Improvement of heat treatment process for heavy duty gear

Heavy duty gear has the advantages of large load, large impact force, high safety requirement, excellent wear resistance, high contact fatigue strength and bending fatigue strength, as well as high impact resistance and overload resistance. Heavy duty gears are usually made of low carbon alloy structural steel such as 20CrMnMo, which needs carburizing and quenching heat treatment to meet the performance requirements.

The simplest carburizing heat treatment process of gear is to cool down to quenching temperature after carburizing, and then directly quench after heat preservation. It is easy to use this method to make the material grain coarser, brittleness larger, workpiece structure stress larger, only carrying small module gear with smaller strength. At present, the most commonly used process of 20crmomn steel parts in production is that after carburizing, the steel parts are cooled to 550 ℃ by furnace air cooling, and then put into furnace again for heating and quenching. As the furnace is cooled to a certain temperature after carburizing, the lower the furnace temperature is, the more favorable it is for reducing oxidation and decarburization on the surface of the workpiece, and the lower the furnace temperature is, the slower the cooling speed of the workpiece will be; on the other hand, the surface quality of the workpiece after quenching can be ensured only when the workpiece enters the furnace for quenching and heating through a period of establishing furnace gas carbon potential. Therefore, the current heat treatment process takes a long time.

In view of the problems of high energy consumption and long production cycle in the current gear heat treatment process, Shanghai heat treatment plant has developed a new carburizing quenching heat treatment process through technological innovation. The process combines carburizing, isothermal and quenching, not only simplifies the process, shortens the process time, reduces the production energy consumption, but also effectively controls the technical indicators of carburizing heat treatment of heavy duty gears.

The technical points of the new process are as follows:

(1) Carburizing stage. The carbon potential, time and other process parameters of each stage of carburizing and diffusion were optimized, and the surface carbon concentration, carburizing depth and carburizing layer carbon concentration gradient and other quality indexes were achieved at a faster carburizing speed. The carburizing temperature is 900 ℃.

(2) Cold stage of carburizing furnace. With the slow decrease of furnace temperature, a small amount of fine mesh cementite is gradually precipitated on the carburized surface. When it is cooled to below 620 ℃, it will stay at the same temperature. At this stage, austenite will change to pearlite, and some spheroidization will take place on the carbides on the carburized surface, so as to prepare for the subsequent quenching. The spheroidizing effect of carbides in isothermal stage mainly depends on the surface carbon content. If the surface carbon concentration is too high, it will form coarse mesh or bulk carbides, so the spheroidizing effect is poor. Therefore, the surface carbon concentration must be controlled within 0.85 ~ 1.00, which is one of the control points of this carburizing composite heat treatment technology.

(3) Quenching heating stage. The key technology of this stage is to divide the quenching and heating process into two stages: the first stage has a higher heating temperature of 840 ~ 860 ℃, which is conducive to the transformation of ferrite in the core of the workpiece. At this time, the pearlite transforms into austenite, and some carbides in the carburized layer melt into austenite, which ensures the high hardness and strength of martensite after quenching, while retaining a proper amount of undissolved carbides. In the second stage, the lower heating temperature is 810 ~ 830 ℃ to reduce the quenching stress and to obtain high hardness.

(4) Tempering stage. Through low temperature tempering at 200 ~ 240 ℃, the quenched martensite is transformed into tempered martensite, and the residual austenite on the surface is decomposed into martensite. In order to make the retained austenite fully transformed and to eliminate the heat treatment stress, twice tempering is adopted.

After years of practice, it has been proved that the new carburizing composite heat treatment process has obvious energy saving and consumption reduction effect, which can shorten the original carburizing heat treatment process cycle by about 20, reduce the energy consumption by at least 10, reduce the consumption of carburizing agent and effectively reduce the production cost of heat treatment; moreover, the process has good repeatability and high quality stability.

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