Method for improving tensile property and stability of ML25 steel cold heading bolt
Patent Information
- Application Number
- CN202611136452.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-07-29
- Publication Date
- 2026-10-09
AI Technical Summary
[0003]针对现有方法的不足，本发明提供一种提高ML25钢冷镦螺栓拉伸性能及稳定性的方法，解决ML25钢螺栓滚丝后抗拉强度下降以及淬火回火后性能不稳定、温度高、生产周期长、螺纹通止不合格的技术难题
1、ML25低碳钢滚丝过程中受到挤压力作用，在螺纹牙底产生残余应力，牙底应力无法及时有效释放，导致螺栓做拉伸试验时，破坏拉力值不达标，抗拉强度不满足要求。本发明螺栓制造过程采用较大牙底圆弧尺寸的滚丝轮，缓慢滚压降低加工应力产生，滚丝加工后增加应力释放处理，彻底消除残余应力，实现ML25钢螺栓力学性能的精准调控，M12×1.5、M8规格螺栓抗拉强度分别可达668MPa、670MPa，远高于630MPa的航空标准，且性能稳定，批量生产合格率为100%。
Smart Images
Figure CN122875663A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of fastener processing technology, specifically relating to a method for improving the tensile properties and stability of ML25 steel cold heading bolts. Background Technology
[0002] ML25 steel, a high-quality carbon structural steel, is widely used in the production of standard bolts, screws, and other fasteners due to its excellent cold-working properties and moderate cost. Currently, aerospace bolts often use ML25 steel with a tensile strength of 630 MPa. After cold heading, it undergoes thread rolling, requiring the finished bolt to maintain the 630 MPa aerospace standard tensile strength. Unlike other materials, ML25 steel does not exhibit significant work hardening after cold working. Instead, the thread rolling process easily generates stress concentration at the thread root, leading to a decrease in tensile properties and making it unsuitable for large-scale industrial production. In actual production, the tensile strength of ML25 steel bolts decreases by approximately 20 MPa after thread rolling, failing to meet the 630 MPa aerospace standard requirement, resulting in substandard bolt breaking tensile strength and failure to meet usage requirements. To compensate for the loss of tensile properties, existing technologies mostly use a quenching and tempering heat treatment process to improve strength. However, this process has many drawbacks: First, the quenching temperature is 890-910℃ and the tempering temperature is 400-600℃. The quenching and tempering temperatures are high, and the total quenching and tempering time is at least 9 hours, resulting in a long production cycle. In addition, ML25 steel has poor hardenability, which causes large performance dispersion in the same batch of bolts, with strength fluctuating and difficult to consistently meet standards. If the strength does not meet the standards, repeated quenching and tempering are required, which not only prolongs the production cycle but also significantly increases energy consumption and production costs. Second, quenching can easily cause deformation of the thread profile, resulting in thread non-pass / non-passing issues, requiring thread calibration to ensure thread pass / non-passing. Summary of the Invention
[0003] To address the shortcomings of existing methods, this invention provides a method for improving the tensile properties and stability of ML25 steel cold heading bolts, solving the technical problems of decreased tensile strength after thread rolling of ML25 steel bolts, unstable performance after quenching and tempering, high temperature, long production cycle, and non-compliant thread opening and closing.
[0004] To achieve the above objectives, the present invention adopts the following technical solution: A method for improving the tensile properties and stability of ML25 steel cold heading bolts includes the following steps: The bolts made of cold-forged ML25 material are thread-rolled using a thread rolling wheel. The rolling parameters are adjusted to slowly roll the bolts. The threaded structure is formed at the compression point of the bolt by using the tooth tip structure of the thread rolling wheel to compress the bolt. Stress relief treatment is performed on the bolts after thread rolling.
[0005] Preferably, the bolts include M12×1.5 and M8.
[0006] Preferably, the rolling parameters include feed rate, spindle speed, and rolling force.
[0007] Preferably, the rolling parameters are adjusted as follows: M12×1.5 thread rolling wheel feed speed 2.0mm / s, spindle speed 20r / min, rolling pressure 10MPa; M8 thread rolling wheel feed speed 2.6mm / s, spindle speed 20r / min, rolling pressure 9MPa.
[0008] Preferably, the bottom arc dimension of the M12×1.5 thread rolling wheel is R0.23-0.25mm; the bottom arc dimension of the M8 thread rolling wheel is R0.19-0.22mm.
[0009] Preferably, the stress relief method is as follows: after the thread rolling, the bolts are laid flat or stacked in the material frame, and then the material frame is placed in a drying oven for heat treatment and air-cooled to room temperature.
[0010] Preferably, the maximum thickness of the bolts stacked in the material frame is 25mm, and the minimum interval between the upper and lower material frames is 50mm.
[0011] Preferably, the heat treatment temperature is 340-360℃, and the holding time is 90-120min.
[0012] The positive and beneficial effects of this invention are: 1. During the thread rolling process of ML25 low-carbon steel, the bolts are subjected to extrusion pressure, resulting in residual stress at the thread root. This stress cannot be effectively released in a timely manner, leading to substandard tensile strength and failure to meet requirements during tensile testing. This invention addresses this issue by using a thread rolling wheel with a larger thread root radius during bolt manufacturing. Slow rolling reduces processing stress, and stress release treatment is applied after thread rolling to completely eliminate residual stress. This allows for precise control of the mechanical properties of ML25 steel bolts. The tensile strengths of M12×1.5 and M8 bolts can reach 668MPa and 670MPa respectively, far exceeding the aerospace standard of 630MPa, with stable performance and a 100% pass rate in mass production.
[0013] 2. The M12×1.5 thread rolling wheel of this invention has a thread root arc size of R0.23-0.25mm, and the M8 thread rolling wheel has a thread root arc size of R0.19-0.22mm. This increases the thread root arc transition, changes the sharp structure of the traditional M-type thread root, effectively reduces the stress concentration coefficient in the thread rolling process, and avoids the problem of fracture caused by rapid stress superposition when the bolt is under force.
[0014] 3. The parameters of the M12×1.5 thread rolling wheel of this invention are: feed speed 2.0 mm / s, spindle speed 20 r / min, and rolling pressure 10 MPa; the parameters of the M8 thread rolling wheel of this invention are: feed speed 2.6 mm / s, spindle speed 20 r / min, and rolling pressure 9 MPa; this invention adopts a thread rolling mode with low speed, low rolling pressure, and low feed amount to further weaken the residual stress generated by the violent flow of metal.
[0015] 4. This invention uses a stress relief treatment method to replace the traditional quenching and tempering process. The heating temperature of the stress relief treatment method is 340-360℃, which is lower than the quenching temperature of 890-910℃ and the tempering temperature of 400-600℃. The stress relief treatment method takes 3 hours, which is shorter than the quenching and tempering process of 9 hours.
[0016] In addition, due to the poor hardenability of ML25 material, if the strength does not meet 630MPa, it is necessary to quench and temper again, resulting in a quenching and tempering time of more than 9 hours. However, the stress relief treatment method of the present invention does not require rework, has a 100% pass rate, greatly shortens the production cycle, and reduces production costs.
[0017] Meanwhile, the stress relief treatment method of this invention does not change the metallographic structure and mechanical properties of the bolt during the process, but only eliminates residual stress, avoiding the problem of unstable performance of traditional quenching process and the problem of thread deformation leading to thread failure. Moreover, it can process up to 100kg in a single process, which greatly improves the processing efficiency. Attached Figure Description
[0018] Figure 1 This is a schematic diagram of the finished bolt structure; Figure 2 Diagram showing the usage status of the thread rolling wheel; Figure 3 This is a schematic diagram of the M12×1.5 thread rolling wheel structure of the present invention; Figure 4 This is a schematic diagram of a standard M12×1.5 thread rolling wheel structure; Figure 5 This is a schematic diagram of the M8 thread rolling wheel structure of the present invention; Figure 6 This is a schematic diagram of a standard M8 thread rolling wheel. Detailed Implementation
[0019] The present invention will be further described below with reference to some specific embodiments.
[0020] Example 1
[0021] See Figure 1-3A method for improving the tensile properties and stability of ML25 steel cold-heading bolts, taking M12×1.5 bolts as an example, includes the following steps: S1. ML25 bolts that have been cold-forged are threaded using a thread rolling wheel. The thread tip structure of the thread rolling wheel is used to compress the bolt, forming a thread structure at the compression part of the bolt. The thread tip structure of the thread rolling wheel reduces the stress generated at the root of the bolt thread. The root radius of the M12×1.5 thread rolling wheel is R0.23-0.25mm. S2. The bolt is slowly rolled with rolling parameters of 2.0 mm / s feed speed, 20 r / min spindle speed and 10 MPa rolling pressure to allow sufficient stress release time for thread forming and reduce stress concentration. S3. Perform stress relief treatment on the threaded bolts to improve tensile strength. The specific steps are as follows: Lay or stack the threaded bolts in the material frame. The maximum thickness of the bolts stacked in the material frame is 25mm, and the minimum interval between the upper and lower material frames is 50mm. Then, put the material frame into an electric heating constant temperature forced air drying oven and heat it to 350℃ with the furnace. The holding time is 100min. After the holding time is completed, open the furnace door and air cool to room temperature. The whole process takes 3 hours. The cycle is short. 100kg can be processed in one batch. The efficiency is high. The temperature is low and will not affect the thread deformation. Use a ring gauge to check the thread go-ahead and the thread go-ahead is qualified. S4. After stress relief treatment, the bolt failure tensile force and tensile strength were tested using an electronic universal testing machine. The test results are shown in Table 1. The failure locations of the tensile test specimens were all on the threads, meeting the tensile performance requirements of aviation standards. The tensile strength fluctuated within 10 MPa, indicating high performance stability.
[0022] Comparative Example 1 A method for improving the manufacturing of ML25 steel cold-forging bolts, taking M12×1.5 bolts as an example, includes the following steps: S1. A standard M-type thread rolling wheel is used to perform thread rolling on cold-forged ML25 bolts. The standard M-type thread rolling wheel has sharp tooth tips. (See attached image.) Figure 4 When the thread is extruded, the stress cannot be released, forming concentrated stress and affecting tensile properties. The root radius of the M12×1.5 conventional thread rolling wheel is R0.19mm. S2. The bolt was rapidly rolled with rolling parameters of 3.0 mm / s feed speed, 26 r / min spindle speed and 13 MPa rolling pressure. This did not allow sufficient stress release time for thread forming, which accelerated stress concentration and affected performance. S3. The bolts after thread rolling are subjected to quenching and tempering. The quenching method is as follows: heat to 650℃ at a heating rate of no more than 25℃ / min, hold for 1.5h, then heat to 890℃ at a heating rate of no more than 25℃ / min, hold for 2h, and then oil cool at 50℃. The tempering method is as follows: heat to 450℃ in the furnace, hold for 3h, and then air cool after removal from the furnace. The entire quenching and tempering process takes 9 hours, and a single furnace can process 20kg. In addition, the quenching and tempering is uneven and the performance test fails, so the quenching and tempering is repeated, extending the cycle to 13h. Due to the high quenching and tempering temperature, thread deformation is easily caused. The thread go / no-go is checked using a ring gauge. If the thread go / no-go is not qualified, the thread needs to be calibrated. S4. After stress relief treatment, the bolt failure tensile force and tensile strength were tested using an electronic universal testing machine. The test results are shown in Table 1. The failure locations of the tensile test specimens were all on the threads, which met the tensile performance requirements of aviation standards. However, the failure tensile force had a large dispersion, and the tensile strength fluctuated by about 100 MPa.
[0023] Example 2
[0024] See Figure 1 , 2 5. A method for improving the tensile properties and stability of ML25 steel cold-heading bolts, taking M8 bolts as an example, includes the following steps: S1. ML25 bolts that have been cold-forged are threaded using a thread rolling wheel. The thread tip structure of the thread rolling wheel is used to compress the bolt, forming a thread structure at the compression part of the bolt. The thread tip structure of the thread rolling wheel reduces the stress generated at the root of the bolt thread. The root radius of the M8 thread rolling wheel is R0.19-0.22mm. S2. The bolt is slowly rolled with rolling parameters of 2.6 mm / s feed speed, 20 r / min spindle speed and 9 MPa rolling pressure to allow sufficient stress release time for thread forming and reduce stress concentration. S3. Perform stress relief treatment on the threaded bolts to improve tensile strength. The specific steps are as follows: lay the threaded bolts flat or stack them in the material frame. The maximum thickness of the bolts stacked in the material frame is 25mm, and the minimum interval between the upper and lower material frames is 50mm. Then put the material frame into the electric heating constant temperature forced air drying oven and heat it to 350℃ with the furnace. The holding time is 100min. After the holding time is completed, open the furnace door and air cool to room temperature. The whole process takes 3 hours. The cycle is short. 100kg can be processed in one batch. The efficiency is high. The temperature is low and will not affect the thread deformation. Use a ring gauge to check the thread go-ahead and the thread go-ahead is qualified. S4. After stress relief treatment, the bolt failure tensile force and tensile strength were tested using an electronic universal testing machine. The test results are shown in Table 1. The failure locations of the tensile test specimens were all on the threads, meeting the tensile performance requirements of aviation standards. The tensile strength fluctuated within 10 MPa, indicating high performance stability.
[0025] Comparative Example 2 A method for improving the manufacturing of ML25 steel cold heading bolts, taking M8 bolts as an example, includes the following steps: S1. A standard M-type thread rolling wheel is used to perform thread rolling on cold-forged ML25 bolts. The standard M-type thread rolling wheel has sharp tooth tips. (See attached image.) Figure 6 When the thread is extruded, the stress cannot be released, forming concentrated stress and affecting performance. The root radius of the M8 conventional thread rolling wheel is R0.16mm. S2. The bolt was rapidly rolled with rolling parameters of 3.2 mm / s feed speed, 25 r / min spindle speed and 10 MPa rolling pressure. This did not allow sufficient stress release time for thread forming, which accelerated stress concentration and affected performance. S3. The bolts after thread rolling are subjected to quenching and tempering. The quenching method is as follows: heat to 650℃ at a heating rate of no more than 25℃ / min, hold for 1.5h, then heat to 890℃ at a heating rate of no more than 25℃ / min, hold for 2h, and then oil cool at 50℃. The tempering method is as follows: heat to 450℃ in the furnace, hold for 3h, and then air cool after removal from the furnace. The entire quenching and tempering process takes 9 hours. A single furnace can process 20kg. In addition, the quenching and tempering is uneven and the performance test is unqualified. The quenching and tempering is repeated, extending the cycle to 15h. Due to the high quenching and tempering temperature, the thread is prone to deformation. The thread go / no-go is checked using a ring gauge. If the thread go / no-go is not qualified, the thread needs to be calibrated. S4. After stress relief treatment, the bolt failure tensile force and tensile strength were tested using an electronic universal testing machine. The test results are shown in Table 1. The failure locations of the tensile test specimens were all on the threads, which met the tensile performance requirements of aviation standards. However, the failure tensile force had a large dispersion, and the tensile strength fluctuated by about 100 MPa.
[0026] Table 1 Tensile property test results
[0027] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solutions of the present invention, as long as they do not depart from the spirit and scope of the technical solutions of the present invention, should be covered within the scope of the claims of the present invention.
Claims
1. A method for improving the tensile properties and stability of ML25 steel cold-heading bolts, characterized in that, Includes the following steps: The bolts made of cold-forged ML25 material are thread-rolled using a thread rolling wheel. The rolling parameters are adjusted to slowly roll the bolts. The threaded structure is formed at the compression point of the bolt by using the tooth tip structure of the thread rolling wheel to compress the bolt. Stress relief treatment is performed on the bolts after thread rolling.
2. The method for improving the tensile properties and stability of ML25 steel cold-heading bolts according to claim 1, characterized in that, The bolts include M12×1.5 and M8.
3. The method for improving the tensile properties and stability of ML25 steel cold-heading bolts according to claim 2, characterized in that, The rolling parameters include feed rate, spindle speed, and rolling force.
4. The method for improving the tensile properties and stability of ML25 steel cold-heading bolts according to claim 3, characterized in that, The rolling parameters are adjusted as follows: M12×1.5 thread rolling wheel feed speed 2.0mm / s, spindle speed 20r / min, rolling pressure 10MPa; M8 thread rolling wheel feed speed 2.6mm / s, spindle speed 20r / min, rolling pressure 9MPa.
5. The method for improving the tensile properties and stability of ML25 steel cold-heading bolts according to claim 2, characterized in that, The bottom arc dimension of the M12×1.5 thread rolling wheel is R0.23-0.25mm; the bottom arc dimension of the M8 thread rolling wheel is R0.19-0.22mm.
6. The method for improving the tensile properties and stability of ML25 steel cold-heading bolts according to claim 1, characterized in that, The stress relief method is as follows: after the thread rolling, the bolts are laid flat or stacked in the material frame, and then the material frame is placed in a drying oven for heat treatment and air-cooled to room temperature.
7. The method for improving the tensile properties and stability of ML25 steel cold-heading bolts according to claim 6, characterized in that, The maximum thickness of the bolts stacked in the material frame is 25mm, and the minimum interval between the upper and lower material frames is 50mm.
8. The method for improving the tensile properties and stability of ML25 steel cold-heading bolts according to claim 6, characterized in that, The heat treatment temperature is 340-360℃, and the holding time is 90-120min.