Wheel bolt heat treatment method

Abstract

The invention provides a wheel bolt heat treatment method, and relates to the technical field of heat treatment. The invention provides a wheel bolt heat treatment method. The wheel bolt heat treatment method comprises the following steps that a bolt blank is subjected to thermal refining; the quenched and tempered bolt is subjected to medium-frequency quenching treatment, so that a hardening layer of a martensite structure is formed on the surface of the bolt; and carrying out low-temperature tempering treatment on the bolt subjected to medium-frequency quenching. According to the wheel bolt heat treatment method, the production efficiency can be improved, and the process difficulty can be reduced.

Description

Technical Field

[0001] This application relates to the field of heat treatment technology, and in particular to a heat treatment method for wheel bolts. Background Technology

[0002] Currently, the proportion of heavy-duty and super-heavy-duty models in the dump truck industry is gradually increasing, and the overloading rate is getting higher and higher. In addition, with the increase in the load capacity of dump trucks and the increasing number of vehicles operating in harsh working conditions such as mining areas, the tire damage rate is gradually increasing. There is a phenomenon of frequent tire disassembly and assembly leading to stripped threads. Therefore, the requirements for the strength and service life of wheel bolts are getting higher and higher.

[0003] Existing surface strengthening methods for wheel bolts involve heating the wheel bolts and a nitriding medium in a pit furnace. The active nitrogen atoms generated from the decomposition of cyanide ions in the medium diffuse into the wheel bolts, forming a compound layer with high wear and corrosion resistance, and a fatigue-resistant diffusion layer on the surface. However, this method is not only slow, resulting in low production efficiency, but also requires extremely strict control over the purity and temperature uniformity of the furnace atmosphere, leading to significant process challenges. Summary of the Invention

[0004] To address at least one of the problems mentioned in the background art, this application provides a heat treatment method for wheel bolts, which can improve production efficiency and reduce process difficulty.

[0005] To achieve the above objectives, this application provides the following technical solution:

[0006] This application provides a heat treatment method for wheel bolts, including the following steps:

[0007] Step 1: Perform heat treatment on the bolt blank;

[0008] Step 2: Perform medium-frequency quenching on the tempered bolts to form a hardened martensitic layer on the bolt surface;

[0009] Step 3: Perform low-temperature tempering treatment on the bolts after medium-frequency quenching.

[0010] As an optional implementation method, the medium-frequency quenching process includes: heating the surface of the tempered bolt to 860℃-920℃ using a medium-frequency induction heating device and then cooling it.

[0011] As an optional implementation method, the bolts are cooled by annular water spray or oil spray during medium frequency quenching.

[0012] As an optional implementation, the heat treatment includes: heating the bolt blank to 850℃-900℃, holding it at that temperature for 15min-30min and then cooling it, then heating the cooled bolt to 500℃-650℃, holding it at that temperature for 1.5h-3h and then cooling it.

[0013] As an optional implementation method, the low-temperature tempering adopts a segmented process, which includes a first stage process and a second stage process. The first stage process includes heating the bolts after medium-frequency quenching to 150℃-180℃, holding them at that temperature for 10min-30min, and then allowing them to cool naturally.

[0014] As an optional implementation, the second stage of treatment includes: heating the bolts after the first stage treatment to 200℃-250℃, holding them at that temperature for 60min-120min, and then allowing them to cool naturally.

[0015] As an optional implementation, the thickness of the hardened layer is 3mm-4mm.

[0016] As an optional implementation, the wheel bolt heat treatment method further includes step 4: coating the bolt surface with a titanium carbide or chromium carbide coating after low-temperature tempering, with a coating thickness of 5μm-20μm.

[0017] As an optional implementation method, the heating temperature for low-temperature tempering is 150℃-250℃, and the holding time is 70min-150min.

[0018] As an alternative implementation, the bolts are made of 40Cr, 35CrMo, or 42CrMo.

[0019] The heat treatment method for wheel bolts provided in this application includes the following steps: quenching and tempering the bolt blank; performing medium-frequency quenching on the quenched and tempered bolt to form a hardened martensitic layer on the bolt surface; and performing low-temperature tempering on the medium-frequency quenched bolt.

[0020] The heat treatment method for wheel bolts provided in this application achieves excellent comprehensive mechanical properties and a uniform and refined microstructure by tempering the bolt blank, laying the foundation for improved overall performance. Subsequently, the tempered bolts undergo medium-frequency quenching, which rapidly forms a hardened martensitic layer on the bolt surface, significantly improving surface hardness and wear resistance. Finally, the medium-frequency quenched bolts are subjected to low-temperature tempering, effectively eliminating internal stresses generated during quenching, reducing material brittleness, stabilizing the martensitic structure, and maintaining high hardness while preserving the toughness of the bolt core, thus improving the bolt's toughness and fatigue resistance. This effectively solves the problems of low production efficiency due to slow nitriding in traditional methods and the high process difficulty caused by stringent requirements for furnace atmosphere and temperature in the prior art. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of this application 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 some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a flowchart of the heat treatment method for wheel bolts provided in the embodiments of this application;

[0023] Figure 2 This is a schematic diagram of the bolts used in the heat treatment method for wheel bolts provided in the embodiments of this application;

[0024] Figure 3 This is a schematic diagram of the medium-frequency quenching process in the heat treatment method for wheel bolts provided in the embodiments of this application.

[0025] Explanation of reference numerals in the attached figures:

[0026] 100-Power supply;

[0027] 200 - Heating coil;

[0028] 300-Tooling;

[0029] 400 - Cooling spray device;

[0030] 500-bolt;

[0031] 510-core;

[0032] 520 - Hardened layer;

[0033] 521 - Hardened layer on the smooth shaft;

[0034] 522 - Thread hardened layer. Detailed Implementation

[0035] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] In this application, the terms “upper,” “lower,” “left,” “right,” “front,” “back,” “top,” “bottom,” “inner,” “outer,” “vertical,” “horizontal,” “lateral,” and “longitudinal” indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. These terms are primarily for the purpose of better describing this application and its embodiments, and are not intended to limit the indicated devices, elements, or components to having a specific orientation, or to be constructed and operated in a specific orientation.

[0037] Furthermore, in addition to indicating location or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in some cases to indicate a certain dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0038] Furthermore, the terms "installation," "setup," "equipped with," "connection," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0039] Furthermore, the terms "first," "second," etc., are primarily used to distinguish different devices, elements, or components (which may be the same or different in specific type and construction), and are not intended to indicate or imply the relative importance or quantity of the indicated devices, elements, or components. Unless otherwise stated, "a plurality of" means two or more.

[0040] Existing surface strengthening methods for wheel bolts involve heating the wheel bolts and a nitriding medium in a pit furnace. The active nitrogen atoms generated from the decomposition of cyanide ions in the medium diffuse into the wheel bolts, forming a compound layer with high wear and corrosion resistance, and a fatigue-resistant diffusion layer on the surface. However, this method is not only slow, resulting in low production efficiency, but also requires extremely strict control over the purity and temperature uniformity of the furnace atmosphere, leading to significant process challenges.

[0041] In view of this, this application provides a heat treatment method for wheel bolts, comprising the following steps: quenching and tempering the bolt blank; subjecting the quenched and tempered bolt to medium-frequency quenching to form a martensitic hardened layer on the bolt surface; and subjecting the medium-frequency quenched bolt to low-temperature tempering. By quenching and tempering the bolt blank, the bolt matrix can obtain good comprehensive mechanical properties and a uniform and refined microstructure, laying the foundation for improved overall performance. Subsequently, medium-frequency quenching of the quenched and tempered bolt enables the rapid formation of a martensitic hardened layer on the bolt surface, which significantly improves the surface hardness and wear resistance of the bolt. Finally, low-temperature tempering of the medium-frequency quenched bolt effectively eliminates the internal stress generated during quenching, reduces the brittleness of the material, stabilizes the martensitic structure, and maintains the toughness of the bolt core while retaining high hardness, thus improving the bolt's toughness and fatigue resistance. This effectively solves the problems in the prior art, such as low production efficiency due to slow nitriding in traditional processing methods and high process difficulty caused by stringent requirements for furnace atmosphere and temperature.

[0042] Figure 1 This is a flowchart of the heat treatment method for wheel bolts provided in the embodiments of this application; Figure 2 This is a schematic diagram of the bolts used in the heat treatment method for wheel bolts provided in the embodiments of this application; Figure 3 This diagram illustrates the medium-frequency quenching process in the heat treatment method for wheel bolts provided in this embodiment. (See also...) Figures 1 to 3 This application provides a heat treatment method for wheel bolts 500, including the following steps:

[0043] S100. The bolt blank is subjected to heat treatment;

[0044] S200. The tempered bolts are subjected to medium-frequency quenching to form a hardened martensitic layer on the bolt surface.

[0045] S300, perform low-temperature tempering treatment on bolts after medium-frequency quenching.

[0046] The heat treatment method for wheel bolts 500 provided in this application involves quenching and tempering the bolt 500 blank, which enables the bolt 500 matrix to acquire good comprehensive mechanical properties and a uniform and refined microstructure, laying the foundation for improved overall performance. Subsequently, the quenched and tempered bolt 500 undergoes medium-frequency quenching, which rapidly forms a hardened martensitic layer on the bolt 500 surface. This hardened layer significantly improves the surface hardness and wear resistance of the bolt 500. Finally, the medium-frequency quenched bolt 500 undergoes low-temperature tempering, effectively eliminating internal stress generated during quenching, reducing material brittleness, stabilizing the martensitic structure, and maintaining the toughness of the bolt 500 core while retaining high hardness, thus improving the bolt 500's toughness and fatigue resistance. This effectively solves the problems in the background technology, such as low production efficiency due to slow nitriding in traditional processing methods and high process difficulty caused by stringent requirements for furnace atmosphere and temperature.

[0047] The bolt may include a screw head and a stud. The stud includes a core 510 in the middle and a hardened layer 520 on the surface. The hardened layer 520 includes a threaded hardened layer 522 and a plain hardened layer 521 along the screw axis. The hardness of the core 510 of the screw can reach 39 HRC-44 HRC after heat treatment in this embodiment. The hardness of the threaded hardened layer 522 can reach 50 HRC-53 HRC. The hardness of the plain hardened layer 521 can reach 48 HRC-50 HRC.

[0048] In the above embodiments, the medium-frequency quenching process may include: heating the surface of the tempered bolt 500 to 860℃-920℃ using a medium-frequency induction heating device and then cooling it. It is understood that medium-frequency induction heating can quickly heat the surface of the bolt 500 to 860℃-920℃, thereby rapidly reaching the phase transformation temperature. During the subsequent cooling process, a martensitic hardened layer 520 can be formed on the surface of the bolt 500. This hardened layer 520 significantly improves the hardness, wear resistance, and fatigue resistance of the bolt 500 surface, effectively strengthening the threaded portion and enhancing its anti-slipping ability during frequent tire removal and installation operations. If the heating temperature is less than 860℃, the austenitic phase transformation cannot fully occur on the surface of the bolt 500, resulting in insufficient martensite structure after quenching. Consequently, the hardness and thickness of the hardened layer 520 do not meet requirements, leading to a decrease in the wear resistance and deformation resistance of the bolt 500 surface, and making the threads prone to wear and slippage. When the heating temperature exceeds 920℃, the austenite grains on the surface of the bolt 500 will grow excessively, and the martensite structure after quenching will be coarse. This will not only reduce the toughness of the bolt 500, but also cause its hardness and wear resistance to be unstable. It will be more prone to brittle fracture during use and will not be able to meet the strict performance requirements of the wheel bolt 500 for dump trucks under heavy load and harsh working conditions.

[0049] In the above embodiments, the medium-frequency quenching treatment can be performed by cooling the bolt 500 using an annular water spray or oil spray method. It can be understood that the annular design allows for omnidirectional spraying of coolant around the bolt 500, ensuring uniform cooling of all parts of the bolt 500 surface. For example, in actual operation, surrounding the bolt 500 with an annular water spray device and evenly distributed nozzles allows the bolt 500 surface to simultaneously contact the coolant, avoiding microstructural differences caused by uneven cooling, thereby forming a uniform hardened layer 520 and ensuring the overall performance stability of the bolt 500. The cooling intensity can be flexibly adjusted between water spray and oil spray. Water spray cooling is relatively fast, allowing the bolt 500 surface to reach the critical cooling rate in a short time, rapidly forming martensitic structure and significantly improving the surface hardness and wear resistance of the bolt 500. When the bolt 500 is used in heavy-duty dump truck applications where extremely high hardness is required, annular water spray cooling allows the bolt 500 to withstand frequent tire removal and installation and enormous torque. Oil spraying cooling is relatively slow. For some bolts 500 that require a certain level of toughness, it can appropriately control the martensitic transformation rate while ensuring the formation of a hardened layer 520, thus avoiding excessive internal stress due to rapid cooling. The water or oil spraying cooling method can be flexibly selected and adjusted according to different types and specifications of bolts 500 and specific operating conditions, exhibiting strong versatility. It can be widely applied to the heat treatment of various dump truck wheel bolts 500, improving the adaptability and flexibility of the overall production process.

[0050] Specifically, medium-frequency quenching can be performed using medium-frequency quenching equipment. The medium-frequency quenching equipment may include a power supply 100 that provides electrical energy and has adjustable power, a heating coil 200 that is electrically connected to the power supply 100, a fixture 300 for fixing the bolt 500, and a cooling spray device 400 for spraying water or oil in a ring on the bolt 500. During medium-frequency quenching, the bolt 500 can be erected on the fixture 300, and the induction coil can be wrapped around the bolt 500 to heat the bolt 500 quickly and evenly. When the required temperature is reached, the cooling spray device 400 is immediately activated to spray water or quenching oil evenly in a ring onto the surface of the bolt 500 for cooling.

[0051] In the above embodiments, the heat treatment may include: heating the bolt 500 blank to 850℃-900℃, holding it at that temperature for 15min-30min and then cooling it, and then heating the cooled bolt 500 to 500℃-650℃, holding it at that temperature for 1.5h-3h and then cooling it. It is understandable that this quenching and tempering process can achieve a full austenitic transformation of the bolt 500 matrix. The holding time can ensure that the core and surface temperatures of the bolt 500 blank are uniform. After quenching, the bolt 500 matrix initially forms a supersaturated austenitic structure. Subsequent high-temperature tempering at 500℃-650℃ with sufficient holding time can promote the matrix structure to complete a full tempering transformation, forming a uniform and fine sorbite structure. This endows the bolt 500 matrix with excellent strength and toughness matching and comprehensive mechanical properties. It ensures that the bolt 500 has the high strength and tensile strength required for heavy-load conditions, as well as good toughness and impact resistance. It can effectively prevent the bolt 500 from undergoing overall fracture failure during actual service. At the same time, the homogenized matrix structure can lay a good microstructure foundation for the subsequent medium-frequency quenching process, ensuring that the hardened layer 520 on the surface of the bolt 500 is tightly bonded to the matrix and eliminating the risk of the hardened layer 520 falling off.

[0052] If the quenching temperature is below 850℃, the austenitization transformation of the bolt 500 blank will be insufficient, resulting in poor grain refinement of the matrix. After quenching, a sufficient amount of supersaturated austenite structure cannot be formed, and the matrix strength and hardness after tempering will not meet the design requirements, making it difficult to withstand the huge loads under the heavy-duty conditions of dump trucks. If the quenching temperature is above 900℃, the austenite grains in the bolt 500 blank will grow excessively, resulting in a coarse matrix structure after tempering. The toughness of the bolt 500 will be greatly reduced, making it prone to brittle fracture under alternating loads. Furthermore, excessively high temperatures can easily cause oxidation and decarburization on the surface of the bolt 500, deteriorating the surface quality of the matrix. If the quenching holding time is less than 15 minutes, the temperature difference between the inside and outside of the bolt 500 blank will be too large, resulting in incomplete austenitization transformation and poor uniformity of the matrix structure. If the quenching holding time exceeds 30 minutes, it will prolong the process cycle and reduce production efficiency.

[0053] If the high-temperature tempering temperature is below 500℃, the matrix tempering transformation process is slow, and the supersaturated austenite cannot be fully decomposed. The residual internal stress in the bolt 500 matrix is ​​too high, and the toughness improvement effect is not good. If the high-temperature tempering temperature is above 650℃, the matrix structure will be excessively softened, and the overall strength of the bolt 500 will decrease significantly, failing to meet the requirements for heavy-duty use. If the high-temperature tempering holding time is less than 1.5h, the matrix structure tempering transformation is insufficient, and the strength and toughness are unbalanced. If the high-temperature tempering holding time exceeds 3h, it will significantly increase production time and energy consumption, reduce overall production efficiency, and easily cause slight growth of matrix grains, thereby weakening the mechanical properties of the bolt 500 matrix.

[0054] In the above embodiments, the low-temperature tempering can be carried out in stages. The staged process includes a first stage process and a second stage process. The first stage process includes heating the bolt 500 after medium frequency quenching to 150℃-180℃, holding it at that temperature for 10min-30min, and then allowing it to cool naturally. It is understandable that this segmented low-temperature tempering treatment can achieve a gradient-like initial release of internal stress after quenching of bolt 500. Relying on the heating and heat preservation effect in the low-temperature range, the surface martensite structure of bolt 500 is initially relaxed. While preserving the high hardness core characteristics of martensite, the residual stress generated during the medium-frequency quenching and cooling process is first eliminated, avoiding the direct initiation of microcracks in bolt 500 due to stress concentration. At the same time, this temperature range can inhibit the excessive decomposition of martensite structure, ensuring that the hardness and wear resistance of the surface hardened layer 520 of bolt 500 are not compromised, which is suitable for the core application requirements of anti-slippage and anti-wear in the threaded parts. Among them, the heat preservation time of 10min-30min can ensure that the circumferential and threaded complex areas of bolt 500 are heated evenly, and complete the full initial stress release. The natural cooling method can avoid the additional stress generated by rapid cooling, further ensuring the stability of the structure and stress state of bolt 500.

[0055] If the heating temperature is below 150℃, the martensitic lattice cannot be effectively relaxed, the residual stress on the surface of bolt 500 will not be sufficiently eliminated, and there will still be a risk of cracking. If the heating temperature is above 180℃, the martensitic structure will undergo slight decomposition prematurely, the surface hardness of bolt 500 will decrease slightly, and the deformation resistance of the threaded part will be weakened. If the holding time is less than 10 minutes, the stress release reaction in various parts of bolt 500 will be insufficient, and uneven local stress residue is likely to occur. If the holding time exceeds 30 minutes, it will increase the tempering process time, reduce the overall production efficiency, and there is no corresponding performance gain, which does not meet the process economy requirements of industrial mass production.

[0056] In the above embodiments, the second stage of processing may include: heating the bolt 500 after the first stage treatment to 200℃-250℃, holding it at that temperature for 60min-120min, and then allowing it to cool naturally. Based on the initial pressure relief and surface stabilization achieved in the first stage, the second stage further eliminates residual stress within the bolt 500 and simultaneously completes secondary stabilization and regulation of the martensitic structure. Through heating and holding within this temperature range, the high hardness and wear resistance of the bolt 500 surface can be preserved to the greatest extent while significantly improving the brittleness of the martensitic structure. This greatly enhances the fatigue resistance and impact resistance of the bolt 500, effectively preventing brittle fracture and crack propagation failures under heavy alternating loads and the torque impact of frequent tire removal and installation. Furthermore, these process parameters ensure sufficient stress release and uniform structural transformation in the core and surface of the bolt 500, resulting in optimal overall strength and toughness matching of the bolt 500, meeting the requirements of heavy-duty and harsh working conditions for dump trucks.

[0057] If the heating temperature is below 200℃, the deep residual stress inside the bolt 500 cannot be completely eliminated, and the stabilization effect of the martensitic structure is insufficient. During the service life of the bolt 500, the continuous release of stress can easily cause dimensional deformation, resulting in a decrease in the thread fit accuracy. If the heating temperature is above 250℃, the tempered martensitic structure on the surface of the bolt 500 will be excessively decomposed, causing a sharp decrease in the surface hardness and wear resistance of the bolt 500. The threaded parts will be unable to resist disassembly and assembly wear and plastic deformation, and are prone to stripping failure. If the holding time is less than 60 minutes, the internal structure transformation of the bolt 500 will be insufficient, and the stress elimination and performance optimization effects will not meet the design standards, resulting in poor overall mechanical property stability. If the holding time exceeds 120 minutes, it will significantly extend the overall process cycle of low-temperature tempering, reduce the production line turnover efficiency, and increase energy consumption and production control costs. Moreover, after exceeding this time, the mechanical properties of the bolt 500 will not be significantly improved, resulting in process redundancy, which is not conducive to efficient industrial production.

[0058] In the above embodiments, the thickness of the hardened layer 520 ( Figure 2 The marking a) in the figure can be 3mm-4mm. This thickness design allows the surface of the bolt 500 to form a martensitic hardened layer 520 with high hardness, high wear resistance, and structural stability. This achieves full-depth reinforcement of the threaded area, effectively resisting torque wear and plastic deformation caused by frequent tire removal and installation, thus eliminating the problem of thread stripping failure at the root. It also ensures that the hardened layer 520 forms a stable metallurgical bond with the bolt 500 substrate, giving the hardened layer 520 excellent anti-stripping ability, suitable for the continuous stress requirements of heavy-duty and ultra-heavy-duty dump trucks. At the same time, the 3mm-4mm thickness of the hardened layer 520 can balance the surface strengthening effect of the bolt 500 with the overall mechanical coordination, avoiding insufficient wear resistance due to an excessively thin hardened layer 520, or interlayer stress concentration caused by an excessively thick hardened layer 520; if If the hardened layer 520 thickness is less than 3mm, the surface reinforcement depth of the bolt 500 is insufficient. The wear-resistant layer at the threaded part is easily worn away quickly during repeated disassembly and assembly operations, failing to form long-term anti-slip thread protection. Furthermore, the shallow hardened layer 520 is unable to withstand alternating loads under heavy-duty working conditions, and is prone to surface crushing and deformation failure, failing to meet the requirements of harsh working conditions. If the hardened layer 520 thickness is greater than 4mm, it will result in an excessively high proportion of martensitic hardened layer 520 on the surface of the bolt 500, causing an imbalance in the overall strength and toughness of the bolt 500. Significant stress concentration will occur in the transition zone between the hardened layer 520 and the matrix. When the bolt 500 is subjected to impact or torsional loads, cracks are prone to initiation and rapid propagation in the transition zone, leading to the peeling of the hardened layer 520 or even overall fracture failure of the bolt 500.

[0059] In the above embodiments, the heat treatment method for wheel bolt 500 may further include step 4: coating the surface of bolt 500 after low-temperature tempering with a titanium carbide or chromium carbide coating, the coating thickness being 5μm-20μm. Among them, the titanium carbide and chromium carbide coatings possess extremely high hardness, excellent wear resistance, and anti-adhesive wear performance, which can significantly improve the wear resistance level of the bolt 500 surface. It can effectively resist the seizing wear and abrasive wear of the threads during tire installation and removal, further strengthening the anti-slip capability of the threads. At the same time, the coating can isolate the bolt 500 surface from external corrosive media and moisture, giving the bolt 500 excellent corrosion resistance and oxidation resistance, making it suitable for harsh working conditions with high dust and humidity, delaying the corrosion of the bolt 500 surface, and extending the overall service life of the bolt 500. Moreover, the titanium carbide and chromium carbide coatings are tightly bonded to the bolt 500 hardened layer 520, which can work together to strengthen the bolt. While retaining the toughness of the bolt 500 substrate and the high hardness of the hardened layer 520, it achieves a second leap in the wear resistance and corrosion resistance of the bolt 500 surface, fully meeting the stringent requirements of high strength, high wear resistance, and long service life of wheel bolts 500 under heavy and ultra-heavy load conditions of dump trucks.

[0060] The coating thickness design allows the coating to adhere tightly and uniformly to the surface of the hardened layer 520 after the low-temperature tempering of bolt 500. The titanium carbide and chromium carbide coatings themselves have ultra-high hardness and excellent wear resistance and corrosion resistance. This thickness of coating can give full play to the performance advantages of the materials themselves, greatly improve the resistance of bolt 500 threads and surface to abrasive wear and adhesive wear, effectively isolate external moisture and corrosive media from contact with bolt 500 substrate, significantly enhance the corrosion resistance and oxidation resistance of bolt 500, further delay the rust and wear failure of bolt 500 surface under heavy load and harsh working conditions, extend the service life of bolt 500, and take into account both surface strengthening effect and process economy.

[0061] If the coating thickness is less than 5μm, the coating cannot achieve complete and dense coverage of the bolt 500 surface, and coating pores and missed areas are likely to appear. It cannot effectively block corrosive media, and the wear resistance and protection effect is greatly reduced. The surface of the bolt 500 is still prone to rust and wear failure, making it difficult to achieve the purpose of secondary strengthening. If the coating thickness is greater than 20μm, large internal stress is easily generated inside the coating, which leads to increased brittleness of the coating. When the bolt 500 is subjected to torque and impact load, it is very easy to crack and peel off, losing its protective function. At the same time, the excessively thick coating will increase the coating process time and material consumption, increase production costs, and also cause the bolt 500 thread size to be out of tolerance, affecting the assembly accuracy of the bolt 500 and the wheel hub.

[0062] In the above embodiments, the heating temperature for low-temperature tempering is 150℃-250℃, and the holding time is 70min-150min. If the low-temperature tempering temperature is too low, such as below 150℃, it may not be able to fully eliminate the internal stress inside the bolt 500 after medium-frequency quenching, and the brittleness of the martensite will be difficult to effectively improve. In subsequent use, the bolt 500 will still be prone to cracking, and it is also not conducive to stabilizing the metallographic structure of the hardened layer 520, which may result in its hardness and wear resistance not being maintained for a long time. If the low-temperature tempering temperature is too high, exceeding 250℃, it may cause excessive decomposition of the martensite structure on the surface of the bolt 500, resulting in a significant decrease in hardness, which cannot meet the requirements for high hardness and wear resistance of the bolt 500 under heavy-load conditions. If the heat preservation time is less than 70 minutes, the internal parts of bolt 500 will not undergo sufficient tempering reaction, and the internal stress cannot be completely eliminated, affecting the stability and service life of bolt 500. If the heat preservation time exceeds 150 minutes, it will not only reduce production efficiency, but may also lead to grain growth, which will reduce the mechanical properties of bolt 500, such as reduced toughness and strength, which is also not conducive to its performance in actual use.

[0063] In the above embodiments, the bolt 500 can be made of 40Cr, 35CrMo, or 42CrMo. It is understood that compared to ordinary carbon steel, 40Cr, 35CrMo, and 42CrMo, due to their alloying elements such as chromium and molybdenum, have stronger hardenability, ensuring uniform overall mechanical properties of the bolt 500 and preventing core softening. Among these, 35CrMo and 42CrMo exhibit superior high-temperature strength and fatigue resistance, making them suitable for the extreme load conditions of ultra-heavy-duty dump trucks. 40Cr, on the other hand, offers advantages in terms of good processability and controllable cost, making it suitable for conventional heavy-duty conditions. All three base materials can meet the process requirements of this application, and industrial procurement and processing are convenient without adding extra production difficulty, which highly aligns with the core inventive objective of this application to improve production efficiency and reduce process difficulty.

[0064] The following are some specific examples for further explanation:

[0065] Example 1

[0066] First, the bolt 500 blank is heated to 850℃ and held for 15 minutes before cooling. Then, the cooled bolt 500 is heated to 500℃ and held for 1.5 hours before cooling. Next, the surface of the tempered bolt 500 is heated to 860℃ using a medium-frequency induction heating device, and then immediately cooled by annular water spraying or oil spraying. Then, the medium-frequency quenched bolt 500 is heated to 150℃ and held for 10 minutes before natural cooling. Finally, the bolt 500 is heated to 200℃ and held for 60 minutes before natural cooling.

[0067] Example 2

[0068] First, the bolt 500 blank is heated to 875℃ and held for 20 minutes before cooling. Then, the cooled bolt 500 is heated to 600℃ and held for 2 hours before cooling. Next, the surface of the tempered bolt 500 is heated to 900℃ using a medium-frequency induction heating device, and immediately cooled by annular water spraying or oil spraying. Then, the medium-frequency quenched bolt 500 is heated to 165℃ and held for 20 minutes before natural cooling. Finally, the bolt 500 is heated to 230℃ and held for 90 minutes before natural cooling.

[0069] Example 3

[0070] First, the bolt 500 blank is heated to 900℃ and held for 30 minutes before cooling. Then, the cooled bolt 500 is heated to 650℃ and held for 3 hours before cooling. Next, the surface of the tempered bolt 500 is heated to 920℃ using a medium-frequency induction heating device, and then immediately cooled by annular water spraying or oil spraying. Then, the medium-frequency quenched bolt 500 is heated to 180℃ and held for 30 minutes before natural cooling. Finally, the bolt 500 is heated to 250℃ and held for 120 minutes before natural cooling.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A heat treatment method for wheel bolts, characterized in that, Includes the following steps: Step 1: Perform heat treatment on the bolt blank; Step 2: The tempered bolts are subjected to medium-frequency quenching to form a hardened martensitic layer on the bolt surface; Step 3: Perform low-temperature tempering treatment on the bolts after medium-frequency quenching.

2. The heat treatment method for wheel bolts according to claim 1, characterized in that, The medium-frequency quenching process includes: heating the surface of the tempered bolt to 860℃-920℃ using a medium-frequency induction heating device and then cooling it.

3. The heat treatment method for wheel bolts according to claim 2, characterized in that, The medium-frequency quenching process involves cooling the bolts using a ring-shaped water spray or oil spray method.

4. The heat treatment method for wheel bolts according to claim 3, characterized in that, The heat treatment includes: heating the bolt blank to 850℃-900℃, holding it at that temperature for 15min-30min and then cooling it; then heating the cooled bolt to 500℃-650℃, holding it at that temperature for 1.5h-3h and then cooling it.

5. The heat treatment method for wheel bolts according to claim 4, characterized in that, The low-temperature tempering adopts a segmented process, which includes a first stage process and a second stage process. The first stage process includes heating the bolt after medium-frequency quenching to 150℃-180℃, holding it at that temperature for 10min-30min, and then allowing it to cool naturally.

6. The heat treatment method for wheel bolts according to claim 5, characterized in that, The second stage of processing includes heating the bolts after the first stage of processing to 200℃-250℃, holding them at that temperature for 60min-120min, and then allowing them to cool naturally.

7. The heat treatment method for wheel bolts according to any one of claims 1-6, characterized in that, The thickness of the hardened layer is 3mm-4mm.

8. The heat treatment method for wheel bolts according to any one of claims 1-6, characterized in that, The heat treatment method for wheel bolts further includes step 4: coating the surface of the bolts after low-temperature tempering with a titanium carbide or chromium carbide coating, wherein the thickness of the coating is 5μm-20μm.

9. The heat treatment method for wheel bolts according to any one of claims 1-6, characterized in that, The heating temperature for the low-temperature tempering is 150℃-250℃, and the holding time is 70min-150min.

10. The heat treatment method for wheel bolts according to any one of claims 1-6, characterized in that, The bolts are made of 40Cr, 35CrMo, or 42CrMo.

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