Improvement of heat treatment process for heavy-duty gears
2025/7/28 15:23:47 28
Heavy-duty gears have a large load capacity, are subject to significant impact forces, and have high safety requirements. When in use, they are required to have good wear resistance, high contact fatigue strength and bending fatigue strength, as well as high impact resistance and overload resistance. Heavy-duty gears are commonly made of low-carbon alloy structural steels such as 20CrMnMo and need to undergo carburizing and quenching heat treatment to meet their performance requirements.
The simplest carburizing heat treatment process for gears is to cool down to the quenching temperature after carburizing, and then quench directly after holding at that temperature. This method is prone to causing coarse grains in the material, high brittleness, and large stress in the workpiece structure, which can only support small-module gears with relatively low strength. At present, the most commonly used process for 20CrMoMn steel parts in production is to first cool them in the furnace to 550℃ after carburizing and then take them out for air cooling, followed by re-heating and quenching in the furnace. Since the workpiece needs to be cooled to a certain temperature in the furnace after carburizing before it can be taken out, the lower the temperature taken out, the more beneficial it is for reducing oxidation and decarburization on the surface of the workpiece. However, the lower the furnace temperature, the slower the cooling rate of the workpiece will be. On the other hand, since the workpiece needs to go through a period of time to establish the carbon potential of the furnace gas when it is heated for quenching in the furnace, the surface quality of the workpiece after quenching can be ensured. Therefore, this current heat treatment process takes a relatively long time.
In response to the current problems of high energy consumption and long production cycle in gear heat treatment processes, Shanghai Heat Treatment Plant has developed a new carburizing and quenching heat treatment process through technological research and development. This process combines carburizing, isothermal treatment and quenching, which not only simplifies the procedures, shortens the process time and reduces production energy consumption, but also effectively controls various technical indicators of carburizing heat treatment for heavy-duty gears.
The technical key points of the new process are as follows:
(1) Carburizing stage. Optimize the process parameters such as carbon potential and time in each stage of strong infiltration and diffusion during carburizing to achieve quality indicators such as surface carbon concentration, carburizing depth, and carbon concentration gradient of the carburized layer at a relatively fast carburizing speed. The carburizing temperature is 900℃.
(2) The cooling stage of the carburizing furnace. As the furnace temperature gradually decreases, a small amount of fine-grained cementite gradually precipitates from the carburized surface layer. When cooled to below 620℃, it is kept at an isothermal residence. At this stage, the transformation from austenite to pearlite occurs, and the carbides on the carburized surface will undergo partial spheroidization, preparing the microstructure for subsequent quenching. The spheroidization effect of carbides in the isothermal stage mainly depends on the surface carbon content. If the surface carbon concentration is too high, coarse networks or large blocky carbides will form, resulting in poor spheroidization effect. Therefore, the surface carbon concentration must be controlled between 0.85 and 1.00, which is one of the key control points of this carburizing composite heat treatment technology.
(3) Quenching heating stage. The technical key at this stage is to divide the quenching heating process into two sections: The heating temperature in the first stage is 840 to 860℃, which is relatively high and conducive to the transformation of ferrite in the core of the workpiece. At this point, pearlite transforms into austenite, and some carbides in the infiltration layer dissolve into the austenite, ensuring the high hardness and strength of the martensite after quenching, while retaining an appropriate amount of undissolved carbides. The lower heating temperature of 810 to 830℃ in the second stage is to reduce quenching stress and is also conducive to obtaining high hardness on the surface.
(4) Tempering stage. Through low-temperature tempering at 200 to 240℃, quenched martensite is transformed into tempered martensite, and at the same time, the residual austenite on the surface is decomposed into martensite. In order to ensure the complete transformation of residual austenite and facilitate the elimination of heat treatment stress, two tempering processes are adopted.
Years of practice have proved that the above-mentioned new carburizing composite heat treatment process has a relatively obvious energy-saving and consumption-reducing effect. It can shorten the original carburizing heat treatment process cycle by about 20 years, reduce energy consumption by at least 10 years, and also reduce the consumption of carburizing agents, effectively lowering the production cost of heat treatment. Moreover, the process repeatability is good and the quality stability is relatively high.
Xinguang Heat Treatment Industry (Kunshan) Co., LTD., one of the metal heat treatment processing manufacturers, reminds you that for heat treatment processing, please choose Xinguang Heat Treatment. Xinguang is equipped with heat treatment-related equipment and QC (Quality Control) equipment from Japan, Germany, Switzerland and other countries. Xinguang has a production process that ensures processing efficiency and a complete inspection procedure. Xinguang is also certified as a qualified heat treatment plant by major global automotive Oems and top 100 first-tier automotive parts suppliers such as TRW(TRW), BOSCH(Bosch), AUTOLIV(Autoliv), and MAGNA.