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: