Heat treatment is a kind of hot working process, which heats the steel to a predetermined temperature in the solid state, keeps it at this temperature for a period of time, and then cools it down at a certain speed.
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the service performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition on the surface of the workpiece. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye.
In order to make the metal workpiece have the required mechanical, physical and chemical properties, in addition to the reasonable selection of materials and various forming processes, the heat treatment process is often essential. Proper heat treatment can significantly improve the mechanical properties of steel and prolong the service life of machine parts. Proper heat treatment process can also eliminate various defects caused by casting, forging, welding and other hot working processes, refine grains, eliminate segregation, reduce internal stress, and make the structure and properties of steel more uniform.
Throughout the development of heat treatment production technology at home and abroad, the main aspects are clean heat treatment, precision heat treatment, energy-saving heat treatment and less oxidation heat treatment based on atmosphere and vacuum.
1.Clean heat treatment technology
The waste gas, waste water, waste salt, highly toxic substances, dust, noise, electromagnetic wave, etc. formed by heat treatment production will cause pollution to the environment. Both at home and abroad have attached great importance to this, and a series of pollution-free and effective pollution control technologies have emerged. The combustion products of fuel furnace are an important source of pollution. Advanced industrial countries have basically eliminated the use of coal as fuel for heat treatment. In China, a considerable number of township enterprises still use coal furnaces for heating, emitting a large amount of SO2, CO2, Co, dust and cinder. There are fewer and fewer heating furnaces using heavy oil as fuel in the world, and most of them use light oil. Natural gas is still the most ideal fuel. In Japan, light oil and liquefied gas are mainly used as fuels, and propane or butane are used as raw materials for preparing endothermic atmosphere. The combustion waste heat of the combustion furnace has been utilized to a high degree. The improvement of the burner structure and the strict control of the air-fuel ratio can minimize NOx and CO on the premise of ensuring reasonable combustion.
2.Precise heat treatment technology
At present, mechanical manufacturing has stepped into the field of nanotechnology. The progress of physical metallurgy of metals has almost understood the general law of the relationship between material structure and performance and its influencing factors, and can use the achievements of computer and sensor technology to realize the transition from heat treatment production to precision target, that is, various technical conditions for precision heat treatment have become mature.
The so-called precision heat treatment is to strictly control the quality of heat treated products. On the one hand, automatic optimization of process and improvement of product quality; On the other hand, the stable and optimized process conditions are fully guaranteed to obtain uniform product quality with small dispersion. This is the image goal in the 2020 Vision of American heat treatment: the main starting point of zero heat treatment distortion and zero mass dispersion. At present, the quality control of heat treatment production not only depends on the automation of production process (eliminating human factors affecting quality), strict on-line control of process parameters and computer simulation of process effects, but also strives to realize the self adaptation and intelligence of process control. This is the main development direction of quality control technology.
3.Energy saving heat treatment technology
The annual GDP of China’s heat treatment enterprises is one twenty-six times that of the United States, while the electricity consumption is 40% more than that of the United States. The effective utilization of heat treatment energy is an urgent task for heat treatment workers in China. Scientific production and energy management are the most potential factors for the effective utilization of energy. One of the objectives of scientific management is to ensure full production and give full play to equipment capacity. It is widely popular in the world, and the booming specialized production mode in China has proved to be the most effective energy-saving measure.
Based on the careful calculation of energy utilization and production cost, advanced industrial countries have made reasonable choices in the allocation of primary and secondary energy of electricity and fuel, and accumulated rich experience in the energy structure of heat treatment and the full use of waste heat and waste heat. It is unreasonable to use more than 95% of the electricity for heat treatment furnaces in China’s machinery industry. Under the condition that the current natural gas resources can be fully supplied, the energy structure of heat treatment must be adjusted and changed.
Researching, developing and popularizing energy-saving heat treatment process is an energy-saving measure with small investment and quick effect. The type selection, structure and use of heat treatment equipment also have great energy-saving potential. Japan, which basically relies on imports for fuel, has a lot of mature experience in these aspects, which is worth learning from.
4.Heat treatment technology with less oxidation
The popularity of less oxidation heat treatment is still the main symbol to measure whether the heat treatment technology is advanced or not. In this technical field, it includes inert atmosphere, vacuum, salt bath and fluidized bed, induction, packing, stainless steel foil packaging, coating protection, and also includes reductive control of combustion flame. Among them, atmosphere and vacuum are the main development directions.
Heat treatment production requires the continuous development of long-life, clean, efficient, energy-saving and accurate heat treatment technology to achieve high-quality, efficient and low consumption production, continuously save energy consumption, reduce costs, improve labor productivity, and move towards the goal of zero pollution and zero dispersion of quality in heat treatment production.
The popularization of less oxidation heat treatment production technology is still the main direction of the development of heat treatment technology. The key to popularizing this technology is to research, develop, produce and popularize high-quality and high reliability equipment. Controllable atmosphere and vacuum heat treatment should be the development focus.