining it with an alloy sleeve, the difference in thermal expansion coefficient between the alloy sleeve and carbon steel is utilized. Under the cooperation of the tapered deformation hole and the threaded mandrel, the expansion difference is converted into the radial gradient pressure at the root of the rivet nut thread. This reduces the diffusion barrier of carbon atoms during the austenite-martensite phase transformation, promotes the uniform nucleation of carbides in the matrix, and makes the newly precipitated carbides in the material finer and more uniformly distributed at high temperature. This achieves uniform carbide distribution, eliminates the coarse salt-like aggregation of carbides, and improves the shear strength at the root of the thread, thereby eliminating the risk of crushing. Compared with the traditional process, it is more suitable for the repeated load impact of high-speed riveting.

[0025] (3) In the secondary tempering stage of the present invention, the thermal expansion difference of the strengthening device is further amplified to form a higher radial pressure. The softened matrix undergoes plastic flow under high pressure, which forces the hard carbide particles to cut into the ferrite lattice along the slip surface, so that the carbide particles form a hook-like anchor point. At the same time, the high-pressure cooling locks the residual compressive stress layer at the root of the thread, which can increase the effective meshing area and interface biting force of the rivet nut thread, thereby effectively solving the problem of easy disengagement or crushing at the root of the thread, enhancing the meshing stability of the thread and the mating parts, and extending the service life of repeated turning. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the carbon steel rivet nut structure of a preferred embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of the overall exploded structure of the strengthening device according to a preferred embodiment of the present invention;

[0029] Figure 3 This is a front view schematic diagram of the threaded mandrel structure according to a preferred embodiment of the present invention;

[0030] Figure 4 This is a schematic diagram of a half-section of the alloy sleeve according to a preferred embodiment of the present invention;

[0031] In the diagram: 1. Carbon steel rivet nut; 11. Rod body; 12. Deformation section; 13. Upper inner hole; 14. Lower inner hole; 15. Threaded section; 16. Flange; 2. Base; 3. Threaded mandrel; 31. Upper shaft section; 32. Middle shaft section; 33. Lower shaft section; 4. Alloy sleeve; 41. Deformation hole; 5. Locating pin; 51. Locating hole. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0034] Application Overview:

[0035] The heat treatment strengthening of carbon steel rivet nuts requires a balance between strength and toughness. The failure root causes of traditional processes are closely related to the material phase transformation law and micro-stress state. For medium carbon steel with carbon content of 0.40-0.60 wt% and manganese content of 0.60-0.90 wt%, the austenitizing heating at 800-900 ℃ in traditional processes often leads to insufficient dissolution of carbides or coarse grains, while the single tempering at 200-300 ℃ cannot simultaneously achieve ε-carbide precipitation and residual stress elimination.

[0036] The applicant discovered that traditional processes have fundamental defects in strengthening the thread root of this type of medium-carbon steel: the thread root cross section is large during tempering, and the precipitation kinetics of ε-carbide are uneven, resulting in abnormal growth of carbides at grain boundaries, forming a diffuse and uneven structure similar to "unkneaded coarse salt grains in dough". That is, the carbide distribution is uneven, which cannot effectively pin dislocations, greatly reduces the continuity of the matrix, and makes it difficult to effectively resist deformation. At the same time, there is residual tensile stress at the thread root, resulting in insufficient strength and easy derailment or crushing.

[0037] To address the aforementioned problems, this invention proposes a synergistic solution combining a radial gradient pressure thread strengthening fixture with a two-stage tempering process. This solution precisely controls the precipitation morphology and distribution of carbides, effectively controls the distribution state of carbides, and reduces residual stress at the thread root. It not only significantly improves the tensile strength and yield strength of the deformed part and reduces the breakage rate during high-speed riveting, but also effectively enhances the shear strength of the threaded part and the repeated tightening life of the thread, completely solving the failure problem of thin-walled small-size rivet nuts under high stress conditions.

[0038] It should be noted that the strengthening process of the present invention is preferably applicable to carbon steel rivet nuts with a carbon content of 0.4~0.6 wt% and a manganese content of 0.6~0.9 wt%.

[0039] A heat treatment strengthening process for carbon steel rivet nuts includes the following steps:

[0040] S1. Quenching treatment: The cold-forged carbon steel rivet nut 1 is quenched in a heating furnace;

[0041] It should be noted that the heating furnace is preferably a controlled atmosphere continuous mesh belt furnace, wherein the controlled atmosphere is preferably hydrogen, which is used to reduce the surface oxygen potential and prevent the carbon-depleted layer from reducing hardenability.

[0042] S2. Tempering: Temper the quenched carbon steel rivet nut 1 at 396~410 ℃ and cool it to room temperature;

[0043] It should be noted that tempering is performed once within 1 hour after quenching to eliminate quenching stress and obtain tempered troostite structure, which improves toughness while maintaining high strength. Specifically, martensite precipitates ε-carbides (Fe2.4C) with an average size of <20nm and a spacing of 50-100nm, pinning dislocations, while the residual austenite decomposes, i.e., γ→α+θ (cementite), eliminating hydrogen trap sources.

[0044] S3. Secondary tempering: The carbon steel rivet nut 1 after the first tempering is tempered at 272~295 ℃ and cooled to 60 ℃ to obtain the heat-treated and strengthened carbon steel rivet nut 1.

[0045] It should be noted that secondary tempering activates screw dislocation cross-slip, reconstructs the dislocation cell structure, and allows C atoms to diffuse to the dislocation lines to form Cottrell atmospheres, thereby neutralizing tensile stress and eliminating residual stress.

[0046] In some specific implementations, in step S1, the quenching temperature is 840~860 ℃ and the holding time is 5~10 min.

[0047] It should be noted that the quenching temperature of 840~860 ℃ completely dissolves the cementite (Fe3C), and the C atoms are dissolved in the γ-Fe lattice. The Mn element inhibits the precipitation of Fe3C at the grain boundaries.

[0048] In some specific implementations, in step S1, the quenching medium is rapid quenching oil with an oil temperature of 50~70 ℃.

[0049] In some specific implementations, in step S2, the holding time for one tempering is 13 to 25 minutes.

[0050] In some specific implementations, the holding time for the second tempering in step S3 is 8 to 15 minutes.

[0051] In some specific embodiments, the wall thickness of the deformed part 12 of the carbon steel rivet nut 1 is ≤1.0 mm, and the thread part 15 is of specification M3~M6.

[0052] like Figure 1-4As shown, the present invention provides an apparatus for heat treatment strengthening of carbon steel rivet nuts, comprising: a base 2, a plurality of threaded mandrels 3 fixed to the top of the base 2, and a plurality of alloy sleeves 4 disposed on the top of the base 2; the bottom of the alloy sleeves 4 is open, and the top is provided with a deformation hole 41 with a taper of 1:45~52; the threaded mandrels 3, from top to bottom, include: an upper section shaft 31 for clearance fit with the deformation hole 41, a middle section shaft 32 for connecting the threaded portion 15 of the carbon steel rivet nut 1, and a lower section shaft 33 for positioning the carbon steel rivet nut 1; the deformation hole 41 is sleeved on the side of the upper section shaft 31 and has the same cross-sectional shape; the room temperature gap between the upper section shaft 31 and the inner wall of the deformation hole 41 is 0.05-0.08 mm.

[0053] It should be noted that the taper of the deformation hole 41 is preferably 1:50; the room temperature gap is preferably 0.06 mm; the base 2, threaded mandrel 3, alloy sleeve 4 and carbon steel rivet nut 1 are all coaxially arranged; the side of the middle section shaft 32 is threadedly matched with the threaded part 15 of the carbon steel rivet nut 1; the diameter of the lower section shaft 33 is larger than the diameter of the middle section shaft 32, that is, the diameter difference forms a shoulder structure, which contacts the lower end face of the rod body 11 of the carbon steel rivet nut 1 to achieve axial positioning.

[0054] The alloy sleeve 4 is made of a material with a thermal expansion coefficient of 12.7~13.4×10⁻⁶. -6 A nickel-based alloy with a temperature of / ℃, preferably Inconel 718 nickel-based superalloy, in which the γ'' phase (Ni3Nb) and γ' phase (Ni3(Al,Ti)) remain stable at 650℃, imparting a tensile strength as high as 1200 MPa (tested at 700℃), and a coefficient of thermal expansion at room temperature of 13.0 × 10⁻⁶. -6 / ℃, and the coefficient of thermal expansion is 14.5×10 when the temperature rises to 400 ℃. -6 / ℃, significantly higher than the expansion difference required for carbon steel forming process;

[0055] The threaded mandrel 3 is made of martensitic stainless steel, preferably SUS440C high-carbon martensitic stainless steel. Its chemical composition includes 0.95-1.20% carbon, 16.00-18.00% chromium, no more than 1.00% manganese and silicon, and no more than 0.04% phosphorus and 0.03% sulfur, respectively. It also contains no more than 0.75% molybdenum. It possesses an ultra-high hardness of 58±1 HRC and a 99.8% elastic recovery rate, with a yield strength of 1200 MPa. It can maintain a high-temperature hardness of 55 HRC at approximately 280℃. Its coefficient of thermal expansion is 10.2×10⁻⁶. -6 / ℃ is close to that of carbon steel, effectively reducing the difference in thermal deformation.

[0056] Specifically, the base 2 provides stable support. The threaded mandrel 3 achieves circumferential positioning through a threaded connection between the middle section shaft 32 and the threaded portion 15 of the carbon steel rivet nut 1. The lower section shaft 33 achieves axial positioning of the carbon steel rivet nut 1 through a shoulder formed with the middle section shaft 32. The alloy sleeve 4 is fitted onto the upper section of the threaded mandrel 3 through a bottom opening. The upper section shaft 31 of the threaded mandrel 3 and the deformation hole 41 of the alloy sleeve 4 form a 0.05-0.08 mm diameter joint. With a mm-sized conical clearance fit, during the tempering stage of heat treatment, the deformation hole 41 at the top of the alloy sleeve 4 with a taper of 1:45~52 is compressed by the radial contraction (Δα effect) of the alloy sleeve 4 when heated. The difference in thermal expansion between the alloy sleeve 4 and the threaded mandrel 3 generates radial gradient pressure. The upper shaft 31 undergoes elastic deformation under compression. The middle shaft 32 transmits the expansion force to the root of the thread of the carbon steel rivet nut 1 through the thread, so that the rivet nut is subjected to uniform inward compression during the tempering process, providing mechanical conditions for carbide refinement and stress control.

[0057] In some specific embodiments, the top of the base 2 is fixed with several positioning pins 5, and the bottom of the alloy sleeve 4 is provided with several positioning holes 51, and the positioning pins 5 are inserted into the positioning holes 51.

[0058] It should be noted that the positioning pins 5 and positioning holes 51 are evenly distributed circumferentially relative to the bottom of the alloy sleeve 4, and the number of positioning pins 5 and positioning holes 51 is the same for each alloy sleeve 4, and they are coaxially arranged.

[0059] Specifically, the positioning pin 5 at the top of the base 2 is inserted into the positioning hole 51 at the bottom of the alloy sleeve 4 to restrict the circumferential rotation and radial offset of the alloy sleeve 4 during the heating process, ensuring that the sleeve and the threaded mandrel 3 have high coaxiality, avoiding local overpressure or underpressure of the carbon steel rivet nut 1 due to eccentricity, and improving the dimensional consistency of the product after heat treatment.

[0060] When the strengthening device of this invention is used in conjunction with the strengthening process, the carbon steel rivet nut 1 is first quenched. Then, the threaded part 15 of the carbon steel rivet nut 1 is screwed into the middle section shaft 32 of the threaded mandrel 3 until the lower end face of the rod body 11 of the carbon steel rivet nut 1 is in contact with the shoulder of the lower section shaft 33, so that the upper inner hole 13 and the lower inner hole 14 of the carbon steel rivet nut 1 are located on the side of the middle section shaft 32, thus completing the axial positioning. Then, the positioning hole 51 at the bottom of the alloy sleeve 4 is aligned with the positioning pin 5 at the top of the base 2, so that the positioning hole 51 and the positioning pin 5 are inserted and engaged, ensuring that the bottom opening of the alloy sleeve 4 covers the flange 16 and the rod body 11 of the carbon steel rivet nut 1. At this time, the upper section shaft 31 and the inner wall of the deformation hole 41 maintain a room temperature gap of 0.05-0.08 mm. After assembly, the strengthening device, together with the carbon steel rivet nut 1, enters the heat treatment process of primary tempering and secondary tempering, and in conjunction with the strengthening process, utilizes the difference in thermal expansion of materials to achieve automatic pressurization without power.

[0061] To further simplify and make the present invention achieve its objectives and effects, the present invention will be further illustrated in conjunction with the following specific embodiments and comparative examples, but the present invention is not limited to the scope of the embodiments described herein.

[0062] It should be noted that the features not specified in the embodiments and comparative examples regarding materials and specifications are as follows: the deformed part 12 of the carbon steel rivet nut 1 has a wall thickness of 1.0 mm, the threaded part 15 is M5, the wire is AISI 1045 steel with a carbon content of 0.43~0.50 wt% and a manganese content of 0.6~0.9 wt%, purchased from Wuxi Xinfuchang Special Steel; the rapid quenching oil is K25 with a kinematic viscosity of 18~28 mm. 2 The threaded mandrel 3 is made of SUS440C high-carbon martensitic stainless steel and was purchased from Jiangsu Changhu Industry; the alloy sleeve 4 is made of Inconel 718 nickel-based high-temperature alloy with a coefficient of thermal expansion of 13.0 × 10⁻⁶ at room temperature. -6 / ℃, purchased from Jiangsu Guoyan Special Steel; the taper of deformation hole 41 and upper shaft 31 is 1:50.

[0063] Example 1

[0064] A heat treatment strengthening process for carbon steel rivet nuts includes the following steps:

[0065] S1. Quenching treatment: The cold-forged carbon steel rivet nut 1 is quenched in a heating furnace at 850 ℃ for 8 min. The quenching medium is rapid quenching oil and the oil temperature is 60 ℃.

[0066] S2, First tempering: Temper the quenched carbon steel rivet nut 1 at 400 ℃, hold for 20 min, and cool to room temperature;

[0067] S3. Secondary tempering: The carbon steel rivet nut 1 after the first tempering is tempered at 280 ℃, held at that temperature for 10 min, and then cooled to 60 ℃ to obtain the heat-treated and strengthened carbon steel rivet nut 1.

[0068] Example 2

[0069] This embodiment is basically the same as embodiment 1, except that the temperature of the first tempering is different. The specific steps of S2 are as follows: First tempering: The quenched carbon steel rivet nut 1 is tempered at 396 ℃, held for 20 min, and then cooled to room temperature.

[0070] Example 3

[0071] This embodiment is basically the same as embodiment 1, except that the temperature of the first tempering is different. The specific steps of S2 are as follows: First tempering: The quenched carbon steel rivet nut 1 is tempered at 410 ℃, held for 20 min, and then cooled to room temperature.

[0072] Example 4

[0073] This embodiment is basically the same as embodiment 1, except that the temperature of the second tempering is different. The specific steps of S3 are as follows: Second tempering: The carbon steel rivet nut 1 after the first tempering is tempered at 272 ℃, held for 10 min, and cooled to 60 ℃ to obtain the heat-treated and strengthened carbon steel rivet nut 1.

[0074] Example 5

[0075] This embodiment is basically the same as embodiment 1, except that the temperature of the second tempering is different. The specific steps of S3 are as follows: Second tempering: The carbon steel rivet nut 1 after the first tempering is tempered at 295 ℃, held for 10 min, and cooled to 60 ℃ to obtain the heat-treated and strengthened carbon steel rivet nut 1.

[0076] Example 6

[0077] This embodiment is basically the same as Embodiment 1, except that it uses a single quenching and tempering process. Specifically, there is no S3 step, that is, no secondary tempering is performed.

[0078] Example 7

[0079] This embodiment is basically the same as Embodiment 1, except that: in the strengthening process, a strengthening device is used to perform subsequent heat treatment strengthening of primary and secondary tempering; specifically, after the carbon steel rivet nut 1 is quenched, the threaded part 15 of the carbon steel rivet nut 1 is screwed into the middle section shaft 32 of the threaded mandrel 3 until the lower end face of the rod body 11 of the carbon steel rivet nut 1 is in contact with the shoulder of the lower section shaft 33, so that the upper inner hole 13 and the lower inner hole 14 of the carbon steel rivet nut 1 are located on the side of the middle section shaft 32, thus completing the axial positioning; then the positioning hole 51 at the bottom of the alloy sleeve 4 is aligned with the positioning pin 5 at the top of the base 2, so that the positioning hole 51 and the positioning pin 5 are inserted and engaged, ensuring that the bottom opening of the alloy sleeve 4 covers the flange 16 and rod body 11 of the carbon steel rivet nut 1, at which time the upper section shaft 31 and the inner wall of the deformation hole 41 maintain a room temperature gap of 0.06 mm; after assembly, the strengthening device enters the heat treatment process of primary and secondary tempering together with the carbon steel rivet nut 1.

[0080] Example 8

[0081] This embodiment is basically the same as embodiment 7, except that the room temperature gap between the upper shaft 31 and the inner wall of the deformation hole 41 is 0.05 mm.

[0082] Example 9

[0083] This embodiment is basically the same as embodiment 7, except that the room temperature gap between the upper shaft 31 and the inner wall of the deformation hole 41 is 0.08 mm.

[0084] Performance testing: The heat-treated and strengthened carbon steel rivet nuts 1 obtained in Examples 1-9 were subjected to performance tests in sequence, including the breakage rate of the deformed part 12, the strength of the threaded part 15, and the number of times the threaded part 15 was disengaged during the high-speed riveting process. The results are shown in Table 1.

[0085] Tensile strength: The heat-treated carbon steel rivet nut 1 is installed on a 1.5 mm thick 6061 aluminum alloy plate. An axial tensile force is applied at a speed of 30 mm / s using a rivet gun until the deformed part 12 breaks or the effective riveting is completed. 30 samples are tested in each group. The number of broken samples is recorded and the tensile strength is calculated. The test is repeated 3 times. Tensile strength = (number of broken samples / total number of samples) × 100%.

[0086] Strength of threaded section 15: Using a single shear test device, the heat-treated carbon steel rivet nut 1 is screwed into the grooved test block (thread engagement length 1.5 times the pitch). A transverse shear force is applied by a universal testing machine at a loading speed of 5 mm / min. Ten samples are tested in each group, and the maximum force value when the thread undergoes plastic deformation or shear failure is recorded.

[0087] Thread tripping count 15: Using a digital torque wrench, screw the M5 bolt into the rivet nut at a speed of 10 N·m / min. Record the maximum static friction torque when screwing in for the first time and the residual torque when screwing out. Then repeat the screwing operation with the same parameters until the thread strips (torque decrease ≥20%) or the preload cannot be maintained, which is called tripping. 10 samples are tested in each group, and the number of screwing tripping counts before the best failure is recorded.

[0088] Table 1: Performance test results of carbon steel rivet nuts 1 obtained in Examples 1-9

[0089]

[0090] As shown in Table 1:

[0091] A comparison of Examples 1-5 shows that by adopting a two-stage tempering process, not only are the tensile strength and yield strength of the deformed part 12 of the carbon steel rivet nut 1 significantly improved, enabling it to achieve an extremely low breakage rate of 1.67% during high-speed riveting, but the shear strength of the threaded part 15 is also effectively enhanced, and the threaded part 15 has 15 times of disengagement, thus possessing a good thread re-tightening life.

[0092] Specifically, the precise control of the two-stage tempering temperature and the synergy with wires containing specific carbon and manganese contents play a decisive role in the carbide precipitation kinetics and interface strengthening effect. In Example 1, the primary tempering at 400 °C causes uniform precipitation of ε-carbides in the martensitic matrix, which forms a coherent strain field with the α-Fe matrix. This achieves a uniform spacing distribution through the Ostwald ripening mechanism, effectively pinning dislocation movement. The secondary tempering at 280 °C further activates screw dislocation cross-slip, promoting the diffusion of C atoms to dislocation lines to form Cottrell atmospheres, neutralizing lattice distortion energy. When the tempering temperature deviates from the optimal window, the carbide precipitation kinetics become unbalanced: low temperatures lead to insufficient ε-carbide precipitation density, weakening the pinning effect (thread strength decreases to 572-548 MPa); high temperatures induce cementite (Fe3C) coarsening, forming stress concentration sources. Manganese maintains matrix continuity by inhibiting Fe3C coarsening and neutralizing sulfur embrittlement, but temperature deviations still weaken its effect.

[0093] A comparison between Example 1 and Example 6 reveals that: Example 6 employs a traditional single quenching and tempering process, where the retained austenite (γ phase) is not completely decomposed and transforms into brittle martensite during service. The Cottrell atmosphere is not formed, and tensile stress is concentrated around the dislocation line. ε-carbides coarsen and accumulate due to the lack of a secondary tempering high-pressure environment, resulting in insufficient dislocation slip resistance in the deformation region 12. Residual tensile stress remains at the thread root, the tripping life is only 8 times, and the breakage rate is as high as 4.33%. This confirms the inherent limitations of the traditional single quenching and tempering process in adapting to instantaneous overload of thin-walled parts.

[0094] A comparison of Examples 7-9 with Example 1 reveals that in Examples 7-9, the synergistic effect of the radial gradient pressure of the strengthening device significantly improves the uniformity of carbide distribution and the interfacial bonding strength. In Example 7, the expansion difference between the Inconel 718 alloy sleeve 4 and the SUS440C threaded mandrel 3 generates a radial pressure of 50 MPa at 400°C. This expansion difference is converted into a strong force that compresses the threaded mandrel 3 inward, and the radial pressure is transmitted from the middle shaft 32 to the threaded portion 15 of the carbon steel rivet nut 1. This causes newly hardened carbides to precipitate inside the material, which, like kneading dough, makes the carbides inside the material grow finer and more evenly distributed at high temperatures, resulting in uniform carbide distribution and improved shear strength at the root of the thread.

[0095] Secondary tempering at 280 ℃ further amplifies the expansion difference under the taper of the deformation hole 41, softening the matrix (yield strength decreases by about 40%) and causing plastic flow. Under high temperature and pressure, the rigid carbide particles on the surface of the thread root will undergo small plastic deformation, like nails cutting into the relatively soft ferrite lattice along the slip surface, forming barbed anchor points to increase the effective meshing area, thereby improving the thread's ability to resist being pulled out and flattened, enhancing the meshing stability between the threaded part 15 and the mating part, and extending the service life of repeated tightening.

[0096] Compared to Example 1 without the reinforcement device, Example 7 has a reduced tensile strength of 1.35%, an increased thread strength of 634 MPa, and a 113% increase in the number of tripping cycles.

[0097] To further illustrate the present invention, the preferred embodiment 7 is used as the basis for comparison.

[0098] Comparative Example 1

[0099] This comparative example is basically the same as Example 7, except that: the secondary tempering is performed without the strengthening device, that is, the strengthening device is used to perform only one tempering.

[0100] Comparative Example 2

[0101] This comparative example is basically the same as Example 7, except that the wire used for the carbon steel rivet nut 1 is AISI 1035 steel with a carbon content of 0.32~0.38 wt% and a manganese content of 0.6~0.9 wt%, purchased from Changyize Mold in Kunshan.

[0102] Comparative Example 3

[0103] This comparative example is basically the same as Example 7, except that the wire used for the carbon steel rivet nut 1 is AISI 1065, with a carbon content of 0.60~0.70 wt% and a manganese content of 0.6~0.9 wt%, and was purchased from Dongguan Yuewan Metal Materials Co., Ltd.

[0104] Comparative Example 4

[0105] This comparative example is basically the same as Example 7, except that the deformation hole 41 and the upper shaft 31 have no taper, that is, the deformation hole 41 and the upper shaft 31 are both cylindrical.

[0106] Comparative Example 5

[0107] This comparative example is basically the same as Example 7, except that the room temperature gap between the upper shaft 31 and the inner wall of the deformation hole 41 is 0.04 mm.

[0108] Comparative Example 6

[0109] This comparative example is basically the same as Example 7, except that the room temperature gap between the upper shaft 31 and the inner wall of the deformation hole 41 is 0.1 mm.

[0110] Performance testing: The carbon steel rivet nuts 1 obtained in Comparative Examples 1-6 were subjected to the same performance tests as in Examples 1-9, namely, the breakage rate of the deformed part 12, the strength of the threaded part 15, and the number of times the threaded part 15 was disengaged during the high-speed riveting process. The comparison results with Example 7 are shown in Table 2.

[0111] Table 2: Performance test results of carbon steel rivet nuts 1 obtained in Example 7 and Comparative Examples 1-6

[0112]

[0113] As shown in Table 2:

[0114] A comparison between Example 7 and Comparative Example 1 reveals that when no strengthening device is used during the secondary tempering stage, the lack of radial pressure leads to the disappearance of the carbide embedding effect. Comparative Example 1 only applies pressure during the first tempering stage, which can refine the carbides, but during the second tempering, the matrix softens and there is no high pressure to push the carbides into the matrix. The residual compressive stress layer is not formed, and tensile stress may still exist at the root of the thread, causing the carbides to maintain a planar contact state. The shear strength drops to 589 MPa, and the tripping life is sharply reduced to 16 times.

[0115] A comparison of Example 7 with Comparative Examples 2-3 reveals that: In Comparative Example 2, due to insufficient carbon atoms, the density of ε-carbide precipitation is reduced, the dislocation pinning effect is weakened, and at the same time, the Mn element cannot fully suppress Fe3C coarsening, resulting in island-like distribution of carbides at the root of the thread and a tensile strength of only 512 MPa; In Comparative Example 3, the supersaturated carbon leads to an excessively high content of residual austenite, and the undecomposed γ phase transforms into brittle martensite during tempering at 280 ℃, inducing temper brittleness. Simultaneously, cementite precipitates at the grain boundaries, resulting in a tensile strength as high as 3.85%.

[0116] A comparison of Example 7 and Comparative Example 4 reveals that: the deformation hole 41 and the upper section of the threaded mandrel 31 have no taper, which disrupts the pressure gradient transmission. In Comparative Example 4, the sleeve and mandrel are in parallel clearance fit, and the expansion difference during heating basically produces linear displacement, making it difficult to form a gradient pressure field from top to bottom. This results in uneven distribution of carbides at the root of the thread, less tangential force driving the carbide particles, and an inability to cut into the matrix along the sliding surface. The plastic flow of the matrix is ​​insufficient, the anchor point depth is insufficient, the thread strength drops to 577 MPa, and the tripping life is only 16 times.

[0117] A comparison of Example 7 and Comparative Examples 5-6 reveals that: Comparative Example 5, due to its small room temperature gap, experiences premature contact between the sleeve and mandrel at 400°C, resulting in overpressure and excessive elastic deformation of the mandrel, leading to easy collapse of the thread profile. Comparative Example 6, on the other hand, suffers from an excessively large gap, resulting in a delayed and insufficient pressure peak after expansion differential filling, a lower carbide embedding depth than in Example 7, and a reduction in the number of tripping cycles to 20. Neither example achieves the optimal carbide dispersion strengthening and residual stress control effects.

[0118] The above description is based on the preferred embodiments of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description, and all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0119] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A heat treatment strengthening process for carbon steel rivet nuts, characterized in that, Suitable for carbon steel rivet nuts with a carbon content of 0.4~0.6 wt% and a manganese content of 0.6~0.9 wt%, the following steps are included: S1. Quenching treatment: The cold-forged carbon steel rivet nuts are quenched in a heating furnace. S2. First tempering: Screw the threaded part of the quenched carbon steel rivet nut into the middle section of the threaded mandrel. Insert the positioning hole at the bottom of the alloy sleeve into the positioning pin at the top of the base to ensure that the alloy sleeve covers the carbon steel rivet nut and that the deformation hole at the top of the alloy sleeve is fitted onto the side of the upper section of the threaded mandrel. Temper at 396~410 ℃ and cool to room temperature. S3. Secondary tempering: After cooling after primary tempering, the rivet is tempered at 272~295 ℃ and then cooled to 60 ℃ to obtain a heat-treated and strengthened carbon steel rivet nut.

2. The heat treatment strengthening process for carbon steel rivet nuts according to claim 1, characterized in that: In step S1, the quenching temperature is 840~860 ℃ and the holding time is 5~10 min.

3. The heat treatment strengthening process for carbon steel rivet nuts according to claim 1, characterized in that: In step S1, the quenching medium is rapid quenching oil, and the oil temperature is 50~70 ℃.

4. The heat treatment strengthening process for carbon steel rivet nuts according to claim 1, characterized in that: In step S2, the holding time for the first tempering is 13-25 minutes.

5. The heat treatment strengthening process for carbon steel rivet nuts according to claim 1, characterized in that: In step S3, the holding time for the secondary tempering is 8 to 15 minutes.

6. The heat treatment strengthening process for carbon steel rivet nuts according to claim 1, characterized in that: The deformed part of the carbon steel rivet nut has a wall thickness of ≤1.0 mm, and the thread specification is M3~M6.

7. An apparatus for heat treatment strengthening of carbon steel rivet nuts based on any one of claims 1-6, characterized in that, include: A base, several threaded mandrels fixed to the top of the base, and several alloy sleeves disposed on the top of the base; The bottom of the alloy sleeve is open, and the top is provided with a deformation hole with a taper of 1:45~52; The threaded mandrel, from top to bottom, includes: an upper section for clearance fitting with the deformation hole, a middle section for connecting the threaded portion of the carbon steel rivet nut, and a lower section for positioning the carbon steel rivet nut. The deformation hole is fitted onto the side of the upper shaft and has the same cross-sectional shape; the room temperature gap between the upper shaft and the inner wall of the deformation hole is 0.05-0.08 mm.

8. The apparatus according to claim 7, characterized in that: The alloy sleeve is made of a material with a thermal expansion coefficient of 12.7~13.4×10⁻⁶. -6 The material of the threaded mandrel is a nickel-based alloy at / ℃; the material of the threaded mandrel is martensitic stainless steel.

9. The apparatus according to claim 7, characterized in that: The side of the middle section shaft is threaded to match the threaded portion of the carbon steel rivet nut; the diameter of the lower section shaft is larger than the diameter of the middle section shaft.

10. The apparatus according to claim 7, characterized in that: The top of the base is fixed with several positioning pins, and the bottom of the alloy sleeve is provided with several positioning holes, and the positioning pins are inserted into the positioning holes.

Images