Basic knowledge of heat treatment of hottest metal

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Basic knowledge of metal heat treatment

metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium, maintained at this temperature for a certain time, and then cooled at different speeds

1. metal structure

metal: a substance with opaque, good metal luster, heat conduction and conductivity, and its conductivity decreases with the increase of temperature, rich in ductility and expansibility. A solid (i.e., a crystal) in which the atoms in a metal are regularly arranged

alloy: a substance with metallic properties, which is composed of two or more metals or metals and nonmetals

phase: the component with the same composition, structure and performance in the alloy

solid solution: it is a solid metal crystal in which the atoms (compounds) of one (or several) component are dissolved into the lattice of another component while maintaining the lattice type of the other component. Solid solution can be divided into interstitial solid solution and displacement solid solution

solid solution strengthening: as the solute atoms enter the gap or node of the solvent lattice, the lattice is distorted and the hardness and strength of the solid solution are increased. This phenomenon is called solid solution strengthening phenomenon

compound: the alloy components combine to form a new crystal solid structure with metal properties

mechanical mixture: an alloy composition composed of two crystal structures. Although it is two kinds of crystals, it is a component with independent mechanical properties

Ferrite: interstitial solid solution of carbon in a-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 composed of ferrite and cementite (f+fe3c carbon 0.8%)

ledeburite: mechanical mixture composed of cementite and austenite (carbon 4.3%)

2 The role of heat treatment

heat treatment role metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the service performance of the workpiece by changing the microstructure inside the workpiece or the chemical composition on the surface of the workpiece. Its characteristic is to improve the internal quality of the workpiece, which is generally not visible to the naked eye. In order to make the metal workpiece have the required mechanical, physical and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in the mechanical industry. The microstructure of steel 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 other CAE calculation and analysis capabilities are weak, and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different service properties

in the process of developing from the stone age to the bronze age and the iron age, the goal of heat treatment is to gradually realize its role under low-cost conditions. As early as 770 BC to 222 BC, Chinese people have found in production practice that the properties of copper and iron will change due to the influence of temperature and pressure deformation. Softening treatment of white cast iron is an important process for manufacturing agricultural tools. In the 6th century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, the quenching process developed rapidly. Martensite exists in the microstructure of two swords and a halberd unearthed in yanxiadu, Yi County, Hebei Province, China, indicating that they have been quenched. With the development of quenching technology, people gradually find the influence of Quenchant on quenching quality. Pu Yuan, a Shu man of the Three Kingdoms, once made 3000 knives for Zhugeliang in today's xiegu, Shaanxi Province. According to legend, he sent someone to Chengdu to get water for quenching. This shows that China paid attention to the cooling capacity of different water quality in ancient times, and also paid attention to the cooling capacity of oil and urine. The carbon content of the sword in the tomb of King Jing in Zhongshan of the Western Han Dynasty (206 BC ~ 24 AD) unearthed in China is 0.15 ~ 0.4%, while the carbon content of the surface is more than 0.6%, indicating that the carburization process has been applied. But at that time, the secret of personal "craft" refused to be spread, so its development was very slow

In 1863, British metallographists and geologists showed six different metallographic structures of steel under the microscope, which proved that the internal structure of steel would change when it was heated and cooled, and the phase at high temperature in the steel would change into a harder phase when it was quenched. The isomorphism theory of iron established by French Osmond and the iron carbon phase diagram first prepared by British Austin, which will be established by five vertical research laboratories including thermal analysis of thin film materials, preparation and processing of thin film material samples, electromagnetic analysis of thin film materials, mechanical analysis of thin film materials and optical analysis of thin film materials, have laid a preliminary theoretical foundation for modern heat treatment technology. At the same time, people also studied the protection methods of metal in the heating process of metal heat treatment to avoid the oxidation and decarburization of metal in the heating process. From 1850 to 1880, there were a series of patents for the application of various gases (such as hydrogen, gas, carbon monoxide, etc.) for protective heating. From 1889 to 1890, British Lake obtained a patent for bright heat treatment of various metals

since the 20th century, the development of metal physics and the transplantation and application of other new technologies have led to greater development of metal heat treatment process. A remarkable development was the application of rotary hearth furnace for Gas Carburization in industrial production from 1901 to 1925; In the 1930s, the dew point potentiometer appeared, which made the carbon potential of the atmosphere in the furnace controllable. Later, the method of further controlling the carbon potential of the atmosphere in the furnace with carbon dioxide infrared instrument, oxygen probe and so on was developed; In the 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

3. Metal heat treatment process

heat treatment process generally includes three processes: heating, insulation and cooling, and sometimes there are only two processes: heating and cooling. These processes are interconnected 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 then liquid and gas fuels were used. The application of electricity makes heating easy to control and no environmental pollution. These heat sources can be directly heated, or indirectly heated by molten salt or metal, or even floating particles When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of 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 a controllable atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected and heated by coating or packaging methods. Heating temperature is one of the important process parameters of heat treatment process. 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 materials and the purpose of heat treatment when the electronic universal testing machine is usually used, but it is generally heated to above the phase transition temperature to obtain high-temperature structure. In addition, the transformation takes a certain time. Therefore, when the surface of the metal workpiece reaches the required heating temperature, it must be maintained at this temperature for a certain time to make the internal and external temperatures consistent and complete the transformation of the microstructure. This period of time is called the holding time. When using high-energy density heating and surface heat treatment, the heating speed is very fast, and generally there is no holding time, while the holding time of chemical heat treatment is often longer

cooling is also an indispensable step in the process of heat treatment. The cooling method varies with different processes, mainly controlling the cooling rate. Generally, the cooling speed of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. However, there are different requirements due to different steel types. 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: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Using different heat treatment processes, the same metal can obtain different structures and thus have different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many kinds of steel heat treatment processes. 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

there are four basic processes for the overall heat treatment of steel: annealing, normalizing, quenching and tempering. Annealing is to heat the workpiece to an appropriate temperature, adopt different holding time according to the material and workpiece size, and then slowly cool it. The purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and service performance, or prepare the structure for further quenching. Normalizing 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 microstructure obtained is finer. It is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements as the final heat treatment

quenching refers to the rapid cooling of the workpiece in water, oil or other inorganic salts, organic aqueous solutions and other quenching media after heating and insulation. After quenching, the steel parts become hard, but at the same time become brittle. In order to reduce the brittleness of steel parts, the quenched steel parts are kept at a suitable temperature higher than room temperature and 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 overall heat treatment. Among them, quenching and tempering are closely related and often used together. "Four fires" evolved into different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a certain strength and toughness, the process of combining quenching and high temperature tempering is called quenching and tempering. After quenching to form an over saturated solid solution, some alloys are kept at room temperature or a slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloy. Such heat treatment process is called aging treatment

the method of effectively and closely combining pressure machining deformation and heat treatment to make the workpiece obtain a good combination of strength and toughness is called thermomechanical treatment

the heat treatment carried out in negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only make the workpiece not oxidized and decarburized, keep the surface of the workpiece bright and clean after treatment, improve the performance of the workpiece, but also carry out chemical heat treatment with infiltrating agent

surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece without too much heat entering the interior of the workpiece, the heat source used must have a high energy density, that is, a large amount of heat energy is given to the workpiece per unit area, so that the surface layer or part of the workpiece can reach high temperature in a short time or instantaneously. The main methods of surface heat treatment are flame quenching and induction heating heat treatment. The common heat sources include oxyacetylene or oxypropane flame, induced current, laser and electron beam

chemical heat treatment is through changing the chemical composition of the surface layer of the workpiece

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