Heat treatment is an indispensable process in mechanical manufacturing, especially for heat treatment of metal materials such as steel, which is the key to modern manufacturing technology. Its main function is to improve the strength and plasticity of materials through three processes: heating, insulation, and cooling. Since the microstructure of the material surface or interior needs to change during these three processes, the energy consumption of the three heat treatment processes is relatively high. According to relevant surveys, the total capacity of electric resistance furnaces for heat treatment in China is currently 500,000 kW higher than that of the European Union, but the output value is far less than that of the European Union. It can be seen that optimizing the energy conservation of heat treatment in the manufacturing industry has become an important work that cannot be ignored in the current heat treatment process.
I. Current Status of Heat Treatment Production in China
(1) Energy Consumption Scale of Heat Treatment
As an essential process in mechanical manufacturing, heat treatment is an industry with a large production scale in the mechanical manufacturing industry. According to the survey results at the end of the "Eleventh Five-Year Plan", there are about 18,000 heat treatment-related production plants and workshops in China, with an installed capacity of 15 million kW and an annual production capacity of over 45 million tons. At the same time, the annual total consumption is as high as 23 billion kWh, accounting for about one-third of the total annual electricity consumption of China's mechanical manufacturing industry. It can be seen that high energy consumption of heat treatment is a prominent feature of China's mechanical manufacturing industry, and there is a huge space and potential for energy conservation and consumption reduction.
(2) Main Types and Sources of Pollutants in Heat Treatment
Heat treatment is not only a major energy consumer in China's mechanical manufacturing industry but also a major polluter. According to the relevant survey at the end of the "Eleventh Five-Year Plan", China is currently the world's second-largest energy consumer, with an annual energy consumption of about 1.42 billion tons of standard coal. In particular, the energy consumption per unit of product is 20% to 60% higher than that of developed countries, indicating that China's heat treatment efficiency is not high, and the energy consumption loss during the heat treatment process is relatively large. Therefore, the environmental pollutants produced are relatively higher, and these pollutants mainly come from three major processes: heating, quenching cooling, and surface treatment, which have seriously threatened human health and the social environment.
II. The Main Problems Faced by Energy Conservation in Heat Treatment in China
From the current status of heat treatment production in China, it can be seen that high energy consumption and heavy pollution have become prominent features of China's heat treatment, and they are also the two major problems that need to be solved in China's heat treatment energy conservation. This is ultimately due to the low level of heat treatment technology in China, which is prominently reflected in the following two points:
(1) High energy consumption and low energy utilization of heat treatment equipment
At present, the upgrading speed of heat treatment equipment in China is relatively slow. In the past 30 years, the unit energy consumption of heat treatment in China has only decreased by 600 kW·h/t, dropping to an average level of 1000 kW·h/t. Among them, the energy-saving effect of large and medium-sized enterprises is slightly better, basically controlled at 500-800 kW·h/t, but there is still a considerable gap compared with the average unit energy consumption of 300-450 kW·h/t in developed countries, which is about 2-3 times that of developed countries.
(2) Low application rate of low-oxidation heat treatment equipment, serious pollution and resource waste
Moreover, due to the slow speed of equipment upgrading, the application of low-oxidation heat treatment equipment is not widespread in China. By the end of the "Eleventh Five-Year Plan", the application rate of low-oxidation heat treatment equipment in China's mechanical manufacturing industry was only 30%, while the application rate of low-oxidation heat treatment in developed manufacturing countries was as high as 80%-90%. It can be seen that China's heat treatment process is still dominated by combustion heating, which has caused serious environmental pollution due to the carbon black, smoke, and a large amount of CO2 and residual ammonia produced by combustion. In addition, the metal oxidation and burning loss caused by heat treatment combustion is usually about 3% of the weight of the metal body. Plus the steel wasted by the surface decarburization treatment of the steel, it exceeds one million tons every year, causing a huge waste.
III. Measures to Optimize Energy Conservation in Heat Treatment
Although the high energy consumption and heavy pollution caused by heat treatment in China are mainly caused by the technical level, the author believes that optimizing energy conservation in heat treatment is not limited to improving the technical level, but also requires improving the management ability of enterprises.
(1) Management Measures for Optimizing Energy Conservation in Heat Treatment
Management is the foundation of enterprise development and the basis of technological innovation. If the management is not in place, even with the most advanced technology and processes, the technical level may not be fully utilized due to human operation factors. The primary task of optimizing the management of energy conservation in heat treatment is to establish a complete energy conservation system, formulate a modern heat treatment enterprise system, production management system, shift efficiency assessment system, etc., so that there are "laws" to follow. It can not only control the energy consumption flow through strict control of heat treatment operation parameters such as heat balance parameters, but also mobilize employees' enthusiasm for energy conservation and consumption reduction through assessment and reward and punishment measures. In addition, it is also necessary to hold a "five small achievements" technological transformation activity with the theme of saving electricity, coal, gas, water, and oil, and give spiritual and material rewards to employees who have made contributions, encouraging employees to carry out technological innovation.
(2) Realizing Energy Conservation and Emission Reduction by Applying New Technologies and New Energy
Applying new energy and using clean energy as fuel
In view of the current situation in China that combustion is still the main heating method for heat treatment and it is difficult to change in the short term, clean energy should be used as fuel as much as possible in the combustion process to replace coal fuel with higher pollution. Based on this, natural gas can be considered as fuel. Under normal technical conditions, its thermal efficiency can reach 50.5%, which is nearly twice that of coal-fired thermal efficiency. In addition to directly combustion, which can obtain a thermal efficiency of at least 30%, the waste heat from flue gas of the heat treatment furnace can also be used for preheating combustion, air, heating of tempering furnace, heating of cleaning liquid and quenching oil, and other living purposes, so that the absolute heat utilization rate can reach more than 80%, with the dual advantages of energy conservation and environmental protection.
Strengthening scientific and technological research and development, testing and promoting new energy-saving equipment and processes
It is necessary to develop new processes, strengthen the research of heat treatment energy-saving theory, test and promote new energy-saving equipment and processes, and adopt new energy-saving technologies as much as possible.
(3) Using new technologies to improve the heat treatment production environment
From the above analysis, it can be seen that China's heat treatment heating still adopts the combustion method and uses oil cooling for heat exchange and cooling, which has caused great pollution to the environment. Clean and energy-saving heat treatment technologies need to be used to replace it. The new technologies that can be used at present are as follows.
Vacuum heat treatment technology
Vacuum heating is a method of heating the heat-treated parts in a space with a negative pressure of less than or equal to 10-5. The use of vacuum heating technology can not only avoid the oxidation and decarburization brought to the heat-treated parts by combustion, but also perform high-pressure gas quenching after vacuum carburizing or carbon-nitrogen co-infiltration, thereby improving the heating efficiency and performance of heat treatment. According to the survey, the application rate of vacuum heat treatment technology in developed countries can reach about 20%, while in China it is less than 5%. Therefore, increasing the use rate of vacuum heat treatment technology is one of the important ways to improve the efficiency of heat treatment at present.
Ion nitriding and ion carburizing technology
Ion nitriding and ion carburizing are chemical heat treatment methods in which the workpiece placed in a low-pressure container is bombarded by charged particles under the action of glow discharge, causing its temperature to rise and achieving the atomic penetration into the surface layer. Compared with conventional chemical heat treatment, it features fast penetration speed, small workpiece deformation, easy control, high thermal efficiency, energy saving, and no smoke or waste gas pollution.
Induction heating heat treatment technology
Induction heating utilizes the principle of electromagnetic induction to generate eddy currents on the workpiece surface, causing it to be heated. Its characteristics are fast heating speed, easy control, less oxidation and decarburization of the workpiece surface, energy saving, and less pollution.
Laser beam, electron beam, ion beam surface technology
This technology mainly includes surface quenching, remelting, and coating, which relies on the rapid heating of the heat-treated part surface by high-energy beams to achieve heat treatment. Compared with combustion heating, this technology does not produce smoke and other pollution, and is a very clean and environmentally friendly technology.
CAD technology for heat treatment
In today's era, we have entered the information age. With the continuous development of IT technology, the use of modern information technology to achieve energy saving in heat treatment is becoming more and more widely used. CAD technology is one of them. It can control the use of quenching agents and quenching methods through computer simulation of the heat treatment process to achieve the optimal configuration of the two, speed up the carburizing process, and reduce the emission of waste gas. Moreover, intelligent CAD technology can also use continuously developed advanced calculation methods such as three-dimensional temperature fields to recycle and utilize the waste heat of heat treatment, reduce energy loss, and is an important technology for green energy saving and clean production.
In summary, to improve the current situation of high energy consumption and heavy pollution in China's heat treatment, it is necessary to start from two aspects: optimizing energy-saving management and applying new technologies and new energy. Only by taking various technical measures on the basis of standardized and strict management can the level of energy conservation in heat treatment be effectively improved.
