Terminology of heat treatment:
Metal: an opaque substance with good thermal conductivity and electrical conductivity, which decreases with the increase of temperature and is rich in ductility and ductility. A solid (i.e., a crystal) in which atoms are regularly arranged in a metal.
Alloy: A substance consisting of two or more metals or metal and nonmetal, with metallic characteristics.
Phase: Components in an alloy with the same composition, structure and properties.
Solid solution: A solid metal crystal in which atoms (compounds) of one (or several) components are dissolved into the crystal lattice of another component, while still maintaining the crystal lattice type of the other component. Solid solution can be divided into interstitial solid solution and displacement solid solution.
Solid solution strengthening: due to solute atoms entering the gaps or nodes of the solvent lattice, the lattice is distorted, and the hardness and strength of solid solution are increased. This phenomenon is called solution strengthening.
Compound: A new crystal solid structure with metallic properties is generated by chemical combination between alloy components.
Mechanical mixture: an alloy composition consisting of two crystal structures. Although it is a double-sided crystal, it is an element with independent mechanical properties.
Ferrite: interstitial solid solution of carbon in α-Fe (iron with body-centered cubic structure).
Austenite: interstitial solid solution of carbon in g-Fe (iron with face-centered cubic structure).
Cementite: A stable compound (Fe3c) formed by carbon and iron.
Pearlite: Mechanical mixture of ferrite and cementite (F+Fe3c contains 0.8% carbon).
Ledeburite: Mechanical mixture of cementite and austenite (containing 4.3% carbon)
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the working 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 intrinsic quality of the workpiece, which is generally invisible to the naked eye.
In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in machinery industry, and its microstructure is complex and can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium and their alloys can also change their mechanical, physical and chemical properties through heat treatment, thus obtaining different properties.
In the process from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment was gradually recognized by people. As early as 770 BC ~ 222 BC, the people of China discovered in production practice that the properties of copper and iron would change due to the influence of temperature and pressure deformation. Softening of white cast iron is an important process for manufacturing farm tools.
In the 6th century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, quenching technology has been developed rapidly. Two swords and a halberd unearthed in Yanxiadu, Yixian County, Hebei Province, China, have martensite in the microstructure, indicating that they have been quenched.
With the development of quenching technology, people have gradually discovered the influence of quenching agent on quenching quality. In the Three Kingdoms period, Shuman Puyuan once made 3,000 knives for Zhuge Liang in this oblique valley in Shaanxi. According to legend, he sent people to Chengdu to get water for quenching. This shows that ancient China paid attention to the cooling capacity of different water quality, as well as the cooling capacity of oil and urine. The sword unearthed in the tomb of Wang Jing in Zhongshan in the Western Han Dynasty (206 BC-24 AD) in China has a carbon content of 0. 15-0.4% and a surface carbon content of more than 0.6%, indicating that carburizing technology has been applied. But at that time, as a personal "craft" secret, it refused to spread, so it developed slowly.
1863, British metallographers and geologists showed six different metallographic structures of steel under a microscope, which proved that the internal structure of steel would change when it was heated and cooled, and the high-temperature phase in steel would change into a harder phase when it was rapidly cooled. The iron isomerism theory established by Frenchman Osmond and the iron-carbon phase diagram first formulated by Englishman Austin laid a theoretical foundation for modern heat treatment technology. At the same time, people have also studied the protection methods of metals during heat treatment to avoid oxidation and decarbonization of metals during heating.
From 1850 to 1880, there are a series of patents on the application of various gases (such as hydrogen, gas, carbon monoxide, etc.). ) for protective heating. From 1889 to 1890, British Lake obtained patents for bright heat treatment of various metals.
Since the 20th century, with the development of metal physics and the transplantation and application of other new technologies, the metal heat treatment technology has been greatly developed. A remarkable progress is 190 1 ~ 1925, which uses rotary hearth furnace for gas carburizing in industrial production; Dew point potentiometer appeared in 1930s, which made the carbon potential in furnace atmosphere controllable. Later, by using carbon dioxide infrared instrument and oxygen probe, a method to further control the carbon potential in the furnace atmosphere was developed. In 1960s, plasma field was used in heat treatment technology, and ion nitriding and carburizing processes were developed. With the application of laser and electron beam technology, new surface heat treatment and chemical heat treatment methods have been obtained for metals.
Metal heat treatment technology
The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes there are only two processes of heating and cooling. These processes are interrelated and uninterrupted.
Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. Charcoal and coal were first used as heat sources, and later liquid and gas fuels were used. The application of electricity makes heating easy to control and has 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 air, which often leads to oxidative decarbonization (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 or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging.
Heating temperature is one of the important technological parameters in heat treatment process, and 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 to be treated and the purpose of heat treatment, but it is generally heated above the phase transition temperature to obtain a high-temperature structure. 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 time, so that the internal and external temperatures are consistent and the microstructure is completely transformed. This time is called holding time. When using high energy density heating and surface heat treatment, the heating speed is extremely fast, and there is generally no holding time, while the holding time of chemical heat treatment is often longer.
Cooling is also an essential step in the heat treatment process, and the cooling mode varies from process to process, mainly controlling the cooling speed. Generally, the cooling rate of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. However, due to different steel grades, the requirements are also different. For example, air hardened steel can be hardened at the same cooling rate as normalizing.
Metal heat treatment processes can be roughly divided into three categories: integral heat treatment, surface heat treatment and chemical heat treatment. According to the difference of heating medium, heating temperature and cooling mode, each type can be divided into several different heat treatment processes. The same metal can obtain different structures through different heat treatment processes, thus having different properties. Steel is the most widely used metal in industry, and its microstructure is also the most complex, so there are many heat treatment processes for steel.
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. The integral heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering.
Annealing is to heat the workpiece to a suitable temperature, adopt different holding time according to the material and size of the workpiece, and then slowly cool it, with the purpose of making the internal structure of the metal reach or approach the equilibrium state, obtaining good technological performance and service performance, or preparing the structure for further quenching.
Normalization 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 obtained microstructure is finer, which is often used to improve the cutting performance of materials and sometimes as the final heat treatment of some parts with low requirements.
Quenching is to heat and insulate the workpiece, and then quickly cool it in quenching media such as water, oil or other inorganic salts, organic water solution, etc. After quenching, steel becomes hard, but at the same time it becomes brittle.
In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature but lower than 650℃ for a long time, and then cooled. This process is called tempering.
Annealing, normalizing, quenching and tempering are the "four fires" in the whole heat treatment, among which quenching and tempering are closely related, and they are often used together and are indispensable.
The "Four Fires" evolved heat treatment processes with different heating temperatures and cooling modes. In order to obtain certain strength and toughness, the process of combining quenching with high temperature tempering is called quenching and tempering. Some alloys are quenched to form supersaturated solid solutions and kept at room temperature or slightly higher temperature for a long time to improve the hardness, strength or electromagnetic properties of the alloys. This heat treatment process is called aging treatment.
The method of combining pressure working deformation with heat treatment effectively and closely to obtain good strength and toughness of workpiece is called thermomechanical treatment. The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make the workpiece not oxidized and decarbonized, but also keep the surface of the workpiece clean and improve the performance of the workpiece, and can also introduce infiltration agent for chemical heat treatment.
Surface heat treatment is a kind of metal heat treatment process that only heats the surface layer of the workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece without transferring excessive heat to the interior of the workpiece, the heat source used must have high energy density, that is, the heat energy given to the workpiece per unit area is large, so that the surface layer or part of the workpiece can reach high temperature in a short time or instantly. The main methods of surface heat treatment are flame quenching and induction heating heat treatment, and the commonly used heat sources are flame such as oxyacetylene or oxypropane, induced 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 layer of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid and solid) containing carbon, nitrogen or other alloying elements, and keep the temperature for a long time, so that the surface of the workpiece is infiltrated with carbon, nitrogen, boron and chromium. After the elements are infiltrated, sometimes other heat treatment processes such as quenching and tempering are needed. The main methods of chemical heat treatment are carburizing, nitriding and metallization.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and molds. 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, so as to facilitate various cold and hot processing.
For example, white cast iron can be annealed for a long time to obtain malleable cast iron and improve its plasticity; Using the correct heat treatment process, the service life of gears can be doubled or even dozens of times longer than that of gears without heat treatment; In addition, cheap carbon steel has some properties of expensive alloy steel by infiltrating some alloying elements, which can replace some heat-resistant steels and stainless steels; Almost all tools and molds need heat treatment before use.
Annealing-quenching-tempering
First, the annealing type
1. Complete annealing and isothermal annealing
Complete annealing, also known as recrystallization annealing, is generally referred to as annealing. This kind of annealing is mainly used for casting, forging and hot rolling profiles of various carbon steels and alloy steels with sub-* * composition, and sometimes it is also used for welding structures. It is generally used for the final heat treatment of some light workpieces or the preheating treatment of some workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for carbon steel and alloy tool steel that have been analyzed (such as steel used for manufacturing tools, measuring tools and dies). Its main purpose is to reduce hardness, improve cutting performance and prepare for quenching in the future.
3. Stress relief annealing
Stress relief annealing, also known as low temperature annealing (or high temperature tempering), is mainly used to eliminate the residual stress of castings, forgings, welded parts, hot rolled parts and cold drawn parts. If these stresses are not eliminated, steel parts will be deformed or cracked after a certain period of time or in the subsequent cutting process.
2. When quenching, the most commonly used cooling media are salt water, water and oil. The workpiece quenched by salt water is easy to obtain high hardness and smooth surface, and it is not easy to produce unhardened soft spots, but it is easy to cause serious deformation and even cracking of the workpiece. However, using oil as quenching medium is only suitable for quenching some alloy steel or small carbon steel workpieces with high undercooled austenite stability.
Three. The purpose of tempered steel
1. Reduce brittleness, eliminate or reduce internal stress. After quenching, the steel parts have large internal stress and brittleness. If not tempered in time, steel parts often deform or even crack.
2. Obtain the required mechanical properties of the workpiece. After quenching, the workpiece has high hardness and brittleness. In order to meet the requirements of different properties of various workpieces, the hardness can be adjusted by appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.
3. Stabilize the workpiece size
4. For some alloy steels that are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing) to make carbides in the steel gather properly and reduce the hardness, which is beneficial to cutting.
Several common heat treatment concepts
1. Normalization: A heat treatment process in which steel or steel parts are heated to an appropriate temperature higher than the critical point AC3 or ACM for a certain time, and then cooled in air to obtain pearlitic structure.
2. Annealing: It is a heat treatment process. In this process, what we have to do is
3. Solution heat treatment: the heat treatment process of heating the alloy to a high-temperature single-phase region and maintaining a constant temperature, so that the excess phase is fully dissolved in the solid solution, and then rapidly cooling to obtain a supersaturated solid solution.
4. Aging: the phenomenon that the properties of the alloy change with time after solution heat treatment or cold plastic deformation when it is placed at room temperature or kept slightly higher than room temperature.
5. Solution treatment: fully dissolve all phases in the alloy, strengthen solution, improve toughness and corrosion resistance, relieve stress and soften, so as to continue processing and molding.
6. Aging treatment: heat and keep the temperature at which the strengthening phase precipitates, so as to precipitate and harden the strengthening phase and improve the strength.
7. Quenching: a heat treatment process in which the steel is austenitized and then cooled at an appropriate cooling rate, so that unstable structures such as martensite of the workpiece change in the whole cross section or in a certain range.
8. Tempering: a heat treatment process in which the quenched workpiece is heated to an appropriate temperature below the critical point AC 1 for a certain time, and then cooled in a satisfactory way to obtain the required structure and properties.
9. Carbonitriding of steel: Carbonitriding is the process of simultaneously infiltrating carbon and nitrogen into the steel surface. Traditionally, carbonitriding is also called cyanidation. At present, medium temperature gas carbonitriding and low temperature gas carbonitriding (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, the main purpose of which is to improve the wear resistance and bite 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. Tempered sorbite structure is obtained after quenching and tempering, and its mechanical properties are better than normalized sorbite structure with the same hardness. Its hardness depends on the tempering temperature at high temperature, and is related to the tempering stability of steel and the cross-sectional size of workpiece, generally between HB 200 and 350.
1 1. Brazing: a heat treatment process in which two kinds of workpieces are bonded together with solder.
Types and applications of tempering
According to the performance requirements of the workpiece and the tempering temperature, tempering can be divided into the following categories:
(1) Low temperature tempering (150-250 degrees)
The microstructure obtained by tempering at low temperature is tempered martensite. Its purpose is to reduce the internal stress and brittleness of hardened steel and avoid cracking or premature damage in use on the premise of maintaining high hardness and high wear resistance. It is mainly used for various high-carbon tools, measuring tools, cold stamping tools, rolling bearings and carburized parts, and the hardness after tempering is generally HRC 58-64.
(2) tempering at medium temperature (350-500 degrees)
The microstructure obtained by tempering at medium temperature is tempered troostite. Its purpose is to obtain high yield strength, elastic limit and high toughness. Therefore, it is mainly used for the treatment of various springs and hot working dies, and the hardness after tempering is generally HRC35-50.
(3) High temperature tempering (500-650 degrees)
The microstructure obtained by high temperature tempering is tempered sorbite. Traditionally, the heat treatment combining quenching with high temperature tempering is called tempering, and its purpose is to obtain better comprehensive mechanical properties such as strength, hardness, plasticity and toughness. Therefore, it is widely used in important structural parts of automobiles, tractors and machine tools, such as connecting rods, bolts, gears and shafts. The hardness after tempering is generally HB 200-330.
heat treatment
(1): Annealing: refers to the heat treatment process of heating a metal material to an appropriate temperature, holding it for a certain time, and then slowly cooling it. Common annealing processes include recrystallization annealing, stress relief annealing, spheroidizing annealing, complete annealing and so on. The main purpose of annealing is to reduce the hardness and improve the plasticity of metal materials, so as to facilitate cutting or pressure processing, reduce residual stress, improve the uniformity of weaving and composition, or prepare for subsequent heat treatment.
(2): Normalization: refers to the heat treatment process of heating steel or steel parts to (the upper critical temperature of steel) or above, keeping the temperature at 30 ~ 50℃ for a proper time, and then cooling in still air. The purpose of normalizing is to improve the mechanical properties of low carbon steel, improve the machinability, refine the grains, eliminate the structural defects, and prepare for the subsequent heat treatment.
(3): Quenching: refers to the heat treatment process of heating steel to a certain temperature above Ac3 or Ac 1 (the lower critical temperature of steel) for a certain time, and then obtaining martensite (or bainite) structure at an appropriate cooling rate. The common quenching processes are salt bath quenching, martensite graded quenching, bainite isothermal quenching, surface quenching and local quenching. The purpose of quenching is to obtain the martensite structure required by steel parts, improve the hardness, strength and wear resistance of the parts, and prepare for the subsequent heat treatment.
(4): Tempering: refers to the heat treatment process of hardening steel parts, then heating them below a certain temperature, keeping them for a certain time, and then cooling them to room temperature. The common tempering processes are: low temperature tempering, medium temperature tempering, high temperature tempering and multiple tempering.
The purpose of tempering is mainly to eliminate the stress of steel parts during quenching, so that steel parts have higher hardness and wear resistance, as well as the required plasticity and toughness.
(5): Quenching and tempering: refers to the composite heat treatment process of quenching and high-temperature tempering of steel or steel parts. Steel used for quenching and tempering treatment is called quenched and tempered steel. Generally refers to medium carbon structural steel and medium carbon alloy structural steel.
(6): Carburizing: Carburizing refers to the process of carbon atoms infiltrating into the steel surface. It makes the low-carbon steel workpiece have the surface layer of high-carbon steel, and then after quenching and low-temperature tempering, the surface layer of the workpiece has higher hardness and wear resistance, while the central part of the workpiece still maintains the toughness and plasticity of the low-carbon steel.
Prevent heat treatment deformation
[Edit this paragraph]
The reasons for the deformation of precision and complex molds are often complicated, but as long as we master the deformation law, analyze the reasons and take different methods to prevent the deformation of molds, it can be reduced and controlled. Generally speaking, the heat treatment deformation of precision and complex molds can be prevented by the following methods.
(1) Reasonable material selection. Fine micro-deformation die steel (such as gas quenching steel) should be selected for precision and complex dies, die steel with severe carbide segregation should be forged and quenched and tempered reasonably, and die steel with large size and unforgeable can be treated with solid solution and double refinement.
(2) The mold structure design should be reasonable, the thickness should not be too different, and the shape should be symmetrical. For molds with large deformation, it is necessary to master the deformation law and reserve the machining allowance. For large, precise and complicated molds, the combined structure can be adopted.
(3) Precision and complicated molds should be preheated to eliminate the residual stress generated in the processing.
(4) Reasonable selection of heating temperature and control of heating speed. For precise and complicated molds, balanced heating methods such as slow heating and preheating can be adopted to reduce the heat treatment deformation of the mold.
(5) On the premise of ensuring the hardness of the mold, precooling, graded cooling quenching or warm quenching processes shall be adopted as far as possible.
(6) For precise and complicated molds, vacuum heating quenching and cryogenic treatment after quenching shall be adopted as far as possible when conditions permit.
(7) For some precise and complicated molds, preheating treatment, aging heat treatment, tempering and nitriding heat treatment can be used to control the accuracy of the molds.
(8) When repairing mould defects such as sand holes, air holes and wear, repair equipment with small thermal influence such as cold welding machine should be selected to avoid deformation during repair.
In addition, correct heat treatment operations (such as hole plugging, hole punching, mechanical fixation, proper heating mode, correct selection of cooling direction of the mold and moving direction in the cooling medium, etc.). ) and reasonable tempering heat treatment process are also effective measures to reduce the deformation of precision and complex dies.