Steel is an alloy of iron with carbon, the concentration of which does not exceed 2.14%. In general-purpose steel its content ranges from 0.05 to 1%. No sphere of human activity can do without this alloy. It is used both for the creation of volumetric structures and for the production of high-precision equipment.
Carbon and other impurities in the composition of steel
The alloying of iron with carbon consists of two stages. At the first, 6.67% of carbon is added to the iron, resulting in the formation of iron carbide, or cementite. Under normal conditions, conventional steel consists of two homogeneous phases - cementite and ferrite. When heated, cementite dissolves in the iron to form an austenite. The concentration of carbon affects the basic mechanical properties of steel. Its increase helps reduce the ductility and viscosity, increase the hardness and strength of the substance. In addition, carbon increases the casting properties, but impairs the weldability and workability of the material in question.
There are also various impurities in the steel, the presence of which is due to production technology or from iron ore. Manganese and silicon are special additives introduced into the steel composition to remove sulfur compounds of iron and bivalent oxide. The concentration of silicon is within 0.4%, and manganese - 0.8%. Manganese and silicon also increase the strength and elasticity, respectively (just below is a graph of the heat treatment of steel).
Phosphorus increases the strength of the substance, reducing the ductility and viscosity. The negative effect of the element is to give the steel a cold-brittle character, so the production is not allowed to exceed its content by 0.045%. Sulfur causes the redness of the alloy, its concentration is limited to 0.05%.
There are several classifications of steel.
1. Depending on the chemical composition:
- carbon, they contain iron, carbon and impurities;
- alloyed with various special additives.
2. Depending on the concentration of carbon:
- high-carbon (over 0.7%);
- medium-carbon (0.25 - 0.7%);
- low-carbon (up to 0,25%).
3. By appointment:
- special purpose.
4. Depending on the quality:
- ordinary quality,
- especially high-grade.
Heat treatment of steel. basic information
The purpose of heat treatment of steel is to change the structure of the alloy, and
consequently, and its properties, for example, making the product hard and brittle or, conversely, soft and plastic.
The essence of the processes is the heating of the steel billet, its aging and cooling. All this happens with strict observance of certain parameters, in particular, temperature and speed. The modes are also affected by the classification of steels. Heat treatment of steels of certain types requires different conditions for achieving the same result.
The hardness of austenite is 2-2.5 times higher than that of ferrite. The latter is more plastic. When cooled, the alloy structure changes in reverse order.
The main types of heat treatment of steel - hardening, normalization, tempering, annealing.
The technology of this process consists of heating steel preforms, soaking with subsequent slow cooling, after which an equilibrium structure is achieved in the metal. Its task is to reduce the internal stress in the alloy, as well as an increase in ductility. This heat treatment of steel is divided into two types. They have significant differences. In the first case, the heat treatment of steel does not imply structural changes based on phase transformations.
Annealing of the 1st kind
This type of heat treatment is divided into 4 groups:
Recrystallization annealing. It is used to remove the effect of hardening of steel associated with cold plastic deformation, as a result of which crystal lattice defects, called dislocations and vacancies, are formed. In the formation of such a structure, the metal grains are flattened and pulled out, which causes hardening and decreases the plasticity of the alloy.
This technology of heat treatment of steel implies heating to temperatures higher by 100-200 ° C of the onset of crystallization (approximately 500-550 ° C).
The duration of the exposure varies from 0.5 to 2 hours, then the cooling is slow. The structure changes due to the formation of new grains and the gradual disappearance of deformed ones. Thus, the crystal lattice defects decrease.
- Annealing to remove residual stresses. Internal stresses in steel parts arise as a result of such processes as welding, casting, cutting, grinding, hot deformation. They reach quite large values. In the end, together with the workers subsequently cause the destruction of metal.
To eliminate this phenomenon, annealing is performed at a temperature below the crystallization temperature (727 ° C). With a process lasting 20 hours at 600 ° C, the voltages are almost completely eliminated. To reduce the duration of the process, the temperature is increased to 680-700 ° C.
Annealing of the 2nd kind
With the help of this process, the equilibrium structure of the material is achieved during phase transformations. The structure of the steel after the heat treatment is partially or completely changed. The cardinal change in the structure of the alloy is due to a double recrystallization, as a result of which the grain size decreases, the internal stresses are removed, and the hardening is removed. Types of heat treatment of steel - full (softening) and incomplete annealing.
As a result of this process, a large ferritic-perlite structure transforms into a fine austenite structure, which, upon slow cooling (30-50 ° C), is transformed into a fine ferritic-pearlitic structure. In this way, structural steel is treated to increase ductility and reduce hardness.
As a result of incomplete annealing, the lamellar perlite is converted into granular ferritocencite, passing through the austenite stage (about 780 ° C). This process is used for tool steels.
Since annealing is a sufficiently long operation (up to 20 hours), the normalization of the substance is used as an alternative. This is the heat treatment of steel, as a result of which its machinability is improved by cutting, the structure of welded joints is corrected, and also the alloy is prepared for quenching. The process temperature exceeds the point Asz or Aart. depending on the type of steel at 30-50 ºС.
Normalization is, as a rule, thermal treatment of carbon steels. As a result, further hardening of medium carbon steels and some special ones is not required, since the strength required for the use of parts is achieved through normalization. The structure of normalized steel is sorbitol.
This is the heat treatment of steel, due to which there is an increase in its strength, wear resistance, hardness, elastic limit, and a decrease in ductile properties. The quenching technology consists of heating to a certain temperature (approximately 850-900 ° C), soaking and quenching, thanks to which these properties are achieved. Hardening is the most common way to improve the physical and mechanical properties of the alloy. Types of heat treatment of steel: with polymorphic transformation and without it.
Hardening with polymorphic transformation is used for steels, in which an element capable of polymorphic transformations is present.
The alloy is heated to the temperature of the change in the crystal lattice of the polymorphic element. As a result of heating, the solubility of the alloying component increases. When the temperature is lowered, the lattice type changes in the opposite direction, but since it occurs at a high speed, the excess concentration of the element with the changed lattice remains in the alloy. Thus, a nonequilibrium structure appears that is thermodynamically unstable. The needle-like microstructure of the steel, after heat treatment formed in the alloy, is called martensite. To remove residual stresses, the metal is further subjected to tempering.
Hardening without polymorphic transformations is used in cases where one of the alloy components is bound to dissolve in the other. When the alloy is heated above the solidus line, the component will dissolve. And with rapid cooling, the secondary phase will not have time to return to its original state, because for the onset of the phase interface, the formation of the initial lattice and the diffusion process, there is not enough time. The result is a metastable solid solution with an excessive content of the component. The process leads to an increase in the ductility of the metal. Thermodynamic stability is achieved in the process of spontaneous or thermal aging.
Since the heat treatment regimes of steel quenching have such an important decisive parameter as the cooling rate, it is necessary to mention the environments in which the process takes place (air, water, inert gases, oil, aqueous solutions of salts).
The cooling rate of water became 6 times higher at a temperature of 600 ° C and 28 times at 200 ° C (in comparison with a technical oil). It is used to cool carbon alloys with a high critical quenching rate. A disadvantage of water is a sufficiently high cooling rate in the areas of occurrence of martensite (200-300 ° C), which can lead to the formation of cracks. Salts are added to the water to increase its hardening ability. Thus, for example, heat treatment of steel 45 takes place.
Alloys with a low critical quench rate, which are alloyed, are cooled with oil. Its use is limited to easy flammability and the ability to burn to the surface of parts. Responsible parts of carbon steel are cooled in two environments: water and oil.
Martensitic steels, which should not have an oxide film, for example, used for medical equipment, are cooled in a discharged atmosphere or in air.
In order to convert the residual austenite, which gives steel brittleness, to martensite, additional cooling is used.
For this purpose, the parts are placed in a refrigerator with a temperature of -40 - -100 ° C or covered with a mixture of carbon dioxide and acetone. Special additional processing with low temperatures helps to increase the hardness of cutting tools, the material of which is alloy steel, the stabilization of dimensional parameters of high-precision parts, the increase of the magnetic qualities of the metal.
For some parts, for example, shafts, cams, axles, gears, fingers for clutches working on abrasion, use surface hardening. In this case, a wear-resistant coating of the part is formed, the core of which is viscous, with increased fatigue strength. In order to produce such a hardening, high-frequency currents generated by a transformer from a special generator are used. They heat the surface of the part covered by the inductor. Then the part is cooled in the air. The thickness of the surface layer subjected to hardening can vary from 1 to 10 mm.
This is the heat treatment of steel, aimed at weakening internal stresses that arise during quenching, and also to increase the viscosity. This treatment is applied to steels that have undergone polymorphic transformations. Modes of heat treatment of steel include heating to a temperature of 150-650 ° C, holding and cooling, the speed of which does not play a role. During the tempering process, harder but unstable structures are converted to more plastic and stable. Leave is high, medium and low.
With a low tempering, it heats up to 150-250 ° C, followed by up to 1.5 hours and cooling in air or oil. The crystal lattice of martensite changes, which does not affect the hardness, increases viscosity and removes internal stresses. In this way, the cutting and measuring tools are machined.
With average tempering, it heats up to 300-500 ° C. The structure of steel is represented by troostite tempering. Steel parts after processing are characterized by high elastic properties and strength characteristics. This is how the springs, membranes, springs are treated.
For high tempering, a heating temperature of 450-650 ° C is typical, which leads to the formation of sorbitol. Products become less rigid, plastic, have high impact strength. He is subjected to gear transmissions, axles, rolls and other critical parts of the mechanisms.
Chemical-thermal treatment of steel
It contributes to the strength and hardness of the alloy, its corrosion resistance, imparting antifriction and wear-resistant properties. This process includes both thermal and chemical effects on the composition, structure, and properties of the surface layer of the alloy.
The chemical-thermal treatment of steel is based on such processes as dissociation, diffusion and adsorption. Depending on the saturating element, it is divided into nitriding, cementation, cyanidation, etc.
The task of cementation is to obtain a hard surface on parts made of low-carbon steel with a sufficiently viscous core. The process is carried out in the carburettor at 930-950 ° C, because at this temperature the most stable austenite. In this way, both low-carbon and alloyed alloys are treated. The treatment of the classification of steel. Heat treatment of steels of certain types requires special parameters to achieve the result.
Cementation is divided into solid and gas. At the second it becomes possible to obtain a certain carbon content in the surface layer, shorten the duration of the process, automate. This is a more perfect method than hard carburizing.
Heat treatment is carried out to reduce the grain size of the core and the cemented layer, and consequently, to improve the mechanical properties. The temperature treatment consists of double quenching and low tempering at temperatures of 160-180 ° C.
It involves the saturation of the surface layers of parts of alloyed steels by nitrogen atoms through diffusion. As a result, a reaction of nitrogen with the alloying elements (molybdenum, chromium, aluminum) occurs to form solid and stable compounds - nitrides.
Advantage is the lower processing temperature compared to the carburizing process - 500-600 ºС. In addition, the nitrided layer has higher mechanical properties and corrosion resistance (these properties are maintained at temperatures up to 500 ° C). The characteristics of the cemented layer are stable at temperatures up to 220 ºС.
This is the process of a one-time filling of the steel surface with nitrogen and carbon atoms. The technology assumes the use of both a liquid and a gas phase. Cyanidation can also be low- and high-temperature.
In the case of a liquid, special baths filled with cyanide and neutral salts are used. After saturation of the surface with nitrogen, the process actually turns into carburization. In the case of low-temperature cyanidation, the parts are further subjected to additional heat treatment.
Gas cementing takes place in a medium containing nitriding and cementing gases. With this method of cyanidation, the depth of the treated layers reaches 1.8 mm.