Brief introduction of distortion control technology in gear heat treatment

The distortion of gear heat treatment, especially the distortion of carburizing and quenching, makes the gear accuracy greatly reduced. In the gear industry, it is not surprising that the accuracy is reduced by 2-3 grades. The distortion of gear heat treatment not only affects the accuracy, but also affects the strength of gear. For non grinding gears, excessive distortion will cause eccentric load; for grinding gears, eccentric wear due to distortion will cause tooth surface burn, grinding steps and early pitting and fatigue tooth fracture.

The distortion of gear heat treatment, especially the distortion of carburizing and quenching, is a worldwide problem. The reason why the distortion is difficult to control is that there are too many influencing factors. If the transformation of quenching structure is understood as a state function, the distortion has distinct process function characteristics, that is, starting from the raw materials, through a series of processing procedures, Until the gear products are manufactured, every operation will have external and internal factors that affect the distortion. The well-known fish skeleton diagram of heat treatment distortion factors can illustrate this point.

The analysis of the influencing factors of heat treatment distortion also has the following characteristics: (1) from the perspective of technology, it involves many subjects; (2) from the perspective of production, it involves many processes; (3) from the perspective of management, it involves many departments; (4) from the Perspective of measures, it involves economy. Therefore, distortion control of heat treatment is a complex project. No wonder a large-scale distortion research project organized by Germany in 2001 is called “distortion Engineering”.

Because there are too many and complex factors that affect the distortion, we are often at a loss in the face of the “fish skeleton map”. In view of this, whether we can use the Oriental way of thinking to study and find operable methods and ways of macro-control distortion, therefore, In this paper, the concept of “micro analysis macro control” and the principles of “mass balance phase change approach” and “heat transfer uniformity temperature difference reduction” are used to discuss the problem of gear distortion.

It should be a fact that there are 26 factors and 77 sub factors listed in foreign literature that affect the distortion of heat treatment. We can imagine how difficult it is to deal with all the influencing factors one by one from the micro perspective.

For this kind of gear distortion problem, which has many influencing factors and is difficult to be determined in detail, on the one hand, macro control method can be used to make various micro distortion potential unable to play, so as to reduce distortion; on the other hand, through balanced process measures, the heat dissipation of gear wheel in heating and cooling is relatively uniform, The temperature distribution on the surface of the gear and its parts tends to be balanced, and the change of the step and volume of the phase transformation tends to be closer, thus reducing the distortion.

Through the discussion and analysis of this paper, the following measures to control the distortion of gear heat treatment are put forward:

1) The hardenability steel is fully used, and different hardenability bandwidth is adopted according to the gear type;

2) The preparation heat treatment of gear blank should adopt isothermal normalizing or other preparation heat treatment methods which can ensure the uniformity of structure and hardness;

3) The effect of gear heating on distortion should not be ignored. The reasonable clamping, preheating and step heating methods are adopted, and the strip temperature can be heated up in the carburizing furnace after stress relief annealing if possible. These measures are to minimize the temperature difference of each part of the gear during heating;

4) Gear quenching is an important part of distortion. Suitable clamps shall be used for proper clamping and placement; various balance compensation pads, washers and mandrel shall be used according to the structural shape of the gear; the cooling medium shall be stirred reasonably, hot oil shall be used as far as possible, and nitrates isothermal quenching and high-pressure air quenching shall be used if possible. These measures are to make all parts of the gear cool evenly as much as possible;

5) For the bevel gear and thin-walled gear sleeve produced in batch, press forced quenching shall be used as far as possible.

The above measures are not new, but they are operable and effective in gear production.