Tue, 31 Jul 2018 01:42:50 +0000

Heavy duty gear heat treatment process

Heavy duty gear has large bearing capacity, large impact force and high safety requirement. When used, it is required to have excellent wear resistance, high contact fatigue strength and bending fatigue strength, as well as high impact resistance and overload resistance. Heavy duty gear is usually made of low carbon alloy structural steel such as 20CrMnMo.
The simplest carburizing heat treatment process for gears is to cool down to quench temperature after carburizing and then quench directly after heat preservation. The method is easy to make the material grain thick, brittle, the workpiece structure stress, can only bear the small modulus gear with small strength. 20 crmomn steel parts of the most commonly used in the production process is first after carburizing furnace cooling to 550 ℃ from air cooling, then charging heating quenching. The lower the furnace temperature is, the more favorable the workpiece surface is to reduce oxidation and decarbonization. The lower the furnace temperature is, the slower the workpiece temperature will be. On the other hand, the surface quality of the workpiece after quenching can be ensured only after a period of time to establish the gas carbon potential of the furnace. Therefore, the present heat treatment process takes a long time.
In order to solve the problems of high energy consumption and long production cycle of gear heat treatment technology, Shanghai heat treatment plant has developed a new carburizing and quenching heat treatment technology through technological innovation. Combined with carburizing, isothermal and quenching, the process not only simplifies the process, shortens the process time, reduces the production energy consumption, but also effectively controls various technical indicators of carburizing heat treatment for heavy duty gear.
The technical points of the new process are as follows:
(1) carburizing stage. The process parameters such as the carbon potential and time of strong permeability and diffusion in each stage of carburizing were optimized, and the surface carbon concentration, carburizing depth and carburizing concentration gradient and other quality indicators were reached at a fast carburizing speed. Carburizing temperature is 900 ℃.
(2) carburizing furnace cooling stage. As the temperature down slowly, carburized surface precipitation a small amount of fine network cementite, cold to below 620 ℃, the isothermal stopover, at this stage the transformation from austenite to perlite, carburizing carbide spheroidizing effect will occur on the surface, for subsequent quenching is ready for organization. The spheroidization effect of carbide at the isothermal stage mainly depends on the surface carbon content. If the surface carbon concentration is high, coarse net or large block carbide will be formed.
(3) quenching and heating stage. And the key technologies of this stage is to quench heating process is divided into two sections: the first phase of heating temperature of 840 ~ 860 ℃ is higher, is conducive to the center of workpiece ferrite transformation. At this time, the pearlite changes into austenite, and some carbides in the percolation layer dissolve into austenite, which ensures high hardness and strength of martensite after quenching, and at the same time, some undissolved carbides are retained. The second stage low heating temperature is 810 ~ 830 ℃ in order to reduce the quenching stress, at the same time is helpful to obtain high surface hardness.
(4) tempering stage. Through 200 ~ 240 ℃ low temperature tempering, quenching martensite into tempered martensite, and the surface residual austenite into martensite. In order to transform the retained austenite sufficiently and to eliminate heat treatment stress, two tempering is adopted.
After years of practice, it has been proved that the new process of carburized composite heat treatment has obvious energy-saving and consumption reduction effects, which can shorten the process cycle of primary carburized heat treatment by about 20, reduce energy consumption by at least 10, reduce the consumption of carburized agents and effectively reduce the production cost of heat treatment. The process has good repeatability and high quality stability.