High-speed gear transmission is prone to fatigue pitting, so materials with higher hardness and thicker hard layer should be selected. In gear transmission with impact load, gear teeth are easy to break, so materials with good toughness should be selected; In low-speed and heavy-load gear transmission, gear teeth are easy to break and tooth surfaces are easy to wear, so materials with high mechanical strength and hardness of tooth surfaces should be selected.
45 steel has good comprehensive mechanical properties after heat treatment. Normalization or tempering can improve the machinability of metallographic structure and materials, reduce the surface roughness after machining and reduce the deformation during quenching. Because of the poor hardenability of 45 steel, the material becomes brittle and deformed greatly after overall quenching, so the surface quenching of tooth surface is generally adopted, and the hardness can reach HRC52-58. Gear suitable for machine tool industry, with accuracy below grade 7.
40Cr is a medium carbon alloy steel. Compared with 45 steel, adding a small amount of chromium alloy can refine metal grains, improve strength, improve hardenability and reduce deformation during quenching.
Carburizing quenching is a method to obtain high tooth surface hardness, sufficient toughness and high bending fatigue strength at the center of gears. Generally, low-carbon alloy steel 18CrMnTi is selected, which has good cutting performance, small workpiece deformation during carburizing, quenching hardness reaching HRC56-62, and less retained austenite, and is mostly used for carrying large and impact gears in automobiles and tractors.
After nitriding, 38CrMoAlA nitrided steel has higher wear resistance and corrosion resistance than carburized and quenched gears, and has less deformation, so it can be used as a gear material that needs wear resistance in high-speed transmission.
Cast iron is easy to cast into complex shapes, easy to cut and low in cost, but its bending strength, impact resistance and wear resistance are poor. Therefore, it is often used for gears with small stress, no impact and low rotation speed.
The nonferrous metals used as gear materials include brass HPB59- 1 bronze QNP10-1and aluminum alloy LC4.
Non-metallic materials such as bakelite, nylon and plastic are also commonly used to make gears. These materials have the advantages of easy processing, low transmission noise, wear resistance and good vibration reduction, and are used in places with light load, vibration reduction, low noise and poor lubrication conditions.
1. Heat treatment of gear blank
The most common heat treatment of steel gear blank is normalizing or quenching and tempering. Normalization is arranged after casting or forging and before cutting. In this way, the residual internal stress in steel casting or forging can be eliminated, and the microstructure inhomogeneity after casting or forging can be refined or even through recrystallization, thus improving the cutting performance and surface roughness and reducing the tendency of deformation and cracking during quenching. Quenching and tempering also plays a role in refining the grain and making the structure uniform, but it can make the gear blank more tough, but its cutting performance is poor.
For bar gear blanks, normalizing or tempering is generally arranged after rough turning, which can eliminate the internal stress formed by rough turning.
2. Heat treatment of gear teeth
Common heat treatments of gear teeth include high frequency quenching, carburizing and nitriding.
High-frequency quenching can form a surface layer with slightly higher hardness than ordinary quenching, and maintain the strength and toughness of the center.
Carburizing can make the surface of gear have higher hardness and wear resistance after quenching, and the core still maintains a certain strength and higher toughness.
Nitriding is to heat the theory in ammonia to 520-560 degrees, so that active nitrogen atoms penetrate into the surface layer of gear teeth, forming a thin nitrided layer with high hardness.
In gear production, the quality of heat treatment has a great influence on gear machining accuracy and surface roughness. Often due to the unstable quality of heat treatment, the deformation of gear positioning base surface and tooth surface is too large or the surface roughness is too large, which becomes the key problem in gear production.
3. Manufacture of gear blank
The main forms of gear blanks are bars, forgings and castings. Bar is used for gears with small size, simple structure and low strength requirements. Forgings are mostly used in gears, which require high strength, impact resistance and wear resistance. When the gear diameter is more than 400-600 mm, the gear blank is usually cast by casting method. In order to reduce the machining amount, gear teeth can be directly cast for large-size and low-precision gears; New technologies such as die casting, precision casting, powder metallurgy, thermal bonding and cold extrusion can be used to manufacture gear blanks with pulley teeth, which can improve labor productivity and save raw materials.
Reasonable selection of gear materials
A meet the mechanical properties of materials.
The mechanical properties of materials include strength, hardness, plasticity and toughness, which reflect the characteristics of materials in use. When gears are engaged, there is contact stress at the tooth surface contact and maximum bending stress at the tooth root, which may lead to the failure of tooth surface or tooth body strength. There is relative sliding at all points on the tooth surface, which will cause wear. The main failure forms of gears are pitting corrosion, tooth surface gluing, tooth surface plastic deformation and gear tooth breakage. Therefore, it is required that the gear material has high bending fatigue strength and contact fatigue strength, the tooth surface has enough hardness and wear resistance, and the core has certain strength and toughness.
For example, when determining the hardness of large and small gears, we should pay attention to make the tooth surface hardness of small gears 30-50HBS higher than that of large gears, because the loading times of small gears are more than that of large gears, and the tooth roots of small gears are thinner and the strength is lower than that of large gears. In order to make the teeth of two gears close to equal strength, the tooth surface of the pinion is harder than that of the big gear.
On the other hand, after determining the material grade according to the performance of the material. It is necessary to define the mechanical properties or hardness of the material, and then we can achieve the required hardness range through different heat treatment processes, thus giving the material different mechanical properties. For example, the gear made of 40Cr alloy steel can be quenched in oil at 840-860℃ and tempered at 540-620℃, and the hardness of quenching and tempering can reach 28-32HRC, which can improve the structure and improve the comprehensive mechanical properties. After oil quenching at 860-880℃ and tempering at 240-280℃, the hardness can reach 46-5 1HRC, so the steel has good surface wear resistance, good toughness and small deformation. Nitriding at 500-560℃, the nitrided layer is 0. 1.5-0.6 mm, and the hardness can reach 52-54HRC, so the steel has high surface hardness, high wear resistance, high fatigue strength, high corrosion resistance and adhesion resistance, and minimal deformation. After electroplating or surface alloying treatment, the friction performance and corrosion resistance of gear working surface can be improved.
B. meet the technological properties of materials.
The technological performance of materials refers to the ability of materials to adapt to various processing requirements. Gear manufacturing has to go through several processes, such as forging, cutting and heat treatment, so we should pay attention to the technological properties of materials when selecting materials. Generally speaking, the forging and cutting of carbon steel have good technological properties, and its mechanical properties can meet the requirements of general working conditions. But the strength is not high enough and the hardenability is poor. Alloy steel has good hardenability and high strength, but poor forging and cutting properties. We can improve the technological properties of materials by changing the technological procedures and heat treatment methods.
For example, the gears in the automobile gearbox are made of 20CrMnTi steel, which has high mechanical properties. After carburizing, quenching and tempering at low temperature, the surface hardness is 58-62HRC and the core hardness is 30-45HRC. The technological performance of 20CrMnTi is good, and its cutting performance is improved by normalizing after forging. In addition, 20 CrMnTi also has good hardenability. Due to the influence of titanium, it is insensitive to overheating and can be quenched directly after carburization. The carburizing speed is fast, the transition layer is uniform and the deformation after carburizing and quenching is small. It is suitable for manufacturing important parts bearing high-speed and moderate load, impact and friction, so it is more suitable to choose 20CrMnTi steel according to the working conditions of gears.
C. Economic requirements of materials
The so-called economy, refers to the minimum cost to achieve maximum economic benefits. On the premise of satisfying the performance, we should also pay attention to reducing the total cost of parts when selecting gear materials. We can consider the following aspects:
Considering the price of the material itself. The prices of carbon steel and cast iron are relatively low, so choosing carbon steel and cast iron on the premise of meeting the mechanical properties of parts not only has better processability, but also can reduce the cost. From the perspective of metal resources and supply, the import of materials and the use of expensive materials should be reduced as much as possible.
Consider the cost of gear production process. First of all, different heat treatment methods have different relative processing costs. For example, the cost of carburizing surface quenching of 12CrNi3A steel is much less than that of nitriding treatment, and carbonitriding has the characteristics of short production cycle and low cost. Secondly, the cost can be reduced by improving the heat treatment process. For example, when a gear works under the conditions of high speed, medium load and moderate impact, the high-grade carburized steel of medium alloy 18cr2Ni4WA is initially selected. After carburizing at 9 10-940℃, quenching at 850℃ and tempering at 180-200℃, the tensile strength of mechanical properties is ≥ 16544. Although it can meet the service performance and process performance of gears, the price of parts is higher. At present, low carbon medium alloy and medium hardenability carburizing steel 20CrMnTi with relatively low price is selected. After carburizing at 9 10-940℃, quenching at 870℃ and tempering at 180-200℃, the tensile strength, yield strength, elongation and area shrinkage of mechanical properties are ≥ 1 100Mpa. Only in this way can the cost of materials be greatly reduced, and the usability and process performance can be satisfied. Third, choose as few steel grades as possible and concentrate on procurement and management. With the development of gear shape, size and material to multi-variety, multi-series and individualization, especially in the case of multi-model and small output, there are many disadvantages in gear forging, machining and heat treatment, such as large design, long production cycle, low efficiency, high cost, high energy consumption, difficult management and difficult quality assurance. Therefore, the selection, optimization and compression of material grades and specifications in gear material selection will help to improve the generalization, serialization and standardization of material selection, improve the utilization rate of materials and improve the planning of material procurement, thus reducing inventory backlog, speeding up capital turnover, facilitating storage and preservation, and reducing the cost consumption of materials. Finally, we can improve the economic benefits by improving the process. For example, the die forging process of die forgings breaks through the requirements of traditional processes. When providing shaped blanks, we can use less cutting technology, combine die forging with mechanical finishing, partially or completely replace cutting processing to directly produce parts, or adopt group technology and technology in production, which can also improve product quality, production efficiency and reduce costs.