Metal heat treatment

Metal heat treatment is a process in which the metal workpiece is heated to a suitable temperature in a certain medium and then cooled at different speeds after it is kept at a certain temperature for a certain time.

Metal heat treatment is one of the important technology in mechanical manufacturing, compared with other processing technology, heat treatment generally does not change the shape of the workpiece and the chemical composition of the whole, but by changing the workpiece internal microstructure, or change the surface chemical composition, giving, or improve the metal heat treatment is metal artifacts in a certain medium heated to proper temperature, and keep a certain time in the this temperature, and cooling of a process at different speeds.

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 of the workpiece and the overall chemical composition, but by changing the internal microstructure of the workpiece, or change the surface chemical composition of the workpiece, give or improve the performance of the workpiece. Its feature is to improve the intrinsic quality of the workpiece, which is generally not visible to the naked eye.

In order to make the metal workpiece with the required mechanical properties, physical properties and chemical properties, in addition to reasonable selection of materials and a variety of forming technology, heat treatment technology is often essential. Steel is the most widely used material in mechanical industry, steel microstructure is complex, can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. In addition, the mechanical, physical and chemical properties of aluminum, copper, magnesium, titanium and their alloys can also be changed through heat treatment to obtain different performance.

During the development from the Stone Age to the Bronze Age and the iron age, the role of heat treatment was gradually recognized. As early as 770 b. c. to 222 b. c., the Chinese discovered in production practices that the properties of copper and iron varied with the effects of temperature and pressure deformation. The 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, quenching technology was developed rapidly. Two swords and a halberd unearthed in yanxiadu, yi county, hebei province, China, both contain martensite in their microstructure, indicating that they were quenched.

With the development of quenching technology, the influence of quenching agent on quenching quality is gradually discovered. Pu yuan, a native of shu of The Three Kingdoms, used to make 3000 dao for zhuge liang in the valley of shaanxi. This shows that the cooling capacity of different water quality was noticed in ancient China, as well as the cooling capacity of oil and urine. The sword unearthed in the tomb of king jing of zhongshan during the western han dynasty (206bc ~ AD 24) had carbon content of 0.15 ~ 0.4% in the heart, but more than 0.6% in the surface, indicating that the carburizing process had been applied. But at that time as a personal "craft" secret, refused to spread, so the development was slow.

In 1997, British metallographers and geologists showed that steel has six different metallographic structures under the microscope, proving that the internal microstructure of steel changes as it is heated and cooled, and that the phase in steel at high temperature changes into a harder phase during sharp cooling. The isomeric theory of iron established by osmond, a Frenchman, and the iron-carbon phase diagram first developed by Austin, an Englishman, laid the theoretical foundation for the modern heat treatment process. At the same time, the protection of metal during heat treatment is also studied to avoid oxidation and decarbonization of metal during heat treatment.

In 1880, a series of patents were issued for the use of various gases (such as hydrogen, gas, carbon monoxide, etc.) for protective heating. In 1889 ~ 1890, the Englishman lacker obtained the patent of various metal bright heat treatment.

Since the 20th century, the development of metal physics and the application of other new technologies have made the metal heat treatment process more developed. A significant development was the application of rotary furnace for gas carburizing in industrial production from 1901 to 1925. The dew point potential difference meter appeared in the 1930s, 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 meter and oxygen probe was studied. In the 1960s, the heat treatment technology used the role of plasma field, developed the ion nitriding, carburizing process; With the application of laser and electron beam technology, new surface heat treatment and chemical heat treatment methods have been obtained.

Heat treatment process generally includes heating, heat preservation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and cannot be interrupted.

Heating is one of the important processes of heat treatment. Metal heat treatment heating methods are many, the first use of charcoal and coal as heat sources, and then the use of liquid and gas fuel. The application of electricity makes heating easy to control without 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 (that is, the carbon content on the surface of steel parts decreases) often occur, which has a very adverse effect on the surface performance of the parts after heat treatment. Therefore, metal should be heated in controlled atmosphere or protective atmosphere, in molten salt and in vacuum.

The heating temperature is one of the important parameters in the heat treatment process. The heating temperature varies according to the metal being treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain high-temperature microstructure. In addition, the transformation needs 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 temperature is the same, so that the microstructure changes completely, this period of time is called the holding time. When high energy density heating and surface heat treatment are adopted, the heating speed is extremely fast, so there is usually 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. In general, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. However, there are also different requirements for different types of steel, such as hollow steel can be quenched with normalized cooling rate.

Metal heat treatment process can be divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the difference of heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. With different heat treatment processes, different tissues can be obtained for the same metal, which has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is the most complex, so there are many kinds of 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 rate to change its overall mechanical properties. There are four basic processes for the heat treatment of steel: annealing, normalizing, quenching and tempering.

Annealing is to heat the workpiece to the appropriate temperature, according to the material and the size of the workpiece with different holding time, and then slow cooling, the purpose is to make the metal internal organization to achieve or close to equilibrium state, to obtain good process performance and service performance, or for further quenching for the organization preparation. 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 resulting microstructure is finer. It is often used to improve the cutting properties of materials, and sometimes used as the final heat treatment for some parts with low requirements.

Quenching is to heat the workpiece after heat preservation, in water, oil or other inorganic salts, organic water solution and other quench cooling medium rapidly cooling. After quenching, the steel pieces harden, but at the same time become brittle. In order to reduce the brittleness of steel pieces, steel pieces after quenching at a suitable temperature higher than room temperature but lower than 650℃ for a long time, and then cooling, this process is called tempering. Annealing, normalizing, quenching and tempering are the "four handles of fire" in the whole heat treatment.

With the different heating temperature and cooling methods, different heat treatment processes have evolved. In order to obtain a certain strength and toughness, the combination of quenching and high temperature tempering process, called conditioning. After quenching to form supersaturated 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 alloys. Such a heat treatment process is called aging treatment.

The deformation of pressure machining and heat treatment are effectively and closely combined to make the workpiece get good strength, toughness matching method is called deformation heat treatment; The heat treatment in the negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make the workpiece not oxidized, not decarburized, keep the surface of the treated workpiece clean, improve the performance of the workpiece, but also through the infiltration agent for chemical heat treatment.

Surface heat treatment is a metal heat treatment process that heats only the surface of the workpiece to change its mechanical properties. In order to only heat the surface of the workpiece without too much heat into the workpiece, the use of heat source must have a high energy density, that is, in the unit area of the workpiece to give greater thermal energy, so that the surface of the workpiece or local can temporarily or instantaneously reach high temperature. The main methods of surface heat treatment include flame quenching and induction heat treatment, the commonly used heat source such as flame, induced current, laser and electron beam, etc.

Chemical heat treatment is a metal heat treatment process by changing the surface chemical composition, structure and properties of the workpiece. Chemical heat treatment differs from surface heat treatment in that the latter changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to put the workpiece in the medium containing carbon, nitrogen or other alloying elements (gas, liquid, solid) to heat, heat preservation for a long time, so that the surface of the workpiece infiltration of carbon, nitrogen, boron and chromium and other elements. After infiltration of elements, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding and metalizing.

Heat treatment is one of the important processes in the manufacture of mechanical parts and moulds. Generally speaking, it can guarantee and improve the various properties of the workpiece, such as wear resistance, corrosion resistance and so on. The microstructure and stress state of the blank can also be improved to facilitate various cold and hot processing.

For example, malleable cast iron can be obtained after long annealing treatment to improve plasticity. Gear with the right heat treatment technology, the service life can be doubled or dozens of times higher than the gear without heat treatment; In addition, cheap carbon steel through the infiltration of some alloy elements have some high price of alloy steel properties, can replace some heat resistant steel, stainless steel; The mold is almost all need to undergo heat treatment before use.

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