Thermo-mechanical processing of metals is a set of operations of deformation, heating and cooling, as a result of which the formation of the final structure and properties of the material occurs under conditions of increased density and optimal distribution of structural imperfections created by plastic deformation.
Thermo-mechanical processing of steel is carried out mainly according to three schemes: high-temperature (HTMT), low-temperature (LTMT) and preliminary thermo-mechanical treatment (PTMT).
main idea high temperature treatment consists in selecting rolling and cooling modes after rolling, which ensures the production of fine and uniform grain in the finished product.
Low temperature processing consists of heating steel to 1000..L 100 °C, rapid cooling to the temperature of the metastable state of austenite (400...600 °C) and a high degree (up to 90% and above) of deformation at this temperature. After this, hardening for martensite and tempering at 100...400 °C are performed. The result is a significant increase in strength compared to HTMO, but lower ductility and toughness. This method is applicable almost only to alloy steels.
Preliminary thermomechanical treatment characterized by the simplicity of the technological process: cold plastic deformation (increases the dislocation density), pre-recrystallization heating (provides polygonization of the ferrite structure), quenching and tempering.
19. Copper and copper-based alloys. Marking of bronze and brass. Application of copper-based alloys in sanitary engineering.
Copper- a viscous, viscous metal of red color (pink when broken), in very thin layers it looks greenish-blue when viewed through light.
The properties of the resulting product depend on the purity, and the level of impurity content determines its grade: MOOk - at least 99.99% copper, MOK - 99.97%, M1K - 99.95%, M2k - 99.93% copper, etc. marks after the letter M (copper) indicate the conditional number of purity, and then the letter the method and conditions for obtaining copper: k - cathode; b - oxygen-free; p - deoxidized; f - deoxidized by phosphorus. Harmful impurities that reduce the mechanical and technological properties of copper and its alloys are lead, bismuth, sulfur and oxygen. Their content in copper is strictly limited: bismuth - no more than 0.005%, lead - 0.05%, etc.
Copper is a heavy non-ferrous metal. The density is 8890 kg/m 3, the melting point is 1083 °C. Pure copper has high electrical and thermal conductivity.
Copper has high ductility and excellent workability under cold and hot pressure, good casting properties and satisfactory cutting machinability. The mechanical properties of copper are relatively low: tensile strength is 150...200 MPa, relative elongation is 15...25%.
Binary or multicomponent alloys of copper with zinc and other elements are called brasses.
Brasses are marked with the letter L (brass), followed by numbers indicating the percentage of copper. For example, L68 brass contains 68% copper, the rest is zinc. If the brass is multi-component, then after the letter L they put the symbol for other elements (A - aluminum, F - iron, N - nickel, K - silicon, T - titanium, Mts - manganese, O - tin, C - lead, C - zinc and etc.) and numbers indicating their average percentage in the alloy. The order of letters and numbers is different in wrought and cast brasses. In cast brasses, the average content of the alloy component is indicated immediately after the letter indicating its name.
Bronze- an alloy of copper with tin, aluminum, lead and other elements, of which zinc and nickel are not the main ones. Zinc and nickel can be introduced into bronzes only as additional alloying elements. According to the chemical composition, bronzes are divided into tin to tinless.
Bronze is marked with the letters Br, followed by alphabetic and numerical designations of the elements contained except copper. The designation of elements in bronzes is the same as when marking brass. The presence of copper in the brand is not indicated, and its content is determined by the difference. In grades of bronzes processed by pressure, the names of alloying elements are indicated in descending order of their concentration, and at the end of the grade their average concentrations are indicated in the same sequence. For example, bronze grade BrOTsS4-4-2.5 contains 4% tin and zinc, 2.5% lead, the rest is copper. In grades of cast bronzes (GOST 613 and 493), after each designation of an alloying element its content is indicated. If the compositions of casting and pressure-processed bronzes overlap, for example BrA9ZZL.
20. Aluminum and aluminum-based alloys. Application of aluminum-based alloys in sanitary technology.
Aluminum is a silvery-white light metal with a density of 2.7 g/cm3 and a melting point of 660 °C. It is characterized by high thermal and electrical conductivity and good corrosion resistance in many aggressive environments. The purer the aluminum, the higher its corrosion resistance.
Depending on the content of impurities, aluminum is divided into groups and grades: high purity aluminum A999 - 99.999% aluminum, high purity grades: A995 - 99.995%, A99 - 99.99%, A97 - 99.97%, A95 - 99.95 % aluminum, technical purity with impurity content OD5...1.0%: A85, A8, A7, A6, A5, AO. For example, the A85 grade means that the metal contains 99.85% aluminum, and the AO grade means 99% aluminum. Technical deformable aluminum is marked ADO and AD1. Fe, Si, Cu, Mn, Zn, etc. may be present as impurities in aluminum.
According to technical characteristics, all aluminum alloys are divided into 2 classes:
Castable and non-deformable.
Duralumins are the most common alloys of this group, based on aluminum, copper and magnesium. Duralumin is characterized by a combination of high strength and ductility, and is easily deformed in hot and cold states.
Silumins is the general name for a group of casting alloys based on aluminum containing silicon (4... 13% and in some brands up to 23%) and some other elements. Silumins have high casting properties, fairly high strength, increased corrosion resistance, and can be easily machined.
The degree of influence of the liquid metal medium on the deformed material depends on its thermal and thermomechanical treatment. To a large extent, this influence is determined by the level of strength and grain size that the materials acquire as a result of processing. However, the effect of thermal and thermomechanical treatment is also associated with some features of the structural state of the material.V. G. Markov investigated the effect of liquid tin on pearlitic chromium-molybdenum-vanadium steels tempered at various temperatures. In all cases, hardening was carried out at 990° C, and tempering at 270, 370, 470, 570, 670 and 770° C; the duration of tempering at each temperature was 1.5 hours. From steel blanks that had undergone the specified heat treatment conditions, samples with a cylindrical working part with a diameter of 6 mm were made, which were then tested in tension at a speed of 1.25 mm/min. The samples were tested in a bath of liquid tin and in air at a temperature of 250/650° C.
It has been established that steel is exposed to the greatest impact of liquid metal after low and medium tempering (at a temperature of 270/470 ° C). Samples that have undergone such heat treatment fail brittlely, without plastic deformation, their tensile strength is 1.5-2 times lower than the yield strength in air. Samples tempered at 570°C are destroyed in the tin by some plastic deformation; their tensile diagram ends in the region of uniform deformation. Tempering at 670° C leads to a further weakening of the influence of tin on steel. In this case, the yield strength, tensile strength and uniform elongation of the samples tested in air and in tin are the same; the influence of liquid metal is expressed only in a decrease in concentrated elongation. Samples tempered at 770° C did not reveal any influence of the liquid metal medium.
Thus, an increase in the tempering temperature leads to a decrease in the effect of liquid metal on the mechanical properties of pearlitic steel. The main reason for the weakening of the effect is due in this case, apparently, to a decrease in the strength of the steel. Thus, the tensile strength in air changes continuously from approximately 130 kg/mm2 after tempering at 270°C to 55 kg/mm2 after tempering at 670°C.
Similar patterns of influence of heat treatment of 30KhGSA steel on the magnitude of the effect of liquid tin and tin-lead solder were established in the works, their results are discussed above (see Table 35). The work noted that high-temperature tempering of pearlitic chromium-nickel and carbon steels reduces their sensitivity to the effects of molten solders.
The authors of the work investigated the effect of mercury at room temperature on the mechanical properties of dispersion-hardening aluminum alloys depending on the duration of aging. In Fig. 88 shows the test results of an aluminum alloy alloyed with 4.5% Cu, 0.6% Mn and 1.5% Mg. It can be seen that an increase in the duration of aging of the alloy, accompanied by hardening in air, leads to a sharp drop in its strength in a liquid mercury environment. It is interesting that even a slight strengthening of the alloy at the beginning of the aging process causes a strong influence of the liquid metal. This indicates the dependence of the influence of the liquid metal medium on the structural state of the material.
A slightly different nature of the influence of the liquid metal (mercury with 2% Na) was observed during the aging of the Cu - 2% Be alloy. From Fig. 89 it follows that testing an alloy in liquid metal does not distort (qualitatively) the nature of the effect of aging on its yield strength. In this case, the usual stages of hardening and then softening (with increasing exposure) associated with overaging of the alloy are observed. As for the influence of liquid metal on the relative elongation of the material, it was similar to the effect on strength established in the work, i.e. the effect of the environment, expressed in a decrease in the relative elongation, increases as the alloy hardens and is greatest at maximum hardening. Overaging of the alloy leads to a decrease in the embrittlement effect of the liquid metal coating.
In Fig. 89 also shows the results of testing a copper-beryllium alloy subjected to work hardening after quenching. This treatment promotes even greater strengthening of the alloy during aging, but the decrease in relative elongation is much less pronounced. For example, the greatest reduction in elongation after quenching and work hardening was about 60%, while after quenching alone it was close to 100%.
The use of cold hardening after heat treatment of the alloy, as shown in the works, usually does not cause a change in the degree of exposure to the liquid metal. Thus, hardening of a copper-beryllium alloy after quenching and aging at 370° C for 0.5 and 12 hours, i.e., before and after the peak of hardening (see Fig. 89), does not lead to either strengthening or weakening the influence of the liquid metal medium. The alloy that underwent maximum hardening during heat treatment (quenching and aging at 370°C for 1 hour) showed increased exposure to the environment with increasing degree of hardening.
Thermo-mechanical processing of a material in some cases makes it possible to increase its strength in a liquid metal environment. The work investigated the effect of thermomechanical treatment on the mechanical properties of 40X steel in air and in contact with Pb-Sn eutectic. Cylindrical samples with a diameter of 10 mm with a circular cut were tested. The material was processed in the area of the stress concentrator. The sample was installed on a special machine and heated by passing an electric current through it to the austenitization temperature; then it was cooled to a temperature of 400/600 ° C, at which the concentrator was rolled in with profile rollers. The initial depth of the cut made on the lathe was 1 mm, the radius at the apex was 0.2 mm, and the angle was 0.8 rad. By rolling in with rollers, the depth of the cut increased to 1.5 mm, the radius remained unchanged. After running in, the sample was quenched in oil and then tempered. In addition to thermomechanical treatment with rolling rollers, treatment with torsional deformation of the sample was also used. The influence of cold hardening at room temperature on the effect of liquid metal on steel after quenching and normalization was also assessed.
From those shown in Fig. 90 tensile diagrams show that at temperatures of 400 and 500 ° C, hardened samples are destroyed under the action of liquid metal in the elastic region, experiencing a multiple decrease in strength. Some increase in strength is achieved by cold hardening of samples, rolling with rollers at room temperature and thermomechanical treatment using torsion. The greatest increase in strength is achieved by thermomechanical processing using rolling of samples with rollers. However, although when tested in air such treatment gives a sharp increase in the ductility of the samples, when tested in the melt the samples fail brittlely. It should be noted that the thermomechanical treatment method, which turned out to be effective for 40X steel, did not give a positive result for 2X13 steel either when tested in air or in a Pb-Sn eutectic melt. The degree of influence of the liquid metal in this case was approximately the same as after quenching and tempering, imparting the same level of strength and ductility to the steel.
The above data show that increasing the strength of a material as a result of thermal or thermomechanical treatment usually leads to increased exposure to liquid metal. The effect of strengthening of 40X steel in the Pb-Bi eutectic after rolling the stress concentrator with rollers is obviously associated mainly with the appearance of compressive stresses in the surface layer of the sample, since thermomechanical treatment in the same mode, but with deformation of the sample by torsion, does not lead to similar results. The structural factor apparently influences the degree of influence of the liquid metal medium in the case of testing dispersion-strengthened alloys. One should expect an increase in the influence of the environment on these alloys, since significant stress concentrations may appear in them in the area of finely dispersed precipitates, which are serious obstacles to the movement of dislocations.
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Thermo-mechanical processing involves plastic deformation, which affects the formation of structure during thermal exposure of the metal. Plastic deformation changes the nature of the distribution and increases the density of crystal lattice defects, which in turn greatly affects the nature of structure formation during phase transformations. Thus, after TMT, a structure with an increased density of defects in the crystalline structure is formed in the alloy, which leads to the acquisition of new mechanical properties.
For steel, mainly two types of thermomechanical processing are used: low-temperature and high-temperature.
During LTMT, supercooled austenite is deformed in the region of its increased stability, but necessarily below the temperature at which recrystallization begins. After this, it turns into martensite (Fig. 53). Low tempering is used as the final heat treatment.
The reason for the strengthening of steel during LTMT is the inheritance of the dislocation structure of deformed austenite by martensite. Dislocations do not disappear during the formation of martensite, but are transferred from the original phase to the new one, i.e. martensite inherits the substructure of deformed austenite. The high density of dislocations fixed by carbon atoms and carbide inclusions results in high strength with an acceptable level of ductility.
Rice. 53 Low temperature circuit (LTMO)
thermomechanical processing of steel
LTMT is applicable only for alloy steels with a sufficient level of stability of supercooled austenite. In addition, carrying out scientific and technical treatment requires the presence of powerful deforming equipment.
During HTMT, austenite is deformed in the region of its high-temperature stability, and then hardened to martensite (Fig. 54). Hardening is followed by low tempering.
Rice. 54 High temperature circuit (HTMO)
thermomechanical processing of steel.
The HTMT mode is chosen so that by the beginning of the martensitic transformation the austenite has a developed polygonized structure. The degree of deformation should not be too great, so as not to cause recrystallization, which reduces hardening. After completion of deformation, immediate hardening is necessary to prevent static recrystallization and maintain the deformed structure at the beginning of the martensitic transformation. Martensitic crystals do not extend beyond the austenite subgrains, which causes their significant refinement and a high range of properties.
The most important advantage of HTMO is the ability to simultaneously increase both strength and fracture toughness. In addition, powerful specialized equipment is not required to perform VTMO.
6. Chemical-thermal treatment of steel
6.1. general characteristics chemical-thermal treatment of steel
Chemical-thermal treatment (CHT) is the surface saturation of steel with certain chemical elements, namely non-metals and metals (for example, carbon, nitrogen, aluminum, chromium, etc.) through their diffusion in the atomic state from the external environment at high temperature. During these processes, the chemical composition, microstructure and properties of the surface layers of products necessarily change. During chemical treatment, the parts being processed are heated in some chemically active environment. The main processing parameters are heating temperature and holding time. CTO is usually carried out over a long period of time. The process temperature is chosen specifically for each type of processing.
The primary processes of any type of CTO are dissociation, absorption and diffusion.
Dissociation - decomposition chemical compound to obtain chemical elements in a more active, atomic state. Absorption is the absorption of atoms of the specified non-metals by the surface of the part. Diffusion is the movement of an absorbed element deep into the product. The speeds of all three processes must be consistent with each other. For absorption and diffusion, it is necessary that the saturating element interact with the base metal to form either a solid solution or a chemical compound, since in the absence of this, chemical-thermal treatment is impossible.
The main types of chemical-thermal treatment of steel are carburization, nitriding, nitrocarburization, cyanidation and diffusion metallization.
The rate of diffusion of atoms into the iron lattice is not the same and depends on the composition and structure of the resulting phases. When saturated with carbon or nitrogen, which form interstitial solid solutions with iron, diffusion proceeds faster than when saturated with metals forming interstitial solid solutions. Therefore, in this case, higher temperatures and longer processing times are used, but despite this, a smaller layer thickness is obtained than with nitriding and especially carburizing.
When determining the thickness of the diffusion layer obtained by saturating steel with one or another element, usually not its full value with a changed composition is indicated, but only the depth to a certain hardness or structure (effective thickness).
In contrast to thermal treatment itself, chemical-thermal and thermomechanical treatment, in addition to thermal effects, include, respectively, chemical and deformation effects on the metal. This complicates the overall picture of changes in structure and properties during heat treatment.
Equipment for chemical-thermal and thermomechanical treatments is, as a rule, more complex than for heat treatment itself. In addition to conventional heating devices, it includes, for example, installations for creating a controlled atmosphere, equipment for plastic deformation.
Below we consider the general patterns of changes in structure and properties during chemical-thermal and thermomechanical treatments and their varieties.
"Theory of heat treatment of metals",
I.I.Novikov
During HTMT, austenite is deformed in the region of its thermodynamic stability and then hardened to martensite (see figure for alloy steel processing scheme). After hardening, a low tempering is carried out. The main goal of conventional heat treatment with deformation (rolling forging) heating is to eliminate special heating for hardening and thereby obtain an economic effect. The main goal of HTMT is to improve the mechanical properties...
Of great interest is the phenomenon of inheritance (“reversibility”) of hardening from HTMT discovered by M. L. Bernstein during repeated heat treatment. It turned out that the hardening from HTMT is preserved if the steel is re-hardened with a short exposure at the heating temperature for quenching or if the HTMT-strengthened steel is first subjected to high tempering and then re-hardened. For example, the tensile strength of steel 37XH3A after HTMT according to the regime...
The processes of TMT of steels began to be intensively studied in the mid-50s in connection with the search for new ways to increase structural strength. Low-temperature thermomechanical treatment (LTMT) During LTMT, supercooled austenite is deformed in the region of its increased stability, but always below the temperature of the onset of recrystallization and then (transforms into martensite. After this, low tempering is carried out (not shown in the figure). Processing scheme...
The use of HTMO is limited by the following factors. The alloy may have such a narrow range of heating temperatures for quenching that it is practically impossible to maintain the temperature of hot forming within such narrow limits (for example, within ± 5 °C for D16 duralumin). The optimal temperature range for hot deformation may be significantly lower than the heating temperature range for hardening. For example, when pressing aluminum alloys...
The essence of PTMT is that the semi-finished product obtained after hot deformation in a non-recrystallized state retains a non-recrystallized structure even when heated for quenching. PTMT differs from HTMT in that the operations of hot deformation and heating for hardening are separated (see figure Schemes of thermomechanical processing of aging alloys). PTMO is widely used in the production technology of semi-finished products from aluminum alloys. It was a long time ago...
During HTMT, hot deformation, hardening with deformation heating and aging are carried out (see figure Schemes of thermomechanical processing of aging alloys). During hot deformation, the dislocation density increases and hot hardening occurs, which during the deformation itself can be partially or completely removed as a result of the development of dynamic polygonization and dynamic recrystallization. The stress-strain curve has a section of increasing flow stress,...
The figure shows the main diagrams of TMT of aging alloys. Jagged lines indicate plastic deformation. Schemes for thermomechanical treatment of aging alloys Low-temperature thermomechanical treatment (LTMT) LTMT of aging alloys is the first thermomechanical treatment (30s) and the most widely used in industry. The main purpose of NTMO is to increase strength properties. In HTMT, the alloy is first subjected to conventional quenching,...
Let us first consider the effect of cold deformation on zone aging. It would seem that deformation, by increasing the dislocation density and vacancy concentration, should accelerate zone aging. But, firstly, zones nucleate homogeneously, and not on dislocations, and, secondly, dislocations are effective sites for the drain of vacancies. Very strong plastic deformation increases the vacancy concentration (the ratio of the number of vacancies to the number of atoms) by only 10–6....
The effectiveness of using LTMO is determined by which strengthening phase is released during aging. So, for example, additional strengthening from the introduction of deformation before artificial aging for Al - Cu - Mg alloys (hardening agent - phase S) is greater than for Al - Cu alloys (hardening agent - phase θ´). When heated for aging after cold deformation, recrystallization, as a rule, does not occur, but...
in order to change specifications metal, you can create an alloy based on it and add other components to it. However, there is another way to change the parameters of a metal product - heat treatment of the metal. With its help, you can influence the structure of the material and change its characteristics.
Heat treatment of metal is a series of processes that allow you to remove residual stress from a part, change the internal structure of the material, and improve performance. The chemical composition of the metal does not change after heating. When the workpiece is uniformly heated, the grain size of the material structure changes.
Story
The technology of heat treatment of metal has been known to mankind since ancient times. During the Middle Ages, blacksmiths heated and cooled sword blanks using water. By the 19th century, people learned to process cast iron. Blacksmith putting metal into container full of ice, and sprinkled sugar on top. Next, the process of uniform heating begins, lasting 20 hours. After this, the cast iron billet could be forged.
In the mid-19th century, Russian metallurgist D.K. Chernov documented that when a metal is heated, its parameters change. From this scientist came the science of materials science.
Why is heat treatment needed?
Equipment parts and communication units made of metal are often subjected to severe loads. In addition to exposure to pressure, they may be exposed to critical temperatures. To withstand such conditions, the material must be wear-resistant, reliable and durable.
Purchased metal structures are not always able to withstand loads for a long time. To make them last much longer, metallurgy masters use heat treatment. During and after heating, the chemical composition of the metal remains the same, but the characteristics change. The heat treatment process increases the corrosion resistance, wear resistance and strength of the material.
Benefits of Heat Treatment
Heat treatment of metal blanks is a mandatory process when it comes to the manufacture of structures for long-term use. This technology has a number of advantages:
- Increased wear resistance of the metal.
- Finished parts last longer, and the number of defective workpieces is reduced.
- Improves resistance to corrosion processes.
After heat treatment, metal structures can withstand heavy loads and their service life increases.
Types of heat treatment of steel
In metallurgy, three types of steel processing are used: technical, thermomechanical and chemical-thermal. Each of the presented methods of heat treatment must be discussed separately.
Annealing
A type or another stage of technical metal processing. This process involves uniform heating of a metal workpiece to a certain temperature and its subsequent cooling naturally. After annealing, the internal stress of the metal and its heterogeneity disappear. The material softens under the influence of temperature. It is easier to process in the future.
There are two types of annealing:
- First kind. There is a slight change in the crystal lattice in the metal.
- Second kind. Phase changes in the structure of the material begin. It is also called complete annealing of the metal.
The temperature range during this process is from 25 to 1200 degrees.
Hardening
Another stage of technical processing. Metal hardening is carried out to increase the strength of the workpiece and reduce its ductility. The product is heated to critical temperatures and then quickly cooled by dipping into a bath with various liquids. Types of hardening:
- Two-stage cooling. Initially, the workpiece is cooled to 300 degrees with water. After this, the part is placed in a bath filled with oil.
- Using one liquid. If small parts are processed, oil is used. Large workpieces are cooled with water.
- Stepped. After heating, the workpiece is cooled in molten salts. After this, it is laid out in fresh air until it cools completely.
You can also distinguish the isothermal type of hardening. It is similar to the step method, but the holding time of the workpiece in the molten salts changes.
Thermo-mechanical treatment
This is a typical mode of heat treatment of steels. This technological process uses equipment that creates pressure, heating elements and cooling tanks. At different temperatures, the workpiece is heated, and after this plastic deformation occurs.
Vacation
This is the final stage of technical heat treatment of steel. This process is carried out after hardening. The viscosity of the metal increases and internal stress is relieved. The material becomes more durable. Can be carried out at different temperatures. This changes the process itself.
Cryogenic treatment
The main difference between heat treatment and cryogenic exposure is that the latter involves cooling the workpiece. At the end of this procedure, the parts become stronger, do not require tempering, and are better ground and polished.
When interacting with cooling media, the temperature drops to minus 195 degrees. The cooling rate may vary depending on the material. To cool the product to the desired temperature, a processor is used that generates cold. The workpiece is cooled evenly and remains in the chamber for a certain period of time. After that, take it out and allow it to warm up to room temperature on its own.
Chemical-thermal treatment
Another type of heat treatment, in which the workpiece is heated and exposed to various chemical elements. The surface of the workpiece is cleaned and coated with chemical compounds. This process is carried out before hardening.
The master can saturate the surface of the product with nitrogen. To do this, they are heated to 650 degrees. When heated, the workpiece must be in a cryogenic atmosphere.
Heat treatment of non-ferrous alloys
The presented types of heat treatment of metals are not suitable for various types of alloys and non-ferrous metals. For example, when working with copper, recrystallization annealing is carried out. Bronze heats up to 550 degrees. They work with brass at 200 degrees. Aluminum is initially hardened, then annealed and aged.
Heat treatment of metal is considered a necessary process in the manufacture and further use of structures and parts for industrial equipment, cars, aircraft, ships and other equipment. The material becomes stronger, more durable and more resistant to corrosion processes. The choice of technological process depends on the metal or alloy used.