11 major energy-saving measures for heat treatment processes of steel pipes

The energy-saving potential of heat treatment is very large. How to take measures to strengthen energy saving is an important issue facing every heat treatment worker. The following is a brief discussion on energy saving in heat treatment processes.

First, reduce the heating temperature.

Generally, the quenching heating temperature of hypereutectoid carbon steel is 30-50℃ above Ac3, and the quenching heating temperature of eutectoid and hypereutectoid carbon steel is 30-50℃ above Ac1. However, recent studies have confirmed that heating and quenching of hypereutectoid steel in the α+γ two-phase region slightly lower than Ac3 (i.e., sub-temperature quenching) can improve the strength and toughness of steel, reduce the brittle transition temperature, and eliminate temper brittleness. The heating temperature of quenching can be reduced by 40℃. Using low-temperature, fast, and short-time heating and quenching for high-carbon steel can reduce the carbon content of austenite, which is conducive to obtaining lath martensite with good strength and toughness matching, which can not only improve its toughness but also shorten the heating time. For some transmission gears, carbonitriding is used instead of carburizing, which can improve wear resistance by 40% to 60%, fatigue strength by 50% to 80%, and co-diffusion time is equivalent, but the co-diffusion temperature (850℃) is 70℃ lower than the carburizing temperature (920℃), and heat treatment deformation can also be reduced.

Second, shorten the heating time.

Production practice shows that the traditional heating time determined by the effective thickness of the workpiece is conservative, so the heating coefficient α in the heating and insulation time formula τ＝α·K·D should be corrected. According to the traditional processing parameters, when heated to 800-900℃ in an air furnace, the α value is recommended to be 1.0-1.8min/mm, which is conservative. If the α value can be reduced, the heating time can be greatly shortened. The heating time should be determined by experiment according to the steel type, workpiece size, furnace loading, etc. Once the optimized process parameters are determined, they must be implemented carefully to achieve significant economic benefits.

Third, cancel tempering or reduce the number of tempering times.

Cancel the tempering of carburized steel. For example, the fatigue limit of the double-sided carburized piston pin for 20Cr steel loaders can be increased by 16% compared with tempering; cancel the tempering of low-carbon martensitic steel, simplify the bulldozer pin sleeve to 20 steel quenching state (low-carbon martensite), the hardness is stable at about 45HRC, the product strength and wear resistance are significantly improved, and the quality is stable; reduce the number of tempering times for high-speed steel, such as W18Cr4V steel machine saw blade uses one tempering (560℃×1h) instead of the traditional 560℃×1h three temperings, and the service life is increased by 40%.

Fourth, use low-medium temperature tempering instead of high-temperature tempering

Medium-carbon or medium-carbon alloy structural steel uses medium and low-temperature tempering instead of high-temperature tempering to obtain higher multi-impact resistance. W6Mo5Cr4V2 steel Φ8mm drill bit, after quenching, is subjected to secondary tempering at 350℃×1h＋560℃×1h, which increases the cutting life of the drill bit by 40% compared with the drill bit that is tempered three times at 560℃×1h.

Fifth, reasonably reduce the depth of the infiltration layer.

The chemical heat treatment cycle is long and consumes a lot of power. If the depth of the infiltration layer can be reduced to shorten the time, it is an important means of energy saving. The necessary hardening layer depth is obtained by stress measurement, which shows that the current hardening layer is too deep, and only 70% of the traditional hardening layer depth is sufficient. Studies have shown that carbonitriding can reduce the layer depth by 30% to 40% compared with carburizing. At the same time, if the depth of the infiltration layer is controlled at the lower limit of its technical requirements in actual production, it can also save 20% of energy, and at the same time shorten the time and reduce deformation.

Sixth, use high temperature and vacuum chemical heat treatment

High-temperature chemical heat treatment is to increase the chemical heat treatment temperature when the equipment operating temperature allows and the austenite grains of the infiltrated steel do not grow, thereby greatly accelerating the carburizing speed. Increasing the carburizing temperature from 930℃ to 1000℃ can increase the carburizing speed by more than 2 times. However, due to many problems, future development is limited. Vacuum chemical heat treatment is carried out in a negative-pressure gas phase medium. Because the surface of the workpiece is purified under vacuum and a higher temperature is used, the penetration rate is greatly increased. For example, vacuum carburizing can increase productivity by 1 to 2 times; the penetration rate of aluminum and chromium can be increased by more than 10 times at 133.3×(10-1～10-2)Pa.

Seventh, ion chemical heat treatment

It is a chemical heat treatment process that uses glow discharge between the workpiece (cathode) and the anode to infiltrate the elements to be infiltrated in a gas phase medium containing the elements to be infiltrated at a pressure lower than one atmosphere. Such as ion nitriding, ion carburizing, ion sulfurizing, etc., with the advantages of fast penetration rate, good quality, and energy saving.

Eighth, use induction self-tempering

Use induction self-tempering instead of furnace tempering. Because induction heating is used to transfer heat outside the quenching layer, the residual heat is not completely taken away during quenching cooling, and short-time tempering is achieved. Therefore, it has high efficiency and energy saving, and in many cases (such as high carbon steel and high carbon high alloy steel) it can avoid quenching cracking. At the same time, once the process parameters are determined, it can be mass-produced, and the economic benefits are significant.

Ninth, use post-forging preheating quenching.

Post-forging preheating quenching can not only reduce the energy consumption of heat treatment, and simplify the production process, but also improve product performance. Using post-forging residual heat quenching + high-temperature tempering as a pretreatment can eliminate the shortcomings of coarse grains and poor impact toughness when post-forging residual heat quenching is used as the final heat treatment. It is shorter than spheroidizing annealing or general annealing and has high productivity. In addition, the temperature of high-temperature tempering is lower than annealing and political activity, so it can greatly reduce energy consumption, and the equipment is simple and easy to operate. Compared with general normalizing, post-forging residual heat normalizing can not only improve the strength of steel but also improve plastic toughness, and reduce cold brittle transition temperature and notch sensitivity. For example, 20CrMnTi steel was cooled at 730-630℃ at a cooling rate of 20℃/h after forging and achieved good results.

Tenth, replace carburizing quenching with surface quenching.

A systematic study of the properties of medium-high carbon steel with a carbon content of 0.6% to 0.8% after high-frequency quenching (such as static strength, fatigue strength, multiple impact resistance, and residual internal stress) shows that it is entirely possible to partially replace carburizing quenching with induction quenching. We have successfully used 40Cr steel high-frequency quenching to manufacture gearbox gears, replacing the original 20CrMnTi steel carburizing quenching gears.

Eenth. Replace overall heating with local heating.

For some parts with local technical requirements (such as wear-resistant gear shaft diameter, roller diameter, etc.), local heating methods such as bath furnace heating, induction heating, pulse heating, flame heating, etc. can be used instead of overall heating such as box furnace, etc., which can achieve proper coordination between the friction and bite parts of each part, improve the service life of the parts, and because it is local heating, it can significantly reduce quenching deformation and reduce energy consumption.