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Vacuum heat treatment technology of heavy duty gear

Gear is widely used in manufacturing industry. It should have excellent wear resistance, contact fatigue and bending fatigue strength. There are many kinds of gear. For large gear, it can be divided into soft gear, hard gear and hard gear. In the 1950s, the large gear with low speed and heavy load was mostly soft gear with normalizing or tempering treatment. With the progress of cutting technology, the early 1960s began to use hard tooth face gear. According to the strength calculation, the allowable contact stress and bending stress of alloy structural steel are 850 MPa and 300 MPa after tempering to 300 HBW, while the allowable contact stress and bending stress of carburizing and quenching hardness up to 60 HRC are 1 600 MPa and 500 MPa respectively. Therefore, after the 1960s, the third generation of heavy-duty gear, namely carburized or carburitriding, hardening gear has been widely used. Generally speaking, hard gear has the advantages of good bearing performance and long service life. Aircraft gear, automobile gear, tractor gear and engineering machinery gear all belong to hard gear. Carburizing and quenching have become the heat treatment process of gear.

The flexural fatigue resistance, contact fatigue resistance and meshing accuracy of gears are the three main factors that determine the service life of gears. In order to make the bending fatigue strength and contact fatigue strength of gears meet the requirements, it is necessary to make them hard enough to ensure the contact fatigue strength. High contact fatigue strength, surface hardness and wear resistance can make the tooth surface difficult to damage. The residual compressive stress on gear surface is beneficial to increase the bending fatigue strength. The root must be strong enough to avoid fatigue failure due to bending stress generated during gear operation. In order to improve the service life of gear and prevent fatigue damage, the strength distribution of gear hardening layer should be higher than the stress distribution. To this end, measures such as increasing hardening layer depth and improving matrix strength of gears can be taken. In order to make the gear with impact resistance, vibration resistance, low noise, high strength, good wear resistance and good resistance to contact fatigue and bending fatigue, it is required that the gear surface hardness is high, the heart strength and toughness is good, the hardening layer distribution is reasonable, improve the gear load and impact fracture resistance performance. Gear should be easy to cut to obtain low roughness and high machining accuracy. In addition, the gear is required to have high raw material quality.

Gears are made from a variety of materials, such as cast iron, steel and powder metallurgy materials. Carburized or nitriding steel is generally selected for heavy duty gear used in automobiles, tractors and construction machinery. 2OCrMo, 20CrMnTi, 20CrMnMo steel and so on are commonly used carburizing steel. Commonly used nitriding steel 38CrMoAlA,35CrMo,42CrMo steel, etc., after nitriding has excellent wear resistance and corrosion resistance. 20Cr2Ni4A and 18Cr2Ni4WA steel are generally used for gear with more demanding service conditions such as aircraft, tanks and diesel locomotive. Using medium carbon steel gear and high or medium frequency induction quenching can reduce the cost, suitable for working contact stress and bending stress is not too big gear.

For bulldozer gear, it is found that the main failure form is broken tooth, so improving the bending fatigue performance of gear is the key to improve the life of gear.

(1) Gear fatigue fracture morphology can be divided into three parts: crack source, crack steady-state propagation zone and transient fracture zone. The steady-state crack propagation zone has obvious fatigue striations. The fatigue crack is mainly caused by microholes and microcracks in the subsurface layer, and machining defects on the gear surface are also one of the crack sources.

(2) The fatigue life of carburized and quenched gear is generally higher than that of the middle carbon structural steel gear with ion nitriding, which is related to the shallower ion nitriding.

(3) Among the carburized steel gears tested, the fatigue life of 22CrNi2MoNb steel gear is the best, followed by 22CrMoH steel and 20CrMnTi steel gear,2OCrMnMo steel gear is the worst. This is related to the fine grain of 22CrNi2MoNb steel gear, high strength and toughness, and the reason why 20CrMnTi steel gear is better than 20CrMnMo steel gear is related to the finer grain.

(4) For the gear with low working stress, it is a feasible scheme to select the low cost medium carbon S48C steel for medium frequency induction quenching.