Metal heat treatment is a process in which the metal workpiece is heated to a suitable temperature in a certain medium, maintained at this temperature for a certain time, and then cooled at different speeds.
Sometimes there are only three processes: heat preservation and cooling. These processes are interconnected and uninterrupted.
Heating is one of the important processes of metal heat treatment. There are many heating methods for metal heat treatment. Charcoal and coal were first used as heat sources, and then liquid and gas fuels were used. The application of electricity makes heating easy to control and no environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salt or metal or even floating particles.
When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse impact on the surface properties of parts after heat treatment. Therefore, metals should usually be heated in controlled atmosphere or protective atmosphere, molten salt and vacuum, or protected by coating or packaging. Heating temperature is one of the important process parameters of metal heat treatment process. The selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material being treated and the purpose of heat treatment, but it is generally heated above the phase transformation temperature to obtain high-temperature structure. In addition, the transformation takes a certain time. Therefore, when the metal workpiece surface reaches the required heating temperature, it must be maintained at this temperature for a certain time to make the internal and external temperatures consistent and complete the transformation of microstructure. This period of time is called holding time. When using high-energy density heating and surface heat treatment, the heating speed is very fast, and generally there is no holding time, while the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the process of metal heat treatment. The cooling method varies with different processes, mainly controlling the cooling rate. Generally, the cooling speed of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. However, there are different requirements due to different steel types. For example, air hardening steel can be hardened at the same cooling rate as normalizing.
Metal cooling and metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Different structures can be obtained by different heat treatment processes for the same metal, so it has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many kinds of steel heat treatment processes.
Integral heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to change its overall mechanical properties. There are four basic processes for overall heat treatment of iron and steel: annealing, normalizing, quenching and tempering.
Annealing is to heat the workpiece to an appropriate temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly. The purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and service performance, or prepare the structure for further quenching. Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the microstructure is finer. It is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements as the final heat treatment.
Quenching refers to the rapid cooling of the workpiece in water, oil or other inorganic salts, organic aqueous solutions and other quenching media after heating and insulation. After quenching, the steel becomes hard but brittle at the same time. In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at an appropriate temperature higher than room temperature and lower than 650 ℃, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the “four fires” in the overall heat treatment. Among them, quenching and tempering are closely related and often used together.
“Four fires” evolve different heat treatment processes with different heating temperatures and cooling methods. In order to obtain certain strength and toughness, the process of combining quenching and high temperature tempering is called quenching and tempering. After some alloys are quenched to form supersaturated solid solutions, they are kept at room temperature or slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloys. Such heat treatment process is called aging treatment.
The method of effectively and closely combining pressure machining deformation and heat treatment to make the workpiece obtain a good combination of strength and toughness is called thermomechanical treatment; The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only prevent the workpiece from oxidation and decarburization, keep the surface of the workpiece clean after treatment, and improve the performance of the workpiece, but also carry out chemical heat treatment with infiltrating agent.
Heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece and not transfer too much heat into the interior of the workpiece, the heat source used must have high energy density, that is, large heat energy is given to the workpiece per unit area, so that the surface layer or part of the workpiece can reach high temperature in a short time or instantaneously. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. The common heat sources include oxyacetylene or oxypropane flame, induced current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process by changing the surface chemical composition, structure and properties of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in the medium (gas, liquid and solid) containing carbon, nitrogen or other alloy elements for a long time, so as to make the surface of the workpiece infiltrate elements such as carbon, nitrogen, boron and chromium. After infiltration of elements, other heat treatment processes, such as quenching and tempering, are sometimes carried out. The main methods of chemical heat treatment include carburizing, nitriding and metalizing.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools and dies. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance and so on. It can also improve the microstructure and stress state of the blank, which is conducive to various cold and hot processing.
For example, malleable cast iron can be obtained by annealing white cast iron for a long time to improve plasticity; With the correct heat treatment process, the service life of the gear can be doubled or dozens of times longer than that of the gear without heat treatment; In addition, cheap carbon steel has some expensive alloy steel properties by infiltrating some alloy elements, which can replace some heat-resistant steel and stainless steel; Almost all tools and dies need heat treatment before they can be used.
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