Due to its high hardness and small deformation, the nitriding process is increasingly applied to the surface treatment of hardened gears. For example, more than 40% of the hardened gears of Philadelphia Gear Company use nitriding process, Germany, France, Japan, the United Kingdom and the former Soviet Union have also used a large number of nitriding processes on important gears, such as rolling mills, diesel engines, hoists and automobile gears, with a diameter of up to 4 m.
China is one of the countries where the research and application of ion nitriding technology is very early. The gear nitriding process has also been successfully applied in China. The industrial fields of application include steel, chemical, machine tools, automobiles and energy, especially at some high speeds. , heavy duty, precision gear applications. In recent years, due to its small size, small mass, compact structure and high transmission efficiency, planetary transmissions are being used more and more, including internal ring gears, and yaw teeth in wind power of new clean energy industry. Nitriding heat treatment is required for the ring and the like. Other processing methods are difficult to meet the requirements of the inner ring gear for deformation. The nitriding process is generally preferred for surface hardening of worms. In particular, the deep nitriding process of the gear can replace the carburizing and quenching process in a certain range, thereby eliminating the process of grinding the teeth, saving manufacturing costs and construction period. With the development of the economy, the industrial application of the gear nitriding process has great potential for development.
The early nitriding process is limited in practical applications due to shallow infiltration layer, low hardness of the core, and low bearing capacity of the tooth surface. Especially due to the influence of the concept of "shell effect", the application of nitriding in gears is relatively conservative. For example, the contact fatigue limit and bending fatigue limit of nitriding gears in product design are determined by a foreign institution. 70% and 75%. Therefore, how to further improve the bearing capacity of the nitriding gear has become the direction of the relevant engineering and technical personnel.
Through a large number of experimental studies, the key to determine the bearing capacity of the hardened gear is the ratio of the orthogonal shear stress to the microhardness of the transition zone of the hardened layer. The size depends on the load, the radius of curvature of the tooth surface, and the hardened layer. Depth, core hardness and residual stress in the transition zone. In order to prevent contact fatigue failure of the gear, the relevant conditions must be met from the tooth surface to the core. Therefore, increasing the effective hardened layer depth and improving the distribution of the hardness gradient of the transition zone can effectively increase the fatigue strength of the gear. The contact fatigue test in the literature shows that under the test conditions, the hardness of the core is increased from 240-260 HBW to 310-330 HBW, which can increase the contact fatigue strength by about 30%; the thickness of the layer increases from 0.5 mm to 0.8-1.0 mm, which can be improved. The contact fatigue strength is about 25%; the compound layer with the γ' phase as the main surface can increase the contact fatigue strength by nearly 40% compared with the ε+γ' dual phase layer. Therefore, the use of medium hard tempering + tough deep nitriding is an important way to improve the bearing capacity of nitriding gears.
In the past ten years, with the development of related technologies, ion nitriding equipment has been greatly developed, new technologies and components are constantly being adopted, and the furnace body tends to be low in energy consumption and multifunctional. The marketing application of pulsed power supply has also achieved great results, and the level of automation control has been continuously improved, so that the implementation of the gear nitriding process has a solid equipment guarantee. This paper discusses the technical points of the deep ion nitriding process, and introduces the characteristics and application effects of the device through an example of equipment development.
1. Selection and process of deep controlled ion nitriding
The deep nitriding process is adopted on the gears, and both domestic and foreign have been explored and studied. Examples of successful early use are: Philadelphia Gear Company of the United States, which conducts gas nitriding up to 1 mm in depth, with a process cycle of 150 h, and then removes the white bright layer on the surface; the Zhengzhou Machinery Research Institute of China has carried out research on the deep ion nitriding process. Only 60 ~ 70h can make the nitriding layer depth of 0.8 ~ 1.2mm, the surface obtained γ' single-phase structure, no need to wear off the white bright layer processing, and obtained the national patent. Domestic scholars have carried out in-depth and extensive research on the process, organization and mechanism of deep rapid nitriding, and carried out cyclic temperature-changing ion nitriding, thermal cycle ion nitriding, pressurized nitriding, low-pressure pulse nitriding, and rare earth osmosis. Process tests such as deformation-induced osmosis and high-temperature nitriding, together with research and test on the bearing capacity and mechanical properties of nitriding gears, promoted the application of deep nitriding process on gears.
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