Introduction to heat treatment and energy saving process
1, reduce the heating temperature problem
Generally, the quenching heating temperature of the hypoeutectic carbon steel is 30 to 50 ° C above AC3, and the quenching heating temperature of the eutectoid and hypereutectoid carbon steel is 30 to 50 ° C above AC1. However, recent studies have confirmed that the hypoeutectic steel is heated and quenched in the α+γ two-phase region slightly lower than Ac3 (ie, sub-temperature quenching) to improve the toughness of the steel, reduce the brittle transition temperature, and eliminate temper brittleness. The heating temperature for quenching can be lowered by 40 °C.Low-temperature rapid short-time heating and quenching of high-carbon steel can reduce the austenite carbon content, which is beneficial to obtain a good toughness of lath martensite, which not only improves the toughness, but also shortens the heating time.
For some transmission gears, carbonitriding replaces carburizing, wear resistance is increased by 40% to 60%, fatigue strength is increased by 50% to 80%, and co-penetration time is equivalent, but co-infiltration temperature (850 °C) is more carburizing. The temperature (920 ° C) is 70 ° C lower, while also reducing the heat treatment deformation.
2, shorten the heating time
Production practice shows that the traditional heating time determined according to the effective thickness of the workpiece is conservative, so the heating coefficient α in the heating and holding time formula τ=α·K·D is corrected. According to the traditional processing parameters, when heated to 800-900 ° C in an air furnace, the α value is recommended to be 1.0-1.8 min / mm, which is obviously conservative. If the alpha value can be reduced, the heating time can be greatly shortened. The heating time should be determined experimentally according to the size of the steel workpiece, the amount of furnace loading, etc. After the optimized process parameters are determined, the significant economic benefits can be achieved.
3. Cancel tempering or reduce the number of tempering
Cancel the tempering of carburized steel, such as 20Cr steel loader with double-sided carburizing piston pin to cancel the tempering fatigue limit of 16% compared with tempering; cancel the tempering of low carbon martensitic steel, the bulldozer pin The set is simplified to 20 steel quenched state (low carbon martensite), the hardness is stable at about 45HRC, the product strength and wear resistance are significantly improved, the quality is stable; the high speed steel reduces the number of tempering, such as W18Cr4V steel machine saw blade used once Fire (560 ° C × 1h) instead of the traditional 560 ° C × 1h three times tempering, the service life increased by 40%.
4, use low and medium temperature tempering instead of high temperature tempering
Medium or low-carbon structural steels can be replaced by high-temperature tempering in medium and low temperature tempering to obtain higher multi-impact resistance. W6Mo5Cr4V2 steel Φ8mm drill bit, after quenching, 350 °C × 1h + 560 ° C × 1h secondary tempering, compared with 560 ° C × 1h three tempered bit cutting life increased by 40.
5. Reasonably reduce the depth of the seepage
The chemical heat treatment cycle is long and consumes a large amount of electricity. If the depth of the layer is reduced to shorten the time, it is an important means of energy saving. The necessary depth of the hardened layer is determined by stress measurement, indicating that the current hardened layer is too deep, and only 70% of the depth of the conventional hardened layer is sufficient. Studies have shown that carbonitriding can reduce the layer depth by 30% to 40%. At the same time, if the depth of the layer is controlled to the lower limit of its technical requirements in actual production, it can also save 20%, and also shorten the time and reduce the deformation.
6. Using high temperature and vacuum chemical heat treatment
The high-temperature chemical heat treatment is to allow the austenite grains of the infiltrated steel to grow in a narrow condition and increase the chemical heat treatment temperature, thereby greatly accelerating the rate of carburization. Increasing the carburizing temperature from 930 ° C to 1000 ° C can increase the carburizing speed by more than 2 times. However, due to many problems, the future development is limited.The vacuum chemical heat treatment is carried out in a negative pressure gaseous medium. The surface speed is greatly improved due to the surface cleaning of the workpiece under vacuum and the use of a higher temperature. For example, vacuum carburizing can increase the productivity by 1 to 2 times; in 133.3×(10-1～10-2) Pa, aluminizing and chromium can increase the permeation rate by more than 10 times.
7. Ion chemical heat treatment
It is a chemical heat treatment process in which a glow discharge is generated between a workpiece (cathode) and an anode while infiltrating into an element to be infiltrated in a gas phase medium containing an element at a pressure lower than one atmosphere. Such as ion nitriding, ion carburizing, ion osmosis, etc., has the advantages of fast permeation speed, good quality and energy saving.
8. Using induction self-tempering
Induction tempering is used instead of tempering in the furnace. Because induction heating is used to transfer heat to the quenching layer, the residual heat is not taken away during quenching and cooling to achieve short-time tempering, so it is energy efficient and many In the case (such as high carbon steel and high carbon high alloy steel) can avoid quenching cracking, and at the same time, once the process parameters are determined to be mass production, the economic benefits are significant.
9. Using pre-heating and quenching after forging
Pre-heating quenching after forging not only reduces heat treatment energy consumption, simplifies the production process, but also improves product performance.After forging after heat quenching + high temperature tempering as pretreatment, it can eliminate the shortcomings of grain coarsening and poor impact toughness after forging heat quenching, which is shorter than spheroidizing annealing or general annealing and high productivity. The high temperature tempering temperature is lower than the annealing and political activities, so the energy consumption can be greatly reduced, and the equipment is simple and easy to operate.
After forging, the residual heat normalizing can not only improve the strength of the steel, but also improve the plastic toughness and reduce the cold-brittle transition temperature and the notch sensitivity. For example, 20i steel is forged at 730-630 °C at 20 °C/h. The cold cooling has achieved good results.
10. Surface hardening instead of carburizing and quenching
Systematic studies on the properties of medium-high carbon steels with carbon content between 0.6% and 0.8% after high-frequency quenching (such as static strength, fatigue strength, multiple impact resistance, residual internal stress) indicate that the induction hardening is used instead of carburizing. Quenching is entirely possible. We used 40Cr steel for high-frequency quenching to manufacture gearbox gears, which replaced the original 20i steel carburizing and quenching gears.
11. Replace local heating with local heating
For some local and technical requirements (such as wear-resistant shaft diameter, roll diameter, etc.), local heating such as bath heating, induction heating, pulse heating, flame heating, etc. can be used instead of integral heating such as box furnace. It can realize the proper cooperation between the frictional occlusion parts of each part, improve the service life of the parts, and because it is local heating, it can significantly reduce the quenching deformation and reduce the energy consumption.
We deeply understand that a company can use energy reasonably and achieve maximum economic benefits with limited energy resources, including the efficiency of energy-using equipment, whether the process technology route is reasonable, and whether management is scientific or not. This requires us to consider the system perspective comprehensively, and every link can not be ignored. At the same time, we must also have a global concept when formulating the process, and we must closely integrate with the economic benefits of the enterprise, and we cannot formulate the process for the formulation of the process. This is especially important today when the market economy is developing at a high speed.
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