Reasonable selection of hardening heat treatment method for low speed and heavy load gears

It is very important to choose the hardening heat treatment method of low speed and heavy load gear reasonably. The general surface hardening methods, including flame surface hardening, high frequency and medium frequency induction hardening, are not suitable for improving the bearing capacity of large heavy duty gears

(1) The hardness of the transition zone and the core of the hardened layer is too low to meet the requirements, especially when the transition zone is heated, there is a low hardness tempering zone;

(2) The hardening layer is shallow and can not meet the requirements,

(3) The residual tensile stress in the transition zone of hardened layer is large.

We have carried out the medium frequency surface hardening test on the herringbone gear shaft of 650 Rolling Mill. Although the medium frequency surface hardening layer along the tooth groove is 4.5-5mm after quenching and tempering, and the hardness of the center is hb260, the application effect is not ideal. Under the 70% stress of the design load,

After 2 years, the tooth surface spalled.

The disadvantage of surface hardening method makes the depth of hardened layer must be twice as deep as that of carburized layer, so that the shear stress in transition zone can be less than 0.55, which obviously makes it more difficult to use surface hardening for large heavy duty gears. Therefore, the surface hardening process can not be used for large low speed and heavy load gears. Of course, this does not include low load or small gears.

Although the residual tensile stress in the transition zone can be reduced by nitriding hardening method, it is not suitable for large heavy duty gear hardening because the hardening layer is too shallow and the hardness of the center can not be guaranteed. With the increase of curvature radius of gear, the nitriding time should be increased correspondingly, that is, deeper nitriding depth can be obtained. Therefore, large module gear is not suitable for nitriding treatment. However, the nitrided gear has small deformation, low total production cost and certain anti contact fatigue performance. At present, the methods to improve the core hardness of nitrided materials and deepen the depth of nitrided layer are being studied at home and abroad, but they are rarely used in production.

Deep carburizing and quenching, as the hardening treatment of large low speed and heavy load hardened gears, is the most ideal method. It can obtain high core hardness, small residual tensile stress in transition zone, and enough depth of hardened layer. According to the development of foreign countries, carburizing and quenching have been widely used. Philadelphia gear company of America and large rolling mill gears of Japan have been carburized and quenched for more than ten years, and good results have been achieved.

Under the current domestic production conditions, according to the different service conditions and stress values of large heavy-duty gears, soft tooth face gear, medium hardness gear or hard tooth face gear can be selected in production. However, it should be noted that the economic benefit of using hardened gears is very high by giving full play to the role of heat treatment. With the improvement of production technology, large heavy-duty gear adopts deep carburizing hard tooth surface, medium and small gear or large gear with lower load adopts surface quenching and nitriding treatment to replace soft tooth surface gear, which will achieve obvious economic benefits in prolonging service life, reducing weight and volume of reducer.