Heat treatment process of heavy duty gear improvement

Heavy duty gear has high bearing capacity, high impact force, high safety requirements, good wear resistance, high contact fatigue strength and bending fatigue strength, and high impact resistance and overload resistance. Heavy duty gear is commonly used in the manufacture of low carbon alloy structural steel such as 20CrMnMo, which needs to meet its performance requirements through carburizing and quenching heat treatment.

Simple carburizing heat treatment process of gear zui is in the carburizing after cooling to quenching temperature, after the heat preservation of direct quenching. This method is easy to make the material grain coarse, brittleness larger, the workpiece organization stress is large, can only bear the small modulus gear with less strength. At present, the most commonly used process of 20CrMoMn steel parts in production is after carburizing, the furnace is cooled to 550℃ air cooling, then re-furnace heating and quenching.

Aiming at the problems of high energy consumption and long production cycle in the current gear heat treatment process, Shanghai thermal treatment plant has developed a new carburizing and quenching heat treatment process through technological innovation. The process combines carburizing, isothermal and quenching, which not only simplifies the process, shortens the process time, reduces the production energy consumption, but also effectively controls the technical indexes of carburizing heat treatment of 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 each stage of carburizing and diffusion were optimized, and the quality indexes such as surface carbon concentration, carburizing depth, and carbon concentration gradient of carburizing layer were achieved at a faster carburizing speed. The carburizing temperature is 900℃.

(2) cold stage of carburizing furnace. With the slow decrease of the furnace temperature, a small amount of fine mesh cementite is gradually precipitated out of the cementite surface, and when it is cooled to less than 620℃, the cementite stays at an isothermal state. At this stage, the austenite is transformed into pearlite, and the carbides on the cementite surface will undergo partial spheroidal action to prepare for the subsequent quenching. The spheroidization effect of carbides in isothermal stage mainly depends on the surface carbon content. If the surface carbon concentration is too high, a thick net or large block carbides will be formed, and the spheroidization effect is poor. Therefore, the surface carbon concentration must be controlled at 0.85~1.00, which is one of the control points of this carburized composite heat treatment technology.

(3) quenching heating stage. The key technology of this stage is to divide the quenching heating process into two stages: the heating temperature of the first stage is higher from 840 to 860℃, which is conducive to the transformation of ferrite at the center of the workpiece. At this time, the pearlite is transformed into austenite, and part of the carbide in the permeated layer dissolves into austenite, which ensures the high hardness and strength of martensite after quenching, while retaining an appropriate amount of undissolved carbide. In the second stage, the lower heating temperature of 810~830℃ is to reduce the quenching stress and to facilitate the surface to obtain a high hardness.

(4) tempering stage. By tempering at 200~240℃, the hardened martensite was transformed into tempered martensite, and the surface residual austenite was decomposed into martensite. In order to fully transform the residual austenite and eliminate the heat treatment stress, two tempering was used.

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 energy consumption by at least 10, reduce the consumption of carburizing agent, and effectively reduce the production cost of heat treatment. And the process repeatability is better, the quality stability is higher.

Induction heat treatment technology has the advantages of fast, accurate, environmental protection, energy saving, controllable and repeatable. The induction heat treatment equipment adopts mature induction heating technology, high quality stainless steel and closed-loop temperature control system.