Taking the processing of high-precision shaft parts of 40CrNi2Si2MoVA steel as an example, the processing characteristics of the product are ensured through reasonable arrangements of heat treatment, cold working, grinding and fault detection. Normalizing and high-temperature tempering are first used to improve the machining performance of the material, and then carbide tools are used to perform roughing and semi-finishing according to the recommended processing parameters to reduce the machining performance of the material. tool wear. After the semi-finished product is completed, high temperature quenching is used to increase the strength and rigidity of the material. At the same time, grinding is selected to ensure product precision and surface quality. Additional inspection by acid etching and magnetic particle detection. surface defects of the material are included. After grinding, stress relief quenching is performed to remove residual stresses caused by grinding. Throughout the process, the materials and mechanical processing are highly consistent to ensure the characteristics of high strength, high hardness, high precision and high surface quality of the product.
1 Preface
With the development of national defense technology, a large number of difficult-to-process materials have appeared, among which ultra-high strength low alloy steel 40CrNi2Si2MoVA (300M) has gradually been pushed to the forefront.[1，2]domestic and foreign researchers have invested in research on this topic.[3-5]. Due to the low alloy content of the material, heat dissipation is poor during cutting, and it is easy to accumulate high temperatures and large residual stress. High temperature causes the metallographic structure of the workpiece surface to change and the internal grains of the material to mutate, which cannot guarantee the mechanical properties of the material. At the same time, high temperature chip nodules accumulate and stick to the cut. edge of the tool, worsening tool wear and causing irreversible damage to the tool life; machining Large residual stresses in the machine can have a serious impact on the quality of the machined surface, leading to surface cracks, dimensional instability and other problems. .[6-9]。
This paper conducts a comprehensive study on the processing technology of 40CrNi2Si2MoVA steel high-precision shaft parts in aerospace. Considering current problems such as high cutting force, severe tool wear and low dimensional accuracy of parts, appropriate processes are selected in real time accordingly. to the processing characteristics of the material. , rationally organize cold working, heat treatment, defect detection and grinding to avoid difficult processing due to high material strength. At the same time, cleaning and oiling during processing can effectively protect the surface quality of parts. To further ensure part surface integrity, processed parts are inspected for internal defects. The overall standardized operation can not only effectively reduce tool wear during processing, but also ensure the requirements of high precision and high strength use of parts in aerospace.
2 Material properties
The chemical composition of ultra high strength low alloy steel 40CrNi2Si2MoVA is shown in Table 1. The contents of Cr, Mn, Ni and Si are relatively large, which plays an important role in improving the processing properties of the material. The element content is proportional to the hardenability of steel, and the element Si can improve the yield strength and tensile strength of steel. The Mn element can improve the hot processing performance of steel, although the Ni element can improve the overall strength and hardness of steel. of the material, it is inversely proportional to the thermal conductivity. In summary, it can be found that the high strength and high hardness of the material make the processing performance poor, and the material is easy to harden during processing.
Table 1 Chemical composition of 40CrNi2Si2MoVA steel (mass fraction) (%)
The mechanical properties of ultra-high strength low alloy steel 40CrNi2Si2MoVA are shown in Table 2. The ultra-high tensile strength and yield strength ensure the high strength and high hardness characteristics of the material. Processing this material requires significant mechanical cutting force. Due to the low thermal conductivity of the material, the high cutting force and low thermal conductivity make the workpiece surface hardened, which not only increases the cutting difficulty, but also reduces the cutting time. quality of the machined surface. At the same time, the accumulated heat accelerates the wear of tool edges, these unfavorable factors make this material a typical representative of difficult-to-machine materials in the aerospace industry.
Table 2 Mechanical properties of 40CrNi2Si2MoVA steel
3 process paths
When processing high precision shaft parts of 40CrNi2Si2MoVA steel, in order to avoid large tool wear and low processing surface precision caused by difficult materials, cold processing, heat treatment, detection defects and grinding methods will be reasonably arranged during processing to effectively improve the processing efficiency of parts and dimensional accuracy. The process flow is shown in Figure 1 and the parts are shown in Figure 2.
Figure 1 Process route
Figure 2 pieces
Firstly, the blank is cut according to the outer dimensions of the high-precision shaft parts made of 40CrNi2Si2MoVA steel. Due to the difficulty of cutting the material itself, the blank is preheated by normalizing + high temperature tempering to refine the surface grains of the material. and improve the cutting ability of materials and the stabilizing role of the structural dimensions of the blank. The preheated blank is cold treated and carbide tools are used to turn and mill the part. A nozzle-type cooling device is attached to completely cool the workpiece during processing to avoid heat accumulation and burning the surface layer of the workpiece. After the part is cold worked, roughing and semi-finishing, it should be isothermal quenched according to the mechanical properties of the part to obtain the characteristics of high strength and high hardness of the material to ensure performance stable. heat treated material, the part quench transfer time should be as fast as possible unless otherwise noted. In addition to regulations, the quenching time should be controlled within 15 seconds to effectively ensure the stability and controllability of the internal structure of the part. after heat treatment and quenching.
Once the part is quenched, the strength and hardness of the material are significantly improved. According to the local surface quality and dimensional accuracy of the workpiece, the grinding method is selected for processing. The cutting fluid must completely cool the workpiece during grinding. avoid dry grinding and burning the workpiece. After grinding and finishing is completed, the parts are stress relieved and quenched within 4 hours to eliminate residual stresses inside the parts. Due to the high hardness and high strength of the material, grinding can cause microscopic damage to the interior of parts. Magnetic particle inspection is required on parts, and the wet continuous method in magnetic particle inspection is used to detect defects on the surface. or near the surface of the material. When performing magnetic particle inspection, consider whether to perform acid etching inspection based on the grinding tolerance of the material. When the material grinding tolerance does not exceed 0.0127m, magnetic particle inspection will be carried out directly after grinding the workpiece. Otherwise, an acid etch inspection will be carried out. Then perform a magnetic particle test.
4 process parameters
4.1 Heat treatment
Heat treatment can effectively improve the cutting performance of materials, making materials that are initially difficult to process easier to process. It has a protective effect on the cutting tools used for processing, significantly improving the processing efficiency. internal residual stress during material processing and prevents stress release, resulting in dimensional deformation, the accuracy of parts cannot be guaranteed.
In this paper, the blank is treated with normalizing + high temperature quenching. The quenching temperature should not exceed 650°C and be maintained for a certain period of time. It is then cooled in air or cooled to room temperature in the atmosphere.
After the parts are finished and ground, stress relief and tempering should be carried out within 6 hours (190±10)℃ for ≥4 hours. Strictly control the time interval, static charge should be used for correction, and knocking on the hardened parts is strictly prohibited.
4.2 Cold working
For cold mechanical processing of standardized + hardened parts at high temperatures, the machine tool must have sufficient rigidity, as well as sufficient power and cooling device. The installation rigidity of the tool should be as great as possible, and there should be no vibration during use. . During the machining process, it is necessary to ensure that there are no scratches on the surface of the workpiece. It is recommended to use carbide tools with a tip radius of 0.4 to 1.6 mm, lead angle of 45° to 100°. and a cutting speed of 24.4 to 40 m/min during rough turning. Cutting speed is 0.1 ~ 0.5mm/r, cutting depth is 0.38 ~ 3.0mm; during finishing, the cutting speed is 24.4 ~ 40m/min, the feed is 0.1 ~ 0.15mm/r, and the cutting depth is 0.2 ~ 0.38mm. When milling grooves and bosses simultaneously, it is recommended to use carbide tools with positive cutting angle, the cutting direction is down milling, the cutting speed is 9.1 ~ 15, 2m/min, the feed rate for finish milling is 0.05~0.1mm/z, and the cutting depth is up to 5mm.
During the whole processing process, dehydrated anti-rust oil should be applied promptly after the end of each process to avoid surface corrosion caused by cutting fluid adhering to the workpiece surface.
4.3 Grinding
When grinding, the grinding wheel must have sufficient rigidity. The gaps between the abrasive grains on the surface of the grinding wheel make the grinding wheel sharp is not allowed, thereby reducing surface damage. During grinding, the parts must be completely cooled by the cutting fluid. The recommended grinding wheel particle size is 46#~80#, the grinding wheel speed is 15~35 m/s, the workpiece speed is 9~3131 m/min, and the cross feed is 3 ~ 6mm/r.
5Conclusion
This article takes 40CrNi2Si2MoVA high-precision shaft parts as an example to conduct process research, considering the existing problems in processing, such as high cutting force, large tool wear and low dimensional accuracy. , through reasonable provisions for heat treatment, cold working and grinding. and defect detection, the product is guaranteed to have processing characteristics. Analyze and summarize the processing characteristics and appropriate processing parameters of different processes, and the overall process meets the processing requirements of aviation parts. The following conclusions are drawn in terms of the arrangement of each process.
1) Three heat treatments on ultra high strength low alloy steel 40CrNi2Si2MoVA can effectively avoid the difficult-to-process characteristics of the material, not only guarantee the dimensional accuracy of processing, but also reduce tool loss and improve processing efficiency. treatment.
2) Through the reasonable arrangement of heat treatment, cold working, grinding, flaw detection and acid etching inspection, the whole process flow is highly consistent with mechanical processing , guaranteeing the high strength, high hardness, high precision and high quality of 40CrNi2Si2MoVA steel. shaft parts. Surface quality characteristics.
Expert commentary
The ultra high strength low alloy steel 40CrNi2Si2MoVA in this example exhibits poor cutting heat dissipation and is prone to accumulation of high temperatures and large residual stresses, which accelerates tool wear and causes cracking superficial on the parts. Taking high-precision shaft parts as an example to conduct process research, considering the existing problems in processing such as high cutting force, large tool wear and low dimensional accuracy, methods appropriate processing methods are selected according to the processing characteristics of the material, and the cold processing, heat treatment, defect detection and grinding process, through the overall process improvement, reduce tool wear and improve machining precision.
The highlight of the article is the arrangement of process routes and the generally standardized operation of shaft parts made of difficult-to-machine materials. Throughout the process, a high degree of compatibility between materials, heat treatment and mechanical treatment is achieved, ensuring the precision and surface quality of the product.
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