Thermomechanical processing of metals and alloys. Thermomechanical treatment Purpose and types of heat treatment

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SP 16.13330.2011 Steel structures;SP 128.13330.2012 Aluminum structures;

1. General information

Metals, as materials, have a complex of properties valuable for construction equipment - high strength, ductility, weldability, endurance; the ability to harden and improve other properties under thermomechanical and chemical influences.

This is the reason for their wide application in construction and other fields of technology.

In its pure form, metals are rarely used due to insufficient strength, hardness and high ductility. They are mainly used as alloys with other metals and non-metals, such as carbon.

Iron and its alloys (steel C2.14%, cast iron C>2.14%) are called ferrous metals, the rest (Be, Mg, Al, Ti, Cr, Mn, Ni, Cu, Zn, etc.) and their alloys - non-ferrous.

Ferrous metals are most widely used in construction.

Their cost is much lower than colored ones.

However, the latter have a number of valuable properties - high specific strength, ductility, corrosion resistance and decorative effect, expanding the scope of their application in construction, primarily architectural and construction parts and structures made of aluminum.

Metal classification

The raw material for the production of ferrous metals is iron ores, represented by minerals of the oxide class - magnetite (FeFeO), hematite (FeO), chromite (FeCrO), etc.

For the production of non-ferrous metals, bauxites are used; sulfide and carbonate ores of copper, nickel, zinc, etc.

2. Atomic-crystal structure of metals

Metals and alloys in the solid state are crystalline bodies.

The atoms in them are located regularly in the nodes of the crystal lattice and vibrate with a frequency of about 10 Hz.

The bond in metals and alloys is electrostatic, due to the forces of attraction and repulsion between positively charged ions (atoms) in the nodes of the crystal lattice and itinerant conduction electrons, the density of which is 10-10 electrons per 1 cm, which is tens of thousands of times higher than the content of atoms and molecules in the air.

Electromagnetic, optical, thermal and other properties of metals depend on the specific properties of conduction electrons.

Atoms in the lattice tend to occupy a position corresponding to the minimum of its Energy, forming the densest packings - cubic volume- and face-centered and hexagonal.

Coordination numbers (packing density) of crystal lattices. a)cubic face-centered (K 12); b) body-centered (K8);c) hexagonal (K 12)

The packing density is characterized by the coordination number, which is the number of neighboring atoms that are at an equal and smallest distance from a given atom.

The higher the number, the denser the packing.

For body-centered cubic packing, it is equal to 8 (K8); face-centered - 12 (K12); hexagonal - also 12 (K12).

The distance between the centers of the nearest atoms in the lattice is called the lattice period.

The lattice period for most metals is in the range of 0.1-0.7 nm.

Many metals, depending on the temperature, undergo structural changes in the crystal lattice.

So iron at temperatures below 910 ° C and above 1392 ° C has a body-centered packing of atoms with a lattice period of 0.286 nm and is designated -Fe; in the range of these temperatures, the crystal lattice of iron is rearranged into a face-centered one with a period of 0.364 nm, and is designated -Fe.

Recrystallization is accompanied by heat release during cooling and absorption during heating, which is recorded on the diagrams along horizontal sections.

Iron cooling (heating) curve

Metals are polycrystalline bodies, consisting of a large number of small crystals of irregular shape.

Unlike regular-shaped crystals, they are called crystallites or grains.

Crystallites are differently oriented; therefore, the properties of metals are more or less the same in all directions, i.e. polycrystalline bodies are isotropic.

However, for the same orientation of crystallites, such an imaginary isotropy will not be observed.

The crystal lattice of metals and alloys is far from an ideal structure.

It contains defects - vacancies and dislocations.

3. Fundamentals of iron and steel production

Cast iron obtained in the course of a blast-furnace process based on the reduction of iron from its natural oxides contained in iron ores with coke at high temperature.

Coke burns to form carbon dioxide.

When passing through hot coke, it turns into carbon monoxide, which reduces iron in the upper part of the furnace according to the generalized scheme: FeOFeOFeOFe.

As it descends into the lower hot part of the furnace, the iron melts in contact with the coke and, partially dissolving it, turns into cast iron.

The finished cast iron contains about 93% iron, up to 5% carbon and a small amount of impurities of silicon, manganese, phosphorus, sulfur and some other elements that have passed into cast iron from gangue.

Depending on the amount and form of bonding of carbon and impurities with iron, cast irons have different properties, including color, subdivided according to this feature into white and gray.

Steel obtained from cast iron by removing some of the carbon and impurities from it. There are three main methods of steel production: converter, open-hearth and electric melting.

The converter is based on blowing molten iron in large pear-shaped converter vessels with compressed air.

Air oxygen oxidizes impurities, converting them into slag; carbon burns out.

With a low content of phosphorus in cast iron, the converters are lined with acid refractories, for example, dinas, with an increased content, with basic, periclase refractories.

Accordingly, the steel smelted in them is traditionally called Bessemer and Thomas steel.

The converter method is characterized by high productivity, which has led to its wide distribution.

Its disadvantages include increased metal waste, slag contamination and the presence of air bubbles that degrade the quality of steel.

The use of oxygen blast instead of air in combination with carbon dioxide and water vapor significantly improves the quality of converter steel.

The open-hearth method is carried out in special furnaces in which pig iron is fused together with iron ore and scrap metal (scrap).

Burnout of impurities occurs due to oxygen from the air entering the furnace along with combustible gases and iron ore in the composition of oxides.

The composition of the steel lends itself well to regulation, which makes it possible to obtain high-quality steels for critical structures in open-hearth furnaces.

Electric melting is the most advanced way to obtain high-quality steels with desired properties, but it requires an increased consumption of electricity.

According to the method of its supply, electric furnaces are divided into arc and induction.

Arc furnaces are most widely used in metallurgy. In electric furnaces, special types of steels are smelted - medium and high alloyed, tool, heat-resistant, magnetic and others.

4. Mechanical properties of metals

Mechanical properties are determined from the results of static, dynamic and fatigue (endurance) tests.

Static tests are characterized by slow and smooth application of the load. The main ones are: tensile tests, hardness and fracture toughness.

For tensile testsuse standard samples with a calculated lengthI= 10 d and an area of ​​11.3 BUT where (d and BUT- respectively, the diameter and cross-sectional area of ​​a sample of long products of round, square or rectangular section.

The tests are carried out on tensile testing machines with automatic recording of the tensile diagram.

Figure 4 shows such a diagram for medium carbon steel.

Curve 1 characterizes the behavior of the metal under the action of conditional stresses =R/A and the curve 2 - under the action of true stresses, S=R/A, (where BUT and BUT- respectively, the cross-sectional area of ​​the sample before testing and at each stage of loading up to failure).

Usually they use the conditional stress diagram, although the curve is more objective2.

Metal Tensile Diagrams: a) for conditional (solid lines) and true (dashed lines) voltages; / - area of ​​elastic deformation;// - the same plastic; /// - area of ​​crack development; b) conditionally true stresses

The elastic limit is determined by the stress at which the permanent elongation deformation does not exceed 0.05%.

The yield point is characterized by the conditional yield point, at which the residual deformation does not exceed 0.2%.

The physical yield strength corresponds to the stress at which the sample is deformed without further increase in load.

For materials that are brittle in tensile testing, static tests are used for compression (for cast iron), torsion (for hardened and structural steels) and bending (for gray and ductile iron castings).

Hardnessmetals they are tested by pressing a steel ball, diamond cone or pyramid into it under a certain load and evaluated by the amount of plastic deformation (imprint) produced.

Depending on the type of tip used and the evaluation criterion, Brinell, Rockwell and Vickers hardness are distinguished.

Scheme for determining hardness . a) according to Brinell; b) according to Rockwell; c) according to Vickers

Vickers hardness is designated HV 5, HV 10, etc. The thinner and harder the metal and alloy, the lower the test load should be.

To determine the microhardness of small products and structural components of metals, the Vickers method is also used in combination with a metallographic microscope.

The fracture toughness test of metals is carried out on standard notched specimens with three-point bending.

The method makes it possible to evaluate the resistance of a metal to propagation, rather than to the initiation of a crack or a crack-like defect of any origin, which is always present in the metal.

The fracture toughness is estimated by the parameterTO,representing the stress intensity factor or local increase in tensile stresses (MPa) at the crack tip.

Dynamic tests of metals are carried out for impact bending by alternating cyclic loading. For impact bending, metal samples are tested with dimensions (1x1x5.5) 10 m with a stress concentrator (notch) in the middle

The test is carried out on a pendulum impact tester. The resistance of a metal to impact bending is called impact strength and denotedKCU, KV and KST(where KSis the symbol for impact strength, andU, V and T -type and size of the voltage concentrator).

The resistance of a metal to cyclic loading is characterized by the maximum stress that a metal can withstand without destruction for a given number of cycles and is called the endurance limit. Apply symmetric and asymmetric loading cycles.

The endurance limit sharply decreases in the presence of stress concentrators.

5. Crystallization and phase composition of iron-carbon alloys

Crystallization develops only when the metal is supercooled below the equilibrium temperature.

The crystallization process begins with the formation of crystalline nuclei (crystallization centers) and continues with their growth.

Depending on the crystallization conditions (cooling rate, type and amount of impurities), crystals of different sizes from 10 to 10 nm of regular and irregular shape are formed.

In alloys, depending on the state, the following phases are distinguished: liquid and solid solutions, chemical and intermediate compounds (interstitial phases, electronic compounds, etc.).

A phase is a physically and chemically homogeneous part of a system (metal or alloy) that has the same composition, structure, the same state of aggregation and is separated from the rest of the system by a separating surface.

Therefore, a liquid metal is a single-phase system, and a mixture of two different crystals or the simultaneous existence of a liquid melt and crystals, respectively, are two- and three-phase systems.

Substances that form alloys are called components

Solid solutions are phases in which one of the alloy components retains its crystal lattice, while the atoms of another or other components are located in the crystal lattice of the first component (solvent), changing its dimensions (periods).

There are solid solutions of substitution and interstitial.

In the first case, the atoms of the dissolved component replace some of the atoms of the solvent at the sites of its crystal lattice; in the second, they are located in the interstices (voids) of the crystal lattice of the solvent, and in those of them where there is more free space.

In substitutional solutions, the lattice period can increase or decrease depending on the ratio of the atomic radii of the solvent and the dissolved component; in embedding solutions - always increase.

Interstitial solid solutions appear only in cases where the diameters of the atoms of the dissolved component are small.

For example, in iron, molybdenum, chromium, carbon, nitrogen, and hydrogen can dissolve and form interstitial solid solutions. Such solutions have a limited concentration, since the number of pores in the solvent lattice is limited.

6. Modification of the structure and properties of steel

The property of iron-carbon alloys to experience phase transformations during crystallization and reheating-cooling, to change the structure and properties under the influence of thermomechanical and chemical influences and modifier impurities is widely used in metallurgy to obtain metals with desired properties.

When developing and designing steel and reinforced concrete structures of buildings and structures, technological equipment and machines (autoclaves, kilns, mills, pressure and non-pressure pipelines for various purposes, metal molds for the manufacture of building products, construction machines, etc.), it is necessary to take into account climatic, technological and emergency their working conditions.

Low negative temperatures lower the cold brittleness threshold, impact strength and fracture toughness.

Elevated temperature reduces the modulus of elasticity, tensile strength, yield strength, which is clearly manifested, for example, during fires

At 600 °C, steel, and at 200 °C, aluminum alloys, completely pass into a plastic state, and structures under load lose their stability.

That is why unprotected metal structures have relatively little fire resistance.

Technological equipment - boilers, pipelines, autoclaves, metal molds, as well as reinforcement of reinforced concrete structures, constantly subjected to cyclic heating - cooling in the temperature range of 20-200 ° C and more during the production process, experience thermal aging and low-temperature tempering, often aggravated by corrosion, which is necessary be taken into account when choosing steel grades for specific purposes.

The main methods of modifying the structure and properties of steel used in metallurgy are:

Introduction into the molten metal of substances that form refractory compounds, which are centers of crystallization;

The introduction of alloying elements that increase the strength of the crystal lattices of ferrite and austenite, slowing down the diffusion processes of carbon and carbide release and the movement of dislocations;

Thermal and thermomechanical treatment of steel.

They are aimed mainly at grinding the grains of chilled steel, relieving residual stresses and increasing its chemical and physical homogeneity.

As a result, the hardenability of steel is increased; hardness, cold brittleness threshold, temper brittleness, tendency to thermal and deformation aging are reduced, plastic properties of steel are improved.

The specific features of these methods are discussed below.

Alloying elements are introduced into structural steels.

Being carbide-forming elements, they simultaneously serve as modifier additives that ensure the nucleation and refinement of steel grains during melt crystallization.

In alloy steel grades, the type and content of alloying elements are indicated by letters and numbers to the right of the letters.

They indicate the approximate content (%) of the alloying element; the absence of figures means that it does not exceed 1.5%.

Accepted designations of alloying elements: A - nitrogen, B - niobium, C - tungsten, D - manganese, D - copper, E - selenium, K - cobalt, H - nickel, M - molybdenum, P - phosphorus, P - boron, C - silicon, T - titanium, F - vanadium, X - chromium, C - zirconium, H - rare earth, Yu - aluminum.

Alloying elements, dissolving in ferrite and austenite, reduce the grain size and particles of the carbide phase.

Being located along the grain boundaries, they hinder their growth, diffusion of carbon and other alloying elements and increase the resistance of austenite to supercooling.

Therefore, low-alloy steels have a fine-grained structure and higher quality indicators.

Thermal and thermomechanical processing are common ways to modify the structure and improve the properties of steel.

There are the following types of them: annealing, normalization, hardening and tempering. Annealing includes the processes of homogenization, recrystallization and removal of residual stresses.

Temperature ranges for different types of annealing: 1 - homogenization; 2 - low-temperature recrystallization annealing (high tempering) to reduce hardness; 3 - annealing (tempering) for stress relief; 4 - complete annealing with phase recrystallization; 5, 6 - normalization of sub- and hypereutectoid steel; 7 - spheroidization; 8 - incomplete annealing of hypoeutectoid steel

Ingots of alloyed steel are subjected to homogenization at 1100–1200 °C for 15–20 h to equalize the chemical composition, reduce dendritic and intracrystalline segregation, which causes a brittle fracture during pressure treatment, anisotropy of properties, the formation of flocks and a coarse-grained structure.

Recrystallization annealing is used to remove hardening of a deformed metal by heating it above the temperature of the recrystallization threshold, soaking at this temperature and cooling.

There are cold and hot (warm) deformations.

Cold is carried out at a temperature below the recrystallization threshold, and hot - above.

Recrystallization during cold deformation is called static, and during hot - dynamic, characterized by residual "hot work hardening", useful for hardening from rolling heating.

Annealing to remove residual stresses is carried out at 550...650 °C for several hours. It prevents warping of welded products after cutting, straightening, etc.

Normalization provides for the heating of long products to - and hypereutectoid structural steel, a short exposure and cooling in air.

It causes complete phase recrystallization of steel, relieves internal stresses, increases ductility and impact strength.

Accelerated cooling in air leads to the decomposition of austenite at lower temperatures.

Normalization is widely used to improve the properties of low carbon building steels, replacing annealing. For medium carbon and alloy steels, it is combined with high tempering at temperatures below the recrystallization threshold.

Quenching and tempering provide for the improvement of the strength and plastic-viscous properties of steel, the reduction of the threshold of cold brittleness and sensitivity to stress concentrators.

Hardening consists in heating the steel, holding it until the steel is completely austenitized and cooling it at a rate that ensures the transition of austenite to martensite.

Therefore, the crystal lattice of martensite is highly distorted and experiences stresses due to structural features and an increase in the specific volume of martensite compared to austenite by 4...4.25%.

Martensite is brittle, hard and strong. However, a sufficiently complete martensitic transformation is possible only for high-carbon and alloy steels, which have an increased stability of supercooled austenite.

In low-carbon and low-alloy structural building steels, it is small and therefore, during quenching, even with rapid cooling with water, martensite either does not form, or is formed in a smaller amount in combination with bainite.

During rapid cooling of low-carbon building steels (C0.25%) (quenching from rolling heating), austenite decomposes and a highly dispersed ferrite-cementite structure of perlite-sorbite and troostite or low-carbon martensite and cementite is formed.

This structure is called bainite.

It has increased strength, hardness and endurance compared to the decomposition products of austenite in the pearlite region - sorbitol and proostite, while maintaining high plasticity, viscosity and a reduced cold capacity threshold.

Hardening of steel by quenching from rolling heating is due to the fact that dynamic recrystallization during rolling heating is incomplete and bainite inherits a high density of dislocations formed in deformed austenite.

The combination of plastic deformation of steel in the austenitic state with quenching and tempering can significantly increase its strength, ductility and toughness, eliminate the tendency to temper brittleness, which is observed during medium-temperature tempering of alloyed steel at 300...400 °C.

Tempering is the final operation of heat treatment of steel, after which it acquires the required properties.

It consists in heating hardened steel, holding it at a given temperature and cooling it at a certain rate.

The purpose of tempering is to reduce the level of internal stresses and increase the resistance to destruction.

There are three types of it: low-temperature (low) with heating up to 250 °C; medium-temperature (medium) with heating in the range of 350-500 °C and high-temperature (high) with heating at 500-600 °C.

The aging of carbon steel is manifested in a change in its properties over time without a noticeable change in the microstructure.

The strength and cold brittleness threshold increase, plasticity and impact strength decrease.

There are two types of aging - thermal and deformational (mechanical).

Deformation (mechanical) aging proceeds after plastic deformation at a temperature below the recrystallization threshold.

The main reason for this type of aging is also the accumulation of C and N atoms on dislocations, which hinders their movement.

Builders encounter the facts of temper brittleness and aging of steel during the electrothermal method of tensioning reinforcement in the process of manufacturing prestressed reinforced concrete structures.

7. Cast iron

As mentioned above, iron-carbon alloys containing more than 2.14% C are called cast iron.

The presence of eutectics in the structure of cast iron determines its use exclusively as a casting alloy. Carbon in cast iron can be in the form of cementite and graphite, or both.

Cementite gives the fracture a light color and a characteristic sheen; graphite - gray color without shine.

Cast iron, in which all carbon is in the form of cementite, is called white, and in the form of cementite and free graphite - gray

Depending on the form of graphite and the conditions of its formation, there are: gray, high-strength with nodular graphite and malleable cast irons.

The phase composition and properties of cast iron are decisively influenced by the content of carbon, silicon and other impurities in it, as well as the mode of cooling and annealing.

Influence of carbon and silicon content on the structure of cast iron (shaded area - the most common cast irons):

I - area of ​​white cast iron; II - half cast iron; III - pearlitic gray cast iron; IV - ferritic-pearlitic cast iron; V - ferritic gray cast iron;L - ledeburite; P - perlite; C - cementite; G - graphite; F - ferrite

White cast iron has high hardness and strength (HB 4000-5000 MPa), is poorly processed by cutting, and is brittle.

It is used as conversion to steel or ductile iron.

Chilled cast iron has a structure of white cast iron in the surface layer, and gray cast iron in the core, which gives products made from it increased wear resistance and endurance.

Approximate composition of white cast iron: C=2.8-3.6%; Si=0.5-0.8%; Mn=0.4-0.6%.

Gray cast iron is an alloy of Fe-Si-C, with the inevitable impurities of Mn, P and S.

The best properties are hypoeutectic cast irons containing 2.4-3.8% C, part of which, up to 0.7%, is in the form of cementite.

Silicon contributes to the graphitization of cast iron, manganese, on the contrary, prevents it, but increases the tendency of cast iron to bleach.

Sulfur is a harmful impurity that degrades the mechanical and casting properties of cast iron.

Phosphorus in an amount of 0.2-0.5% does not affect graphitization, increases fluidity, but increases the brittleness of cast iron.

The mechanical and plastic properties of cast iron are determined by its structure, mainly by the graphite component. The fewer graphite inclusions, the smaller, more branched and more isolated from each other, the stronger and more ductile cast iron.

The structure of the metal base of cast iron is hypoeutectoid or eutectoid steel, i.e. ferrite + pearlite or pearlite. The greatest strength, hardness and wear resistance has gray cast iron with a pearlite structure of the metal base of the approximate composition: C = 3.2-3.4%; Si - 1.4-2.2%; Mn=0.7-1.0%; P, S 0.15-0.2%.

Influence of the metal base and the shape of graphite inclusions on the mechanical and technological properties of cast irons

Physical and mechanical properties of cast irons of various structures

Name of cast iron	Cast iron grade	Structure of the metal base	graphite shape	Hardness HB, MPa	Tensile strength, MPa	Relative extension, %
Grey	MF-10; SCH-15		Large and medium sized plates	1200-2400	100-150	-
	MF-18; MF-21; MF-24; MF-25; MF-30; SCH-40	Perlite (sorbitol)	Small swirling plates	2550-2900	180-400	-
Highly durable	VCh35-22; VCh40-15; VCh45-10	Ferritic and ferritic-pearlitic	spherical	1400-1700; 1400-2020; 1400-2250;
	VCh50-8;	pearlite	spherical	1530-2450;
	VCh60-3;			1920-2770;
	VCh70-2;			2280-3020;
	VCh80-2;			2480-3510;
	VCh100-2			2700-3600	1000
Malleable	KCh30-6;	ferritic	flaky	1630
	KCH33-8
	KCh35-10
	KCH37-12
	KCh50-4;	pearlite	flaky	2410-2690
	KCh56-4;
	KCh60-3;
	KCh63-2

Graphite inclusions, sharply reducing the tensile strength of gray cast iron, practically do not affect its compressive strength, bending and hardness; make it insensitive to stress concentrators, improve machinability.

Gray cast iron is marked with the letters C - gray and H - cast iron.

The numbers after them indicate the average tensile strength (kg/mm).

Pearlitic cast irons include modified cast iron grades SCH30-SCH35, containing modifier additives - graphite, ferrosilicon, silicocalcium in the amount of 0.3-0.8%, etc.

To relieve internal stresses, castings are annealed at 500–600°C, followed by slow cooling.

Modification and annealing increase the ductility, toughness and endurance of cast iron

With the introduction of gray cast iron during its smelting of magnesium in an amount of 0.03-0.07%, graphite in the process of crystallization acquires a spherical shape instead of a lamellar one.

Such cast iron has high strength, comparable to that of cast steel, good casting properties and ductility, machinability and wear resistance.

Ductile iron grades are designated by letters and numbers.

The latter means tensile strength (kg/mm) and relative elongation (%).

Ductile iron is obtained by long-term heating (annealing) of white cast iron castings.

Annealing is carried out in two stages with exposure to each of them until the complete decomposition of ledeburite (stage I), austenite and cementite (stage II) and the formation of ferrite and graphite.

The latter stands out in this case in the form of flakes, giving cast iron high ductility.

Its fracture is velvety black.

If cooling is accelerated, malleable cast iron is formed with a pearlitic base, which reduces ductility and gives the fracture a light (steel) appearance. Mark it in the same way as high-strength cast iron.

The term "ductile iron" is conditional and characterizes the plastic, and not the technological properties of cast iron, since products from it, like from other cast irons, are obtained by casting, and not by forging.

In construction, all types of the considered cast irons with graphite inclusions are used.

Gray cast irons are used in structures operating under static load (columns, foundation slabs, base plates for trusses, beams, sewer pipes, manholes, valves); Ductile and malleable cast irons with increased strength, ductility and toughness are used in structures subjected to dynamic and vibrational loading and wear (floors of industrial buildings, foundations of heavy forging and pressing equipment, truss supports of railway and road bridges, tubing for fastening critical transport tunnels underground , in the mountains).

8. Non-ferrous metals

Of the non-ferrous metals, aluminum has the greatest use in construction, having high specific strength, ductility, corrosion resistance and economic efficiency.

Silver, gold, copper, zinc, titanium, magnesium, tin, lead and others are mainly used as alloying additives and alloy components and therefore have a special and limited use in construction (special types of glass, unique objects - memorials on Mamaev Kurgan in Volgograd, on Poklonnaya Gora, an obelisk in honor of the conquest of space in Moscow and others, in which titanium, copper, and their alloys are widely used; shut-off and control valves and devices for plumbing and heating, electrical systems of buildings and structures).

In its pure form, non-ferrous metals, like iron, are rarely used due to their low strength and hardness.

Aluminum- silver-white metal, density 2700 kg/m3 and melting point 658 °C. Its crystal lattice is a face-centered cube with a period of 0.40412 nm.

Real aluminum grains, like iron grains, have a block structure and similar defects - vacancies, interstitial atoms, dislocations, low- and high-angle boundaries between grains.

An increase in strength is achieved by alloying Mg, Mn, Cu, Si, Al, Zn, as well as plastic deformation (hard work), hardening and aging. All aluminum alloys are divided into wrought and cast.

Wrought alloys, in turn, are divided intothermally hardened and non-hardened .

The heat-hardened alloys include Al-Mg-Si, Al-Cu-Mg, Al-Zn-Mg; thermally non-hardened - technical aluminum and two-component alloys Al-Mn and Al-Mg (maglia).

Copper- the main alloying addition of alloys - duralumin, increases strength, but reduces the ductility and anti-corrosion properties of aluminum.

Manganese and magnesium increase strength and anti-corrosion properties; silicon - fluidity and fusibility, but worsens plasticity.

Zinc, especially with magnesium, increases strength but reduces stress corrosion resistance.

To improve the properties of aluminum alloys, they introduce a small amount of chromium, vanadium, titanium, zirconium and other elements. Iron (0.3-0.7%) is an undesirable but inevitable impurity.

The ratio of components in the alloys is selected based on the conditions for their achievement after heat treatment and aging of high strength, machinability and corrosion resistance.

Alloys are designated by grades that have an alphabetic and numerical designation characterizing the composition and state of the alloy: M - annealed (soft); H - cold-worked; H2 - semi-hardened; T - hardened and naturally aged; T1 - hardened and artificially aged; T4 - not fully hardened and artificially aged.

Hard-working and semi-hard-working are typical for thermally hardened alloys; hardening and aging - for thermally hardened.

Grades of technical aluminum: AD, AD1 (A - aluminum, D - duralumin type alloy, 1 - characterizes the degree of purity of aluminum - 99.3%; in the AD brand - 98.8 A1); high-strength - B95, B96, forging - AK6, AK8 (the numbers indicate the total content of the main and additional alloying elements in the alloy (%).

Brands of thermally non-hardened aluminum alloys: AD1M, AMtsM, AMg2M, AMg2N2 (M - soft, Mts - manganese, Mg2 - magnesium with a content of 2% in the alloy).

Numerical designation of aluminum alloy grades: 1915, 1915T, M925, 1935T (the first digit indicates the base of the alloy - aluminum; the second - the composition of the components; 0 - commercially pure aluminum, 1 - Al-Cu-Mg, 3 - Al-Mg-Si, 4 - Al-Mn, 5- Al-Mg, 9 - Al-Mg-Zn; the last two are the serial number of the alloy in its group).

The main types of heat treatment of aluminum alloys are annealing, hardening and aging (tempering)

Annealing occurs without phase transformations and is used for stress relief, homogenization, recrystallization and recovery.

In the latter case, there is a restoration of the initial physical and mechanical properties of the alloy, a decrease in strength, an increase in ductility and impact strength, which are necessary for technological purposes.

9. Steel reinforcement for reinforced concrete structures

To reinforce reinforced concrete structures, bar and wire reinforcement of a smooth and periodic profile and ropes made of low-carbon and low-alloy steels, hardened by quenching from rolling heating, cold or warm deformation, are used.

These requirements are more satisfied by high-strength rod (A-1V - AV1; At-1VC (K) - At-V1C (K), etc.), wire (B-II, Vr-II) and rope (K-7, K-9) reinforcement with a yield strength of 590-1410 MPa and a relative elongation of 8-14%, respectively, used for the manufacture of prestressed reinforced concrete structures.

At the same time, along with an increase in the strength and crack resistance of structures by 20-30%, the consumption of reinforcing steel is reduced compared to non-tensioned A-I (A-240), A-II (A-300), A-III (A-400) , Vp-I.

However, from the point of view of corrosion behavior, high-strength, especially prestressed reinforcement, is potentially more vulnerable.

The corrosion behavior of reinforcement in concrete is characterized mainly by the change in strength, plasticity and the nature of its fracture, as well as the depth of corrosion damage (mm/year) or weight loss (g/m day or g/m h)

The passive state of reinforcement in concrete, which is thermodynamically prone to oxidation reactions, is provided by the highly alkaline nature of the medium (pH12) and a sufficiently thick (0.01-0.035 m) and dense protective layer of concrete.

In accordance with the oxide-film theory, the passive state of reinforcement in an oxidizing environment occurs due to the formation of a thin oxide film on the metal surface.

The equilibrium potential for the formation of such a film is positive and is approximately 0.63 V, and iron in the active state is about - 0.4 V.

As soon as the polarization of the anode sections of the metal reaches the potential for the formation of an oxide film, the dissolution current density decreases sharply and the metal passes into a passive state.

This characteristic potential is called the Flade potential..

The passivation of reinforcement in concrete at a temperature of 20 ± 5 ° C is completed in 32-36 hours, not only with a clean surface, but also with rust.

However, the pH value of the medium ambiguously characterizes the state of reinforcement in concrete; it is largely determined by the presence of activating ions, which shift the dissolution potential of the metal in the negative direction; the metal then goes into an active state.

It is possible to objectively judge the electrochemical state of reinforcement in concrete only by its polarizability, i.e. change in the electrode potential and current density.

Not all concretes are characterized by a high pH value.

In autoclaved, gypsum and concretes with active mineral additives from the moment of their manufacture pH<12.

In such concretes, reinforcement requires a protective coating.

Reinforcement depassivation can also occur in the carbonized protective layer of concrete (where the reinforcement is located), especially in places of cracks, which must be taken into account when assigning the thickness and density of the protective layer, depending on the type, purpose, operating conditions and service life of reinforced concrete structures.

Localized corrosion lesions of the metal surface act similarly to stress concentrators.

In ductile mild steels, near the centers of these lesions, a redistribution of stresses occurs, as a result of which the mechanical properties of the steels practically do not change.

In high-strength low-ductility steels of a smooth and periodic profile, for example, V-II and Vr-II, which experience tensile stresses close to the yield strength (and for this reason are less amenable to anodic polarization), local corrosion damage causes a large concentration of weakly relaxing stresses and the likelihood of brittle fracture become.

Therefore, high-strength reinforcing steels recommended for prestressed structures, as a rule, are complexly alloyed, have undergone thermal and thermomechanical treatment, normalization and high tempering, at 600-650 °C.

The introduction of a small amount of alloying additives Cr, Mn, Si, Cu, P, Al and others into reinforcing steel, along with thermal and thermomechanical treatment, significantly improves the mechanical and 2-3 times the anti-corrosion properties of steels

10. Steel structures

The main structural forms and purpose of steel structures are:industrial buildings, frames and large-span coverings of public buildings, bridges and overpasses, towers and masts, stained-glass windows, window and door fillings, suspended ceilings and etc.

The primary elements of building structures are:

Steel sheet-thick hot-rolled 4-160 mm thick, 6-12 m long, 0.5-3.8 m wide, supplied in the form of sheets and rolls; thin hot and cold rolled, up to 4 mm thick in coils; wide-shelf universal 6-60 mm thick hot-rolled with machined, aligned edges;

Profile steel - angles, channels, I-beams, tees, pipes, etc., from which various symmetrical sections are assembled, providing increased stability and cost-effectiveness of structures;

Hot-rolled seamless round pipes with a diameter of 25-550 mm and a wall thickness of 2.5-75 mm for radio and television poles;

Pipes electrowelded round, with a diameter of 8-1620 mm and a wall thickness of 1-16 mm; square and rectangular section with sides from 60 to 180 mm and wall thickness from 3 to 8 mm. Pipes are used in the construction of lightweight roofs, half-timbered walls, bindings, stained-glass windows;

Cold-formed profiles made from tape or strip with a thickness of 1-8 mm. Their main area of ​​application is light, economical construction of building coverings;

Profiles for various purposes - window, door and lantern frames, crane rails, galvanized profile decking, steel ropes and high-strength wire for hanging and cable-stayed roofs, bridges, masts, prestressed roof structures, pipes, tanks, etc.

The main types of rolling profiles. a) sheet steel; b) corner profiles; c) channel; d), e), f) I-beams with different shelf widths; g) thin-walled I-beams and channels; h) seamless and electric-welded pipes

Types of cold-formed profiles made of steel tape or strip with a thickness of 1 to 8 mm. a) unequal and equal angles; b) channels; c) arbitrary section

The list of rolled profiles indicating the shape, dimensions, mass of the unit and tolerances is called the assortment

The most economical and in it are thin-walled profiles.

Fragments of columns, crane and bridge beams, trusses, girders, arches, cylindrical and hip roofs, and other structures are made from primary elements at the factory, which are then enlarged into blocks and mounted on the construction site.

The production and installation of metal structures is carried out by specialized factories and installation organizations that ensure high productivity and quality of products and installation.

Depending on the purpose and operating conditions of metal structures, the degree of responsibility of buildings and structures, it is recommended to use different categories of steels, taking into account their cold resistance at design winter outdoor temperatures.

All types of structures are divided into 4 groups, the requirements for which and, accordingly, steel grades decrease from the first to the fourth group.

And if in the first three of them, for the main critical structures, mainly complex alloyed steels, well welded and cold-resistant, are recommended, then in the fourth group for auxiliary structures - ordinary steels VSt3sp (ps) (kp).

Alloying steels with small amounts of copper, phosphorus, nickel, chromium (for example, steels of the first and second groups, 15G2AFDps, 10KhSND, 10KhNDP, 12GN2MFAYu, etc.) is especially effective for protecting them from atmospheric corrosion.

The ability of low-alloy steels to form dense protective rust films, consisting of amorphous - FeUN, led to the creation of so-called cartens.

They are used for structures of industrial buildings, bridges, supports and other structures operating in atmospheric conditions. Cardens do not require painting and do not corrode throughout the entire service life of the structures. The protective properties of the film are enhanced by periodic moistening - drying.

Typical carten composition 0.09% C and P; 0.4% Mn and Cu; 0.8% Cr and 0.3% Ni.

11. Aluminum structures

The beginning of the use of aluminum in construction can be considered the installation of an aluminum cornice on the Life Building in Montreal in 1896 and an aluminum roof on two religious buildings in Rome in 1897-1903.

During the reconstruction of the city bridge in Pittsburgh (USA) in 1933, for the first time, the bearing elements of the bridge's carriageway were made of aluminum channels and sheet, which were successfully operated for 34 years.

In domestic construction, aluminum structures were first used in the early fifties in the equipment of the research station "North Pole" and the building of climbers in the Caucasus.

Aluminum is more widely used abroad, and the construction sector uses up to 27% of the total aluminum consumption in these countries.

The production of aluminum building structures in them is concentrated at large specialized plants with a capacity of 30-40 thousand tons per year, which ensure the production of various high-quality products.

The most effective of them are:panels of external walls and coverings of frameless type, suspended ceilings, collapsible and sheet structures.

A significant part of the economic effect is achieved by reducing transportation and operating costs due to the increased corrosion resistance and lightness of aluminum structures compared to similar structures made of steel and reinforced concrete.

In load-bearing structures, the use of aluminum is not economically feasible, with the exception of large-span coatings and cases of increased aggressiveness of the environment.

This is due to the low modulus of elasticity of aluminum, as a result of which it is necessary to increase the dimensions of the sections of the elements and the structures themselves in order to provide them with the necessary rigidity and stability.

At the same time, the strength of aluminum is underused.

In addition, aluminum has a reduced cycle endurance and temperature resistance compared to steel.

These shortcomings can be overcome (taking into account the high plastic properties of aluminum) by creating spatial structures, including rod and hanging structures, using bent elements, stampings and corrugated sheets, which simultaneously perform load-bearing functions along with enclosing ones.

Aluminum bent profiles from sheet metal. a) open simple rods; b) open complex rods; c) corrugated sheets with various forms of corrugation (1 - grooved; 2 - membrane; 3 - wavy; 4 - ribbed; 5 - trough); d), e) closed multi-cavity profiles

Types of extruded profiles. a) solid; b) open; c) half-open; d) hollow (closed); e) pressed panels; f) locking connections of paired profiles; g) snap-on profile connections

Aluminum window blocks and stained-glass windows do not provide a significant economic effect compared to wooden ones, including in the conditions of the Far North.

Despite this, they have the best functional properties, appearance and high durability, which predetermine the expediency of their wide application in all types of construction.

Enclosing aluminum structures of walls and coatings can be made in two ways: from panels of full factory readiness or from profiled or smooth sheets, insulated or not insulated during construction.

The latter belong to unheated industrial buildings and warehouses.

Both methods have their advantages and disadvantages.

Simplicity and speed of installation of prefabricated panels are opposed to the absence of factory redistribution in the case of using flat or profiled tapes. But the installation of a heater becomes more complicated.

In prefabricated construction, there is a problem of reliability of joints, especially profiled sheets; with tape - installation and tension of tapes for large spans.

In domestic construction, the first panel method has received the greatest use so far.

Wall and roof panels usually consist of two thin, smooth or profiled sheets of aluminum with insulation between them.

Along the contour of the panel, in most cases, ribs are installed that create a frame.

One of the aluminum sheets (usually internal) can be replaced with plywood, asbestos-cement or plastic sheets, chipboard and fiberboard.

As a heater, mineral wool boards, PSB, PVC, PSB-S foam and polyurethane foam, foamed between the skins during the technological process, are used. The insulation is glued to aluminum sheets with epoxy or rubber glue and is included in the operation of the panel. Panel dimensions are 6x1.5x(0.05-0.15) m, 6.6x3x(0.05-0.2) m and more.

The thickness of aluminum sheathing sheets is 1-2.5 mm. The recommended grades of aluminum alloys for their manufacture are AMg2M, AMg2N2, AD31T 1(4-5), 1915.

Abroad, glued three-layer frame and frameless panels of the "Sandwich" type are prepared on a stream in individual forms or in a continuous way in the form of a continuous tape, cut at the end of an automatic line into products of a given size.

To increase weather resistance and improve the appearance, aluminum sheets are anodized or painted with polymer compounds in different colors. To improve the rigidity and quality of the panels, aluminum sheets are prestressed mechanically.

This allows you to include the skin in the work of the panel frame, increase the distance between the ribs, eliminate the waviness of the sheets and provide better adhesive contact with the insulation.

In industrial construction, aluminum sheets with longitudinal and transverse profiling are widely used for walls and coatings.

The length of the sheets is 10-30 m or more, the width is 0.58-1.6 m, the thickness is 0.3-1.62 mm.

Sheets with transverse profiling, such as "Furral", Snap-rib, Zip-rib for roofing, are used in construction practice in the USA, England, Germany, Switzerland and other countries.

Soft aluminum alloy AMts is used for this roof.

Sheets are transported in rolls. At construction, they are rolled out and attached to a wooden crate.

Fastening sheets of the "Furral" type to a wooden crate. 1 - wooden crate; 2 - sheets "Furral"; 3 - mounting strip

Insulation of wall fencing from corrugated sheets with slab insulation. 1 - corrugated sheets; 2 - insulation

Domestic experience in the manufacture of sheets with transverse profiling differs from foreign experience in the complete factory readiness of rolled fencing, including insulation.

Particularly effective are the fencing of industrial buildings made of smooth prestressed aluminum sheets.

Their cost is 20-30% less than profiled ones, and the useful area is 25-35% more.

A foam-type insulation with a textured layer that acts as a vapor barrier is glued onto the sheets at the factory or applied to the surface of the sheets during their installation, as, for example, in Italy and Japan, where foamed polyurethane foam or a foamed composition based on bitumen with a thickness of 6 -8 mm.

Three-layer roll panel structure: 1 - corrugated sheet (carrier); 2 - elastic insulation; 3 - decorative sheet (internal); a - the length of the corrugated sheet; b - panel width; R - panel bending radius

Collapsible aluminum structures are used for the construction of industrial, residential and public buildings and urban-type settlements in hard-to-reach areas and in the Far North, where they are delivered by air. Compared to traditional materials and structures, the mass of buildings is reduced by almost 20 times, the construction period is reduced by 4 times, and the estimated cost of 1 m2 of usable area is reduced by 15-20%. With an increase in the turnover of prefabricated structures, the economic effect increases significantly.

Suspended ceilings made of aluminum in terms of technical and economic indicators and a variety of functions performed (decorative and acoustic, architectural planning, ventilation, lighting, sanitary and hygienic, etc.) compare favorably with suspended ceilings made of gypsum, asbestos cement, mineral wool boards such as "Agmigran" and other materials

They are lighter, do not warp, do not generate dust, do not require repair, lend themselves to any shaping and color anodizing, which acts as an anti-corrosion protection.

Aluminum tanks are made of two types: for storage of liquid aggressive substances (sour oil and oil products, acetic, concentrated nitric and other acids); for storage of liquefied gases.

Tanks built at different times in different countries have volumes ranging from 500 m to 3500 m and are in good condition.

Pressure and non-pressure pipelines made of aluminum grades AMg2M, AD31T, 1915, 1915T are used for transportation of oil and gas, semi-products of the food and chemical industries, pumping mortars and concretes.

Duralumin pipes with a diameter of 38-50 mm are used for collapsible scaffolding and scaffolding.

Seamless and electric-welded pipes with a diameter of up to 200 mm are usually used.

When laying in soils, pipes are protected from corrosion by bitumen-rubber mastic and polymeric materials.

The practice of construction has positive examples of the use of aluminum also in ventilation and chimneys for the removal of sulfurous gases, which are aggressive towards steel during condensation.

Connections of elements of aluminum structures are carried out:

Argon-arc electric welding using non-consumable (tungsten) and consumable electrodes;
- electrocontact welding (for thin sheets);

Rivet-mounted for hardened aluminum elements and parts of different thicknesses. The riveting is carried out in a cold state in order to avoid gaps and intercrystalline corrosion observed during hot riveting;

On galvanized and cadmium-plated bolts, screws and gaskets;

On glue in bolted connections, locks and latches.

In contrast to the actual thermal chemical-thermal and thermomechanical treatments, in addition to thermal effects, include, respectively, chemical and deformation effects on the metal. This complicates the overall picture of changes in the structure and properties during heat treatment.

Equipment for carrying out chemical-thermal and thermomechanical treatments, as a rule, is more complicated than for actual heat treatment. 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 the 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 area of ​​its thermodynamic stability and then quenched for martensite (see Figure Scheme of Alloy Steel Processing). After quenching, a low tempering is carried out. The main goal of conventional heat treatment with deformation (rolling forging) heating is to exclude special heating for hardening and thereby obtain an economic effect. The main goal of HTMT is to improve mechanical properties...

Of great interest is the phenomenon of inheritance ("reversibility") of hardening from HTMT discovered by ML Bernstein during repeated heat treatment. It turned out that HTMT hardening is retained if the steel is re-hardened with a short exposure at the heating temperature for quenching or if the HTMT-hardened 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 TMT processes of steels have been intensively studied since the mid-1950s in connection with the search for new ways to increase the structural strength. Low-temperature thermomechanical treatment (LTMT) During LTMT, supercooled austenite is deformed in the region of its increased stability, but necessarily below the temperature of the onset of recrystallization and then (turns into martensite. After that, low tempering is carried out (not shown in the figure). Processing scheme ...

The use of HTMT is limited by the following factors. The alloy may differ in such a narrow range of heating temperatures for quenching that it is practically impossible to maintain the hot working temperature within such narrow limits (for example, within ± 5 ° C for D16 duralumin). The optimal temperature range for hot deformation can be significantly lower than the temperature range for heating for quenching. For example, when pressing aluminum alloys…

The essence of PTMT lies in the fact that a 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 quenching are separated (see Figure Thermomechanical treatment of aging alloys). PTMT is widely used in the technology of production of semi-finished products from aluminum alloys. It's been a long time...

At HTMT, hot deformation, quenching from deformation heating and aging are carried out (see the figure of the Scheme of thermomechanical treatment of aging alloys). During hot deformation, the density of dislocations increases and hot hardening occurs, which can be partially or completely removed during the deformation itself as a result of the development of dynamic polygonization and dynamic recrystallization. The stress-strain curve has a section of flow stress rise, ...

The figure shows the main schemes of TMT of aging alloys. Jagged lines indicate plastic deformation. Schemes of thermomechanical treatment of aging alloys Low-temperature thermomechanical treatment (LTMT) LTMT of aging alloys is the first (30s) and the most widely used thermomechanical treatment in industry. The main purpose of LTMT is to increase the strength properties. With LTMT, the alloy is first subjected to conventional hardening, ...

Let us first consider the effect of cold deformation on zone aging. It would seem that deformation, by increasing the density of dislocations and the concentration of vacancies, should accelerate zone aging. But, firstly, the zones are generated homogeneously, and not on dislocations, and, secondly, dislocations are effective places for vacancy sinks. Very strong plastic deformation increases the concentration of vacancies (the ratio of the number of vacancies to the number of atoms) by only 10-6, ...

The efficiency of LTMT application is determined by which hardening phase is released during aging. So, for example, additional hardening from the introduction of deformation before artificial aging for Al-Cu-Mg alloys (hardener - phase S) is greater than for Al-Cu alloys (hardener - phase θ´). When heated for aging after cold deformation, recrystallization, as a rule, does not proceed, but ...

Thermomechanical 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.

Thermomechanical processing of steel is carried out mainly according to three schemes: high-temperature (HTMT), low-temperature (LTMT) and preliminary thermomechanical treatment (PTMT).

Main idea high temperature processing consists in selecting the modes of rolling and cooling after rolling, which ensures the production of fine and uniform grain in the finished product.

Low temperature processing consists in heating the 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 higher) of deformation at this temperature. After that, quenching for martensite and tempering at 100...400 °C is performed. The result is a significant increase in strength compared to HTMT, but lower ductility and impact strength. This method is applicable practically only to alloyed steels.

Preliminary thermomechanical treatment It is characterized by the simplicity of the technological process: cold plastic deformation (increases the density of dislocations), pre-recrystallization heating (provides polygonization of the ferrite structure), hardening and tempering.

19. Copper and copper-based alloys. Marking of bronze and brass. The use of copper-based alloys in sanitary engineering.

Copper- malleable viscous metal of red (pink in a fracture) color, in very thin layers it looks greenish-blue in the light.

The properties obtained depend on the purity, and the level of impurity content determines its brand: MOOC - at least 99.99% copper, IOC - 99.97%, M1K - 99.95%, M2k - 99.93% copper, etc. grades after the letter M (copper) indicate the conditional number of purity, and then the letter method and conditions for obtaining copper: k - cathode; b - anoxic; p - deoxidized; f - deoxidized with 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 belongs to heavy non-ferrous metals. 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 cold and hot workability, good casting properties and satisfactory machinability. The mechanical properties of copper are relatively low: tensile strength is 150...200 MPa, relative elongation is 15...25%.

Double or multi-component alloys of copper with zinc and other elements are called brasses.

Brass is marked with the letter L (brass), followed by numbers indicating the percentage of copper. For example, brass brand L68 contains 68% copper, the rest is zinc. If the brass is multicomponent, then after the letter L put the symbol of other elements (A - aluminum, F - iron, H - nickel, K - silicon, T - titanium, Mts - manganese, O - tin, C - lead, C - zinc and etc.) and figures indicating their average percentage in the alloy. The order of letters and numbers in wrought and cast brass is different. In foundry brasses, the average content of the alloy component is indicated immediately after the letter denoting its name.

Bronze- an alloy of copper with tin, aluminum, lead and other elements, among which zinc and nickel are not the main ones. Zinc and nickel can only be introduced into bronzes as additional alloying elements. Based on their chemical composition, bronzes are classified into tin to tinless.

Bronze is marked with the letters Br, followed by alphabetic and numeric designations of the contained elements except for copper. The designation of elements in bronze is the same as for marking brass. The presence of copper in the grade is not indicated, and its content is determined by the difference. In pressure-treated bronze grades, the names of alloying elements are listed in descending order of their concentration, and at the end of the grade, in the same sequence, their average concentrations are listed. For example, bronze brand BrOTsS4-4-2.5 contains 4% tin and zinc, 2.5% lead, the rest is copper. In grades of foundry bronzes (GOST 613 and 493), after each designation of an alloying element, its content is indicated. If the compositions of foundry and pressure-treated bronzes overlap, for example, BrA9ZhZL.

20. Aluminum and aluminum-based alloys. The use of aluminum-based alloys in sanitary engineering.

Aluminum is a silvery-white light metal with a density of 2.7 g/cm3 and a melting point of 660°C. 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 an impurity content of 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 wrought aluminum is labeled ADO and AD1. Fe, Si, Cu, Mn, Zn, etc. can be present as impurities in aluminum.

On a technical basis, all aluminum alloys are divided into 2 classes:

Cast and non-deformable.

Duralumin are the most common alloys of this group, which are based on aluminum, copper and magnesium. Duralumins are characterized by a combination of high strength and ductility, they are well deformed in hot and cold states.

Silumins- this is the general name for a group of cast alloys based on aluminum containing silicon (4 ... 13% and in some grades up to 23%) and some other elements. Silumins have high casting properties, sufficiently high strength, increased corrosion resistance, and are well processed by cutting.

Test

Materials Science

On the topic: "Heat treatment of metals and alloys"

Izhevsk

1. Introduction

2. Purpose and types of heat treatment

4. Hardening

6.Aging

7. Cold treatment

8. Thermomechanical processing

9. Purpose and types of chemical-thermal treatment

10. Heat treatment of non-ferrous metal alloys

11.Conclusion

12. Literature

Introduction

Heat treatment is used at various stages of the production of machine parts and metal products. In some cases, it can be an intermediate operation that serves to improve the machinability of alloys by pressure, cutting, in others, it is the final operation that provides the necessary set of indicators of mechanical, physical and operational properties of products or semi-finished products. Semi-finished products are subjected to heat treatment to improve the structure, reduce hardness (improve machinability), and parts - to give them certain required properties (hardness, wear resistance, strength, and others).

As a result of heat treatment, the properties of alloys can be changed over a wide range. The possibility of a significant increase in mechanical properties after heat treatment in comparison with the initial state makes it possible to increase the allowable stresses, reduce the size and weight of machines and mechanisms, and increase the reliability and service life of products. Improvement of properties as a result of heat treatment allows the use of alloys of simpler compositions, and therefore cheaper ones. Alloys also acquire some new properties, in connection with which the scope of their application is expanding.

Purpose and types of heat treatment

Thermal (heat) treatment is a process, the essence of which is the heating and cooling of products in certain modes, resulting in changes in the structure, phase composition, mechanical and physical properties of the material, without changing the chemical composition.

The purpose of heat treatment of metals is to obtain the required hardness, improve the strength characteristics of metals and alloys. Heat treatment is divided into thermal, thermomechanical and chemical-thermal. Heat treatment - only thermal action, thermomechanical - a combination of thermal action and plastic deformation, chemical-thermal - a combination of thermal and chemical effects. Heat treatment, depending on the structural state obtained as a result of its application, is divided into annealing (first and second kind), hardening and tempering.

Annealing

Annealing - heat treatment, which consists in heating the metal to certain temperatures, exposure and subsequent very slow cooling together with the furnace. They are used to improve the processing of metals by cutting, to reduce hardness, to obtain a granular structure, as well as to relieve stress, partially (or completely) eliminates all kinds of inhomogeneities that were introduced into the metal during previous operations (machining, pressure treatment, casting, welding), improves the steel structure.

Annealing of the first kind. This is annealing during which phase transformations do not occur, and if they occur, they do not affect the final results provided for by its intended purpose. There are the following types of annealing of the first kind: homogenization and recrystallization.

Homogenizing- this is annealing with a long exposure at a temperature above 950ºС (usually 1100–1200ºС) in order to equalize the chemical composition.

Recrystallization- this is annealing of hardened steel at a temperature exceeding the temperature of the beginning of recrystallization, in order to eliminate hardening and obtain a certain grain size.

Annealing of the second kind. This is annealing, in which phase transformations determine its intended purpose. The following types are distinguished: complete, incomplete, diffusion, isothermal, light, normalized (normalization), spheroidizing (for granular perlite).

Full annealing produced by heating steel 30–50 °C above the critical point, holding at this temperature and slowly cooling to 400–500 °C at a rate of 200 °C per hour for carbon steels, 100 °C per hour for low-alloy steels and 50 °C for hour for high alloy steels. The steel structure after annealing is balanced and stable.

Partial annealing It is produced by heating steel to one of the temperatures in the range of transformations, holding and slow cooling. Incomplete annealing is used to reduce internal stresses, lower hardness and improve machinability.

Diffusion annealing. The metal is heated to temperatures of 1100–1200ºС, since in this case the diffusion processes necessary to equalize the chemical composition proceed more fully.

Isothermal annealing is as follows: the steel is heated and then rapidly cooled (often by transfer to another furnace) to a temperature below the critical temperature by 50–100ºС. Mainly used for alloy steels. Economical, since the duration of conventional annealing (13 - 15) h, and isothermal annealing (4 - 6) h

Spheroidizing annealing (for granular pearlite) consists in heating the steel above the critical temperature by 20 - 30 ° C, holding at this temperature and slow cooling.

bright annealing is carried out according to the modes of complete or incomplete annealing using protective atmospheres or in furnaces with a partial vacuum. It is used to protect the metal surface from oxidation and decarburization.

Normalization- consists in heating the metal to a temperature of (30–50) ºС above the critical point and subsequent cooling in air. The purpose of normalization is different depending on the composition of the steel. Instead of annealing, low carbon steels are normalized. For medium carbon steels, normalization is used instead of quenching and high tempering. High-carbon steels are subjected to normalization in order to eliminate the cementite network. Normalization followed by high tempering is used instead of annealing to correct the structure of alloyed steels. Normalization is a more economical operation than annealing, as it does not require cooling along with the furnace.

hardening

hardening- this is heating to the optimum temperature, exposure and subsequent rapid cooling in order to obtain a non-equilibrium structure.

As a result of hardening, the strength and hardness increase and the ductility of the steel decreases. The main parameters during hardening are heating temperature and cooling rate. The critical quenching rate is the cooling rate that provides the formation of a structure - martensite or martensite and residual austenite.

Depending on the shape of the part, the steel grade and the required set of properties, various hardening methods are used.

Hardening in one cooler. The part is heated to the hardening temperature and cooled in one coolant (water, oil).

Hardening in two environments (intermittent hardening)- this is hardening in which the part is cooled sequentially in two media: the first medium is a coolant (water), the second is air or oil.

step hardening. The part heated to the hardening temperature is cooled in molten salts, after holding for the time necessary to equalize the temperature over the entire section, the part is cooled in air, which helps to reduce hardening stresses.

Isothermal hardening just like the stepped one, it is produced in two cooling media. The temperature of the hot medium (salt, nitrate or alkaline baths) is different: it depends on the chemical composition of the steel, but it is always 20–100 °C higher than the martensitic transformation point for a given steel. Final cooling to room temperature is carried out in air. Isothermal hardening is widely used for parts made of high-alloy steels. After isothermal hardening, the steel acquires high strength properties, that is, a combination of high toughness with strength.

Self tempering is widely used in the tool industry. The process consists in the fact that the parts are kept in a cooling medium not until completely cooled, but at a certain moment they are removed from it in order to save a certain amount of heat in the core of the part, due to which the subsequent tempering is carried out.

Vacation

Vacation steel is the final operation of heat treatment, which forms the structure and, consequently, the properties of steel. Tempering consists in heating steel to different temperatures (depending on the type of tempering, but always below the critical point), holding at this temperature and cooling at different rates. The purpose of tempering is to relieve internal stresses that arise during the hardening process and obtain the necessary structure.

Depending on the heating temperature of the hardened part, there are three types of tempering: high, medium and low.

high vacation produced at heating temperatures above 350–600 °C, but below the critical point; such tempering is used for structural steels.

Average vacation produced at heating temperatures of 350 - 500 °C; such tempering is widely used for spring and spring steels.

low vacation produced at temperatures of 150–250 °C. The hardness of the part after hardening almost does not change; Low tempering is used for carbon and alloy tool steels where high hardness and wear resistance are required.

The tempering control is carried out by the tempering colors that appear on the surface of the part.

Aging

Aging is a process of changing the properties of alloys without a noticeable change in the microstructure. There are two types of aging: thermal and deformation.

Thermal aging proceeds as a result of changes in the solubility of carbon in iron depending on temperature.

If the change in hardness, ductility and strength occurs at room temperature, then such aging is called natural.

If the process proceeds at an elevated temperature, then aging is called artificial.

Deformation (mechanical) aging proceeds after cold plastic deformation.

Cold treatment

A new kind of heat treatment to increase the hardness of steel by converting the retained austenite of hardened steel into martensite. This is done by cooling the steel to the temperature of the lower martensitic point.

Surface hardening methods

surface hardened called the heat treatment process, which is the heating of the surface layer of steel to a temperature above the critical temperature and subsequent cooling in order to obtain a martensite structure in the surface layer.

There are the following types: induction hardening; quenching in an electrolyte, quenching by heating with high frequency currents (HFC), quenching with flame heating.

induction hardening is based on a physical phenomenon, the essence of which lies in the fact that a high-frequency electric current, passing through a conductor, creates an electromagnetic field around it. Eddy currents are induced on the surface of a part placed in this field, causing the metal to heat up to high temperatures. This makes it possible for phase transformations to occur.

Depending on the heating method, induction hardening is divided into three types:

simultaneous heating and hardening of the entire surface (used for small parts);

sequential heating and hardening of individual sections (used for crankshafts and similar parts);

continuous-sequential heating and hardening by movement (used for long parts).

Gas flame hardening. The flame hardening process consists in the rapid heating of the part surface with an oxy-acetylene, oxy-fuel or oxygen-kerosene flame to the hardening temperature, followed by cooling with water or an emulsion.

Hardening in electrolyte. The process of hardening in an electrolyte is as follows: the part to be hardened is lowered into a bath with an electrolyte (5–10% solution of calcined salt) and a current of 220–250 V is passed through. As a result, the part is heated to high temperatures. The part is cooled either in the same electrolyte (after turning off the current) or in a special hardening tank.

Thermomechanical processing

Thermomechanical treatment (T.M.O.) is a new method of strengthening metals and alloys while maintaining sufficient plasticity, combining plastic deformation and hardening heat treatment (quenching and tempering). There are three main methods of thermomechanical processing.

Low temperature thermomechanical processing (L.T.M.O) is based on stepwise hardening, that is, plastic deformation of steel is carried out at temperatures of relative stability of austenite, followed by hardening and tempering.

High-temperature thermomechanical treatment (H.T.M.O) at the same time plastic deformation is carried out at austenite stability temperatures, followed by quenching and tempering.

Preliminary thermomechanical treatment (P.T.M.O) deformation in this case can be carried out at temperatures N.T.M.O and V.T.M.O or at a temperature of 20ºС. Further, the usual heat treatment is carried out: hardening and tempering.

To change the technical characteristics of a 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 - metal heat treatment. 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 after heating does not change. With uniform heating of the workpiece, 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 blanks for swords with water. By the 19th century, man had learned to process cast iron. The blacksmith placed the metal in a container full of ice, and covered it with sugar on top. Next, the process of uniform heating begins, lasting 20 hours. After that, the cast iron billet could be forged.

In the middle of the 19th century, the Russian metallurgist D.K. Chernov documented that when a metal is heated, its parameters change. From this scientist went science - materials science.

What is heat treatment for?

Equipment parts and communication units made of metal are often subjected to severe stress. In addition to being subjected to pressure, they can 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.

Advantages 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:

1. Increased wear resistance of metal.
2. Finished parts last longer, the number of defective blanks is reduced.
3. Improves resistance to corrosion processes.

Metal structures after heat treatment withstand heavy loads, 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 variation or another stage of the technical processing of metal. This process implies uniform heating of a metal workpiece to a certain temperature and its subsequent cooling in a natural way. After annealing, the internal stress of the metal and its inhomogeneity disappear. The material softens with heat. It is easier to process later.

There are two types of annealing:

1. First kind. There is a slight change in the crystal lattice in the metal.
2. Second kind. Phase changes in the structure of the material begin. It is also called full metal annealing.

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:

1. two-stage cooling. Initially, the workpiece is cooled to 300 degrees with water. After that, the part is placed in a bath filled with oil.
2. Use of one liquid. If small parts are processed, oil is used. Large workpieces are cooled with water.
3. Stepped. After heating, the workpiece is cooled in molten salts. After that, it is laid out in fresh air until it cools completely.

An isothermal type of hardening can also be distinguished. It is similar to stepwise, but the time of holding the workpiece in molten salts changes.

Thermomechanical processing

This is a typical mode of heat treatment of steels. This process uses pressurizing equipment, heating elements and cooling tanks. At different temperatures, the workpiece is heated, and then plastic deformation occurs.

Vacation

This is the final stage of the technical heat treatment of steel. This process is carried out after hardening. The viscosity of the metal increases, the internal stress is removed. The material becomes more durable. Can be carried out at various temperatures. This changes the process itself.

Cryogenic processing

The main difference between heat treatment and cryogenic exposure is that the latter implies the cooling of the workpiece. At the end of this procedure, the parts become stronger, do not require tempering, 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 evenly cooled and remains in the chamber for a certain period of time. After that, it is taken out and allowed 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 heat up 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, machines, aircraft, ships and other equipment. The material becomes stronger, more durable and more resistant to corrosion processes. The choice of process depends on the metal or alloy used.