Production process of 12.9-grade low-temperature-impact-resistant high-strength bolt

Abstract

The invention discloses a production process of a 12.9-grade low-temperature-impact-resistant high-strength bolt, and belongs to the field of bolt production. The process comprises the steps of blank cutting, cold heading and thread rolling, sectional quenching, quenching cooling, tempering and surface treatment. Wherein in the sectional quenching process, step cooling and heating at 810-870 DEG C are adopted, and the total time is 120 minutes; the temperature of the quenching liquid is 65-85 DEG C; tempering is performed at the temperature of 480 DEG C for 7 sections, each section is treated for 20 minutes, and hydrochloric acid dephosphorization, sodium carbonate neutralization and blackening treatment with specific parameters can also be included. According to the process, by optimizing heat treatment and surface treatment, the strength, toughness, low-temperature impact resistance and plasticity of the alloy steel bolt are improved, the fracture risk is reduced, the service life is prolonged, and the reliability is improved.

Description

Technical Field

[0001] This invention relates to the field of bolt manufacturing technology, and in particular to a manufacturing process for a 12.9 grade low-temperature impact resistant high-strength bolt. Background Technology

[0002] In modern industrial manufacturing, alloy steel materials are widely used in mechanical components, automotive parts, aerospace, and engineering machinery, playing a crucial role, especially in applications requiring high strength, wear resistance, and high temperature resistance. SCM435 alloy steel, as an alloy steel material with high strength, good toughness, and wear resistance, has been widely used in numerous industries.

[0003] However, while SCM435 alloy steel meets the high strength requirements, its plasticity (i.e., elongation after fracture) is usually low, which makes it prone to fracture when subjected to sudden impact or long-term repeated load, thereby reducing its service life and reliability. Summary of the Invention

[0004] The main objective of this invention is to provide a manufacturing process for a 12.9 grade low-temperature impact resistant high-strength bolt, which aims to improve its service life and reliability.

[0005] To achieve the above objectives, this invention proposes a manufacturing process for a 12.9 grade low-temperature impact resistant high-strength bolt, comprising the following steps:

[0006] S1. Cut the raw material into blank workpieces of predetermined length;

[0007] S2. The blank is processed into a bolt semi-finished product using the mold of a cold heading machine, and the threads are rolled on it;

[0008] S3. After the thread rolling process in step S2, the semi-finished bolts are placed in the quenching zone for segmented quenching. The quenching zone is equipped with multiple heating zones with different temperatures, and the temperature range is controlled within the range of 810-870℃. The semi-finished bolts are first placed in the heating zone with the highest temperature for heating. As time goes on, the semi-finished bolts are then transferred to the heating zones with lower temperatures in sequence. The entire heating process lasts for 120 minutes.

[0009] S4. Place the semi-finished bolts heated in step S3 into a quenching solution for cooling. The temperature of the quenching solution is controlled within the range of 65-85℃.

[0010] S5. Tempering process is carried out on the bolt semi-finished product after cooling in step S4.

[0011] S6. Perform surface treatment on the semi-finished bolts after the tempering treatment in step S5 to obtain the finished bolts.

[0012] In one possible implementation, in step S3, the temperatures of the heating zones are 870°C, 860°C, 850°C, 840°C and 810°C in sequence. First, the semi-finished bolts are placed in the heating zone at 870°C and heated for 60 minutes. Then, they are taken out and placed in the heating zones at 860°C, 850°C, 840°C and 810°C in sequence, with each heating zone heated for 15 minutes.

[0013] In one possible implementation, the carbon potential of the activated carbon inside each of the heating zones is 0.4%.

[0014] In one possible implementation, the tempering process adopts a multi-stage tempering process, with a total of 7 tempering zones. The temperature of each tempering zone is controlled at 480°C, and the tempering time of each tempering zone is 20 minutes.

[0015] In one possible implementation, in step S2, after the mold of the cold heading machine processes the blank into a bolt semi-finished product, it needs to be dephosphorized: the cold-headed bolt semi-finished product is placed in a 10-12% hydrochloric acid solution at a temperature of 50-60°C for 8-10 minutes.

[0016] In one possible implementation, in step S6, after the bolt semi-finished product has undergone tempering, it needs to be blackened: the finished bolt is immersed in a blackening solution with a sodium hydroxide concentration of 850 g / L and a sodium nitrite concentration of 55 g / L, at a temperature of 137°C for 23 minutes.

[0017] In one possible implementation, after pickling, the semi-finished bolts are placed in a 0.5% sodium carbonate solution for neutralization for 30 seconds.

[0018] This invention employs segmented quenching, allowing the bolt's temperature to decrease gradually during the quenching process. This avoids thermal shock caused by a rapid temperature drop, reducing the risk of crack formation. Secondly, segmented quenching, by controlling the cooling rate, smooths the martensitic transformation process, reducing the formation of brittle materials and thus improving the bolt's toughness and durability. Simultaneously, this method ensures more uniform temperature changes across different parts of the bolt, preventing excessive temperature differences between the surface and interior, and reducing thermal stress. Attached Figure Description

[0019] 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 of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0020] Figure 1 This is in accordance with the requirements for grade 12.9 bolts in the existing technology;

[0021] Figure 2 This is a flowchart of the production process of the present invention.

[0022] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0024] Example 1:

[0025] refer to Figure 2 This embodiment proposes a manufacturing process for a 12.9 grade low-temperature impact resistant high-strength bolt, including the following steps:

[0026] S1. Cut the raw material into blanks of a predetermined length; the raw material can be SCM435 alloy steel.

[0027] S2. The blank is processed into a bolt semi-finished product using the mold of the cold heading machine, and the threads are rolled on it;

[0028] Specifically, cold heading machines utilize the plasticity of metal to form the bolt within a mold, compacting defects such as porosity and air bubbles in the raw material and creating continuous metal flow lines along the direction of force. Compared to machining, this significantly improves the overall strength of the bolt. This strengthening effect allows the bolt to meet the basic strength requirement of grade 12.9 without the need for additional alloying elements. Simultaneously, thread rolling, through cold rolling, forms a work-hardened layer on the thread surface, while the thread root fillets are smoother, avoiding sharp edges that might result from machining. This not only improves the wear resistance of the threads but also reduces the stress concentration factor, thereby increasing the fatigue strength of the bolt and reducing the risk of threaded fracture during use.

[0029] S3. After the thread rolling process in step S2, the semi-finished bolts are placed in the quenching zone for segmented quenching. The quenching zone has multiple heating zones with different temperatures, and the temperature range is controlled within the range of 810-870℃. The semi-finished bolts are first placed in the heating zone with the highest temperature for heating. As time goes on, the semi-finished bolts are then transferred to the heating zones with lower temperatures in sequence. The entire heating process lasts for 120 minutes.

[0030] Specifically, because high-alloy steel typically contains elements such as chromium and molybdenum, the carbides formed from them are highly stable and require relatively high temperatures to fully dissolve into austenite. If heating at a single temperature, insufficient carbide dissolution can easily occur, leading to insufficient strength after quenching. Segmented heating first promotes carbide dissolution at high temperatures, then uses medium-to-low temperature holding to allow carbon and alloying elements to diffuse evenly within the austenite. Simultaneously, if the austenite formed at high temperatures is directly quenched, coarse grains can lead to decreased toughness. The segmented, step-down cooling from high to low temperatures smooths the martensitic transformation process, reduces the formation of brittle materials, and thus improves the toughness and durability of the bolts.

[0031] S4. Place the semi-finished bolts heated in step S3 into a quenching solution for cooling. The temperature of the quenching solution is controlled within the range of 65-85℃.

[0032] A quenching solution with a temperature range of 65-85℃ can meet the critical cooling rate required for martensitic transformation, ensuring sufficient strength for the bolts, while avoiding excessive internal stress caused by overly rapid cooling and reducing the risk of cracking. If the quenching solution temperature is too low (below 65℃) and the cooling rate is too fast, the martensitic transformation will be too intense, generating huge internal stress, which can easily cause cracks or deformation in the bolts. It may also exacerbate oxidation or decarburization on the bolt surface, reducing the surface hardness and corrosion resistance. If the temperature is too high (above 85℃) and the cooling rate is too slow, martensite cannot be fully formed, resulting in insufficient steel strength and making it difficult to meet the high strength requirements of grade 12.9 bolts.

[0033] S5. The semi-finished bolts cooled in step S4 are subjected to a tempering process. After quenching, the bolts have a martensitic structure, high internal stress, and are brittle, making them prone to deformation or fracture if used directly. Tempering decomposes the martensite through heating, causing carbon atoms to diffuse and precipitate to form fine and uniform carbides (i.e., tempered sorbite structure), which can significantly reduce internal stress and improve the dimensional stability of the bolts.

[0034] S6. Perform surface treatment on the semi-finished bolts after the tempering treatment in step S5 to obtain the finished bolts.

[0035] In step S3 of this implementation, the temperatures of the heating zones are 870℃, 860℃, 850℃, 840℃ and 810℃ respectively. First, the semi-finished bolts are placed in the heating zone at 870℃ and heated for 60 minutes. Then, they are taken out and placed in the heating zones at 860℃, 850℃, 840℃ and 810℃ respectively, and each heating zone is heated for 15 minutes.

[0036] This embodiment employs a stepped heating process: 870℃×60 minutes → 860℃×15 minutes → 850℃×15 minutes → 840℃×15 minutes → 810℃×15 minutes. First, the high temperature of 870℃ and the 60-minute holding time ensure that the stable carbides formed by chromium and molybdenum in the SCM435 alloy steel are fully dissolved into the austenite, laying the compositional foundation for obtaining a high-strength martensitic structure after quenching and avoiding insufficient strength or stress concentration due to residual carbides. Based on this, the stepped cooling from 870℃ to 810℃ gradually drives the steel through a temperature gradient. Dynamic recrystallization of austenite refines potentially coarse grains at high temperatures, solving the problem of coarse grains and reduced toughness caused by single high-temperature heating. Simultaneously, the 15-minute holding time per stage ensures uniform austenite composition, preventing martensitic brittleness caused by localized carbon concentration deviations. The reasonable allocation of the total 120-minute heating time guarantees sufficient carbide dissolution and grain refinement while avoiding surface oxidation or decarburization due to excessive heating. Ultimately, this allows the bolts to achieve the required high strength (grade 12.9) while simultaneously improving toughness and low-temperature impact performance, achieving a balance between strength and toughness. It should be noted that the heating equipment can be a pit furnace, box-type resistance furnace, or any other heating equipment capable of reaching the required temperature. These devices are existing technologies well-known to those skilled in the art and will not be described further.

[0037] Carbon potential refers to the carbon concentration at which the heating atmosphere and the bolt surface reach carbon equilibrium at a certain temperature. It represents the atmosphere's ability to supply or absorb carbon to the bolt. Controlling the carbon potential of the active carbon in each heating zone at 0.4% can moderately increase the carbon concentration on the bolt surface through weak carburizing, slightly higher than the original carbon content of SCM435 steel (approximately 0.35%), thereby enhancing the surface hardness and wear resistance after quenching. It can also prevent the precipitation of network carbides caused by high carbon, thus ensuring the toughness of the core. At the same time, it can effectively prevent decarburization problems that may occur when the carbon potential is too low (avoiding a decrease in surface strength and stress concentration). If the carbon potential is lower than 0.4%, decarburization is likely to occur, resulting in a decrease in surface carbon content and insufficient hardness. Furthermore, the difference in microstructure between the decarburized layer and the core will exacerbate stress concentration. If the carbon potential is higher than 0.4%, the surface carbon content will be too high, forming brittle coarse martensite or network carbides after quenching, which will reduce the low-temperature impact performance of the bolt.

[0038] When tempering the bolts, a multi-stage tempering process is used, with a total of 7 tempering zones. The temperature of each tempering zone is controlled at 480℃, and the tempering time is 20 minutes. The specific operation is as follows: the bolts are placed in each tempering zone in sequence, and after each tempering zone is held at that temperature for 20 minutes, they are moved to the next tempering zone. This process is repeated until all tempering zones are completed.

[0039] Multi-stage tempering, compared to single-stage tempering, can more effectively eliminate quenching stress, resulting in a more stable microstructure. At a tempering temperature of 480℃, martensite decomposes into tempered sorbite, which possesses excellent comprehensive mechanical properties, further improving the bolt's toughness and strength. This is similar to kneading dough; kneading multiple times, rather than kneading for a long time in one go, results in more evenly elastic dough. Similarly, stress within the bolt can be eliminated more thoroughly, leading to a more stable internal structure. At 480℃, the bolt develops a structure with excellent comprehensive performance, being both tough and strong.

[0040] Multi-stage tempering can more fully eliminate quenching internal stress and stabilize the microstructure because there are macroscopic and microscopic layered internal stresses inside the material after quenching. Moreover, the microstructure transformation, such as martensite decomposition, retained austenite transformation, and carbide aggregation, is staged. During single-stage tempering, stress release is easily incomplete due to superposition, and the microstructure transformation may also be insufficient due to insufficient time or process stagnation. Multi-stage tempering provides a buffer through multiple heating, first gradually releasing macroscopic stress, and then focusing on eliminating microscopic and residual stress. At the same time, it adapts to each stage of microstructure transformation, allowing atoms to diffuse fully, breaking the transformation stagnation, and promoting the complete decomposition of martensite and the uniform distribution of carbides.

[0041] The finished bolts obtained through the process of this embodiment were tested, and the following data were obtained:

[0042] Therefore, it can be seen that bolts produced using this process are superior to traditional 12.9 grade bolts (refer to...). Figure 1 The elongation after fracture, tensile strength, yield strength, elongation after fracture, and reduction of area were significantly improved, thereby enhancing the overall performance, service life, and reliability of the bolts.

[0043] Example 2:

[0044] Based on Example 1, in step S2 of this example, after the cold heading machine's mold processes the blank into a bolt semi-finished product, it needs to undergo descaling treatment: the cold-headed bolt semi-finished product is placed in a 10-12% hydrochloric acid solution at a temperature of 50-60°C for 8-10 minutes. It should be noted that the 10-12% hydrochloric acid solution here refers to the percentage of pure hydrochloric acid in the total solution mass. For example, if 100 grams of this solution is prepared, the mass of pure hydrochloric acid is 10 to 12 grams, and the remaining 88 to 90 grams is water.

[0045] During the cold heading process, bolts develop a layer of oxide scale and phosphate on their surface. Dephosphorization treatment effectively removes these impurities, resulting in a cleaner bolt surface. A clean surface facilitates subsequent heat treatment and surface treatment processes, improving the effectiveness of heat treatment and the adhesion of surface treatments.

[0046] It is worth noting that if the hydrochloric acid concentration is below 10%, the temperature is below 50℃, or the time is less than 8 minutes, the phosphorus removal effect will be poor, leaving surface impurities that will affect subsequent treatments. If the hydrochloric acid concentration is above 12%, the temperature is above 60℃, or the time is more than 10 minutes, it will excessively corrode the bolt surface, leading to increased surface roughness and even damaging the bolt's dimensional accuracy, thus affecting the bolt's mechanical properties and appearance quality.

[0047] After pickling, pickling residue remains on the bolt surface. If not treated promptly, this residual acid will continue to corrode the bolt surface, affecting its quality and service life. Therefore, in this embodiment, after pickling, the semi-finished bolts are placed in a 0.5% sodium carbonate solution for 30 seconds to neutralize the residual acidic substances on the bolt surface, preventing further corrosion. Simultaneously, the sodium carbonate solution is relatively weakly alkaline, preventing excessive corrosion of the bolt surface and ensuring its integrity. It is important to note that the 0.5% sodium carbonate solution refers to the percentage of pure sodium carbonate by mass in the entire solution. 30 seconds is sufficient for neutralization. If the concentration is below 0.5% or the time is less than 30 seconds, neutralization will be incomplete, and residual acid will continue to corrode the bolt surface. If the concentration is above 0.5% or the time is more than 30 seconds, the alkalinity will be too strong or the reaction time too long, corroding the bolt surface, damaging its condition, and affecting subsequent processing.

[0048] In step S6 of this embodiment, after the bolt semi-finished product has undergone the tempering process, it needs to be blackened: the finished bolt is immersed in a blackening solution with a sodium hydroxide concentration of 850 g / L and a sodium nitrite concentration of 55 g / L, at a temperature of 137°C for 23 minutes.

[0049] After blackening, a black oxide film forms on the surface of the bolt. This oxide film has a certain degree of corrosion resistance, protecting the bolt from oxidation and rusting under normal conditions. It is worth noting that if the sodium hydroxide concentration is below 850 g / L or the sodium nitrite concentration is below 55 g / L, the oxide film is prone to insufficient formation and poor corrosion resistance; while excessively high concentrations may lead to a loose film. At temperatures below 137℃ or for less than 23 minutes, the oxide film thickness is insufficient; at excessively high temperatures or for excessively long times, the film is prone to peeling off, failing to provide effective protection and potentially affecting the bolt dimensions.

[0050] Example 3:

[0051] In this embodiment, the difference between Embodiment 3 and Embodiment 1 is that the temperature of each tempering zone is controlled at 490°C.

[0052] The bolts treated using the above method were tested and found to have a tensile strength of 1311 MPa, a yield strength of 1208 MPa, a reduction in strength after fracture of 56%, an elongation after fracture of 9%, and an impact energy of 37 J at -20℃. Compared to Example 1, the elongation after fracture of the bolts decreased. Compared to the national standard 12.9 grade high-strength bolts, the bolts exhibit improved durability, plasticity, and service life.

[0053] Example 4:

[0054] In this embodiment, the difference between Embodiment 3 and Embodiment 1 is that the temperature of each tempering zone is controlled at 450°C.

[0055] The bolts treated by the above method were tested and found to have a tensile strength of 1300 MPa, a yield strength of 1215 MPa, a reduction in size after fracture of 50%, an elongation after fracture of 7%, and an impact energy of 30 J at -20°C. Compared with Example 1, the bolts showed a decrease in elongation after fracture and impact energy at -20°C.

[0056] In the accompanying drawings of this embodiment, the same or similar reference numerals correspond to the same or similar components. In the description of this application, it should be understood that if terms such as "upper," "lower," "left," and "right" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, they are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0057] The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A manufacturing process for a grade 12.9 low-temperature impact resistant high-strength bolt, characterized in that: Includes the following steps: S1. Cut the raw material into blank workpieces of predetermined length; S2. The blank is processed into a bolt semi-finished product using the mold of a cold heading machine, and the threads are rolled on it; S3. After the thread rolling process in step S2, the semi-finished bolts are placed in the quenching zone for segmented quenching. The quenching zone is equipped with multiple heating zones with different temperatures, and the temperature range is controlled within the range of 810-870℃. The semi-finished bolts are first placed in the heating zone with the highest temperature for heating. As time goes on, the semi-finished bolts are then transferred to the heating zones with lower temperatures in sequence. The entire heating process lasts for 120 minutes. S4. Place the semi-finished bolts heated in step S3 into a quenching solution for cooling. The temperature of the quenching solution is controlled within the range of 65-85℃. S5. Tempering process is carried out on the bolt semi-finished product after cooling in step S4. S6. Perform surface treatment on the semi-finished bolts after the tempering treatment in step S5 to obtain the finished bolts.

2. The manufacturing process of a 12.9 grade low-temperature impact resistant high-strength bolt according to claim 1, characterized in that: In step S3, the temperatures of the heating zones are 870°C, 860°C, 850°C, 840°C and 810°C in sequence. First, the semi-finished bolts are placed in the heating zone at 870°C and heated for 60 minutes. Then, they are taken out and placed in the heating zones at 860°C, 850°C, 840°C and 810°C in sequence, and each heating zone is heated for 15 minutes.

3. The manufacturing process of a 12.9 grade low-temperature impact resistant high-strength bolt according to claim 2, characterized in that: The carbon potential of the activated carbon inside each of the heating zones is 0.4%.

4. The manufacturing process of a 12.9 grade low-temperature impact resistant high-strength bolt according to claim 3, characterized in that: The tempering process adopts a multi-stage tempering method, with a total of 7 tempering zones. The temperature of each tempering zone is controlled at 480℃, and the tempering time of each tempering zone is 20 minutes.

5. The manufacturing process of a grade 12.9 low-temperature impact resistant high-strength bolt according to claim 4, characterized in that: In step S2, after the cold heading machine mold processes the blank into bolt semi-finished products, it needs to be dephosphorized: the cold-headed bolt semi-finished products are placed in a 10-12% hydrochloric acid solution at a temperature of 50-60°C for 8-10 minutes.

6. The manufacturing process of a grade 12.9 low-temperature impact resistant high-strength bolt according to claim 5, characterized in that: In step S6, after the bolt semi-finished product has undergone tempering, it needs to be blackened: the finished bolt is immersed in blackening solution with a sodium hydroxide concentration of 850 g / L and a sodium nitrite concentration of 55 g / L, at a temperature of 137°C for 23 minutes.

7. The manufacturing process of a 12.9 grade low-temperature impact resistant high-strength bolt according to claim 6, characterized in that: After pickling, the semi-finished bolts are placed in a 0.5% sodium carbonate solution for neutralization for 30 seconds.

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