Heat treatment refers to a metal thermal processing process in which materials are heated, kept warm and cooled in a solid state to obtain the desired structure and properties.
1. Heat treatment
1. Normalizing: Heat the steel or steel parts to the appropriate temperature above the critical point AC3 or ACM, keep it for a certain period of time, and then cool it in the air to obtain the pearlite-like structure.
2. Annealing: Heat the hypoeutectoid steel workpiece to 20-40 degrees above AC3, keep it warm for a period of time, and then cool it slowly with the furnace (or bury it in sand or lime) to below 500 degrees and cool it in the air. .
3. Solution heat treatment: Heat the alloy to a high-temperature single-phase zone and keep it at a constant temperature, so that the excess phase can be fully dissolved into the solid solution, and then rapidly cooled to obtain a supersaturated solid solution heat treatment process.
4. Aging: After the alloy is subjected to solution heat treatment or cold plastic deformation, its properties change with time when it is placed at room temperature or slightly higher than room temperature.
5. Solution treatment: fully dissolve various phases in the alloy, strengthen the solid solution, improve toughness and corrosion resistance, eliminate stress and soften, so as to continue processing and forming.
6. Aging treatment: heat and keep warm at the temperature at which the strengthening phase precipitates, so that the strengthening phase precipitates, hardens and increases the strength.
7. Quenching: A heat treatment process in which the steel is austenitized and cooled at an appropriate cooling rate, so that the workpiece undergoes martensite and other unstable structural transformations in the entire cross-section or within a certain range.
8. Tempering: Heat the quenched workpiece to an appropriate temperature below the critical point AC1 for a certain period of time, and then cool it with a method that meets the requirements to obtain the required structure and performance.
9. Carbonitriding of steel: Carbonitriding is the process of infiltrating carbon and nitrogen into the surface of steel at the same time. Traditionally, carbonitriding is also called cyanidation, and medium-temperature gas carbonitriding and low-temperature gas carbonitriding (ie, gas soft nitriding) are widely used. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low-temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and seizure resistance of steel.
10. Quenching and tempering: Generally, the heat treatment combining quenching and high temperature tempering is called quenching and tempering. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts that work under alternating loads. After quenching and tempering treatment, the tempered sorbite structure is obtained, and its mechanical properties are better than the normalized sorbite structure with the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of the steel and the cross-sectional size of the workpiece, generally between HB200-350.
11. Brazing: A heat treatment process in which two workpieces are heated, melted and bonded together with solder.
2. Process characteristics
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing techniques, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the microstructure inside the workpiece or changes the chemical composition of the surface of the workpiece. , to give or improve the performance of the workpiece. It is characterized by improving the intrinsic quality of the workpiece, which is generally not visible to the naked eye. In order to make metal workpieces have the required mechanical properties, physical properties and chemical properties, in addition to reasonable selection of materials and various forming processes, heat treatment processes are often essential. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled by heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance.
3. Process
Thermal treatment process generally comprises heating, insulation, three processes of cooling, has only two processes of heating and cooling sometimes. These processes are interconnected and cannot be interrupted.
Heating is one of the important processes of heat treatment. The heating means of metal heat treatmet are a lot, are the earliest to adopt charcoal and coal as thermal source, and then using liquid and geseous fuel. The application of electricity makes the heating easy to control without environmental pollution. These heat sources can be used for direct heating, but also for 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 the steel part 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 a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging methods.
Heating temperature is one of important technological parameter of heat treatment process, selects and controls Heating temperature, and is the subject matter that guarantees heat treatment quality. Heating temperature is different and different with processed metallic material and the purpose of heat treatment, but generally all is to be heated to more than the transformation temperature, to obtain high-temperature microstructure. In addition, the transformation takes a certain amount of 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 to make the internal and external temperatures consistent and complete the microstructure transformation. This period of time is called holding time. When adopting high-energy-density heating and surface heat to handle, rate of heating is exceedingly fast, and generally just do not have soaking time, and the soaking time of thermo-chemical treatment is often longer.
Cooling is also an indispensable step in the heat treatment process. The cooling method is different due to different processes, mainly to control the cooling rate. General annealed speed of cooling is the slowest, normalizing speed of cooling is very fast, and quenching speed of cooling is faster. However, there are different requirements due to different steel types. For example, empty hard steel can be hardened at the same cooling rate as normalizing.
4. Process classification
Metal heat treatment processes can be roughly divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the difference of heating medium, heating temperature and cooling method, each big class can be divided into some different heat treatment processes again. The same metal adopts different heat treatment processes to obtain different structures and thus have different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are various heat treatment processes for steel.
Overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of steel generally has four basic processes: annealing, normalizing, quenching and tempering.
Process means:
Annealing is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and the size of the workpiece, and then slowly cool it. Get organized.
Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to that of annealing, but the structure obtained is finer. It is often used to improve the cutting performance of materials, and sometimes it is used for some parts with low requirements. as final heat treatment.
Quenching is to rapidly cool the workpiece in quenching media such as water, oil or other inorganic salts and organic aqueous solutions after heating and heat preservation. After quenching, the steel part becomes hard, but at the same time it becomes brittle. In order to eliminate the brittleness in time, it generally needs to be tempered in time.
In order to reduce the brittleness of steel parts, the quenched steel parts are kept for a long time at an appropriate temperature higher than room temperature but lower than 650 ° C, 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 they are often used together, and both are indispensable. "Four fires" have evolved different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a 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 a supersaturated solid solution, they are kept at room temperature or at a slightly higher temperature for a longer period of time to improve the hardness, strength or electrical properties of the alloy. Such heat treatment process is called aging treatment.
The method of combining pressure processing deformation and heat treatment effectively and closely so that the workpiece can obtain good strength and toughness is called deformation heat treatment; heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make The workpiece is not oxidized and decarburized, the surface of the workpiece is kept smooth after treatment, the performance of the workpiece is improved, and the infiltration agent can also be used for chemical heat treatment.
Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change the mechanical properties of the surface. In order to only heat the surface of the workpiece without passing too much heat into the interior of the workpiece, the heat source used must have a high energy density, that is, to give a large amount of heat energy to the workpiece per unit area, so that the surface or part of the workpiece can be short-term or instantaneous. to high temperature. The main methods of surface heat treatment are flame quenching and induction heating heat treatment. The commonly used heat sources are flames such as oxyacetylene or oxypropane, induced current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process that changes the chemical composition, structure and properties of the workpiece surface. The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of the workpiece surface. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, salt medium or other alloying elements, and keep it warm for a long time, so that the surface of the workpiece is infiltrated with elements such as carbon, nitrogen, boron and chromium. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes required. 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 tools and molds. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing.