Heat Treatment Profile
Heat Treatment Profile
Heat treatment is, the material is placed in a certain dielectric heating insulation, cooling, by changing the organizational structure of the material surface or the inside, so as to control the performance of an integrated process.
Heat treatment terms:
Metal: substances having opaque, metallic luster good thermal and electrical conductivity and the conductivity is reduced with the increase of temperature, rich, such as ductility and malleability characteristics. Inside the metal atoms with regularity arranged solid (crystal).
Alloy: composed by two or more metal or metal and non-metallic substance having metallic properties.
Phase: refers to the same as the chemical composition in the metal or alloy, the same structure or the same atom aggregation state, and there is a clear interface between the other portions independent uniform part.
Organization: with the naked eye can be directly observed, or distinguish between the material morphology within the image can be observed with a magnifying glass, microscope.
The solid solution: a (or several) element atom (compound) is dissolved in the lattice of another group, while still maintaining another element of the lattice type of solid state metal crystal, solid solution stars interstitial solid solution and substitutional solutes two.
Solid solution strengthening: Because the gap or junction of solute atoms into the lattice of the solvent, the lattice distortion and increased hardness and strength of the solid solution, this phenomenon is called solid-solution strengthening phenomenon.
Compound: occur between compounds of the alloy component, generating a new crystalline solid structure with metallic properties.
The mechanical mixture of: The composition of the alloy composition consists of two kinds of crystal structures, the crystal on both sides, but it is a composition, having a separate mechanical properties.
Ferritic: interstitial solid solution of carbon in the α-Fe (iron) of the body-centered cubic structure.
Austenitic: interstitial solid solution of carbon in the β-Fe (face-centered cubic structure of iron).
Cementite: The carbon and iron to form a stable compound (Fe3C).
Pearlite: mechanical mixture consisting of the ferrite and cementite (F + Fe3C carbon 0.8%)
Ledeburite: a mechanical mixture of cementite and austenite (carbon 4.3%)
A metal heat treatment is one of the important processes in the mechanical manufacturing, as compared with other processing, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but rather by changing the internal microstructure of the workpiece, or a change in the chemical composition of the surface of the workpiece. , conferred or improve the performance of the workpiece. Improve the intrinsic quality of the workpiece, which is generally not the naked eye can see.
The metal workpiece has a desired mechanical properties, physical properties and chemical properties, in addition to the reasonable selection of materials and a variety of forming processes, the heat treatment process is often essential. Steel is the most widely used material in the machinery industry, the complexity of the steel microstructure can be controlled by heat, so the heat treatment of steel is the main content of metal heat treatment. Further, such as aluminum, copper, magnesium, titanium and its alloys may also be changed by the heat treatment the mechanical, physical and chemical properties, in order to obtain different properties.
In the process of progress from the Stone Age to the Bronze Age and Iron Age, the role of the heat treatment gradually recognized. As early as 770 BC to 222 years ago, the Chinese people in the production practice had been found in the performance of copper and iron will change due to the impact of temperature and pressure deformation. White cast iron soften processing is an important process for the manufacture of agricultural implements.
The sixth century BC, iron and steel weapons gradually adopted quenching process, in order to improve the hardness of steel, then developed rapidly. Yi County, Hebei Province, China Yan unearthed two swords and a halberd, have martensite microstructure after quenching.
With the the quenching the development of technology, people gradually discovered that the quenching agent quenching quality. Three Shu Pu Yuan in Shaanxi Xiegu for Zhuge Liang chipped 3000 knife, according to legend, is to send people to the Chengdu water quenching. This shows that the Chinese in ancient times noticed the cooling capacity of the different water quality, but also pay attention to the cooling capacity of the oil and urine. Jing of Zhongshan tomb sword unearthed in China's Western Han Dynasty (206 BC to AD 24), the heart, the carbon content of 0.15 to 0.4%, while the surface of the carbon content is 0.6% or more, has applied carburizing process. But it was as an individual "craft" secret, refused to rumor, and thus very slow development.
In 1863, British metallographic scientists and geologists shows the microstructure of steel under a microscope six different prove that the change of the steel when the heating and cooling, an internal tissue, the high temperature of the steel phase transformation when the quench for a harder phase. Allotrope of iron Frenchman Osmond established theory, as well as the the British Austin earliest developed iron-carbon phase diagram initially laid the theoretical foundation for modern heat treatment process. At the same time, it is also studied the method of protecting the heating process of the heat treatment of the metal of the metal, in order to avoid the oxidation of the metal in the heating process and decarburization.
1850 to 1880, there have been a series of patent protection of the heating for the application of various gases (such as hydrogen gas, carbon monoxide, etc.). 1889 to 1890 British Lake bright heat treatment of patents in a variety of metals.
20th century, the metal physical development and porting applications of new technology, metal heat treatment process to greater development. A significant progress from 1901 to 1925, the drum furnace gas carburizing applications in industrial production; 1930s, the dew point potentiometer, carbon potential of the furnace atmosphere reached controllable, and later developed with the CO2 infrared analyzer oxygen probe further control the carbon potential of the furnace atmosphere; 60s, the heat treatment technology, the use of the ion field, the development of the ion nitriding, carburizing process; laser, electron beam technology, the metal gained new surface heat treatment and chemical heat treatment method.
The process of the metal heat
The heat treatment process generally includes heating insulation, cooling the three processes, and sometimes only the heating and cooling of the two processes. These processes dovetail with each other and not be interrupted.
Heating is one of the important process of heat treatment. Many heating of metal heat treatment, the earliest use of charcoal and coal as a heat source, and then application of liquid and gaseous fuels. Application of electric heating is easy to control, and no environmental pollution. The use of these sources of heat can be directly heated, can also be through the molten salt or metal, as well as floating particles by indirect heating.
Heating of the metal, the workpiece is exposed in the air, often occurs oxidation, decarburization (i.e., the surface of the steel parts reduced carbon content), for the performance of parts of the surface after heat treatment has a very negative impact. Thus metals are typically in a controlled atmosphere or protective atmosphere, and a vacuum in the molten salt heated to protect the heating can also be used the method of coating or packaging.
The heating temperature is one of the important process parameters of heat treatment process, select and control the heating temperature, the main issues to ensure the quality of the heat treatment. Heating temperature with the treated metal material and the purpose of the heat treatment varies, but generally are heated to above the phase transition temperature, to obtain the high temperature microstructure. Further transition requires a certain amount of time, when the surface of the metal workpiece to achieve the required heating temperature must maintain a certain time at this temperature, so that the inside and outside temperature, the microstructure changes completely, this period is called the holding time. High energy density heating and surface heat treatment, the heating rate is fast, there is no generally holding time, the chemical heat insulation time is often longer.
An indispensable step in the cooling is also the heat treatment process, the method of cooling due to different processes and different, is mainly controlled by the cooling rate. The general annealing cooling slowest normalizing the cooling speed, the faster cooling rate of the quenching. But also because of the different kinds of steel and have different requirements, for example, empty hard steel can be carried out by normalizing the same cooling rate hardened.
Metal heat treatment process can be roughly divided into three categories of overall heat treatment, surface heat treatment and chemical heat treatment. According to different heating medium, heating temperature and the cooling method, each category can be divided into a number of different heat treatment process. The same kind of metal using different heat treatment processes, the availability of different organizations, which have different performance. Steel is the most widely used in the industrial metal and steel microstructure is also the most complex, and therefore a wide range of steel heat treatment process.
The overall heat treatment is heated, and then cooled at the appropriate speed in order to change the overall mechanical properties of the metal heat treatment process on the workpiece as a whole. Steel overall heat-treated roughly annealing, normalizing, quenching and tempering four basic processes.
Annealing is the workpiece is heated to an appropriate temperature, according to the material and the workpiece size using different holding time, and then slowly cooled, and the purpose is to make the metal internal organization reached or approached the equilibrium state, to obtain a good process properties and performance, or for further quenching for organizations to prepare.
Normalizing the workpiece is heated to a suitable temperature after cooling in air, normalizing the effect of the same annealing is similar organization just get finer, commonly used to improve the cutting performance of the material, but also sometimes used for some less demanding parts As a final heat treatment.
Quenching After incubation, the workpiece is heated, rapid cooling in the quenching medium of the water, oil, or other inorganic salt, an organic aqueous solution. After quenching, the steel harden, but brittle.
Order to reduce the brittleness of the steel parts, after quenching steel at above ambient but below 650 ° C a suitable temperature for a long period of insulation, and then cooling, this process is known as tempering.
Annealing, normalizing, quenching and tempering, quenching and tempering of which are closely related and often used in conjunction with, and are indispensable to the overall heat treatment "fire".
Four fire "with the heating temperature and cooling the different ways, but it turns out different heat treatment process. In order to obtain a certain degree of strength and toughness, the combined quenching and high-temperature tempering process, referred to as quenching. Certain alloys quenched after the formation of a supersaturated solid solution, placing it under an appropriate temperature of room temperature or slightly higher to maintain a long time to improve the hardness, strength or electrical magnetic. This heat treatment process called aging treatment.
Effectively close combined pressure processing deformation and heat treatment, so that the workpiece to obtain good strength, toughness with a method referred to as thermomechanical treatment; pressure atmosphere or vacuum heat treatment called a vacuum heat treatment, which not only enables workpiece without oxidation, decarburization, maintain processing workpiece surface finish, to improve the performance of the workpiece can also pass the infiltration agent for chemical heat treatment.
Surface heat treatment is only heating the surface of the workpiece, to change the surface mechanical properties of metal heat treatment process. Incoming inside the workpiece to heating only the workpiece surface without the excessive heat, the heat source used is required to have a high energy density, i.e. to give larger heat in the unit area on the workpiece so that the workpiece surface or a partial short-term or instantaneous reach a high temperature. The surface heat treatment the flame hardening heat treatment and induction heating, common heat source oxy-acetylene or oxy-propane flame, the induced current, laser and electron beam.
The chemical heat treatment by changing the workpiece surface chemical composition, microstructure and properties of metal heat treatment process. The chemical heat treatment and surface heat treatment at which change the chemical composition of the workpiece surface. The chemical heat treatment of the workpiece on a carbon, nitrogen or other alloying elements medium (gas, liquid, solid) was heated, and incubated for a longer time, so that the workpiece surface to penetrate the elements such as carbon, nitrogen, boron and chromium. Infiltrated elements, and sometimes other heat treatment processes, such as quenching and tempering. The main method of chemical heat treatment carburizing, nitriding, surface alloying.
Heat treatment is one of the important processes in the manufacturing process of the mechanical parts and tooling. Broadly speaking, it can guarantee and improve the various properties of the workpiece, such as wear-resistant, corrosion-resistant. You can also improve the microstructure and stress state of the rough, and to facilitate a variety of cold and heat processing.
Such as white cast iron can be obtained after a long period of annealing malleable iron, improve plasticity; gear correct heat treatment process, can life than non-heat-treated gear multiplied or improve a few times; inexpensive carbon steel by infiltration of some alloy elements on the performance of some expensive alloy steel can replace some of the heat-resistant steel, stainless steel; tooling almost all heat-treated before use.
Quenching annealing --- --- tempering
A. Types of annealing
1. Complete annealing and isothermal annealing
The fully annealed also known as recrystallization annealing, generally referred to as annealing, this annealing is mainly used for sub-eutectoid composition of carbon steel and alloy steel casting, forging and hot rolled profiles, sometimes for welded structure. Usually often used as the final heat treatment of the workpiece is not heavy, or as certain pre-heat treatment of the workpiece.
2. Ball annealing
Ball annealing is mainly used for eutectoid carbon steel and alloy tool steel (such as the manufacture of cutting tools, mold steels). Its main purpose is to reduce the hardness, to improve the machinability, and to prepare for the subsequent quenching.
3. To stress annealing
To stress annealing, also known as low-temperature annealing (tempering), this annealing is used to eliminate the castings, forgings, welding, hot-rolled, cold-drawn pieces of residual stress. If these stresses are not removed, will cause the steel after a certain time, or in a subsequent machining process is deformed or cracks.
II. The most commonly used cooling medium is a brine, water, and oil quenching. Brine quenching of the workpiece, and easy to get high hardness and smooth surface, not prone to quench hard soft spot, but easy to make the deformation of the workpiece serious, even cracking. Oil for quenching medium only apply to the stability of supercooled austenite alloy steel or carbon steel workpiece size large quenching.
III. The purpose of tempering steel
1. Brittle reduce, eliminate or reduce internal stress, and there are great internal stress and brittle after quenching steel, if not tempered tends to steel deformation or cracking.
2. Obtain the workpiece to the desired mechanical properties, the workpiece by the high hardness after quenching and brittleness, in order to meet the different performance requirements of the various workpieces, by the mating of the appropriate tempering to adjust the hardness, reduced brittleness, to obtain the required toughness, plasticity.
3. Stabilize the workpiece size
4. Often used for certain alloy steel annealing difficult softening after quenching (or normalizing) the high-temperature tempering steel carbide appropriate aggregation to reduce the hardness to Lice cut processing.
Several common heat treatment concept
1. Normalizing: the steel or the steel is heated to an appropriate temperature above the critical point AC3 or ACM maintain a certain time after the cooling in air, the heat treatment process of the pearlite type of organization.
2. Annealing annealing: sub-eutectoid steel workpiece is heated to 20-40 degrees above AC3, after holding for some time, with the furnace slow cooling (or buried in the sand or lime cooling) to 500 degrees in the air cooling heat treatment process
3. Solution heat treatment: The alloy is heated to a constant temperature of the high temperature single phase region remains, so that the excess phase fully dissolved into the solid solution, and then rapidly cooled to obtain a supersaturated solid solution heat treatment process
4. Limitation: the alloy after solution heat treatment or cold plastic deformation, left at room temperature or slightly higher than the holding time at room temperature, and its performance varies with time the phenomenon.
5. Solution treatment: the various phase fully dissolved in the alloy, and strengthen the solid solution and improve toughness and corrosion resistance, eliminate stress and softening, in order to continue processing molding
6. Aging treatment: heating and insulation in the strengthening phase precipitation temperature, the precipitation strengthening phase, and to be hardened to improve the strength
7. Quenching: steel austenitizing cooled to an appropriate cooling rate, so that all or a certain range of the workpiece within the cross-sectional martensite unstable organizational structure changes in the heat treatment process such as
8. Tempering: after the hardening of the workpiece is heated to the appropriate temperature below the critical point AC1 maintain a certain time, followed by the method to meet the requirements of the cooling heat treatment process in order to obtain the desired microstructure and properties of
9. Steel carbonitriding: the carbonitriding Is infiltration of carbon and nitrogen to the surface of the steel. Habits on the carbon and nitrogen permeation, also known as cyanide, permeation-temperature gas carbonitriding and low-temperature gas carbonitriding (ie gas soft nitriding) is widely used. Temperature gas carbonitriding the main purpose is to improve the hardness, wear resistance and fatigue strength of steel. The cryogenic gas carbonitriding Nitriding mainly, its main purpose is to improve the wear resistance and seizure resistance of the steel.
10. Quenched and quenching and tempering: general habit of combining the quenching and high temperature tempering heat treatment known as quenching and tempering treatment. Quenching and tempering treatment is widely used in a variety of important structural parts, especially those who work in the alternating load rod, bolts, gears and shafts, etc.. Quenched and tempered tempered sorbite organization, its mechanical properties than the same hardness normalizing sorbite organization was excellent. Its hardness depends on high-temperature tempering temperature and tempering of steel stability and workpiece cross-section of size, HB200-350.
11. Brazing: solder two workpiece bonded heat treatment process
Tempering types and applications
Its tempering temperature is different depending on the workpiece performance requirements can be tempered divided into the following:
(A) low-temperature tempering (150-250 degrees)
Low-temperature tempering proceeds tempered martensite. Its purpose is based on the premise of maintaining the hardened steel of high hardness and high wear resistance, and reduce its quenching stress and brittleness, in order to avoid using chipping or prematurely damaged. It is mainly used for a variety of high-carbon cutting and cutting tools, measuring tools, Die, Rolling carburized parts tempered hardness HRC58-64.
(B) in the tempering temperature (350-500 degrees)
Tempering proceeds organization the tempered troostite. Its purpose is to obtain a high yield strength, the elastic limit and a high toughness. Therefore, it is mainly used for a variety of spring and hot mold processing, tempered hardness HRC35-50.
(3) high-temperature tempering (500-650 degrees)
High-temperature tempering proceeds organization sorbite. Customary to quenching and heat treatment known as a combination of high-temperature tempering quenching and tempering treatment, its purpose is to obtain better mechanical properties of strength, hardness and ductility, toughness. Therefore, it is widely used in important structural parts of automobiles, tractors, machine tools, such as rods, bolts, gears and shafts. After tempering hardness HB200-330.
(1): Annealing: refers to the metallic material is heated to a suitable temperature, to keep a certain time, and then slow cooling of the heat treatment process. The annealing process: common recrystallization annealing, stress relief annealed, spheroidized annealing, fully annealed. The purpose of annealing: mainly to reduce the hardness of the metal material, to improve the plasticity, Monselice cutting machining or press working to reduce the residual stress, and to improve the organization and composition of the homogenization, or prepare for the heat treatment of the after tissue preparation.
(2): normalizing: refers to the heating of steel or steel (steel on the critical temperature) or more, from 30 to 50 ° C maintain an appropriate time after the heat treating process of cooling in still air. Normalizing Objective: To improve the mechanical properties of low carbon steel to improve machinability, grain refinement, eliminating the tissue defects after heat treatment to make the organization and preparation.
(3): Quenching: the steel is heated to Ac3 Ac1 (the lower critical point temperature) of the steel above a certain temperature, to keep a certain time, and then to the appropriate cooling rate, obtained martensite (or bainite) Organization of the heat treatment process. Common salt bath quenching quenching process, the martensite Martempering, the Austempering surface hardening and hardening. The purpose of quenching: steel martensite, to improve workpiece hardness, strength and wear resistance, to prepare tissue preparation, after heat treatment.
(4): Tempering: refers to the steel member by after hardening, and then heated to a temperature below, and incubated for a certain time, and then cooled to room temperature, heat treatment process. Common tempering process: low-temperature tempering, tempering, high-temperature tempering and multiple tempering.
Tempering: primarily intended to eliminate the stress generated by the steel pieces in the quenching, the steel member having a high hardness and wear resistance, and having the desired plasticity and toughness.
(5): Quenched: quenching and tempering of steel or steel composite heat treatment process. Used in quenched and tempered steel that quenched and tempered steel. It generally refers to the structure of medium carbon steel and medium carbon alloy steel.
(6): carburizing: carburizing ordered carbon atoms penetrate into the process of the surface layer of the steel. Also the low carbon steel workpiece having a surface layer of high-carbon steel, and then having a high hardness and wear resistance after quenching and tempering, so that a surface layer of the workpiece, while the central portion of the workpiece still maintain the toughness of the low carbon steel, and plasticity.
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Prevention of heat treatment deformation causes deformation of sophisticated mold is often complicated, but as long as we grasp the deformation law analysis of its causes, takes a different approach to the prevention of mold deformation can be reduced, and also be able to control. General, heat treatment deformation of sophisticated mold can take methods to prevent.
(1) a reasonable selection. Of sophisticated mold should choose the material good micro-deformation of steel (such as air quenching steel), reasonable forging die steel severe carbide segregation and transfer of heat treatment, solid solution and unable to forging die steel can be double refine the heat treatment.
(2) mold structure design to be reasonable, the thickness should not be too poor to be symmetrical shape, to master the large deformation mold deformation law set aside allowance, composite structures can be used for large-scale, sophisticated mold.
(3) A sophisticated mold to conduct the pre-heat treatment, the elimination of the residual stress generated in the machining process.
(4) a reasonable choice of heating temperature control heating rate for sophisticated molds can be taken to slow heating, preheating and other balanced heating method to reduce mold heat treatment distortion.
(5) under the premise that ensure mold hardness, and maximize the use of pre-cooling, grading cooling quenching or Austempering of.
(6) sophisticated mold conditions permit, as far as possible cryogenic vacuum heating quenching and quenching treatment.
(7) to some sophisticated mold can be pre-heat treatment, aging heat treatment, nitriding heat treatment of the quenched and control accuracy of the mold.
(8), the choice of defect repair mold trachoma, porosity, wear the the cold welding heat affected small repair equipment in order to avoid the generation of deformation in the repair process.
In addition, proper heat treatment process operation (such as plugging the holes, tying the hole, the mechanical fixing, suitable heating method, the correct choice of the cooling of the mold direction and in the direction of movement of the cooling medium, etc.) and the a reasonable return heat treatment process is also to reduce sophisticated the mold deformation of effective measures.
Heat treatment sub-process:
Vulcanizing heat treatment
Hardening heat treatment
Eliminate stress relief heat treatment grinding 1 mm welding repair or replacement.