Metal heat treatment is a kind of technological method that the metal workpiece is heated to a suitable temperature in a certain medium, kept at this temperature for a certain time, and then cooled at different speeds.
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 gradually discovered the influence of cooling liquid 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.
Bimetallic heat treatment process
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 steps 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 usually heated above the phase transition temperature to obtain the required 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 high energy density heating and surface heat treatment are used, the heating speed is extremely fast, and the heat preservation time is generally short or not, while the heat preservation time of chemical heat treatment is often long.
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 process can be roughly divided into whole heat treatment, surface heat treatment, local 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 and lower than 7 10℃ 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 laser heat treatment, flame quenching and induction heating heat treatment. The commonly used heat sources are oxyacetylene or oxypropane flame, 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, metallization and compound carburizing.
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.
Classification of three steels
Steel is an alloy with iron and carbon as the main components, and the carbon content is generally less than 2. 1 1%. Steel is an extremely important metal material in economic construction. Steel is divided into carbon steel and alloy steel according to chemical composition. Carbon steel is an alloy obtained by melting pig iron. In addition to iron and carbon, it also contains a small amount of impurities such as manganese, silicon, sulfur and phosphorus. Carbon steel has certain mechanical properties, good process performance and low price. Therefore, carbon steel has been widely used. However, with the rapid development of modern industry and science and technology, the properties of carbon steel can not fully meet the needs, so people have developed various alloy steels. Alloy steel is a multi-element alloy obtained by purposefully adding some elements (called alloying elements) on the basis of carbon steel. Compared with carbon steel, the properties of alloy steel have been significantly improved, so it has been widely used.
Because of the variety of steel products, it is necessary to classify steel products so as to facilitate production, storage, selection and research. According to the use, chemical composition and quality of steel, steel can be divided into many types:
(1). Classification by purpose
According to the use of steel, it can be divided into three categories: structural steel, tool steel and special performance steel.
1. Structural steel:
(1). Steel used as various machine parts. It includes carburized steel, quenched and tempered steel, spring steel and rolling bearing steel.
(2) Steel used as engineering structure. It includes A, B, special steel and common low alloy steel in carbon steel.
2. Tool steel: steel used for manufacturing various tools. According to the different uses of tools, they can be divided into cutting tool steel, die steel and measuring tool steel.
3. Special performance steel: It is a kind of steel with special physical and chemical properties. Can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel and so on.
(2) Classification by chemical composition
According to the chemical composition of steel, it can be divided into carbon steel and alloy steel
Carbon steel: according to carbon content, it can be divided into low carbon steel (carbon content ≤ 0.25%); Medium carbon steel (0.25% 10%). In addition, according to the different types of main alloying elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium-nickel steel, chromium-manganese-titanium steel and so on.
(3) Classification by quality
According to the content of harmful impurities phosphorus and sulfur in steel, it can be divided into ordinary steel (phosphorus content ≤0.045%, sulfur content ≤ 0.055%; Or both phosphorus and sulfur content are ≤ 0.050%); High quality steel (phosphorus and sulfur content ≤0.030%).
In addition, according to the type of smelting furnace, steel can be divided into open hearth steel (acid open hearth furnace and alkaline open hearth furnace), air converter steel (acid converter, alkaline converter and oxygen top-blown converter steel) and electric furnace steel. According to the degree of deoxidation in smelting, steel is divided into boiling steel (incomplete deoxidation), killed steel (complete deoxidation) and semi-killed steel.
When naming steel products, steel mills often combine three classification methods: use, composition and quality. For example, steel is called ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, advanced high-quality carbon tool steel, alloy structural steel and alloy tool steel. ≤ 0.040%); High quality steel (phosphorus content ≤0.035%,
Mechanical properties of four metal materials
The properties of metal materials are generally divided into two categories: process performance and service performance. The so-called process performance refers to the performance of metal materials in the process of machining and manufacturing mechanical parts under given cold and hot working conditions. The technological properties of metal materials determine their adaptability in the manufacturing process. Due to different processing conditions, the required technological properties are also different, such as casting performance, weldability, ductility, heat treatment performance, machinability and so on. The so-called service performance refers to the performance of metal materials under the service conditions of mechanical parts, including mechanical properties, physical properties, chemical properties and so on. The performance of metal materials determines its application range and service life.
In the mechanical manufacturing industry, the general mechanical parts are used in normal temperature, normal pressure and non-corrosive media, and each mechanical part will bear different loads during use. The ability of metal materials to resist damage under load is called mechanical properties. The mechanical properties of metal materials are the main basis for the design and selection of parts. External loads with different properties (such as tension, compression, torsion, impact, cyclic load, etc.). ) will require different mechanical properties of metal materials. Commonly used mechanical properties include: strength, plasticity, hardness, toughness, impact resistance and fatigue limit. Various mechanical properties will be discussed separately below.
1. Power
Strength refers to the ability of metal materials to resist failure (excessive plastic deformation or fracture) under static load. Because the load acts in the form of tension, compression, bending and shearing, the strength is also divided into tensile strength, compressive strength, bending strength and shearing strength. There is often a certain relationship between various strengths, and tensile strength is generally used as the most basic strength index in use.
2. Plasticity
Plasticity refers to the ability of metal materials to produce plastic deformation (permanent deformation) without damage under load.
3. Difficulties
Hardness is an index to measure the hardness of metal materials. At present, the most commonly used method to measure hardness in production is indentation hardness method, which uses a indenter with a certain geometry to press into the surface of the measured metal material under a certain load, and determines its hardness value according to the indentation degree.
Commonly used methods include Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC) and Vickers hardness (HV).
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The strength, plasticity and hardness discussed above are the mechanical properties of metals under static load. In fact, many machine parts work under cyclic load, and in this case, the parts will fatigue.
5. Impact toughness
The load acting on machine parts at great speed is called impact load, and the ability of metal to resist damage under impact load is called impact toughness.
Step five, annealing-quenching-tempering.
(1) Type of annealing
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. Generally, it is often used as the final heat treatment of some unimportant workpieces or as 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) Inhibition
In order to improve the hardness, the main methods are heating, heat preservation and rapid cooling. 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.
(3) Tempering
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.
6. Selection of common furnace types
The furnace type should be determined according to different process requirements and workpiece types.
1. For products that cannot be mass-produced, with different workpiece sizes and various types, the technology is required to be universal.
Multi-purpose, box furnace can be selected.
2. When heating long shafts, long screws, pipes and other workpieces, you can choose deep well electric furnace.
3. For small batch of carburized parts, well gas carburizing furnace can be selected.
4. Continuous carburizing production line or box-type multi-purpose furnace can be used for mass production of automobile and tractor gears.
5. When heating the blank of mass-produced stamping parts, it is best to use steel rolling furnace and roller hearth furnace.
6. For parts molded in batches, push rod type or conveyor belt type resistance furnace (push rod furnace or belt casting furnace) can be selected for production.
7. Small mechanical parts, such as screws and nuts, can be selected from vibrating bottom furnace or mesh belt furnace.
8. Steel balls and rollers can be heat-treated by rotating tube furnace with internal screw.
9. Non-ferrous metal ingots can be produced in large quantities by push rod furnace, and small non-ferrous metal parts and materials can be heated by air circulation furnace.