According to the technical requirements of the above parts, the following process route is hereby formulated for 20CrMnTi gear: blanking, forging, normalizing, rough machining, high temperature tempering, carburizing, quenching and low temperature tempering, shot peening, gear grinding.
Forging is a kind of processing method which uses forging machine to exert pressure on metal blank to produce plastic deformation to obtain forgings with certain mechanical properties, certain shape and size. It is one of the two parts of forging (forging and stamping). Through forging, the defects of loose as cast metal produced in smelting process can be eliminated, and the microstructure can be optimized. At the same time, the mechanical properties of forgings are generally better than those of castings of the same material because of the intact metal streamline.
According to the alloy phase diagram, 20CrMnTi, a low carbon alloy steel, should be heated to 950 ℃ ~ 970 ℃ and kept warm for about 3 ~ 4 hours, so that the components of the part can be transformed into uniform austenite structure, and then more sorbite and a small amount of ferrite structure can be obtained by cooling in air. In this way, the suitable hardness (hb179-217) is obtained, and the residual internal stress is eliminated in the process of heat preservation and air cooling, which not only meets the technical requirements of ① and ②, but also facilitates the next process of cutting. The reason why the normalizing heat treatment method is selected in this process is that the carbon content of 20CrMnTi alloy steel is too low, there are a lot of soft ferrites in annealed steel, the ductility of steel is very good, the cutting edge is easy to adhere to form a tool lump during cutting, and the chips are torn and broken, so that the surface roughness is poor, and the tool life is also affected, which is not conducive to cutting. Then carry out rough machining and flaw detection after rough machining. In the technical requirements, forgings shall not have any forging defects, so the subsequent process shall be subject to flaw detection to avoid defects in the next process and waste of processing costs.
3. High temperature tempering
High temperature tempering is to heat the parts to 500 ~ 650 ℃ after quenching, generally to a certain temperature of PS K line (critical point AC1), and cool them at a proper speed after holding for a period of time. The mixture of ferrite and fine cementite can be obtained by high temperature tempering, that is, the tempered sorbite structure. With excellent comprehensive mechanical properties, it is mainly used for tempering of structural parts after quenching, such as connecting rod, bolt, gear and shaft. Quenching + high temperature tempering is called tempering.
After carburizing at 920 ℃ and precooling to 860 ℃, there will be a lot of retained austenite in the carburized layer of 20CrMnTi steel. There are two ways to reduce the amount of retained austenite: one is cold treatment after quenching, the other is high temperature tempering before quenching to decompose retained austenite. High temperature tempering process is commonly used in production. Therefore, 570 ℃ high temperature tempering process is adopted for 3-3.5h.
After the rough machining of bevel gear, carburizing treatment shall be carried out. Carburizing is to put the workpiece into the single-phase austenite area with active carburizing medium, which is heated to 900-950 ℃. After holding for enough time, the active carbon atoms decomposed from the carburizing medium penetrate into the surface of the steel piece, so as to obtain high carbon on the surface and keep the original composition in the center. Similarly, low-temperature nitriding treatment. This is a common heat treatment process for metal materials. It can make the surface of carburized workpiece obtain high hardness and improve its wear resistance. As shown in Fig. 2, 920 ℃ cementation of well furnace is adopted, and the cementation time is 1.0-1.4mm according to the required thickness of cemented layer, and the time is 6 hours. The structure of parts after carburizing is as shown in Figure 3. The surface layer is hypereutectoid structure mixed with pearlite and secondary cementite, wherein the secondary cementite is in network shape, the center part is hypoeutectoid original structure mixed with pearlite and ferrite, and the middle part is transition area, the closer the surface layer is, the less ferrite. After carburizing, it is ready for quenching + low temperature tempering.
5. Quenching and low temperature tempering
The quenching temperature of steel can generally be selected according to the Fe-C phase diagram. According to the distribution of carbon content in the surface layer of carburized gear and practical experience, oil cooling from 920 ℃ to 850 ℃ can achieve good results. If the heating temperature is too high or the holding time is too long, the grain size of austenite will be too large, which will cause overheating or grain boundary oxidation and partial melting. The grain coarseness of austenite during overheating not only reduces the gear, but also causes the deformation and cracking of the gear. The burnt workpiece can only be scrapped. Low heating temperature and insufficient holding time will cause insufficient hardness. In addition, the quenching cooling rate is too fast, the transformation from austenite to martensite is violent, and the volume shrinkage causes great internal stress, which is easy to cause the deformation and cracking of gears. As 20CrMnTi is alloy steel with good hardenability, oil cooling is selected to reduce the cooling speed and prevent gear deformation or cracking caused by quenching. At the same time, high hardness of martensite can be obtained.
After quenching, the structure of steel is martensite and a small amount of retained austenite, which are unstable structures and tend to change to stable structures. At the same time, internal stress is produced during quenching. In order to reduce or eliminate the quenching internal stress, stabilize the structure and size, and obtain the required mechanical properties, 20CrMnTi gear was tempered at 200 ℃ for 4 hours at low temperature. During low temperature tempering, supersaturated carbon atoms in martensite gradually precipitate martensite in the form of carbide, and the internal stress decreases with the decrease of lattice distortion. Tempered martensite is composed of martensite and carbide. Although the decomposition of martensite reduces the degree of supersaturation of carbon in α – Fe and the hardness of steel correspondingly, the precipitated carbides strengthen the matrix, and some of the retained austenite decomposes into tempered martensite, so the gear steel still maintains high hardness, wear resistance and certain toughness.
After quenching and low temperature annealing treatment, the final structure of the part is: the surface is fine sheet tempered martensite and a small amount of retained austenite and carbide, the hardness is about HRC58-62, and the core is composed of tempered low carbon martensite, ferrite and fine pearlite, the hardness is hrc32-45, and it has high strength, enough high toughness and plasticity, which conforms to the technology of the part Requirement.
6. Shot peening
Shot peening is one of the effective methods to reduce the fatigue and improve the service life of parts. Shot peening is to spray the high-speed shot flow to the spring surface and make the spring surface plastic
A certain thickness of strengthening layer is formed, and a higher residual stress is formed in the strengthening layer,
Due to the existence of compressive stress on the spring surface, when the spring bears the load, it can counteract part of the anti stress, so as to improve the fatigue strength of the spring. Shot peening can also be used to clean tooth surfaces.
7. Grinding teeth
The process of grinding a gear tooth with a gear grinder is called gear grinding. It can be divided into internal and external gear grinding of cylindrical gear, internal and external gear grinding of cylindrical helical gear, and bevel gear grinding. The grinding wheel is used as a tool to grind the machined gear surface, so as to improve the gear precision and surface finish.