Vacuum heat treatment manufacturers
Analysis of heat treatment process
Heat treatment process generally includes heating, insulation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterrupted.
Heating is one of the most important processes in 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 without environmental pollution. These sources can be used for direct heating, or indirect heating through molten salts or metals, or even floating particles.
When the metal is heated, the workpiece is exposed to air, and oxidation and decarbonization often occur (that is, the carbon content on the surface of the steel parts is reduced), which has a very adverse effect on the surface properties of the parts after heat treatment. Therefore, metals should usually be heated in controlled or protective atmosphere, molten salt and vacuum, or by coating or packaging.
The heating temperature is one of the important technological parameters in the heat treatment process. Selecting and controlling the heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the object of heat treatment and the metal material being treated, but it is generally heated above the phase change temperature to obtain high temperature microstructure. In addition, the transformation takes a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain period of time, so that the internal and external temperatures are consistent, so that the microstructure of the transformation is complete, this period is called holding time. When high energy density heating and surface heat treatment are used, the heating speed is very fast, and there is no heat preservation time in general, but the heat preservation time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the process of heat treatment. Cooling methods vary with the process, mainly controlling the cooling rate. The cooling rate of general annealing is the slowest, the normalizing cooling rate is faster, and quenching is faster. But there are different requirements for different grades of steel, such as air-hardened steel can be normalized with the same cooling rate for hardening.
Metal heat treatment technology can be broadly divided into three categories: integral 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 microstructure and properties can be obtained by different heat treatment processes for the same metal. 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 heat treatment process of steel.
Integral heat treatment is a metal heat treatment process in which the workpiece is heated as a whole and then cooled at an appropriate rate to change its overall mechanical properties. The overall heat treatment of steel consists of four basic processes: annealing, normalizing, quenching and tempering.
Annealing is to heat the workpiece to a suitable temperature, according to the material and workpiece size with different holding time, and then slow cooling, the purpose is to make the internal structure of the metal to achieve or close to equilibrium, to obtain good technical performance and service performance, or to prepare for further quenching microstructure. Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. Normalizing is similar to annealing, but the resulting microstructure is finer. Normalizing is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements as final heat treatment.
Quenching is the rapid cooling of the workpiece in water, oil or other inorganic salts, organic water solution and other quenching media after heating and holding. Hardened steel hardens but becomes brittle at the same time. In order to reduce the brittleness of steel, the quenched steel is kept for a long time at a suitable temperature above room temperature but below 650 C, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the "four fires" in the whole heat treatment, in which quenching and tempering are closely related, often used together, indispensable.
With the difference of heating temperature and cooling mode, the "four fires" have evolved different heat treatment processes. In order to obtain a certain strength and toughness, the quenching and high temperature tempering process is called quenching and tempering. The supersaturated solid solution formed by quenching of some alloys can be kept at room temperature or a little higher for a long time to improve the hardness, strength or electrical magnetism of the alloys. Such a heat treatment process is called aging treatment.
The method of combining pressure processing deformation with heat treatment effectively and tightly to make the workpiece have good strength and toughness is called thermomechanical treatment; the heat treatment under negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only keep the workpiece free from oxidation and decarbonization, but also keep the surface of the workpiece clean after treatment. To improve the performance of the workpiece, chemical treatment can also be carried out by infiltration agent.
Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change the mechanical properties of its surface. In order to heat only the surface layer of the workpiece without transferring excessive heat into the inner part of the workpiece, the heat source used must have a high energy density, i.e. a large amount of heat 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 are flame quenching and induction heating. The commonly used heat sources are oxyacetylene or oxypropane flame, induction current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process by changing the chemical composition, microstructure and properties of the workpiece surface. 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 a medium containing carbon, nitrogen or other alloy elements (gas, liquid, solid) for a long time, so that the surface of the workpiece into carbon, nitrogen, boron and chromium and other elements. After infiltration, some other heat treatment processes such as quenching and tempering are sometimes needed. The main methods of chemical heat treatment are carburizing, nitriding and metallizing.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and dies. Generally speaking, it can guarantee and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the microstructure and stress state of the blank to facilitate cold and hot processing.
For example, malleable cast iron can be obtained by long-time annealing treatment of white cast iron to improve plasticity; gears with the correct heat treatment process, service life can be times or tens of times longer than the gear without heat treatment; in addition, inexpensive carbon steel by infiltrating certain alloying elements have certain expensive alloy steel properties. It can replace some heat-resistant steel and stainless steel, and almost all the tools and dies need to be heat treated before they can be used.
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