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 field of human activity is complete without this fusion. It is used both to create volumetric structures, and in the production of ultra-precise equipment.
Carbon and other impurities in the composition of the steel
The doping of iron with carbon consists of two stages. At first, 6.67% carbon is added to the iron, resulting in the formation of iron carbide, or cementite. Under normal conditions, ordinary steel consists of two homogeneous phases - cementite and ferrite. When heated, cementite dissolves in the gland to form austenite. The carbon concentration affects the basic mechanical properties of the steel. Its increase contributes to the reduction of ductility and viscosity, increase the hardness and strength of the substance. In addition, carbon increases the casting properties, but affects the weldability and workability of the material in question.
Various impurities are also present in the steel, the presence of which is due to the production technology or the release of iron ore. Manganese and silicon are special additives introduced into the steel in order to remove sulfur compounds of iron and bivalent oxide. The concentration of silicon is in the range of 0.4%, and manganese - 0.8%. Manganese and silicon also increase the tensile strength and elasticity, respectively (a graph of heat treatment of steel is presented below).
Phosphorus increases the strength of a substance, reducing plasticity and viscosity. The negative impact of the element is to give the steel cold brittleness, so the production does not allow the excess of its content in 0,045%. Sulfur causes the alloy redness, its concentration is limited to 0.05%.
There are several classifications of steel.
1. Depending on the chemical composition:
- carbonaceous, they contain iron, carbon and impurities;
- alloyed with various special additives.
2. Depending on carbon concentration:
- 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
- high quality;
- especially high quality.
Heat treatment of steel. basic information
The purpose of heat treatment of steel is to change the structure of the alloy, and
therefore, its properties, for example, giving the product hardness and brittleness or, conversely, softness and plasticity.
The essence of the process lies in the heating of the steel billet, its exposure and cooling. All this happens with strict observance of certain parameters, in particular, temperature and speed. Modes are also influenced by the classification of steels. Heat treatment of certain types of steel requires different conditions to achieve the same result.
Austenite hardness is 2-2.5 times higher than that of ferrite. The latter is more plastic. When cooled, the alloy structure changes in the reverse order.
The main types of heat treatment of steel - quenching, normalization, tempering, annealing.
The technology of this process consists of heating steel billets, holding and then slowly 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 increase ductility. Such heat treatment of steel is divided into two kinds. They have significant differences. In the first case, the heat treatment of steel does not imply structural changes based on phase transformations.
Annealing type I
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 lattice defects are formed, called dislocations and vacancies. When such a structure is formed, flattening and pulling out of the metal grains occur, due to which the work hardening occurs and the ductility of the alloy decreases.
This technology of heat treatment of steel implies heating up to temperatures above 100-200 ° C of the onset of crystallization (approximately 500-550 ° C).
The duration of exposure varies from 0.5 to 2 hours, then slow cooling is performed. The structure changes due to the formation of new grains and the gradual disappearance of the deformed. Thus, there is a decrease in lattice defects.
- Annealing for removing residual stresses. Internal stresses in steel parts arise as a result of processes such as welding, casting, cutting, grinding, hot deformation. They reach quite large quantities. As a result, together with the workers subsequently cause the destruction of the metal.
To eliminate this phenomenon, annealing is carried out at a temperature below the crystallization (727 ° C). During the process of 20 hours at 600 ºС, the stresses are almost completely eliminated. To reduce the duration of the process, the temperature is increased to 680-700 ºС.
Type II annealing
With this process, the equilibrium structure of the material is achieved during phase transformations. The structure of the steel after heat treatment is partially or completely changed. A fundamental change in the structure of the alloy occurs due to double recrystallization, which results in a reduction in grain size, the elimination of internal stresses, and the removal of work hardening. Types of heat treatment of steel - full (softening) and incomplete annealing.
As a result of this process, a large ferritic-pearlitic structure is transformed into a small austenitic structure, which, with slow cooling (30-50 ° C), is converted into a small ferritic-pearlitic one. In this way, structural steel is processed to increase ductility and reduce hardness.
As a result of incomplete annealing, plate perlite is transformed into granular ferritotsementit, passing through the stage of austenite (about 780 ºС). This process is used for tool steels.
Since annealing is a rather long operation (up to 20 hours), substance normalization is used as an alternative. This is heat treatment of steel, as a result of which its machinability is improved, the structure of welds is corrected, and also the alloy is prepared for hardening. Process temperature exceeds point Asz or ast depending on the type of steel at 30-50 ºС.
Normalization is, as a rule, heat 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.
It is a heat treatment of steel, due to which there is an increase in its strength, wear resistance, hardness, elastic limit, as well as a decrease in ductile properties. The quenching technology consists of heating to a certain temperature (approximately 850-900 ºС), holding 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 and without polymorphic transformation.
Hardening with polymorphic transformation is used for steels that contain an element capable of polymorphic transformations.
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. With a decrease in temperature, the type of lattice changes in the opposite direction, but since it occurs at high speed, an excess concentration of the element with a modified lattice remains in the alloy. Thus, a nonequilibrium structure arises, which is thermodynamically unstable. The acicular microstructure of steel, after heat treatment formed in the alloy, is called martensite. To relieve residual stresses, the metal is further subjected to tempering.
Quenching without polymorphic transformations is used in cases where one of the alloy components is partially dissolved in the other. When the alloy is heated above the solidus line, the component will dissolve. And with fast cooling, the secondary phase does not have time to return to its original state, since there is not enough time for the appearance of the phase boundary, the formation of the initial lattice and the diffusion process. As a result, a metastable solid solution with an excess content of the component arises. The process leads to an increase in the plasticity of the metal. Thermodynamic stability is achieved in the process of spontaneous or thermal aging.
Since the heat treatment of hardened steel has such an important decisive parameter as the cooling rate, the environments in which the process takes place (air, water, inert gases, oil, aqueous solutions of salts) should be mentioned.
The rate of cooling became water 6 times higher at a temperature of 600 ºС and 28 times at 200 ºС (in comparison with technical oil). It is used to cool carbon alloys with a high critical quenching rate. The lack of water is a fairly high cooling rate in the areas of martensite (200-300 ºС), which can lead to the formation of cracks. Salts are added to water to increase its hardening ability. Thus, for example, heat treatment of steel 45 occurs.
Alloys with a low critical quenching rate, which are alloyed, are cooled with oil. Its use is limited to easy flammability and the ability to stick to the surface of parts. Responsible carbon steel parts are cooled in two media: water and oil.
Martensitic steels on which there should be no oxide film, for example, used for medical equipment, are cooled in a discharged atmosphere or in air.
In order to convert residual austenite, which makes the steel brittle, into martensite, additional cooling is used.
To this end, the parts are placed in a refrigerator with a temperature of -40 - -100 ºС 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 alloyed steel, stabilize the dimensional parameters of high-precision parts, and increase the magnetic qualities of the metal.
For some parts, such as shafts, cams, axles, gears, fingers for abrasion couplings, 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 quenching, high-frequency currents are used, which are formed by a transformer from a special generator. They heat the surface of the part covered by the inductor. Then the part is cooled in air. The thickness of the quenched surface layer can vary from 1 to 10 mm.
This is heat treatment of steel, aimed at easing internal stresses that occur during quenching, as well as at increasing viscosity. Such processing is applied to steels that have undergone polymorphic transformations. The modes of heat treatment of steel include heating to a temperature of 150-650 ºС, holding and cooling, the speed of which does not matter. During the tempering process, the harder, but unstable structures are transformed into more plastic and stable ones. Vacation is high, medium and low.
At low tempering, heating to 150-250 ºС occurs, followed by exposure to 1.5 hours and cooling in air or in oil. The crystal lattice of martensite changes, which has no effect on hardness, increases viscosity and relieves internal stresses. In this way, the cutting and measuring tools are processed.
With an average vacation, heating to 300-500 ºС occurs. The steel structure is represented by a troost of holidays. Steel details after processing are characterized by high elastic properties and strength characteristics. This is the processing of springs, membranes, springs.
High tempering is characterized by a heating temperature of 450-650 ºС, which leads to the formation of sorbitol. Products become less hard, plastic, have a high impact strength. Gears, axles, rollers and other critical parts of mechanisms are subjected to it.
Chemical heat treatment of steel
It helps to increase the strength and hardness of the alloy, its corrosion resistance, imparting anti-friction 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.
Chemical-heat treatment of steel is based on such processes as dissociation, diffusion and adsorption. Depending on the saturating element, it is divided into nitriding, cementation, cyanation, etc.
The task of cementation is to obtain a hard surface on low carbon steel parts with a sufficiently viscous core. The process is carried out in the carburizer at 930-950 ºС, since at this temperature austenite is most stable. In this way is treated as low-carbon and alloyed alloys. The processing is influenced by the classification of steel. Heat treatment of certain types of steel requires special parameters to achieve a result.
Cementation is divided into solid and gas. In the second case, it becomes possible to obtain a certain carbon content in the surface layer, reduce the duration of the process, automate. This is a better way than hard cementation.
Heat treatment is carried out to reduce the granularity of the core and the cemented layer, and therefore, to improve the mechanical properties. Temperature treatment consists of double hardening and low tempering at temperatures of 160-180 ºС.
It assumes the saturation of the surface layers of the parts made of alloyed steels with nitrogen atoms by diffusion. As a result, the nitrogen reacts with alloying elements (molybdenum, chromium, aluminum) with the formation of solid and stable compounds - nitrides.
The advantage is a lower processing temperature compared with the cementation 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 ºС). Characteristics of the cemented layer are stable at temperatures up to 220 ºС.
This is the process of one-time filling of the surface of steel with nitrogen and carbon atoms. The technology involves the use of both liquid and gas phase. Cyanide can also be low and high temperature.
When the liquid used special baths filled with cyanide and neutral salts. After saturation of the surface with nitrogen, the process actually turns into cementation. At low-temperature cyanidation, the parts are further subjected to additional heat treatment.
Gas cementing occurs in a medium containing nitrating and cementing gases. With this method of cyanidation, the depth of the treated layers reaches 1.8 mm.