Vacuum Heat Treatment For Gear Manufacturing

Gears are one of the most widely used important parts in the machinery industry. They are mainly used to transmit power and change the speed and direction of motion. The selection of gear materials should be comprehensively considered from the aspects of gear working conditions, failure modes and performance requirements for materials.Working Conditions of Automotive Gears

Automotive gears are mainly installed in transmissions and differentials. In the transmission, the speed ratio of the engine, crankshaft and main shaft gear is changed through the gear; in the differential, the torque is increased through the gear, and the speed of the left and right wheels is adjusted. The power of the engine is transmitted to the main shaft through the gears to drive the car to run. Therefore, automotive gears are subjected to large forces and frequent impacts, and the requirements for wear resistance, fatigue strength, core strength, and impact toughness are higher than those of general machine tool gears. When the gear is working, the stress is as follows: due to the transmission of torque, the root of the tooth bears a large alternating bending stress; when shifting, starting or uneven bite, the gear teeth bear a certain impact load; The poor contact of the tooth surface caused by shaft deformation, as well as the intrusion of hard particles such as foreign dust and metal chips will become additional loads and deteriorate the working conditions. Therefore, the working conditions and stress conditions of gears are more complicated.

Performance requirements for automotive gears

According to the analysis of working conditions and failure modes, the following performance requirements can be put forward for gear materials.

① High bending fatigue strength, especially at the tooth root must have sufficient strength.

② High tooth surface hardness and wear resistance.

③ High core strength and sufficient impact toughness.

④ Heat treatment deformation is small.

Material Selection for Vacuum Heat Treatment of Automobile Gears

The most widely used automotive gear materials are alloy carburizing steels 20Cr and 20CrMnTi, which are used after vacuum heat treatment such as vacuum carburizing, vacuum quenching and vacuum tempering. After vacuum carburizing, the surface carbon content is greatly increased to ensure high hardness, high wear resistance and high contact fatigue strength after vacuum quenching. Since the alloying elements improve the hardenability, the core can obtain higher strength and sufficient impact toughness after vacuum quenching and vacuum tempering. In order to further improve the service life of gears, after vacuum carburizing, vacuum quenching and vacuum tempering, shot blasting can also be used to increase the surface compressive stress, which is beneficial to improve the fatigue strength of the tooth surface and tooth root, and remove scale.

The production process of alloy carburized steel gears is generally: blanking→forging→normalizing→cutting→vacuum carburizing, vacuum quenching and vacuum tempering→shot peening→grinding.

Does vacuum heating have quenching and carburization?

When analyzing the carburization phenomenon of vacuum heat treatment workpieces, there are two misunderstandings: first, it is considered that the workpiece is carburized in quenching oil; second, it is believed that the graphite parts in the heating chamber cause carburization. In fact, in many cases, it is not these two reasons, but the cleanliness of the heating chamber is not high. A large amount of quenching oil is brought into the hot chamber when the workpiece enters and exits the furnace, the material basket is polluted, and the feeding trolley enters and exits. It remains on the cold wall of the hot chamber. , Form a volatile reducing atmosphere when heated, and increase the carburization of the workpiece.

In addition to directly entering the oil at a temperature above 1050°C. When the workpiece is heated below 1050°C and quenched with oil, a little pre-cooling into the oil will not cause obvious carburization. 

The carburization of workpieces such as graphite parts in the heating chamber cannot be ruled out, but it is not as serious as the atmosphere of residual quenching.

The carburization phenomenon of vacuum heating and quenching is more serious because of the pollution of the furnace by quenching oil, not the cause of quenching in oil or graphite parts as people say.

Vacuum heat treatment (quenching) small deformation?

There are two concepts in heat treatment deformation: tissue deformation and shape structure deformation. The result of the research is that when the vacuum heat treatment obtains the same structure and hardness compared with other furnace heat treatments, the deformation is the smallest. That is: tissue deformation is minimal.

For shape and structure deformation, vacuum heat treatment is often not as small as heat treatment deformation of other furnace types. For heat treatment of other furnace types, such as quenching, it is easy to use methods such as classification, isothermal, and alignment outside the furnace to control the amount of deformation. Vacuum quenching is due to these functions. Imperfect, sometimes it will increase.

The confusion of these two concepts gives people the impression that the deformation of vacuum heat treatment is small, which is a wrong or incomplete understanding.