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Aviation gear vacuum furnace application

In order to meet the reliability of aviation products, the requirements for aviation spiral bevel gears are high precision, high strength, good wear resistance and light weight, and the complex structure and various forms of gears will inevitably make the processing of spiral bevel gears difficult. Spiral bevel gears have a long processing cycle and high processing difficulty. After vacuum heat treatment, vacuum carburizing, vacuum quenching, vacuum tempering and other processes, the bevel gears will be greatly deformed. Helicopter spiral bevel gears have strict requirements on the depth of the carburized layer after vacuum heat treatment of the parts, and also have requirements on the final grinding allowance after heat treatment. When the deformation exceeds the grinding allowance, the parts will be out of tolerance and scrapped, and the scrap rate remains high . Through the research on the deformation compensation technology of typical spiral bevel gear after vacuum heat treatment, the influence of vacuum heat treatment deformation on the product is solved.

Aviation gear vacuum furnace application

The spiral bevel gear is the main device for power transmission in the main reduction system of the aviation helicopter. In order to meet the service life and structural requirements of aviation products, aviation spiral bevel gears are required to meet the requirements of high precision, high strength and light weight. However, the complex structure and various forms of the spiral bevel gear will inevitably increase the difficulty of processing the spiral bevel gear. The helicopter rotor drive system needs to rotate at high speed during flight, and its blades with flexible structure will bear severe swing and torsional high-frequency coupling vibration loads in the asymmetric airflow field of rotation and slip. Functional requirements such as forward flight, backward flight, left and right side flight, hovering, turning, and vertical take-off and landing are required, and the load conditions it bears are complex and changeable. The input bevel gear has a complex structure and is a thin-walled part, and the thinnest part of the auxiliary plate is only 4mm. And there are 5 positions in the whole part that require carburizing, including bevel teeth, splines, inner shaft diameter and outer shaft diameter. The requirements for the depth of the carburized layer are also inconsistent, and secondary carburizing is required, which will lead to greater deformation in vacuum heat treatment .

Duplex bevel gear is currently the most difficult part to control deformation in vacuum heat treatment production. This part is a thin-walled part with double tooth structure. The outer diameter of the part is Φ490mm, and the wall thickness is 8.13mm. Different, but also need to carry out secondary carburizing, vacuum heat treatment deformation is larger. The metallurgical quality inspection of the input bevel gear and double bevel gear after vacuum heat treatment is very strict, and there are corresponding regulations on the carburized layer and HRC60 depth respectively, making the processing very difficult. While ensuring the metallurgical quality, it is also necessary to ensure the accuracy of the parts themselves, and the parts are gears that are difficult to process with grade 4 precision.

The bevel gear is the key part of the final reducer, and the design structure of the part is shown in the figure.

Gear Vacuum Heat Treatment

Aiming at the carburizing and quenching vacuum heat treatment process of bevel gears, combined with the structure of parts, material composition, structure and performance characteristics, the changes of temperature field, phase transition and stress field in the process of vacuum carburizing and quenching under the influence of multiple factors were investigated. Study the internal stress change and stress relaxation process in the parts under the cooling conditions of pressure quenching, analyze the distortion characteristics of the final parts, and explore the deformation, hardness and stress gradient distribution control methods of the parts in the actual carburizing and quenching process. Carburizing temperature is an important factor affecting the deformation of the workpiece. Higher temperature can obtain a faster diffusion rate of carbon atoms, but there are disadvantages of large deformation at high temperature. An important factor for deformation is the influence of gravity. Sometimes in order to provide a favorable deformation state for vacuum quenching, the furnace loading method of the workpiece can be comprehensively considered to suppress or induce deformation. At the same time, in the case of a large amount of parts loaded into the furnace, the heating rate can be appropriately reduced, and at the same time, a slower cooling rate can be used during the cooling process, thereby reducing the deformation of the parts.

Gear vacuum quenching

In order to avoid the carburizing deformation caused by the gravity of the part itself, the vacuum quenching and tempering vacuum heat treatment process after carburizing and high temperature tempering is adopted. In the carburizing process, the part can appropriately increase the supporting wall thickness at the auxiliary plate to reduce the deformation caused by the part’s own gravity. The machining process is added between vacuum quenching and vacuum carburizing to remove the allowance that affects hardenability. The process of vacuum quenching determines the vacuum heat treatment state of the parts, and the maximum stress is also generated during this process, which involves thermal stress and structural stress, which affects the change of the volume and shape of the workpiece. At the same time, the quenching process can also realize pressure quenching and change the deformation of parts, so the vacuum quenching process is also particularly important. The factors affecting vacuum quenching are very complicated, mainly including vacuum quenching method, vacuum quenching temperature, quenching medium and stirring.

Gear vacuum carburizing

Vacuum carburizing heat treatment can obtain higher strength on the surface of the product, while retaining excellent mechanical toughness in the core, but carburizing vacuum heat treatment also has its disadvantages due to its high heating temperature. During the vacuum heat treatment process, it is easy to make parts produce complicated out of shape. For products with simple structure, the deformation of vacuum heat treatment is small, but for parts with complex structure, more complex deformation will occur after vacuum carburizing heat treatment. A large amount of deformation will lead to uneven grinding allowances when the parts are ground. Excessive grinding allowances will weaken the surface strength of the gear and increase the failure of the tooth surface, except for the beneficial surface optimum hardness of HRC60. risks of. Duplex bevel gears are key components of transmission systems. Its structure is complex, with two rows of carburized spiral bevel teeth, and the depth requirements of the carburized layer of the two rows of gears are inconsistent, the outer contour and inner hole of the parts are large, and the thickness of the web plate is thin, which increases the difficulty of vacuum carburizing and vacuum quenching deformation control. Therefore, it is necessary to carry out a large number of process tests to study the secondary carburizing process of the part, explore the deformation rules of vacuum carburizing and vacuum quenching, and finally determine the process parameters of cold and hot processing.

Through the research and research of vacuum heat treatment deformation compensation technology, some achievements have been made, but due to the irregular deformation and the influence of various factors, the problem of large deformation of individual parts still occurs. Track the deformation law of vacuum heat treatment and optimize the cold and hot process route. On the basis of controlling the change of parts, the accumulative change of parts and the change of ovality are controlled, and the problem of difficult deformation control of large spiral cone vacuum heat treatment is completely solved.