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    Today, aluminum is used in almost all industries, from food utensils to spacecraft fuselages. For certain production processes, only certain grades of aluminum are suitable, which have certain physical and chemical properties.

    The main properties of the metal are high thermal conductivity, ductility and ductility, corrosion resistance, low weight and low ohmic resistance. They are directly dependent on the percentage of impurities included in its composition, as well as on the technology of production or enrichment. In accordance with this, the main grades of aluminum are distinguished.

    Types of aluminum

    All metal grades are described and included in a unified system of recognized national and international standards: European EN, American ASTM and international ISO. In our country, aluminum grades are defined by GOST 11069 and 4784. All documents are considered separately. Moreover, the metal itself is subdivided into grades, and alloys do not have specific signs.

    In accordance with national and international standards, two types of microstructure of unalloyed aluminum should be distinguished:

    • high purity with a percentage of more than 99.95%;
    • technical purity, containing about 1% of impurities and additives.

    Iron and silicon compounds are most often considered as impurities. In the international ISO standard for aluminum and its alloys, a separate series is allocated.

    Aluminum grades

    The technical type of material is divided into certain grades, which are assigned to the relevant standards, for example, AD0 in accordance with GOST 4784-97. At the same time, high-frequency metal is also included in the classification, so as not to create confusion. This specification contains the following brands:

    1. Primary (A5, A95, A7E).
    2. Technical (AD1, AD000, ADS).
    3. Deformable (AMg2, D1).
    4. Foundry (VAL10M, AK12pch).
    5. For steel deoxidation (AB86, AV97F).

    In addition, there are also categories of ligatures - aluminum compounds that are used to create alloys from gold, silver, platinum and other precious metals.

    Primary aluminum

    Primary aluminum (grade A5) is a typical example of this group. It is obtained by enriching alumina. In nature, the pure metal is not found due to its high chemical activity. Combining with other elements, it forms bauxite, nepheline and alunite. Subsequently, alumina is obtained from these ores, and pure aluminum is obtained from it using complex chemical-physical processes.

    GOST 11069 establishes requirements for grades of primary aluminum, which should be noted by applying vertical and horizontal stripes with indelible paint of various colors. This material has found wide application in advanced industries, mainly where high technical characteristics are required from raw materials.

    Technical aluminum

    Technical aluminum is a material with a percentage of foreign impurities of less than 1%. Very often it is also called unalloyed. Technical grades of aluminum in accordance with GOST 4784-97 are characterized by very low strength, but high corrosion resistance. Due to the absence of alloying particles in the composition, a protective oxide film is quickly formed on the metal surface, which is stable.

    Technical aluminum grades are distinguished by good thermal and electrical conductivity. In their molecular lattice, there are practically no impurities that scatter the flow of electrons. Due to these properties, the material is actively used in instrument making, in the production of heating and heat exchange equipment, and lighting items.

    Deformable aluminum

    Deformable aluminum refers to a material that is subjected to hot and cold working by pressure: rolling, pressing, drawing and other types. As a result of plastic deformations, semi-finished products of various longitudinal sections are obtained from it: aluminum bar, sheet, tape, plate, profiles and others.

    The main brands of deformable material used in domestic production are given in the regulatory documents: GOST 4784, OCT1 92014-90, OCT1 90048 and OCT1 90026. Characteristic feature A deformable raw material is a solid structure of a solution with a high content of eutectic - a liquid phase that is in equilibrium with two or more solid states of matter.

    The area of ​​application of deformable aluminum, like that where an aluminum bar is used, is quite extensive. It is used both in areas requiring high technical characteristics from materials - in ship- and aircraft construction, and on construction sites as an alloy for welding.

    Cast aluminum

    Casting aluminum grades are used for the production of fittings. Their main feature is a combination of high specific strength and low density, which makes it possible to cast products of complex shapes without cracking.

    According to their purpose, foundry brands are conventionally divided into groups:

    1. High-tight materials (AL2, AL9, AL4M).
    2. Materials with high strength and heat resistance (AL 19, AL5, AL33).
    3. Substances with high corrosion resistance.

    Very often, the performance of cast aluminum products increases different kinds heat treatment.

    Aluminum for deoxidation

    The quality of manufactured products is also influenced by the physical properties of aluminum. And the use of low-grade grades of material is not limited to the creation of semi-finished products. Very often it is used to deoxidize steel - removing oxygen from molten iron, which is dissolved in it, and thereby increases the mechanical properties of the metal. For this process the most commonly used brands are AB86 and AV97F.

    1.2.1. general characteristics steels. Steel is an alloy of iron with carbon containing alloying additives that improve the quality of the metal, and harmful impurities that enter the metal from the ore or are formed during the smelting process.

    Steel structure. In the solid state, steel is a polycrystalline body, consisting of many differently oriented crystals (grains). In each crystal, atoms (more precisely, positively charged ions) are arranged in an orderly manner at the sites of the spatial lattice. The steel is characterized by a body-centered (bcc) and face-centered (fcc) cubic crystal lattice (Fig. 1.4). Each grain as a crystalline formation is sharply anisotropic and has different properties in different directions. With a large number of differently oriented grains, these differences are smoothed out, statistically, on average in all directions, the properties become the same, and the steel behaves like a quasi-isotropic body.

    The structure of the steel depends on the crystallization conditions, chemical composition, heat treatment and rolling conditions.

    The melting point of pure iron is 1535 ° C; during hardening, crystals of pure iron are formed - ferrite, the so-called 8-iron with a body-centered lattice (Fig. 1.4, a); at a temperature of 1490 ° C, recrystallization occurs, and 5-iron passes into γ-iron with a face-centered lattice (Fig. 1.4, b). At a temperature of 910 ° C and below, the γ-iron crystals again turn into body-centered ones, and this state remains up to normal temperature. The last modification is called a-iron.

    With the introduction of carbon, the melting point decreases and for steel with a carbon content of 0.2% is approximately 1520 ° C. On cooling, a solid solution of carbon in y-iron is formed, called austenite, in which carbon atoms are located in the center of the fcc lattice. At temperatures below 910 ° C, the decomposition of austenite begins. The resulting -iron with a bcc lattice (ferrite) poorly dissolves carbon. As ferrite is released, austenite is enriched with carbon and at a temperature of 723 ° C turns into pearlite - a mixture of ferrite and iron carbide Fe 3 C, called cementite.

    Rice. 1.4. Cubic crystal lattice:

    a- body-centered;

    b- face-centered

    Thus, at normal temperature, steel consists of two main phases: ferrite and cementite, which form independent grains, and are also included in the form of plates in the composition of pearlite (Fig. 1.5). Light grains - ferrite, dark - perlite).

    Ferrite is extremely plastic and low strength, cementite is hard and brittle. Perlite has properties that are intermediate between those of ferrite and cementite. Depending on the carbon content, one or another structural component predominates. The size of ferrite and pearlite grains depends on the number of crystallization centers and cooling conditions and significantly affects the mechanical properties of steel (the finer the grain, the higher the quality of the metal).



    Alloying additives, entering the solid solution with ferrite, strengthen it. In addition, some of them, forming carbides and nitrides, increase the number of crystallization sites and contribute to the formation of a fine-grained structure.

    Under the influence of heat treatment, the structure, grain size and solubility of alloying elements change, which leads to a change in the properties of steel.

    The simplest type of heat treatment is normalization. It consists in reheating the rolled stock to the temperature of austenite formation and subsequent cooling in air. After normalization, the steel structure is more ordered, which leads to an improvement in the strength and plastic properties of rolled steel and its impact toughness, as well as an increase in homogeneity.

    With rapid cooling of steel heated to a temperature exceeding the phase transformation temperature, the steel is hardened.

    The structures formed after hardening give the steel high strength. However, its plasticity decreases, and the tendency to brittle fracture increases. To regulate the mechanical properties of the hardened steel and the formation of the desired structure, it is tempered, i.e. heating to the temperature at which the desired structural transformation occurs, holding at this temperature for the required time and then slowly cooling 1.



    During rolling, as a result of reduction, the structure of the steel changes. There is a grinding of grains and their different orientation along and across the rolled product, which leads to a certain anisotropy of properties. The rolling temperature and the cooling rate also have a significant influence. At a high cooling rate, the formation of quenching structures is possible, which leads to an increase in the strength properties of the steel. The thicker the rolled stock, the lower the reduction rate and the cooling rate. Therefore, with an increase in the thickness of rolled products, the strength characteristics decrease.

    Thus, by varying the chemical composition, rolling and heat treatment modes, it is possible to change the structure and obtain steel with specified strength and other properties.

    Classification of steels. According to their strength properties, steels are conventionally divided into three groups: conventional (<29 кН/см 2), повышенной ( = 29...40 кН/см 2) и высокой прочности ( >40 kN / cm 2).

    Increasing the strength of steel is achieved by alloying and heat treatment.

    By chemical composition, steels are subdivided into carbon and alloy steels. Common grade carbon steels are composed of iron and carbon with some

    the addition of silicon (or aluminum) and manganese. Other additives are not specially introduced and can get into steel from ore (copper, chromium, etc.).

    Carbon (U) 1, increasing the strength of steel, reduces its ductility and worsens weldability, therefore, only low-carbon steels with a carbon content of not more than 0.22% are used for building metal structures.

    In addition to iron and carbon, alloy steels contain special additives that improve their quality. Since the majority of additives to one degree or another worsen the weldability of steel, as well as increase its cost, low-alloy steels with a total content of alloying additives of no more than 5% are mainly used in construction.

    The main alloying additions are silicon (C), manganese (G), copper (D), chromium (X), nickel (N), vanadium (F), molybdenum (M), aluminum (Yu), nitrogen (A).

    Silicon deoxidizes steel, i.e. binds excess oxygen and increases its strength, but reduces ductility, worsens weldability and corrosion resistance at an increased content. The harmful effect of silicon can be compensated for by the increased content of manganese.

    Manganese increases strength, is a good deoxidizer and, combining with sulfur, reduces its harmful effects. When the manganese content is more than 1.5%, the steel becomes brittle.

    Copper slightly increases the strength of steel and increases its resistance to corrosion. Excessive copper content (more than 0.7%) contributes to the aging of steel and increases its brittleness.

    Chromium and nickel increase the strength of steel without reducing ductility and improve its corrosion resistance.

    Aluminum deoxidizes steel well, neutralizes the harmful effect of phosphorus, and increases toughness.

    Vanadium and molybdenum increase strength with almost no reduction in ductility and prevent softening of heat-treated steel during welding.

    Unbound nitrogen contributes to the aging of steel and makes it brittle, so it should be no more than 0.009%. In a chemically bound state with aluminum, vanadium, titanium and other elements, it forms nitrides and becomes an alloying element, contributing to the formation of a fine-grained structure and improvement of mechanical properties.

    Phosphorus belongs to harmful impurities, since, forming a solid solution with ferrite, it increases the brittleness of steel, especially at low temperatures (cold brittleness). However, in the presence of aluminum, phosphorus can serve as an alloying element that increases the corrosion resistance of steel. The production of weather-resistant steels is based on this.

    Sulfur, due to the formation of low-melting iron sulfide, makes steel red-brittle (prone to cracking at a temperature of 800-1000 ° C). This is especially important for welded structures. The harmful effect of sulfur is reduced with an increased content of manganese. The content of sulfur and phosphorus in steel is limited and should be no more than 0.03 - 0.05%, depending on the type (grade) of steel.

    A harmful effect on the mechanical properties of steel is its saturation with gases that can get from the atmosphere into the metal in a molten state. Oxygen acts like sulfur, but to a greater extent, and increases the brittleness of steel. Unbound nitrogen also reduces the quality of the steel. Although hydrogen is retained in an insignificant amount (0.0007%), concentrating around inclusions in the intercrystalline regions and located mainly along grain boundaries, it causes high stresses in microvolumes, which leads to a decrease in the resistance of steel to brittle fracture, a decrease in the temporary resistance and deterioration of plastic properties. Therefore, molten steel (eg during welding) must be protected from the atmosphere.

    Depending on the type of delivery, steels are divided into hot-rolled and heat-treated (normalized or thermally improved). In the hot-rolled state, steel does not always have an optimal set of properties. During normalization, the structure of the steel is refined, its homogeneity increases, and the toughness increases, but no significant increase in strength occurs. Heat treatment (quenching in water and high-temperature tempering) makes it possible to obtain steels of high strength, which are well resistant to brittle fracture. The costs of heat treatment of steel can be significantly reduced if quenching is carried out directly from the rolling heating.

    Steel used in building metal structures is mainly produced in two ways: in open-hearth furnaces and in oxygen-purged converters. The properties of open-hearth and oxygen-converter steels are practically the same, however, the oxygen-converter method of production is much cheaper and is gradually replacing the open-hearth one. For the most critical parts, where special requirements are required high quality metal, steels obtained by electroslag remelting (ESR) are also used. With the development of electrometallurgy, a wider use in construction of steels obtained in electric furnaces is possible. Elektrostal has a low content of harmful impurities and high quality.

    According to the degree of deoxidation, steels can be boiling, semi-calm and calm.

    Non-deoxidized steels boil during casting into molds due to the release of gases. Such steel is called boiling and turns out to be more polluted with gases and less homogeneous.

    Mechanical properties vary slightly along the length of the ingot due to uneven distribution chemical elements... This especially applies to the head part, which turns out to be the most loose (due to shrinkage and the greatest saturation with gases), the greatest segregation of harmful impurities and carbon occurs in it. Therefore, the defective part is cut off from the ingot, which is approximately 5% of the ingot mass. Boiling steels, having fairly good properties in terms of yield strength and ultimate strength, are less resistant to brittle fracture and aging.

    To improve the quality of low-carbon steel, it is deoxidized by adding silicon from 0.12 to 0.3% or aluminum to 0.1%. Silicon (or aluminum), combining with dissolved oxygen, reduces its harmful effect. When combined with oxygen, deoxidizers form silicates and aluminates in the finely dispersed phase, which increase the number of crystallization sites and contribute to the formation of a fine-grained structure of steel, which leads to an increase in its quality and mechanical properties. Deoxidized steels do not boil when poured into molds, therefore they are called calm m and. A portion of approximately 15% is cut from the head of the quiescent steel ingot. Calm steel is more homogeneous, weld better, better resists dynamic stress and brittle fracture. Calm steels are used in the manufacture of critical structures that are exposed to dynamic influences.

    However, quiescent steels are about 12% more expensive than boiling steels, which makes them limit their use and switch, when it is beneficial for technical and economic reasons, to the manufacture of structures from semi-quiescent steel.

    Semi-quiescent steel is intermediate in quality between boiling and quiescent. It is deoxidized with a smaller amount of silicon - 0.05 - 0.15% (rarely with aluminum). A smaller part is cut from the head of the ingot, equal to about 8% of the mass of the ingot. In terms of cost, semi-calm steels also occupy an intermediate position. Low alloy steels are supplied in mostly calm (rarely semi-calm) versions.

    1.2.2. Standardization of steels. The main standard governing the characteristics of steels for building metal structures is GOST 27772 - 88. According to GOST, structural shapes are made of steels 1 С235, С245, С255, С275, С285, С345, С345К, С375, for sheet and universal rolled products and bent sections, steel С390, С390К, С440, С590, С590К are also used. Steel С345, С375, С390 and С440 can be supplied with a higher copper content (to increase corrosion resistance), while the letter "D" is added to the steel designation.

    The chemical composition of steels and mechanical properties are presented in table. 1.2 and 1.3.

    Rolled steel can be supplied both hot-rolled and heat-treated. The choice of the chemical composition and the type of heat treatment is determined by the plant. The main thing is to provide the required properties. So, sheet steel S345 can be made of steel with chemical composition C245 with thermal enhancement. In this case, the letter T is added to the steel designation, for example, S345T.

    Depending on the operating temperature of structures and the degree of danger of brittle fracture, impact tests for steels C345 and C375 are carried out at different temperatures, therefore they are supplied in four categories, and a category number is added to the designation of the steel, for example, C345-1; S345-2.

    Standardized characteristics for each category are given in table. 1.4.

    The rental is delivered in batches. The batch consists of rolled products of the same size, one melt-ladle and one heat treatment mode. When checking the quality of metal, two samples are taken at random from a batch.

    One sample for tensile and flexural tests and two samples for determining the impact strength at each temperature are made from each sample. If the test results do not meet the requirements of GOST, then carry out

    repeated tests on a doubled number of samples. If repeated tests have shown unsatisfactory results, then the batch is rejected.

    Steel weldability is assessed by carbon equivalent,%:

    where C, Mn, Si, Cr, Ni, Cu, V, P - mass fraction of carbon, manganese, silicon, chromium, nickel, copper, vanadium and phosphorus, %.

    If with,<0,4%, то сварка стали не вызывает затруднений, при 0,4 %< С,< 0,55 % сварка возможна, но требует принятия специальных мер по предотвращению возник­новения трещины. При С э >0.55%, the risk of cracking increases dramatically.

    To check the continuity of the metal and prevent delamination, if necessary, ultrasonic testing is carried out at the request of the customer.

    Distinctive feature GOST 27772 - 88 is the use of statistical control methods for some steels (С275, С285, С375), which guarantees the provision of standard values ​​for the yield strength and ultimate strength.

    Construction metal constructions They are also made of steels supplied in accordance with GOST 380 - 88 "Carbon steel of ordinary quality", GOST 19281-73 "Low-alloy steel graded and shaped", GOST 19282 - 73 "Low-alloy steel plate and broadband universal" and other standards.

    There are no fundamental differences between the properties of steels with the same chemical composition, but supplied according to different standards. The difference is in the methods of control and designations. So, according to GOST 380 - 88 with changes in the designation of the steel grade, the delivery group, the method of deoxidation and the category are indicated.

    When delivered in group A, the plant guarantees mechanical properties, in group B - chemical composition, in group C - mechanical properties and chemical composition.

    The degree of deoxidation is indicated by the letters KP (boiling), SP (calm) and PS (semi-calm).

    The steel category indicates the type of impact strength tests: category 2 - impact strength tests are not carried out, 3 - are carried out at a temperature of +20 ° C, 4 - at a temperature of -20 ° C, 5 - at a temperature of -20 ° C and after mechanical aging , 6 - after mechanical aging.

    In construction, steel grades VstZkp2, VstZpsb and VstZsp5 are mainly used, as well as steel with a high manganese content VstZGps5.

    According to GOST 19281-73 and GOST 19282-73, the content of the main elements is indicated in the designation of the steel grade. For example, the chemical composition of 09G2S steel is deciphered as follows: 09 - carbon content in hundredths of a percent, G2 - manganese in an amount from 1 to 2%, C - silicon up to 1 %.

    At the end of the steel grade, the category is indicated, i.e. type of impact strength test. For low-alloy steels, 15 categories have been established, tests are carried out at temperatures down to -70 ° C. The steels supplied according to different standards are interchangeable (see table 1.3).

    The properties of steel depend on the chemical composition of the feedstock, the method of smelting and the volume of the smelting units, the reduction force and temperature during rolling, the cooling conditions of the finished rolled product, etc.

    With such a variety of factors affecting the quality of steel, it is quite natural that the indicators of strength and other properties have a certain spread and they can be considered as random values. An idea of ​​the variability of characteristics is given by statistical histograms of distribution, showing the relative proportion (frequency) of a particular value of the characteristic.

    1.2.4. High strength steels(29 kN / cm 2< <40 кН/см 2). Стали повышенной прочности (С345 - С390) получают либо введением при выплавке стали легирующих
    additives, mainly manganese and silicon, less often nickel and chromium, or heat-resistant
    low carbon steel (С345Т).

    At the same time, the ductility of the steel decreases slightly, and the length of the yield area decreases to 1-1.5%.

    High strength steels are welded somewhat worse (especially steels with a high silicon content) and sometimes require the use of special technological measures to prevent the formation of hot cracks.

    In terms of corrosion resistance, most steels of this group are close to low-carbon steels.

    Steels with a high copper content (S345D, S375D, S390D) have a higher corrosion resistance.

    The fine grain structure of low alloy steels provides significantly higher brittle fracture resistance.

    The high value of impact strength is maintained at temperatures of -40 ° C and below, which makes it possible to use these steels for structures operated in northern regions. Due to the higher strength properties, the use of high-strength steels leads to metal savings of up to 20 -25%.

    1.2.5 High strength steels(> 40 kN / cm 2). Rolled high strength steel
    (C440 -C590) is obtained, as a rule, by alloying and heat treatment.

    For alloying, nitride-forming elements are used, which contribute to the formation of a fine-grained structure.

    High-strength steels may not have a yield area (at o>,> 50 kN / cm 2), and their ductility (elongation) decreases to 14% and below.

    The ratio increases to 0.8 - 0.9, which makes it impossible to take plastic deformations into account when calculating structures made of these steels.

    The selection of the chemical composition and the heat treatment mode can significantly increase the resistance to brittle fracture and provide high impact toughness at temperatures down to -70 ° C. Certain difficulties arise in the manufacture of structures. High strength and low ductility require more powerful equipment for cutting, straightening, drilling and other operations.

    When welding heat-treated steels, due to uneven heating and rapid cooling, various structural transformations occur in different zones of the welded joint. In some areas, quenched structures are formed with increased strength and brittleness (hard interlayers), in others, the metal undergoes high tempering and has a reduced strength and high plasticity (soft interlayers).

    The softening of steel in the heat-treated zone can reach 5 - 30%, which must be taken into account when designing welded structures made of heat-treated steels.

    The introduction of some carbide-forming elements (molybdenum, vanadium) into the steel composition reduces the softening effect.

    The use of high-strength steels leads to metal savings of up to 25-30% in comparison with structures made of low-carbon steels and is especially advisable in large-span and heavily loaded structures.

    1.2.6. Atmospheric resistant steels. To increase the corrosion resistance of metal
    low-alloy steels are used, containing in a small
    quantities (fractions of a percent) elements such as chromium, nickel and copper.

    In structures exposed to atmospheric influences, steels with the addition of phosphorus (for example, steel S345K) are very effective. On the surface of such steels, a thin oxide film is formed, which has sufficient strength and protects the metal from the development of corrosion. However, the weldability of steel in the presence of phosphorus deteriorates. In addition, in rolled products of large thicknesses, the metal has a reduced cold resistance, therefore, the use of steel S345K is recommended for thicknesses not exceeding 10 mm.

    In structures that combine bearing and enclosing functions (for example, membrane coatings), thin-sheet steel is widely used. To increase the durability of such structures, it is advisable to use stainless chromium steel grade ОХ18Т1Ф2, which does not contain nickel. Mechanical properties of OH18T1F2 steel:

    50 kN / cm 2, = 36 kN / cm 2,> 33 %. At large thicknesses, rolled products made of chromium steels have increased brittleness, however, the properties of thin-sheet rolled products (especially with a thickness of up to 2 mm) make it possible to use it in structures at design temperatures down to -40 ° C.

    1.2.7. Selection of steels for building metal structures. The choice of steel is made on the basis of variant design and technical and economic analysis, taking into account the recommendations of the norms. In order to simplify the ordering of metal, when choosing steel, one should strive for a greater unification of structures, a reduction in the number of steels and profiles. The choice of steel depends on the following parameters that affect the performance of the material:

    temperature of the environment in which the structure is mounted and operated. This factor takes into account the increased risk of brittle fracture at low temperatures;

    the nature of the loading, which determines the peculiarity of the work of the material and structures under dynamic, vibration and variable loads;

    the type of stress state (uniaxial compression or tension, flat or volumetric stress state) and the level of arising stresses (strongly or weakly loaded elements);

    the method of connecting the elements, which determines the level of their own stresses, the degree of stress concentration and the properties of the material in the connection zone;

    thickness of rolled products used in the elements. This factor takes into account the change in steel properties with increasing thickness.

    Depending on the working conditions of the material, all types of structures are divided into four groups.

    TO first group includes welded structures operating in particularly severe conditions or directly exposed to dynamic, vibrational or moving loads (for example, crane beams, work platform beams or overpass elements that directly take the load from rolling stock, gusset trusses, etc.). The stress state of such structures is characterized by high level and high download frequency.

    Structures of the first group operate in the most difficult conditions, contributing to the possibility of their brittle or fatigue failure, therefore, the highest requirements are imposed on the properties of steels for these structures.

    NS second group includes welded structures operating on a static load when exposed to a uniaxial and unambiguous biaxial tensile stress field (for example, trusses, girders, floor and roof beams and other stretched, stretched-bending and bending elements), as well as structures of the first group in the absence of welded joints ...

    Common to the structures of this group is the increased risk of brittle fracture associated with the presence of a tensile stress field. The probability of fatigue failure is less here than for structures of the first group.

    TO third group includes welded structures operating under the predominant effect of compressive stresses (for example, columns, racks, supports for equipment and other compressed and compressed-bending elements), as well as structures of the second group in the absence of welded joints.

    TO fourth group include auxiliary structures and elements (ties, half-timbered elements, stairs, fences, etc.), as well as structures of the third group in the absence of welded joints.

    If for structures of the third and fourth groups it is sufficient to restrict ourselves to the requirements for strength under static loads, then for structures of the first and second groups it is important to assess the resistance of steel to dynamic effects and brittle fracture.

    In materials for welded structures, weldability must be evaluated. The requirements for structural elements that do not have welded joints can be reduced, since the absence of welding stress fields, a lower stress concentration and other factors improve their operation.

    Within each group of structures, depending on the operating temperature, the requirements for impact strength at different temperatures are imposed on steels.

    The norms contain a list of steels depending on the group of structures and the climatic region of construction.

    The final selection of steel within each group should be made based on a comparison of technical and economic indicators (steel consumption and the cost of structures), as well as taking into account the order of the metal and the technological capabilities of the manufacturer. In composite structures (for example, split beams, trusses, etc.), it is economically feasible to use two steels: higher strength for heavily loaded elements (truss chords, beams) and lower strength for lightly loaded elements (truss lattice, beam webs).

    1.2.8. Aluminum alloys. Aluminum differs significantly from steel in its properties. Its density = 2.7 t / m 3, i.e. almost 3 times less than the density of steel. Modulus of longitudinal elasticity of aluminum E = 71 000 MPa, shear modulus G = 27,000 MPa, which is approximately 3 times less than the modulus of longitudinal elasticity and shear modulus of steel.

    Aluminum does not have a yield pad. The straight line of elastic deformations directly transforms into the curve of elastoplastic deformations (Fig. 1.7). Aluminum is very plastic: elongation at break reaches 40 - 50%, but its strength is very low: = 6 ... 7 kN / cm 2, and the conventional yield strength = 2 ... 3 kN / cm 2. Pure aluminum quickly becomes covered with a strong oxide film that prevents further corrosion.

    Due to the extremely low strength, commercially pure aluminum in building structures used quite rarely. A significant increase in the strength of aluminum is achieved by alloying it with magnesium, manganese, copper, silicon. zinc and some other elements.

    The temporary resistance of alloyed aluminum (aluminum alloys), depending on the composition of alloying additives, is 2-5 times higher than that of commercially pure aluminum; however, the relative elongation is, respectively, 2 - 3 times lower. With an increase in temperature, the strength of aluminum decreases and at temperatures above 300 ° C is close to zero (see Fig. 1.7).

    A feature of a number of multicomponent alloys A1 - Mg - Si, Al - Cu - Mg, Al - Mg - Zn is their ability to further increase the strength during aging after heat treatment; such alloys are called thermally hardened.

    The ultimate strength of some high-strength alloys (Al - Mg - Zn systems) after heat treatment and artificial aging exceeds 40 kN / cm 2, while the relative elongation is only 5-10%. Heat treatment of alloys with a double composition (Al-Mg, Al-Mn) does not lead to hardening, such alloys are called thermally non-hardened.

    An increase in the conventional yield point of products made of these alloys by a factor of 1.5 - 2 can be achieved by cold deformation (autofrettage), while the relative elongation is also significantly reduced. It should be noted that the indicators of all major physical properties alloys, regardless of the composition of alloying elements and state, practically do not differ from those for pure aluminum.

    Corrosion resistance of alloys depends on the composition of alloying additives, the state of delivery and the degree of aggressiveness of the external environment.

    Semi-finished products from aluminum alloys are manufactured at specialized plants: sheets and strips - by rolling on multi-roll mills; pipes and profiles - by extrusion on horizontal hydraulic presses, allowing to obtain profiles of the most diverse cross-sectional shapes, including those with closed cavities.

    On the semi-finished products sent from the factory, the grade of the alloy and the state of delivery are indicated: M - soft (annealed); H - cold-worked; H2 - semi-standardized; T - hardened and naturally aged for 3 - 6 days at room temperature; T1 - hardened and artificially aged for several hours at elevated temperatures; T4 - not fully hardened and naturally aged; T5 - not fully hardened and artificially aged. Semi-finished products delivered without processing have no additional designation.

    Of the large number of aluminum grades, the following are recommended for use in construction:

    Thermally unhardened alloys: AD1 and AMtsM; AMg2M and AMg2MH2 (sheets); AMg2M (pipes);

    Thermally hardened alloys: AD31T1; AD31T4 and AD31T5 (profiles);

    1915 and 1915T; 1925 and 1925T; 1935, 1935T, AD31T (profiles and pipes).

    All of the above alloys, with the exception of 1925T, which is used only for riveted structures, weld well. Casting alloy of AL8 grade is used for cast parts.

    Aluminum structures due to low weight, corrosion resistance, cold resistance, anti-magnetic, non-sparking, durability and good looking have broad application prospects in many areas of construction. However, due to the high cost, the use of aluminum alloys in building structures is limited.

    Aluminum and stainless steel may look similar, but in reality they are completely different. Remember these 10 differences and guide them when choosing the type of metal for your project.

    1. Strength to weight ratio. Aluminum is usually not as strong as steel, but it is also much lighter. This is the main reason why aircraft are made of aluminum.
    2. Corrosion. Stainless steel is composed of iron, chromium, nickel, manganese and copper. Chromium is added as an element to provide corrosion resistance. Aluminum is highly resistant to oxidation and corrosion, mainly due to a special film on the metal surface (passivation layer). When aluminum oxidizes, its surface turns white and sometimes pits appear on it. In some extreme acidic or alkaline environments, aluminum can corrode at a catastrophic rate.
    3. Thermal conductivity. Aluminum has much better thermal conductivity than stainless steel. This is one of the main reasons it is used for automotive radiators and air conditioners.
    4. Price. Aluminum is generally less expensive than stainless steel.
    5. Manufacturability. Aluminum is quite soft and easier to cut and deform. Stainless steel is a more durable material, but harder to work with as it is more difficult to deform.
    6. Welding. Stainless steel is relatively easy to weld, while aluminum can be problematic.
    7. Thermal properties. Stainless steel can be used for much more high temperatures than aluminum, which can become very soft already at 200 degrees.
    8. Electrical conductivity. Stainless steel is a really poor conductor compared to most metals. Aluminum, on the other hand, is a very good conductor of electricity. Due to their high conductivity, low weight and corrosion resistance, high voltage overhead lines power lines are usually made of aluminum.
    9. Strength. Stainless steel is stronger than aluminum.
    10. Impact on food. Stainless steel reacts less with food. Aluminum can react with foods that can affect the color and odor of the metal.

    Still not sure which metal is right for your goals? Contact us by phone, email or come to our office. Our account managers will help you make the right choice!

    Currently, the most common illegal armed groups on the Russian market can be divided into three large groups:

    • systems with substructure made of aluminum alloys;
    • systems with substructure made of galvanized steel with polymer coating;
    • systems with stainless steel substructure.

    The best strength and thermophysical parameters are undoubtedly provided by stainless steel sub-facing structures.

    Comparative analysis of physical and mechanical properties of materials

    * Properties of stainless and galvanized steel are slightly different.

    Thermal and strength characteristics of stainless steel and aluminum

    1. Considering 3 times lower bearing capacity and 5.5 times higher thermal conductivity of aluminum, the aluminum alloy bracket is a stronger "cold bridge" than the stainless steel bracket. An indicator of this is the coefficient of heat engineering uniformity of the enclosing structure. According to research data, the coefficient of heat engineering uniformity of the enclosing structure when using a stainless steel system was 0.86-0.92, and for aluminum systems it is 0.6-0.7, which makes it necessary to lay a large thickness of insulation and, accordingly, increase the cost of the facade ...

    For Moscow, the required resistance to heat transfer of walls, taking into account the coefficient of thermal homogeneity, is for a stainless bracket - 3.13 / 0.92 = 3.4 (m2. ° C) / W, for an aluminum bracket - 3.13 / 0.7 = 4.47 (m 2. ° C) / W, i.e. 1.07 (m 2. ° C) / W higher. Hence, when using aluminum brackets, the thickness of the insulation (with a thermal conductivity coefficient of 0.045 W / (m. ° C) should be taken almost 5 cm more (1.07 * 0.045 = 0.048 m).

    2. Due to the greater thickness and thermal conductivity of aluminum brackets, according to calculations carried out at the Research Institute of Building Physics, at an outside air temperature of -27 ° C, the temperature on the anchor can drop to -3.5 ° C and even lower, because in calculations area cross section the aluminum bracket was taken as 1.8 cm 2, while in reality it is 4-7 cm 2. When using a stainless steel bracket, the temperature at the anchor was +8 ° C. That is, when using aluminum brackets, the anchor works in a zone of alternating temperatures, where moisture condensation on the anchor is possible, followed by freezing. This will gradually destroy the material of the structural layer of the wall around the anchor and, accordingly, reduce its bearing capacity, which is especially important for walls made of material with a low bearing capacity (foam concrete, hollow brick, etc.). At the same time, heat-insulating gaskets under the bracket, due to their small thickness (3-8 mm) and high (relative to insulation) thermal conductivity, reduce heat loss by only 1-2%, i.e. practically do not break the "cold bridge" and have little effect on the temperature of the anchor.

    3. Low thermal expansion of the guides. The thermal deformation of aluminum alloy is 2.5 times greater than that of stainless steel. Stainless steel has a lower coefficient of thermal expansion (10 10 -6 ° C -1) than aluminum (25 10 -6 ° C -1). Correspondingly, the elongation of 3-meter rails at a temperature difference from -15 ° C to +50 ° C will be 2 mm for steel and 5 mm for aluminum. Therefore, to compensate for the thermal expansion of the aluminum guide, a number of measures are required:

    namely, the introduction of additional elements into the subsystem - movable sleds (for U-shaped brackets) or oval holes with bushings for rivets - not rigid fixation (for L-shaped brackets).

    This inevitably leads to the complication and rise in the cost of the subsystem or incorrect installation (since it very often happens that installers do not use bushings or incorrectly fix the assembly with additional elements).

    As a result of these measures, the weight load falls only on the bearing brackets (upper and lower), while others serve only as a support, which means that the anchors are not evenly loaded and this must be taken into account when developing project documentation, which is often simply not done. In steel systems, the entire load is distributed evenly - all nodes are rigidly fixed - insignificant thermal expansion is compensated by the work of all elements in the stage of elastic deformation.

    The design of the cleat allows to make a gap between the plates in stainless steel systems from 4 mm, while in aluminum systems - not less than 7 mm, which, moreover, does not suit many customers and spoils appearance building. In addition, the cleat must ensure free movement of the cladding plates by the amount of extension of the guides, otherwise the plates will collapse (especially at the junction of the guides) or unbend the cleat (both can lead to the falling of the cladding plates). In the steel system, there is no danger of unbending the cleat legs, which can occur over time in aluminum systems due to large temperature deformations.

    Fire protection properties of stainless steel and aluminum

    Melting temperature of stainless steel 1800 ° C, and aluminum 630/670 ° C (depending on the alloy). The temperature during a fire on the inner surface of the tile (according to the results of tests by the Regional Certification Center OPYTNOE) reaches 750 ° C. Thus, when using aluminum structures, melting of the substructure and the collapse of a part of the facade (in the area of ​​the window opening) can occur, and at a temperature of 800-900 ° C, aluminum itself supports combustion. Stainless steel, on the other hand, does not melt in case of fire, therefore it is most preferable according to requirements fire safety... For example, in Moscow, during the construction of high-rise buildings, aluminum substructures are generally not allowed for use.

    Corrosive properties

    To date, the only reliable source about the corrosion resistance of a particular substructure, and, accordingly, the durability, is the expert opinion of "ExpertKorr-MISiS".

    The most durable are stainless steel structures. The service life of such systems is at least 40 years in an urban industrial atmosphere of moderate aggressiveness, and at least 50 years in a relatively clean atmosphere of weak aggressiveness.

    Aluminum alloys, due to the oxide film, have high corrosion resistance, but under conditions of increased concentration of chlorides and sulfur in the atmosphere, rapidly developing intergranular corrosion may occur, which leads to a significant decrease in the strength of structural elements and their destruction. Thus, the service life of a structure made of aluminum alloys in an urban industrial atmosphere of average aggressiveness does not exceed 15 years. However, according to the requirements of Rosstroy, in the case of the use of aluminum alloys for the manufacture of elements of the substructure of illegal armed groups, all elements must necessarily have an anode coating. The anodic coating extends the service life of the aluminum alloy substructure. But during the installation of the substructure, its various elements are connected with rivets, for which holes are drilled, which causes a violation of the anode coating at the attachment site, i.e., areas without an anode coating are inevitably created. In addition, the steel core aluminum rivets together with the aluminum medium of the element, it forms a galvanic pair, which also leads to the development of active processes of intergranular corrosion at the points of attachment of the substructure elements. It is worth noting that often the low cost of one or another IRF system with an aluminum alloy substructure is due precisely to the absence of a protective anode coating on the system elements. Unscrupulous manufacturers of such substructures save on expensive electrochemical anodizing processes.

    Galvanized steel has insufficient corrosion resistance from the point of view of the durability of the structure. But after applying a polymer coating, the service life of a substructure made of galvanized steel with a polymer coating will be 30 years in an urban industrial atmosphere of moderate aggressiveness, and 40 years in a relatively clean atmosphere of weak aggressiveness.

    Comparing the above indicators of aluminum and steel substructures, we can conclude that steel substructures in all respects are significantly superior to aluminum.

    Description of aluminum: Aluminum has no polymorphic transformations, has a face-centered cube lattice with a period of a = 0.4041 nm. Aluminum and its alloys lend themselves well to hot and cold deformation - rolling, forging, pressing, drawing, bending, sheet stamping and other operations.

    All aluminum alloys can be joined spot welding, and special alloys can be fusion welded and other types of welding. Wrought aluminum alloys are divided into hardenable and non-hardened by heat treatment.

    All properties of alloys are determined not only by the method of obtaining a semi-finished product and heat treatment, but mainly by the chemical composition and especially by the nature of the phases - hardeners of each alloy. The properties of aging aluminum alloys depend on the types of aging: zone, phase or coagulation.

    At the stage of coagulation aging (T2 and TZ), corrosion resistance significantly increases, and the most optimal combination characteristics of strength, resistance to stress corrosion, exfoliating corrosion, fracture toughness (K 1s) and plasticity (especially in the high-altitude direction).

    The state of the semi-finished products, the nature of the cladding and the direction of cutting the samples are indicated as follows - Legend for rolled aluminum:

    M - Soft, annealed

    T - Tempered and naturally aged

    T1 - Hardened and artificially aged

    T2 - Hardened and artificially aged for higher fracture toughness and better resistance to stress corrosion

    ТЗ - Hardened and artificially aged according to the mode providing the highest stress corrosion resistance and fracture toughness

    N - Cold-worked (cold-worked sheets of alloys such as duralumia about 5-7%)

    P - Semi-standardized

    H1 - Reinforced work-hardened (sheet work-hardening about 20%)

    TPP - Hardened and naturally aged, increased strength

    GK - Hot rolled (sheets, plates)

    B - Technological cladding

    A - Normal plating

    UP - Thickened cladding (8% per side)

    D - Longitudinal direction (along the fiber)

    P - Transverse direction

    B - Altitude direction (thickness)

    X - Chord direction

    P - Radial direction

    PD, DP, VD, VP, ХР, РХ - The direction of specimen cutting, used to determine the fracture toughness and the rate of growth of a fatigue crack. The first letter characterizes the direction of the sample axis, the second characterizes the direction of the plane, for example: PV - the sample axis coincides with the width of the semi-finished product, and the crack plane is parallel to the height or thickness.

    Analysis and obtaining of aluminum samples: Ores. Currently, aluminum is obtained from only one type of ore - bauxite. Usually used bauxite contains 50-60% А 12 О 3,<30% Fe 2 О 3 , несколько процентов SiО 2 , ТiО 2 , иногда несколько процентов СаО и ряд других окислов.

    Samples from bauxite are taken according to general rules, paying special attention to the possibility of moisture absorption by the material, as well as to the different ratio of the proportions of large and small particles. The mass of the sample depends on the size of the tested supply: from every 20 tons, at least 5 kg must be taken into the total sample.

    When sampling bauxite in cone-shaped piles, small pieces are chipped off from all large pieces weighing> 2 kg lying in a circle with a radius of 1 m and taken into a shovel. The missing volume is filled with small particles of material taken from the lateral surface of the tested cone.

    The selected material is collected in tightly closed vessels.

    All sample material is crushed in a crusher to particles with a size of 20 mm, poured into a cone, reduced and again crushed to particles with a size<10 мм. Затем материал еще раз перемешивают и отбирают пробы для определения содержания влаги. Оставшийся материал высушивают, снова сокращают и измельчают до частиц размером < 1 мм. Окончательный материал пробы сокращают до 5 кг и дробят без остатка до частиц мельче 0,25 мм.

    Further preparation of the sample for analysis is carried out after drying at 105 ° C. The particle size of the sample for analysis should be less than 0.09 mm, the amount of material should be 50 kg.

    Prepared bauxite samples are very prone to delamination. If samples consisting of particles with a size<0,25 мм, транспортируют в сосудах, то перед отбором части материала необходимо перемешать весь материал до получения однородного состава. Отбор проб от криолита и фторида алюминия не представляет особых трудностей. Материал, поставляемый в мешках и имеющий однородный состав, опробуют с помощью щупа, причем подпробы отбирают от каждого пятого или десятого мешка. Объединенные подпробы измельчают до тех пор, пока они не будут проходить через сито с размером отверстий 1 мм, и сокращают до массы 1 кг. Этот сокращенный материал пробы измельчают, пока он не будет полностью проходить через сито с размером отверстий 0,25 мм. Затем отбирают пробу для анализа и дробят до получения частиц размером 0,09 мм.

    Samples from liquid fluoride melts used in the electrolysis of aluminum melt as electrolytes are taken with a steel scoop from the liquid melt after removing the solid deposit from the bath surface. The liquid sample of the melt is poured into a mold and a small ingot with dimensions of 150x25x25 mm is obtained; then the entire sample is crushed to a laboratory sample particle size of less than 0.09 mm ...

    Melting aluminum: Depending on the scale of production, the nature of the casting and the energy capabilities, the melting of aluminum alloys can be carried out in crucible furnaces, in electric resistance furnaces and in induction electric furnaces.

    Melting of aluminum alloys should ensure not only high quality of the finished alloy, but also high productivity of the units and, in addition, the minimum cost of casting.

    The most progressive method of melting aluminum alloys is the method of induction heating by currents of industrial frequency.

    The technology for the preparation of aluminum alloys consists of the same technological stages as the technology for the preparation of alloys based on any other metals.

    1. When carrying out melting on fresh pig metals and ligatures, first of all load (completely or in parts) aluminum, and then dissolve the ligatures.

    2. When carrying out melting using a preliminary pig alloy or pig silumin in the charge, first of all, the pig alloys are loaded and melted, and then the required amount of aluminum and ligatures is added.

    3. In the event that the charge is composed of waste and pig metals, it is loaded in the following sequence: primary aluminum pigs, rejected castings (ingots), waste (first grade) and refined remelting and ligatures.

    Copper can be introduced into the melt not only in the form of a ligature, but also in the form of electrolytic copper or waste (introduction by dissolution).

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