Chemical-thermal and thermomechanical treatment. Thermo-mechanical processing of metals and alloys Thermo-mechanical processing of metals and alloys

:

SP 16.13330.2011 Steel structures;SP 128.13330.2012 Aluminum structures;

1. General information

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

This determines their widespread use in construction and other fields of technology.

Pure metals are rarely used due to their lack of strength, hardness and high ductility. They are mainly used in the form of 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 significantly lower than colored ones.

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

Metal classification

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

Bauxite is used to produce non-ferrous metals; 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 arranged regularly at the sites of the crystal lattice and vibrate with a frequency of about 10 Hz.

The bond in metals and alloys is electrostatic, caused by the forces of attraction and repulsion between positively charged ions (atoms) at the nodes of the crystal lattice and collectivized 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.

The 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-centered, 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 located at an equal and shortest distance from a given atom.

The higher the number, the denser the packaging.

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 a 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 temperature, undergo structural changes in their crystal lattice.

Thus, 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 the indicated 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 the release of heat during cooling and absorption during heating, which is recorded in the diagrams along horizontal sections.

Iron cooling (heating) curve

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

In contrast to crystals of regular shape, they are called crystallites or grains.

The crystallites are differently oriented, therefore, in all directions the properties of metals are more or less the same, i.e. polycrystalline solids are isotropic.

However, with the same orientation of the crystallites, such 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. Basics of producing cast iron and steel

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

Coke, when burned, forms 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 general scheme: FeOFeOFeOFe.

Falling into the lower hot part of the furnace, the iron melts in contact with 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 transferred into cast iron from waste rock.

Depending on the amount and form of bonding of carbon and impurities with iron, cast irons have different properties, including color, being divided according to this criterion 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 cast iron in large pear-shaped converter vessels with compressed air.

Oxygen in the air oxidizes impurities, turning them into slag; carbon burns out.

When the phosphorus content in cast iron is low, converters are lined with acidic refractories, such as dinasium; when the phosphorus content is high, they are lined with basic, periclase refractories.

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

The converter method is characterized by high productivity, which has led to its widespread use.

Its disadvantages include increased metal waste, slag contamination and the presence of air bubbles, which deteriorate 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 cast iron is smelted together with iron ore and scrap metal.

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

The composition of the steel can be easily controlled, which makes it possible to produce high-quality steels for critical structures in open-hearth furnaces.

Electromelting is the most advanced method for producing high-quality steels with specified properties, but requires increased energy consumption.

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

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

4. Mechanical properties of metals

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

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

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

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 a metal under the influence of conventional stresses =R/A and the curve 2 - under the influence of true stresses, S=R/A, (Where A And A- respectively, the cross-sectional area of ​​the sample before testing and at each loading stage until destruction).

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

Metal tensile diagrams: a) for conditional (solid lines) and true (dashed lines) stresses; / - 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 residual elongation strain does not exceed 0.05%.

The yield strength is characterized by the conditional yield strength 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 increasing the load.

For materials that are brittle when tested in tension, static tests are used for compression (for cast iron), torsion (for hardened and structural steels) and bending (for castings made of gray and ductile iron).

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

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

Hardness determination scheme . 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.

Testing of metals for fracture toughness is carried out on standard samples with a notch under three-point bending.

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

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

Dynamic Testing of metals is carried out for impact bending under alternating cyclic loading. Metal samples measuring (1x1x5.5)10 m with a stress concentrator (notch) in the middle are tested for impact bending.

The test is carried out on a pendulum pile driver. The resistance of a metal to impact bending is called impact strength and is designatedKSU, KSV And KST(Where KS- symbol of impact strength, andU, V And T -type and size of voltage concentrator).

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

The endurance limit is sharply reduced 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 as they grow.

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 connections, etc.).

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

Therefore, 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 components of the alloy retains its crystal lattice, and the atoms of another or other components are located in the crystal lattice of the first component (solvent), changing its dimensions (periods).

A distinction is made between substitutional and interstitial solid solutions.

In the first case, the atoms of the dissolved component replace part of the solvent atoms 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 substitution solutions, the lattice parameter can increase or decrease depending on the ratio of the atomic radii of the solvent and the dissolved component; in implantation solutions - always increase.

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

For example, in iron, molybdenum, and 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 undergo phase transformations during crystallization and repeated heating-cooling, to change 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 threshold of cold brittleness, impact strength and fracture toughness.

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

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

This 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 exposed during the production process to cyclic heating - cooling in the temperature range of 20-200 ° C or more, experience thermal aging and low-temperature tempering, often aggravated by corrosion, which is necessary take 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;

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

Thermal and thermomechanical treatment of steel.

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

As a result, the hardenability of steel increases; hardness, cold brittleness threshold, temper brittleness, tendency to thermal and deformation aging are reduced, and the 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, B - tungsten, G - manganese, D - copper, E - selenium, K - cobalt, N - nickel, M - molybdenum, P - phosphorus, P - boron, C - silicon, T - titanium, F - vanadium, X - chromium, C - zirconium, Ch - rare earth, Yu - aluminum.

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

Located along the grain boundaries, they impede their growth, diffusion of carbon and other alloying elements and increase the resistance of austenite to overcooling.

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

Thermal and thermomechanical processing are common methods for modifying the structure and improving the properties of steel.

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

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

Alloy steel ingots are subjected to homogenization at 1100-1200 °C for 15-20 hours to level the chemical composition, reduce dendritic and intracrystalline segregation, which causes brittle fracture during pressure treatment, anisotropy of properties, formation of flakes and coarse-grained structure.

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

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 deformation it is called dynamic, characterized by residual “hot work hardening”, useful for hardening from rolling heating.

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

Normalization involves heating long rolled products of pre- and hypereutectoid structural steel, short-term holding and cooling in air.

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

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

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

Hardening and tempering provide for an improvement in the strength and ductile-ductile properties of steel, a decrease in the threshold of cold brittleness and sensitivity to stress concentrators.

Hardening consists of 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 stress 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 durable. However, a fairly complete martensitic transformation is possible only for high-carbon and alloy steels, which have 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 is either not formed or is formed in smaller quantities in combination with bainite.

With rapid cooling of low-carbon building steels (C0.25%) (quenching from rolling heating), austenite decomposes and the formation of a highly dispersed ferritic-cementite structure of pearlite-sorbite and troostite or low-carbon martensite and cementite.

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 lower cold capacity threshold.

Strengthening 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, and eliminate the tendency to temper brittleness, which is observed during medium-temperature tempering of alloy steel at 300...400 °C.

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

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

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

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.

Aging of carbon steel manifests itself in changes in its properties over time without noticeable changes in the microstructure.

Strength and cold brittleness threshold increase, ductility and impact strength decrease.

Two types of aging are known - thermal and deformation (mechanical).

Strain (mechanical) aging occurs 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 impedes their movement.

Builders are faced with the occurrence of temper brittleness and aging of steel when using 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 eutectic 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 in both forms simultaneously.

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

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

Depending on the shape of graphite and the conditions of its formation, they are distinguished: gray, high-strength with nodular graphite and malleable cast iron.

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 cooling and annealing mode.

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

I - white cast iron area; II - half cast iron; III - pearlitic gray cast iron; IV - ferritic-pearlite 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 difficult to machine and is brittle.

Used as a conversion agent for steel or ductile iron.

Bleached cast iron has a structure of white 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 a Fe-Si-C alloy, with inevitable impurities of Mn, P and S.

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

Silicon promotes 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 impairs 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 fragility of cast iron.

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

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

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

Physico-mechanical properties of cast irons of various structures

Graphite inclusions, while sharply reducing the tensile strength of gray cast iron, have virtually no effect on its compressive strength, bending strength 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 iron includes modified cast iron of the SCh30-SCh35 grades, containing modifier additives - graphite, ferrosilicon, silicocalcium in an 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

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

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

Grades of high-strength cast iron are designated by letters and numbers.

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

Malleable cast iron is produced by prolonged heating (annealing) of white cast iron castings.

Annealing is carried out in two stages with holding in 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 is released in the form of flakes, giving the cast iron high ductility.

Its fracture is velvety black.

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

The term “malleable cast iron” is conditional and characterizes the plastic, not the technological properties of cast iron, since products made from it, like other cast irons, are produced by casting and not forging.

All types of cast irons with graphite inclusions considered are used in construction.

Gray cast irons are used in structures subject to static loads (columns, foundation slabs, support slabs for trusses, beams, sewer pipes, manholes, valves); high-strength and malleable cast irons, which have increased strength, ductility and toughness, are used in structures subject to dynamic and vibration loads and wear (floors of industrial buildings, foundations of heavy forging and pressing equipment, under-truss supports of railway and road bridges, tubings for fastening critical transport tunnels underground , in the mountains).

8. Non-ferrous metals

Of the non-ferrous metals, aluminum is most widely used in construction because it has high specific strength, ductility, corrosion resistance and economic efficiency.

Silver, gold, copper, zinc, titanium, magnesium, tin, lead and others are used mainly as alloying additives and components of alloys and therefore have a special and limited use in construction (special types of glass, unique objects - memorials on the Mamayev Kurgan in Volgograd, on Poklonnaya Hill, 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, heating, electrical systems of buildings and structures).

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

Aluminum- silver-white metal, density 2700 kg/m 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 by plastic deformation (frettening), hardening and aging. All aluminum alloys are divided into wrought and cast.

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

Thermally hardenable alloys include Al-Mg-Si, Al-Cu-Mg, Al-Zn-Mg; thermally non-hardening - technical aluminum and two-component alloys Al-Mn and Al-Mg (magnalium).

Copper- the main alloying additive 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 is fluid and fusible, but impairs ductility.

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

To improve the properties of aluminum alloys, small amounts of chromium, vanadium, titanium, zirconium and other elements are introduced into them. Iron (0.3-0.7%) is an undesirable but inevitable impurity.

The ratio of components in alloys is selected based on the conditions for them to achieve high strength, workability and corrosion resistance after heat treatment and aging.

Alloys are designated by grades that have alphabetic and numerical designations characterizing the composition and condition of the alloy: M - annealed (soft); N - hard-worked; H2 - semi-hardened; T - hardened and naturally aged; T1 - hardened and artificially aged; T4 - not completely hardened and artificially aged.

Hardening and semi-hardening are characteristic of thermally non-hardening alloys; hardening and aging - for thermally hardened ones.

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

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

Digital designation of grades of aluminum alloys: 1915, 1915T, M925, 1935T (the first digit indicates the base of the alloy - aluminum; the second - the composition of components; 0 - technically 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 their group).

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

Annealing occurs without phase transformations and is used to relieve residual stress, homogenization, recrystallization and recovery.

In the latter case, the initial physical and mechanical properties of the alloy are restored, strength is reduced, and ductility and toughness, necessary for technological purposes, are increased.

9. Steel reinforcement for reinforced concrete structures

For the reinforcement of reinforced concrete structures, rod and wire reinforcement of smooth and periodic profiles and ropes made of low-carbon and low-alloy steels, strengthened by hardening with rolling heating, cold or warm deformation, are used.

These requirements are largely satisfied by high-strength rod (A-1V - AV1; At-1VC(K) - At-V1C(K), etc.), wire (B-II, BP-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 increasing the strength and crack resistance of structures by 20-30%, the consumption of reinforcing steel is reduced compared to non-prestressed A-I (A-240), A-II (A-300), A-III (A-400) , VR-I.

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

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

The passive state of reinforcement in concrete, thermodynamically prone to oxidation reactions, is ensured by the highly alkaline nature of the environment (pH12) and a fairly 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 for iron in the active state it is about 0.4 V.

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

This characteristic potential is called the Flade potential..

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

However, the pH value of the environment ambiguously characterizes the state of the reinforcement in concrete; it is largely determined by the presence of activating ions, which shift the metal dissolution potential to the negative side; 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. changes in electrode potential and current density.

Not all concretes are characterized by a high pH value.

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

In such concrete, the reinforcement requires a protective coating.

Depassivation of reinforcement 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 damage to the metal surface acts similarly to stress raisers.

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

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

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

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

10. Steel structures

The main structural forms and purposes of steel structures are:industrial buildings, frames and long-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:

Thick hot-rolled sheet steel 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 rolls; wide-flange universal, 6-60 mm thick, hot-rolled with machined, aligned edges;

Profile steel - angles, channels, I-beams, T-beams, pipes, etc., from which various symmetrical sections are assembled, ensuring increased stability and efficiency 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 supports;

Electric-welded round pipes, with a diameter of 8-1620 mm and a wall thickness of 1-16 mm; square and rectangular section with side dimensions from 60 to 180 mm and wall thickness from 3 to 8 mm. Pipes are used in lightweight roof structures, half-timbered walls, frames, stained glass windows;

Cold-formed profiles made from tape or strip 1-8 mm thick. Their main area of ​​application is lightweight, economical building roofing structures;

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 roofing, bridges, masts, prestressed roofing structures, pipes, tanks, etc.

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

Types of cold-formed profiles made of steel strip 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, unit weight and tolerances is called assortment

Thin-walled profiles are the most economical.

Fragments of columns, crane and bridge beams, trusses, purlins, arches, cylindrical and tent coverings, and other structures are made from primary elements in 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 steel, 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, mainly complex alloy steels, which are well weldable and cold-resistant, are recommended for the main critical structures, 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, 10HSND, 10KHNDP, 12GN2MFAYu, etc.) is especially effective in protecting them from atmospheric corrosion.

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

They are used for the structures of industrial buildings, bridges, supports and other structures operating in atmospheric conditions. Karten 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 and drying.

Typical carthene composition is 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 load-bearing elements of the roadway of the bridge were made of aluminum channels and sheets, which were successfully used for 34 years.

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

Aluminum has become more widely used abroad, with up to 27% of total aluminum consumption in these countries being used in the construction sector.

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

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

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

The use of aluminum in load-bearing structures is not economically feasible, with the exception of long-span coatings and cases of increased environmental aggressiveness

This is due to the low modulus of elasticity of aluminum, as a result of which it is necessary to increase the cross-sectional dimensions of the elements and the structures themselves in order to ensure their necessary rigidity and stability.

This underutilizes the strength of aluminum.

In addition, aluminum has 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, including rod and hanging structures, using bent elements, stampings and corrugated sheets, which simultaneously perform enclosing and power functions.

Aluminum bent profiles from rolled sheets. a) open simple rods; b) open complex rods; c) corrugated sheets with different corrugation shapes (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) profile connections with snaps

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

Despite this, they have the best functional properties, appearance and high durability, which determine the feasibility of their widespread use in all types of construction.

Enclosing aluminum structures of walls and roofs can be made in two ways: from fully prefabricated panels or from profiled or smooth sheets, insulated or not insulated during the construction process.

The latter refer to unheated industrial buildings and warehouses.

Both methods have their advantages and disadvantages.

The simplicity and speed of installation of factory-ready panels are contrasted with the absence of factory processing in the case of using flat or profiled tapes. But installation of insulation becomes more complicated.

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

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

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

In most cases, ribs are installed along the contour of the panel, creating a frame.

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

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

The thickness of the aluminum sheathing sheets is 1-2.5 mm. 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-line in individual forms or in a continuous manner in the form of a continuous strip, cut at the end of an automatic line into products of given sizes.

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

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

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

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

Sheets with transverse profiles, such as "Furral", Snap-rib, Zip-rib for roofing coverings, 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. During construction, they are rolled out and attached to a wooden sheathing.

Fastening Furral type sheets to wooden sheathing. 1 - wooden sheathing; 2 - sheets "Furral"; 3 - fastening strip

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

Domestic experience in manufacturing sheets with transverse profiles differs from foreign ones in the complete factory readiness of roll fencing, including insulation.

Fencing of industrial buildings made of smooth prestressed aluminum sheets is especially effective.

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

Insulation such as foam rubber with a textured layer that acts as a vapor barrier is glued to the sheets in 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 foam composition based on bitumen with a thickness of 6 is used for this. -8 mm.

Three-layer roll panel design: 1 - corrugated sheet (load-bearing); 2 - elastic insulation; 3 - decorative sheet (inner); a is the length of the corrugated sheet; b - panel width; R - panel bending radius

Prefabricated aluminum structures are used for the construction of industrial, residential and public buildings and urban settlements in remote areas and in the Far North, where they are delivered by air. Compared to traditional materials and structures, the weight of buildings is reduced by almost 20 times, the construction period is reduced by 4 times, and the estimated cost of 1 m 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 compare favorably with suspended ceilings made of gypsum, asbestos cement, mineral wool slabs such as "Agmigran" and other materials

They are lighter, do not warp, do not generate dust, do not require repairs, can be shaped in any way and can be colored anodized, which acts as anti-corrosion protection.

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

The 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 transporting oil and gas, semi-products of the food and chemical industries, pumping mortars and concrete.

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

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

When laid in soil, pipes are protected from corrosion with bitumen-rubber mastic and polymer materials.

Construction practice has positive examples of the use of aluminum also in ventilation and chimneys for the removal of sulfur dioxide gases, which are aggressive towards steel upon condensation.

Connections of elements of aluminum structures are carried out:

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

On rivets for elements made of hardened aluminum and parts of different thicknesses. Riveting is done in a cold state 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.

General information. Heat treatment of steel and other structural materials is the technological process of heat treatment of workpieces, machine parts and tools, as a result of which the microstructure of the material changes, and with it the mechanical, physico-chemical and technological properties. The processes of heat treatment of structural materials are associated with allotropic transformations (polymorphism), as well as with changes in the chemical composition of the product material.

Blanks, forgings, stampings, as well as finished parts and tools are subjected to heat treatment to give them the necessary properties: hardness, strength, wear resistance, elasticity, removal of internal stresses, and improvement of workability.

The essence of heat treatment is to heat the metal to a temperature slightly higher or lower than critical temperatures, hold it at these temperatures and quickly or slowly cool it. During the cooling process, allotropic changes occur in the metal structure, as a result of which the mechanical properties sharply change. With rapid cooling, hardness, wear resistance, elasticity, etc. increase, with slow cooling - ductility, impact strength, and workability. In addition, there is heat treatment associated with a change in the chemical composition of the product material, the so-called chemical-thermal treatment.

Depending on the heating method and the depth of heating, allotropic transformations occur throughout the entire cross-section or only in the surface layers of the workpiece. When heated to a certain temperature, held at this temperature and cooled at a certain rate, the microstructure of the parts changes over the entire cross section.

A change in the chemical composition in the surface layers of processed parts is accompanied by their strengthening or changes in other properties.

The following methods of heat treatment of steels exist:

• volumetric heat treatment of steels, carried out with the aim of changing the microstructure of metal alloys in the solid state and giving them the necessary properties throughout the entire volume of processed parts (hardening, tempering, annealing, normalization);
• surface heat treatment of steel, causing a change in the structure and properties only in the surface layer of the product;
• chemical-thermal treatment, which consists in heating metal products together with substances that can change the composition and structure, mainly of the surface layer of the product being processed;
• electrothermal treatment, carried out using induction heating with high frequency currents, as well as by contact heating and heating in electrolytes;
• thermomechanical treatment associated with heating products undergoing, for example, rolling, drawing and similar operations, in order to eliminate work hardening caused by plastic deformations.

Transformation into steel when heated. Transformations in steel when heated are associated with the alloys reaching critical temperatures at which phase transformations occur.

In the system of iron-carbon alloys, the following designations for critical temperatures are accepted: the PSK line temperature (see Fig. 3.6) is designated A 1 (727 °C), the MO line temperature is A 2 (768 °C), the GOS line temperature is A 3 (727 ... 911 °C), line temperature ES - A m(727 … 1,147 °C). To distinguish the critical temperature obtained during cooling from the critical temperature obtained during heating, the letter r (Ar 1, Ar 2) is placed in front of the digital index when cooling, and c (Ac 1, Ac 2) when heating.

The transformation of pearlite into austenite, in full accordance with the Fe-Fe 3 C diagram, can be completed at a temperature of 727 ° C

(Ac 1) with slow heating. The rate of transformation of pearlite into austenite is directly dependent on the carbon content of the steel.

At a temperature of 768 °C (Curie point - Ac 2), steel loses its magnetic properties.

The end of the transformation process is characterized by the formation of austenite and the disappearance of pearlite.

When heating steels with a carbon content of less than 0.8%, i.e. hypoeutectoid, with an initial structure consisting of ferrite and pearlite, the following structural transformations occur. At a temperature of 727 °C, pearlite transforms into austenite. At the same time, the two-phase structure is preserved - austenite and ferrite. With further heating, ferrite transforms into austenite, which ends when the critical temperature Ac 3 is reached, i.e. on the GOS line.

In hypereutectoid steels, when heated above the temperature Ac 1, cementite dissolves in austenite (in accordance with the SE line), which ends at a critical temperature Ac m, i.e. on the SE line.

For a more complete understanding of the processes of structural transformations, let us consider the diagram of the isothermal transformation of pearlite into austenite upon heating (Fig. 1).

Rice. 1. t - temperature; τ - time; A - austenite; P - perlite; C - cementite; v 1 and v 2 - heating rates; Ac 1 - critical temperature (eutectoid)

Since pearlite is a mixture of cementite and ferrite in a ratio of approximately 1: 6, when heated, austenite grains form at the interface between ferrite and cementite. Upon subsequent heating, cementite dissolves in austenite and further growth of austenite grains occurs. As austenite grains grow, the mass fraction of carbon in austenite gradually increases. The heating rate also affects the transformation of pearlite to austenite. In the diagram, rays v 1 and v 2 graphically depict different heating rates. The lower the heating rate, the lower the temperatures the entire process of phase transformations occurs.

An important characteristic of steel is its tendency to grow austenite grains when heated. When grain growth occurs with slight overheating above the critical point, the steel is considered to be hereditarily coarse-grained. If the grain begins to grow with greater overheating, it is hereditarily fine-grained. Grain growth is greatly influenced by various impurities that enter the steel during the smelting process. The tendency for austenite grains to grow is a melting characteristic.

Grain size affects the mechanical properties of steels. Fine-grained steel has significantly higher impact strength than coarse-grained steel, so this factor should be taken into account when heat treating steels.

The actual grain size is the grain size under normal temperature conditions after a certain type of heat treatment. To determine the grain size, a standard scale has been adopted. GOST 5639-82* presents a scale for assessing grain size using a ten-point system (Fig. 2).

Rice. 2. Standard steel grit scale (100x):1-10 - grain points

The grain size is determined at a hundredfold magnification by comparison with a standard scale. To determine the grain size, the steel must be heated to a temperature of 930 °C. If at this temperature the grain number is 1 - 4, then this steel is hereditarily coarse-grained. Steels with a grain number of 5 - 8 or more are hereditarily fine-grained. Alloying elements (vanadium, tungsten, molybdenum, titanium, etc.) contribute to the formation of a hereditarily fine-grained macrostructure. At high temperatures, such steel lends itself well to any type of deformation treatment (rolling, forging, stamping, etc.). In this case, grain coarsening and mechanical properties do not decrease. As a rule, most alloy steels, as well as mild steels, are inherently fine-grained. All boiling steels are hereditarily coarse-grained, have low impact strength and high cold brittleness.

Transformation into steel upon cooling. When cooling steels with an austenitic structure, various transformations can occur, depending on the cooling rate. Let us consider the diagram of the isothermal transformation of austenite into pearlite (Fig. 3). Austenite transformation curves have a C-shaped characteristic and show that the transformation rate is not the same. The maximum transformation rate corresponds to cooling below Ac 1 (727 °C) by 170 °C. The curves for the beginning and end of transformations are shifted to the right and correspond to the greatest stability.

Rice. 3. t - temperature; τ - time; A - austenite; P - perlite; B - bainite; M - martensite; And the rest is retained austenite; T - troostite; F - ferrite; C - cementite; C - sorbitol; v 1 and v 2 - cooling rates; M n and M k are, respectively, the temperatures of the beginning and end of the martensitic transformation; A p - half austenite; v cr - critical speed

The left curve in the diagram corresponds to the boundary of the beginning of transformations, the right curve shows the end of the austenite transformation. The transformation of austenite into pearlite is of a diffusion nature.

The rate of diffusion depends on the degree of supercooling or the cooling rate. The products of pearlite transformation have a lamellar structure, are defined as pearlite, sorbitol and troostite and differ in the degree of dispersion. But if perlite is an equilibrium structure, then sorbitol and troostite are nonequilibrium structures, their carbon content is more or less than 0.8%. There is also an intermediate (bainite) transformation in the temperature range 500 ... 350 °C. With a greater degree of supercooling (up to 230 °C), austenite is in an unstable state, there are no diffusion processes, and a solid solution supersaturated with carbon is formed.

Martensitic transformation in steel has three features. Firstly, the martensitic transformation has a diffusion-free character. Secondly, martensite crystals are oriented. The third feature is that the martensitic transformation occurs during continuous cooling in the range of certain temperatures for each steel. The temperature at which the martensitic transformation begins is called the martensitic point and is designated Mn, and the end temperature is designated Mk. The position of the points Mn and Mk on the diagram depends on the amount of carbon in the steel and the presence of alloying elements. As a rule, a high carbon content and the presence of alloying elements lower the position of the points.

Let's superimpose the cooling rate graphs on the diagram and draw a diagram of the effect of the cooling rate on the temperature of austenite transformations. From the diagrams we see that the higher the cooling rate, the more dispersed the resulting structure. At a low speed v 1, perlite is formed, at a higher speed v 2 - sorbitol and an even higher speed v cr - troostite. At a cooling rate greater than vcr, part of the austenite transforms into martensite. The minimum cooling rate at which all austenite is supercooled to the Mn point and turns into martensite is called the critical quenching rate. This process of transformation into martensite is of great practical importance and forms the basis of heat treatment.

Pearlitic transformation in steels is used in the annealing process; martensitic - during hardening; intermediate - during isothermal hardening.

The mechanical properties of steel with pearlite, sorbite and troostite structures depend on the degree of decrease in the decomposition temperature and the dispersity of the ferrite-cementite structure. At the same time, hardness, strength limits, fluidity and endurance increase.

The structure of martensite has higher hardness and strength, and also depends on the carbon content of the steel. A negative factor of the martensitic structure is increased brittleness. As already mentioned, alloying elements affect the position of the points M n and M k and, accordingly, affect the practical hardening rate, usually in a decreasing direction.

Heat treatment mode. The heat treatment process for the purpose of changing the structure and mechanical properties consists of the operations of heating the product, holding it at a given temperature and cooling at a certain speed. The parameters of the heat treatment technological process will be the maximum heating temperature of the alloy, the holding time at a given temperature and the heating and cooling rates.

Heating steel is one of the main heat treatment operations, on which phase and structural transformations, changes in physical and mechanical properties depend, therefore the heating mode is decisive for obtaining specific characteristics of the alloy. In practice, a distinction is made between technically possible and technically permissible heating rates for each part or batch of parts.

The technically possible heating rate depends on the heating method, the type of heating devices, the shape and location of the products, the mass of simultaneously heated parts and other factors.

The technically permissible, or technological, heating rate depends on the chemical composition of the alloy, structure, configuration of the product and the temperature range at which heating is carried out. The holding time is the time required to completely equalize the temperatures throughout the entire volume of the product and, accordingly, to complete all phase and structural transformations.

Cooling is the final process carried out in order to obtain the desired structure with the necessary mechanical properties.

Depending on the heating temperature and cooling rate, the following main types of heat treatment are distinguished: annealing, normalization and hardening followed by tempering.

Rice. 4. 1 - pearlite + ferrite; 2 - austenite; 3 - martensite; 4 - troostite; 5 - sorbitol; 6 - ferrite + pearlite

In Fig. Figure 4 shows the microstructures obtained as a result of heating and cooling grade 40 steel at different rates. The characteristics of these microstructures are discussed in Table. 1.

2. Annealing and normalization

Annealing. Annealing is a softening treatment of parts and workpieces, which consists of heating to a certain temperature within critical points and subsequent slow cooling with the furnace. The main purpose of annealing is to eliminate structural heterogeneity in parts and workpieces obtained by pressure treatment, casting, forging and welding, and to recrystallize the structures of parts (including obtaining the microstructure of granular pearlite and cementite). With the elimination of structural heterogeneity, a change in mechanical and technological properties occurs, the removal of internal stresses, the elimination of fragility, a decrease in hardness, an increase in strength, ductility and toughness, and an improvement in stampability and machinability. In practice, annealing of the first and second kind is distinguished.

Annealing of the first kind - this is the heating of parts and workpieces with a nonequilibrium structure to obtain a stably equilibrium structure.

Annealing of the second kind - this is the heating of parts and workpieces above critical temperatures, followed by slow cooling to obtain a stable state of the structure. Heating parts and workpieces above critical temperatures ensures complete recrystallization of the metal structure. For example, carbon structural steel grade 40 in a casting or forging will have a deformed structure in the form of large grains of ferrite and pearlite (Fig. 5, a). When this steel is heated to a temperature above Ac 3, the deformed structure transforms into austenite, and upon slow cooling, into an equilibrium structure in the form of small grains of regular shape ferrite and pearlite (Fig. 5, b). This structure is characterized by high hardness, brittleness and low machinability. After the complete annealing operation, the structure becomes larger, the pearlite grains are evenly distributed, the hardness decreases and the workability improves. This is the essence of the process of annealing parts and workpieces.

Rice. 5. Microstructure of steel grade 40 obtained as a result of casting and forging (a) and after normalization (b)

An important factor determining high-quality annealing is the correct choice of heating temperature, which is determined from the iron-carbon (cementite) diagram depending on the steel grade and the mass fraction of carbon. Thus, hypoeutectoid steels are heated to the critical temperature Ac 3 + (20 ... 30 ° C), hypereutectoid steels are heated to the critical temperature Ac 1 + (20 ... 30 ° C) for partial annealing. When heating steel above the critical temperature Ac 3 or Ac m(depending on the grade) the microstructure of pearlite turns into the microstructure of fine-grained austenite.

For high-quality annealing, it is necessary to correctly select the heating speed and temperature, as well as the cooling rate.

Types of annealing. In practice, the following types of annealing are used: complete, incomplete, low-temperature, isothermal, leveling, or diffusion (Fig. 6).

Full annealing subjected to stamping, forging and casting from hypoeutectoid and hypereutectoid steel to recrystallize their deformed microstructure. The heating temperature for complete annealing is chosen 20 ... 30 °C above the critical point Ac 3 (Fig. 7, a) and is cooled to a temperature of 500 °C together with the furnace, then cooled in air. After complete annealing, the deformed structure is corrected, the grain is refined and pearlite and ferrite grains are evenly distributed over the entire cross-section of the parts. At the same time, hardness decreases, impact strength, strength and ductility increase, machinability improves and, most importantly, internal stresses are relieved.

Rice. 6.

Rice. 7. Scheme of complete (a) and incomplete (b) annealing of carbon steels:

Partial annealing used mainly for parts and workpieces made of hypereutectoid steels. For hypoeutectoid steels, this type of annealing is used for forgings, stampings and castings, the microstructure of which has received the correct equilibrium fine-grained shape. During incomplete annealing (Fig. 7, b), the parts are heated to the critical temperature Ac 1 + (20 ... 30 ° C), maintained at this temperature and cooled together with the furnace to temperature Ac 1 - (20 ... 30 ° C), maintained at at this temperature and then cooled together with the furnace to a temperature of 500 ° C, then the part is cooled in air.

Incomplete annealing results in a microstructure of granular (spheroidized) pearlite or granular cementite. At the same time, internal stresses are also reduced. The newly obtained microstructure of granular pearlite reduces hardness, increases ductility and toughness. Improves machinability.

With the help of incomplete annealing, internal stresses are relieved, warping and the formation of microcracks are prevented, and the machinability of parts and workpieces is improved. When heated, the workpieces are kept in the oven for a long time to completely warm them up and cooled together with the oven (at a rate of no more than 60 °C/h). In its purpose and the physicochemical processes occurring in the parts, incomplete annealing is similar to spheroidizing annealing.

Low temperature annealing used for parts and workpieces obtained by forging, stamping and casting, the structure of which has not undergone any particular deformation, is in an equilibrium state and does not require correction, there is no need for its recrystallization. In this regard, workpieces are subjected to low-temperature annealing in order to relieve internal stresses and improve machinability by cutting and drawing. For these purposes, the parts are heated below the critical point Ac 1. Heating is carried out slowly at a rate of up to 150 °C/h, maintained at this temperature, after a long exposure the parts are cooled together with the furnace or in air.

Isothermal annealing subject parts of small sections made of alloy and carbon steels. In this case, structural steels are heated to a temperature 30 ... 40 °C above the critical point Ac 1, and tool steels - to a temperature 50 ... 100 °C above the critical point Ac 3. After heating and warming up (holding), the parts are transferred to another furnace (bath), where they are cooled to a temperature 50 ... 100 ° C lower than that obtained initially

process. At this temperature, the parts are kept until complete (isothermal) decomposition of austenite into granular pearlite. During this thermal operation, hardness decreases, strength and ductility increases, and machinability by various technological operations improves. The diagram of isothermal annealing of forgings made of alloy steel grade KhVG is shown in Fig. 8, a.

As can be seen from the diagram, the forging is heated after forging using a stepwise method. First, it is cooled 50 ... 100 ° C below the critical point Ac 1, maintained at this temperature, then heated above the critical point Ac 1 by 20 ... 50 ° C, maintained for a long time at this temperature and cooled together with the furnace.

A type of isothermal annealing is annealing on granular pearlite (Fig. 8, b). Annealing of granular pearlite is carried out by stepwise heating and cooling until the complete decomposition of austenite into granular pearlite. First, they are heated to the critical point Ac 1 + (20 ... 30 ° C), then cooled to a temperature below Ac 1 (700 ° C) and then heated again to a temperature of 500 ... 660 ° C. After a long exposure at the last temperature, the parts are cooled in air.

Rice. 8. Scheme of isothermal annealing (a) and annealing on granular pearlite (b) forgings made of alloy steel grade HVG:t - temperature; τ - time; Ac 1, Ac 3 - critical temperatures

In most castings, including those made of iron-carbon alloys, heterogeneity in the chemical composition of the crystals (grains) is obtained - the so-called intercrystalline ionic (dendritic or zonal) liquation. In order to eliminate this chemical heterogeneity, it is used in practice leveling , or diffusion , annealing (homogenization). For this type of annealing, the castings are heated to a high temperature, usually up to 1,000 ... 1,100 ° C, kept at this temperature for a long time and then slowly cooled along with the furnace. At high temperatures, atoms of some chemical elements, concentrated unevenly, become more mobile and diffuse from one crystal to another. There is a chemical alignment of the chemical composition of both large crystals (dendrites) and small crystals.

After diffusion annealing, a coarse-grained structure is obtained, which requires additional complete or incomplete annealing. If workpieces requiring further pressure treatment were subjected to this annealing, then these workpieces are not subjected to additional annealing before processing. Such parts undergo one type of annealing only after pressure treatment (forging, stamping, drawing).

Defects during annealing. During annealing, due to violation of technological conditions, the following defects can form: overheating, burnout, decarburization and oxidation of parts and workpieces.

Overheat occurs when the temperature regime is not observed at high temperatures and during technologically unjustified long exposure in the oven. In this case, a coarse-grained structure appears, which is called the overheating structure.

The coarse-grained structure has reduced ductility, a tendency to form cracks, tensile stresses and warping of parts. Overheating can also occur when heating workpieces for hot deformation, when annealing products of complex configurations, heating to a temperature significantly higher than the critical temperature or prolonged exposure at a technologically justified temperature.

Overheating is a correctable defect. To correct it, complete annealing must be carried out in compliance with all temperature conditions.

Significant overheating is accompanied by rapid grain growth, which damages the boundaries of these grains. Damage to grain boundaries is called burnout . Burnout occurs when the metal is kept at high temperatures for a long time. In this case, sometimes partial melting of the grain boundaries or their active oxidation occurs. The part becomes fragile.

Overburning is an irreparable defect and is an annealing defect.

Decarbonization And oxidation parts and workpieces undergo annealing in salt baths, electric and flame furnaces. With these heating methods, the surface of the parts interacts with various gases. Based on the degree of exposure and chemical interaction with the surfaces of parts, reagents are divided into oxidizing (oxygen, carbon monoxide, water vapor) and decarburizing (oxygen, hydrogen, water vapor).

The nature of oxidation in the furnace is determined by the fuel and its chemical composition, the atmosphere of the furnace, the time the parts are in the furnace and the type of construction material. Oxidation causes metal scale on the surface of the part, a change in its size and leads to the cost of additional technological operations to clean the scale.

Decarburization as an annealing defect is caused by the fact that oxygen, present in the furnace atmosphere, oxidizes carbon earlier than iron, i.e., carbon burns out to a small depth from the surface of the part. If oxygen simultaneously oxidizes carbon and iron, scale formation and metal waste occur. If there is steam in the furnace atmosphere, then decarbonization occurs very actively. Decarburization reduces hardenability or generally causes immunity to hardening, reduces fatigue strength, and worsens the chemical properties of the surfaces of parts.

To prevent decarburization of parts, the furnace atmosphere must contain dry hydrogen, carbon monoxide or inert neutral gases. In addition, during annealing, parts are heated in hermetically sealed boxes coated with clay, charcoal or cast iron shavings.

Normalization. Normalization is the process of heat treatment of parts and workpieces, in which they are heated to a critical temperature Ac 3 or Ac m+ (30 … 50 °C), maintained at this temperature and cooled in air. In the process of normalization, the microstructure of fine (dispersed) pearlite is obtained. At the same time, hardness and strength are slightly reduced, ductility and impact strength are increased, and machinability is improved.

The heating temperature for normalization is selected depending on the grade of steel and the mass fraction of carbon in it according to the steel part of the iron-carbon diagram. The purpose of normalization depends on the composition of the steel, the specific post-forming treatment and the design of the part.

For example, low-carbon steels are normalized instead of annealed to improve machinability. Before hardening, tool carbon steels are also subjected to normalization to eliminate the cementite network and obtain a fine pearlite structure. Steel grade 30 after complete annealing (as delivered) has the following properties: strength - 440 MPa; plasticity - 17%; hardness - 179 HB; impact strength KSV - 62 J/cm2. After normalization, these same properties change somewhat: strength is 390 MPa; plasticity - 23%; hardness - 143 ... 179 HB; impact strength KSV - 49 J/cm2. The example is taken for forgings with a diameter of up to 100 mm. As you can see, after normalization the mechanical properties will be slightly lower than in the delivered state due to the stabilization of the metal structure of the parts. This factor significantly improves the machinability of cutting workpieces.

During the normalization process, defects similar to annealing defects arise, but in a less pronounced form. For example, slight overheating of the metal does not lead to burnout. Partial decarburization does not lead to the formation of scale and metal waste.

3. Quenching and tempering

Hardening. Hardening is the heating of steel to a temperature above critical, holding at this temperature and subsequent rapid cooling. As a result of hardening, hardness, strength, elasticity, wear resistance and other mechanical properties increase.

The cooling rate must be significantly higher than the critical rate at which the austenite microstructure breaks down into a metastable martensite microstructure. As is known, this microstructure, like the microstructure of austenite, has a uniform solubility of carbon. Maintaining uniform carbon solubility by fixing the microstructure is the main purpose of hardening.

At a critical cooling rate or significantly higher than it, the physicochemical state of austenite is fixed in its uniform solubility of carbon.

During the hardening process, with a change in the microstructure, mechanical properties (hardness, impact strength), physical properties (magneticity, electrical resistance, etc.) and chemical properties (uniformity in chemical composition, corrosion resistance) change.

The main purpose of hardening is to obtain high hardness, wear resistance, increased strength, elasticity and reduced ductility. All these properties are formed by observing the following technological heat treatment regimes:

• heating temperature;
• heating rate and holding time;
• heating medium;
• cooling rate.

Choice of hardening temperature. The heating temperature for hardening is theoretically determined from the Fe - Fe 3 C diagram. For carbon steels, it should be 30 ... 50 ° C above the GSK line (see Fig. 3.6), i.e. for hypoeutectoid steels it coincides with the critical temperature Ac 3 + (30 ... 50 °C), for eutectoid and hypereutectoid steels - with a critical temperature Ac 1 + (50 ... 70 °C).

For alloy steels, the heating temperature for hardening is determined by three methods: diametric, magnetic or test hardening.

It has been established that the more complex the alloy steel in terms of chemical composition and the nature of the microstructure, the higher the heating temperature for hardening should be, since only at elevated temperatures do carbides of vanadium, tungsten, molybdenum, titanium and chromium successfully dissolve in austenite. In this case, as a basis, as when choosing hardening temperatures for carbon steels, the critical points Ac 1, Ac 3 and Ac m. Heating temperatures for hardening alloy steels increase by 250 ... 300 °C above the critical ones, and for high-speed steels - by 400 ... 450 °C.

Heating and cooling modes. The heating time depends on the cross-section of parts and workpieces, the design and power of heating devices. For example, when heating in electric air furnaces, the heating time is determined on average at the rate of 1 minute per 1 mm of section of the part. The heating time in salt baths is 2 times lower than in electric furnaces, since the heating rate in these baths is 2 times higher. After heating the parts to a given temperature, they are held until complete phase transformation and heating throughout the entire cross-section. An indicator of the holding time is the transformation of the initial pearlite + ferrite structure into the austenite structure. Practice has shown that the specified heating temperature for parts occurs when the color of the parts becomes equal to the color of the furnace (underneath, walls, roof).

Both the heating rate and side (negative) phenomena depend on the environment in heating devices (forges, furnaces, baths). Negative phenomena include decarburization and oxidation of hardened parts. The forge and electric (muffle) furnaces contain an air environment, the oxygen of which oxidizes the parts being hardened. In salt baths, salts not only oxidize, but also decarbonize parts. Baths with molten metal (lead) do not have a negative effect on heated parts for hardening.

Before the austenite structure is completely obtained, the time required is 1/5 of the heating time of the part. Subject to technologically sound heating, holding and cooling regimes, the appearance of large internal stresses, the formation of cracks and other hardening defects is eliminated. On the other hand, the technological time regime eliminates surface oxidation and decarbonization of parts.

The structure and properties of the parts being hardened depend on the cooling rate during hardening. The cooling rate at which the austenite structure transforms into a hardening structure (martensite) is called the critical hardening rate. This time mode is selected depending on the required microstructure of the part. The highest cooling rate gives the microstructure of martensite, the lowest (natural) - sorbitol.

Quenching media. The quenching medium and its cooling ability ensure the fixation of uniform dissolution of carbon in the newly formed microstructure of austenite decomposition. In the temperature range of austenite decomposition into martensite, slow cooling is necessary in order to reduce internal stresses. To obtain complete hardening, coolers with different cooling capacities are used. This ability depends on several factors: lowering the temperature of the coolant, the heat capacity of the metal, its thermal conductivity, maintaining a constant temperature of the cooling medium, circulation speed, reducing the vaporization temperature and reducing the viscosity of the coolant. All these factors increase the cooling rate.

The following solutions and liquids are used as quenching media: water, aqueous solution of table salt, oil, air, minerals and other materials.

Based on their strength, coolers are divided into the following groups:

• weak - a stream of air, molten salts, hot and soapy water;
• moderate - spindle oil, transformer oil, molten salt baths with 1% water;
• medium-acting - solutions in cold water of lime, glycerin and liquid glass;
• strong - clean cold water, table salt in a solution of cold water, distilled water and mercury.

The cooling rate also depends on the method of cooling (immersion) of the part being hardened. In this case, when the part to be hardened is immersed in water or oil, three cooling stages are distinguished:

• the appearance of a steam jacket that prevents further heat transfer (film boiling);
• destruction of the steam jacket and an increase in the cooling rate (nucleate boiling);
• convection of coolant, which occurs at a temperature below the boiling point.

For all these stages, the cooling rate is faster, the lower the temperature conditions from stage to stage. It also depends on the nucleate boiling range.

This or that type of cooling medium is selected depending on technological feasibility, the chemical composition of the metal of the part, and the required physical and mechanical properties.

Water and its solutions are stronger coolants. However, water has significant disadvantages. As the water temperature rises during the quenching process, its cooling capacity drops sharply. In addition, water has a high cooling rate in the martensitic transformation temperature range.

Aqueous solutions of salts, alkalis, and soda increase the cooling rate and also increase the nucleate boiling range. Various types of oils as cooling media reduce the cooling rate, the processes of martensitic transformation are more stable. The disadvantages of oils include their flammability and the formation of burns on the surface of parts.

Hardenability and hardenability. Hardenability depends on the mass fraction of carbon in steel. The greater the mass fraction of carbon in steel, the higher the hardenability of this steel. Steels with a mass fraction of carbon up to 0.3%, as well as carbon structural steels of ordinary quality according to GOST 380-2005, cannot be hardened, since in this group of steels carbon varies widely. Considering that the choice of temperature for hardening is carried out depending on the mass fraction of carbon, and in steels of ordinary quality we cannot accurately determine its content, this group of steels is not subject to hardening.

High-quality carbon structural and alloy steels with a carbon mass fraction of 0.3% and higher, as well as all tool steels, are subject to hardening.

The hardenability of steels refers to the depth of hardening, i.e., the ability to form microstructures of martensite, troostite or sorbite during the hardening process.

Hardenability depends on the critical cooling rate and, as a consequence, on the stable ability of austenite not to change its microstructure. The structure of austenite that remains cold is called supercooled austenite.

If the critical cooling rate of the part over the entire cross-section is equal, then the part will have through-hardenability, i.e., there will be a martensite structure over the entire cross-section. If the cooling rate across the entire cross-section decreases towards the core, then the core will contain ferrite, ferrite + pearlite, sorbitol or troostite. The core of large cross-section parts practically does not accept hardening, since the cooling rate of the core will be slow and natural.

All alloying elements increase hardenability. For example, nickel contributes to a significant increase in hardenability and hardenability. Manganese, chromium, tungsten and molybdenum increase the quenching and tempering temperature, and also increase the hardenability and hardenability of parts and tools, so all alloy steels subjected to hardening have high hardenability, and carbon steels have lower hardenability. With through hardening over the entire cross-section, the hardness of the part will be the same. With non-through hardening, it will decrease from the surface to the core. The part will have a martensite structure at the surface, and a troostite structure at the core. The lower the mass fraction of carbon in steel, the greater the troostite structure and the lower the hardness, and vice versa.

The hardenability of parts during hardening is assessed as a critical parameter. This parameter represents the maximum diameter (section) of the parts, the core of which will have a semi-martensitic hardening structure. Typically, for carbon structural and tool steels the critical parameter is 10 ... 20 mm, and for alloy steels - up to 100 mm or more (depending on the mass fraction of carbon and alloying elements). In addition, hardenability depends on the cooling medium. Water gives higher hardenability than oil.

Steel with a carbon mass fraction of 0.2% (cooling in water) after quenching will have a hardness of 25 HRC, and steel with a carbon mass fraction of 0.5% after quenching will have a hardness of 45 HRC. Consequently, the more carbon in the steel, the higher the hardness of the part obtained during hardening, and, consequently, the greater the depth of hardenability. To determine the depth of hardenability of carbon tool steels, samples with a length of 100 mm are prepared after high tempering of square or round sections (21 ... 23 mm). A cut with a depth of 5 ... 7 mm is made in the middle of the samples. Finished samples are hardened at the following temperatures: 760; 800; 840 °C. Hardened samples are destroyed on pendulum pile drivers (or in a press). The depth of hardenability (hardened layer) or non-hardenability (non-hardened layer), overheating or hardening cracks are determined by the state and type of fracture.

Using a standard scale, the group (or score) of the depth of hardenability of hardened samples at different temperatures is determined. In the standard scale, each group (from 0 to V) corresponds to hardenability depths from 0.3 mm to 9 mm, through hardenability, tough core, unhardened zone and hardening cracks. All this is determined visually by the fracture of the samples. In addition, by the fracture of the samples, it is possible to determine the hardening structure (martensite, semi-martensite, troostite, sorbite) or the unhardened zone (pearlite or ferrite + pearlite).

In Fig. 9, and conventionally shows samples of steel grade 40 (GOST 1050-88*) with a diameter of 12 ... 60 mm after quenching and cooling in water. Samples 1 - 4 receive complete hardening with the formation of a martensite structure (continuous hardenability). As the diameter increases, continuous hardening is formed, but the structures will depend on the critical hardening rate: martensite, semi-martensite, troostite and sorbitol. The cross-sectional hardness of the sample will also vary and range from 25 to 46 HRC depending on the structure. As the sample diameter increases, the critical quenching rate decreases. The cross-sectional structure of the sample will be as follows: martensite, semi-martensite, troostite, sorbitol and pearlite (or pearlite + ferrite). The hardness along the cross section of the sample will be 25 ... 46 HRC. The core of the sample, having a sorbitol + perlite structure, will have high impact strength and strength.

Rice. 9. a - after quenching and cooling in water; b - after quenching and cooling in oil; - martensite; - semi-martensite; - troostitis; - sorbitol; - perlite (or perlite + ferrite)

During continuous hardening (cooling in water), samples 1 - 4 will be brittle.

In practice, the following methods are used to determine hardenability:

• by the fracture structure of the sample;
• on a hardness tester type TK along a cross-section at several points (from the surface to the core);
• by end hardening method.

To determine the diameter of parts requiring continuous hardening, the following condition must be met: the critical hardening diameter must be greater than the diameter of the product.

When determining the hardenability of steel using the end-hardening method, it is recommended to determine the depth of hardenability using various diagrams.

Hardening defects. Violation of hardening conditions (heating temperature, cooling methods, etc.) can cause various types of defects in parts and tools:

• deformation, warping and cracks;
• insufficient hardness;
• increased fragility;
• formation of soft spots;
• resizing;
• internal stresses;
• oxidation and decarbonization.

Vacation. Tempering is the technological process of heating parts after hardening to low temperatures (150 ... 650 ° C), i.e. below the critical point Ac 1, holding at this temperature and slow natural cooling in air.

The purpose of tempering is to eliminate internal stresses in parts after hardening, increase impact strength, reduce brittleness and partially reduce hardness. These indicators are achieved in connection with obtaining a stable metal structure of the part. The tempering temperature depends on the type of parts being hardened and the purpose of the tempering. In practice, low, medium and high holidays are used.

Low Vacation used to relieve internal stresses and increase the impact strength of tools made of alloy and carbon steels. During low tempering, parts are heated to a temperature of 150 ... 250 ° C, maintained at this temperature and cooled in air. At the same time, the hardness and wear resistance of the cutting tool obtained after hardening are preserved.

Low tempering is applied to cutting and measuring tools, parts of ball and roller bearings, permanent magnets, and machine parts made from alloyed structural case-hardened and high-strength steels.

Average holiday used for elastic parts: springs, springs, impact and stamping tools, torsion bars, etc. With this type of tempering, parts are heated to a temperature of 300 ... 500 ° C, heated over the entire cross section and cooled in air. After cooling, the tempered troostite structure is obtained. The hardness of parts obtained during hardening after tempering decreases noticeably. Impact strength increases sharply, which leads to an increase in cyclic toughness (this property is necessary for elastic parts).

High holiday are produced for machine parts from high-quality carbon structural and alloy steels operating under heavy loads: shafts, spindles, gear blocks, claw couplings, ratchet mechanisms, etc. The hardness of parts after hardening and high tempering, depending on the steel grade, is 35 ... 47 HRC.

During high tempering, parts are heated to a temperature of 500 ... 650 ° C, maintained at this temperature and cooled in air (in some cases, together with the furnace). After tempering, the structure of the parts will be tempered sorbitol. The part will have high wear resistance, strength, impact strength and relative ductility. In practice, high tempering with deformation of parts during heating is also used (Fig. 10). The part is deformed between the critical temperatures Ac 1 and Ac 3. After deformation, the parts are slowly cooled to a temperature below Ac 1, then heated, held and slowly cooled.

Rice. 10. t - temperature; τ - time; Ac 1, Ac 3 - critical temperatures; M n - temperature of the beginning of martensitic transformation

Improvement - This is the hardening of steel followed by high tempering. This thermal operation is used for machine parts operating under significant, including alternating, loads and made from structural steel grades 30, 35, 40, 45, 50, 40X, etc.

Aging is the process of changing the properties of alloys without noticeably changing the microstructure. If a change in hardness, strength and ductility occurs under normal conditions (18 ... 20 ° C), then such aging is called natural. If the process occurs at elevated temperatures (120 ... 150 ° C), then aging is called artificial.

With natural aging, parts last for several months, with artificial aging - 24 ... 36 hours. During the aging process, the solubility of chemical elements (carbon, silicon and manganese, as well as alloying additives) in the structure of parts is stabilized and, along with them, the structures are stabilized.

Tempering as a heat treatment is a mandatory operation after hardening and is carried out simultaneously with hardening immediately after cooling the parts.

4. Chemical-thermal treatment

Surface hardening. During operation of machine parts, mechanisms and tools, the working (friction) surfaces of parts and tools wear out and require re-sharpening or complete replacement.

Wearing of working surfaces even to a small depth can lead to serious consequences. In order to give working surfaces high wear resistance, reliability and durability, various technological methods of hardening these surfaces are used. The following types of coatings exist:

• one-component coatings - saturation of surfaces with one chemical element (metal or non-metal): carbon, nitrogen, chromium, tantalum, manganese, etc.;
• two-component coatings - saturation of surfaces with two chemical elements (metal and non-metal): carbon + chromium, carbon + boron, carbon + nitrogen, carbon + manganese, carbon + sulfur, etc.;
• multi-component coatings: carbon + chromium + nitrogen, carbon + boron + nitrogen, carbon + phosphorus + nitrogen, chromium + ammonium + silicon, etc.

A separate group consists of coatings made from chemical compounds: carbides, nitrides and oxides.

With visible differences in technological processes, hardening of working (rubbing) surfaces consists of saturating them with any metals or non-metals under the influence of temperature or other physical and chemical processes.

Chemical-thermal treatment according to its intended purpose is divided into two groups:

• chemical-thermal treatment designed to increase wear resistance and surface hardness of the working surfaces of parts. This type of processing includes carburization, nitriding, nitrocarburization and diffusion metallization;

• chemical-thermal treatment used to obtain high anti-friction (extreme pressure) properties. A chemical element that saturates the surface of parts prevents scuffing and sticking of rubbing surfaces. This type includes sulfidation, lead plating, telluration, etc.

Thus, chemical-thermal treatment is usually called a technological process that consists of saturating the surface layer of parts at high temperatures with metals or non-metals using the diffusion method.

Chemical-thermal treatment is used to increase hardness, wear resistance, corrosion and fatigue resistance, and for decorative finishing.

Chemical-thermal treatment of parts is carried out in some medium (carburizer), the atoms of which can diffuse into the surface of these parts. Chemical-thermal treatment processes consist of three stages: dissociation, adsorption and diffusion. Dissociation - is the release of atoms of chemical elements (metals and non-metals) capable of dissolving in metals (alloys) of parts by diffusion. This process takes place in a gaseous environment. Adsorption - this is the contact of isolated (dissociated) atoms of chemical elements (metals and non-metals) with the surfaces of parts and the formation of a chemical bond with the metal atoms of the parts.

Diffusion is the process of penetration of a saturating element into the atomic lattices of metal parts.

The higher the heating temperature of the parts, the faster all three stages pass. The process is especially active at temperatures equal to the critical ones, since at these temperatures a restructuring of the atomic lattices of the metal of the parts occurs. During the restructuring process, the atoms of the diffusing element are successfully introduced into the atomic lattices or replace the metal atoms of the parts in them.

Chemical-thermal treatment has a number of advantages compared to heat treatment:

• the ability to process parts and tools of any shape, complexity and configuration;
• difference in the mechanical properties of the working part of the parts and their core;
• the possibility of eliminating overheating defects by subsequent heat treatment;
• possibility of hardening low-carbon steels.

Rice. eleven. 1 - solid carburizer; 2 - witnesses; 3 - cementation box; 4 - cemented parts

Cementation. Cementation is a chemical-thermal operation during which the surface layer of parts is saturated with carbon. Cementation is carried out in order to obtain high hardness and wear resistance of the surface of parts with high impact strength of the core. They cement parts made of steel with a mass fraction of carbon up to 0.25%, operating under friction and under alternating loads: gear wheels, gear blocks, distribution and cam rollers, cams, valve tappets and other parts, as well as measuring instruments - gauges, templates, probes, etc. The surface of parts and tools is saturated with carbon in some cases to a depth of 1.4 mm, usually this layer is 0.8 mm. The mass fraction of carbon saturated into the surface of the parts reaches 0.8 ... 1.0%. The carbon concentration decreases from the surface of the part to the core. Thus, parts made of structural carbon and low-alloy steels, which do not respond to improvement by hardening, are subjected to carburization.

The working fluid in which the chemical-thermal treatment is carried out is called a carburizer. There are carburization in solid, liquid and gas carburizers. For carburization in a solid carburizer, parts to be carburized are placed in a steel box (Fig. 11), which are evenly poured over with the carburizer. Control samples, the so-called witnesses, are placed simultaneously with the carburizer. During the heating and holding process, control samples are removed, and the progress of the technological process is determined from them.

In Fig. Figure 12 shows the dependence of carbon concentration on saturation depth. Thus, at a saturation depth of 0.1 mm, the carbon concentration reaches 1%, 0.2 mm - 0.9%, 1 mm - 0.6%, 1.6 mm - 0.16%. This concentration of carbon in the surfaces of rubbing parts (gears, gears, shafts, axles, etc.) ensures the reliability and durability of the contact pair.

Rice. 12.

During cementation, depending on the depth of carbon saturation, various microstructures are formed (Fig. 13). Before heat treatment, at a depth of up to 1 mm there will be a cementite structure, more than 1 mm - perlite and further - ferrite. After heat treatment (hardening), at a depth of up to 1 mm there will be a martensite structure, then troostite and sorbitol. At a depth of more than 2 ... 3 mm - the original structure.

Rice. 13. Various microstructures formed during cementation, depending on the depth of carbon saturation:1 - hypereutectoid zone (P+C); 2 - eutectoid zone (P); 3 - hypoeutectoid zone (P + F); 4 - core

Cementation in a gas environment is the main chemical-thermal process in mass production. Gas carburization is carried out in muffle or shaft furnaces in a carburized atmosphere. The furnace atmosphere is carbonized with methane, kerosene or benzene. After gas carburization, hardening followed by low tempering is used. Gas cementation makes it possible to control the process, which in turn creates conditions for mechanization and automation of production.

During cementation, the following defects are formed:

• corrosion of the surface layer by barium sulfate salts;
• reduced mass fraction of carbon in the cemented layer;
• decarburization that occurs during the cooling process due to cracks or burns in the boxes;
• uneven depth of the cemented layer due to temperature changes in the furnace;
• supersaturation with carbon in the cemented layer due to violations of temperature and time regimes, as well as due to the high content of carbonates in the carburizer;
• small depth of the cemented layer, which occurs at low temperatures and exposures;
• internal oxidation that occurs during gas carburization due to the high oxygen content in the furnace atmosphere.

The appearance of these defects can be avoided by observing the chemical composition of carburizers, thermal and time conditions. Correction of defects in machine parts is carried out through additional normalization and subsequent chemical-thermal treatment.

Nitriding. Nitriding is a chemical-thermal treatment process in which the surfaces of parts are saturated with nitrogen. Nitriding is carried out to obtain high surface hardness, wear resistance, fatigue strength and resistance to scuffing, increasing the endurance limit, corrosion resistance in the atmosphere, fresh water and water vapor, as well as cavitation resistance of various parts and tools. Nitriding is also used for decorative finishing. The nitrided layer can be up to 0.5 mm deep and have a hardness of 1,000 ... 1,100 HV, which is much harder than cementite. Due to the duration of the process (up to 90 hours) and high cost, nitriding is used less frequently than carburization. The nitriding process is carried out in an ammonia environment at temperatures of 500 ... 600 °C. When heated, atomic nitrogen is released from ammonia, which diffuses into the surface of the parts. In order to speed up the nitriding process, a two-stage cycle is used (Fig. 14). This nitriding technology speeds up the process by 1.5 - 2 times. First, the part is heated to a temperature of 500 ... 520 °C, then rapid heating is carried out to a temperature of 580 ... 600 °C and then - long-term exposure and cooling together with the furnace or in air.

Rice. 14. t - temperature; τ - time

Liquid nitriding is carried out at a temperature of 570 °C in a melt of nitrogen-containing salts. Liquid nitriding speeds up the process tenfold and significantly increases the viscosity of the part. The disadvantage of liquid nitriding is the use of toxic cyanide salts.

Thus, nitriding is a multi-purpose technological operation of chemical-thermal treatment, carried out to increase the strength and other properties of various carbon and alloy structural, tool and special steels (corrosion-resistant, heat-resistant and heat-resistant), refractory and sintered materials, as well as galvanic and diffusion coatings.

Defects may occur during the nitriding process. Deformation and changes in the dimensions of parts occur due to large internal stresses due to an increase in the volume of the nitrided layer. To eliminate this defect during machining, it is necessary to reduce the dimensions by 4 ... 6% of the depth of the nitrided layer.

Brittleness and peeling occur when the nitrided layer is oversaturated with nitrogen. A fragile crust forms on the surface to a depth of 0.05 mm and peels off. This defect can be eliminated by grinding.

Reduced hardness, spotty hardness or reduced depth of the nitrided layer are defects that appear when the chemical composition of the environment is not observed, poor preparation of the surface of parts and violation of the thermal regime. To avoid the occurrence of these defects, it is necessary to comply with the technological requirements for preparing parts for nitriding and observe the sequence of the technological process.

Cyanidation and nitrocarburization. Cyanidation is the process of saturating the surface of parts with carbon and nitrogen simultaneously. Parts made of steels with a carbon mass fraction of 0.3 ... 0.4% are subjected to cyanidation. Cyanidation is carried out to increase surface hardness, strength, wear resistance, endurance and other mechanical and operational properties. Cyanidation has a number of advantages over other types of chemical-thermal processing: the ability to process parts of complex shapes, short process duration, and virtually no warping or deformation of parts during processing. As disadvantages, it should be noted the high costs of labor protection due to toxicity and the high cost of cyanide salts. All this significantly increases the cost of cyanidated parts.

There are liquid and gas cyanidation. Gas cyanidation is called nitrocarburization.

Liquid cyanidation is carried out in a medium of molten salts of sodium cyanide. It is carried out at a temperature of 820 ... 850 or 900 ... 950 °C. The process, carried out at a temperature of 820 ... 850 °C, in 30 ... 90 minutes allows you to obtain a layer up to 0.35 mm thick, saturated with carbon and nitrogen, and at 900 ... 950 °C in 2 ... 6 hours - a layer up to 2 mm thick. In Fig. Figure 15 shows the dependence of the thickness of the cyanidated layer on temperature and duration of the process. For example, with a holding time of 2 hours at a temperature of 890 °C, the depth of the cyanidated layer reaches 0.6 mm, and with a holding duration of 4.5 hours at a temperature of 830 °C - also 0.6 mm.

After cyanidation, hardening and low tempering are carried out. The hardness of the cyanidated layer reaches 58 ... 62 HRC.

In practice, low-temperature cyanidation in molten cyanide salts is used to carburize tools made of high-speed steels. It is carried out at a temperature of 540 ... 560 ° C with a holding time of 1.0 ... 1.5 hours. As a result of this treatment, the cyanidated layer will have a hardness of 950 ... 1,100 HV.

Rice. 15.

The mass fraction of carbon in the cyanidation process reaches 1%, nitrogen - 0.2%. These indicators depend on the cyanidation temperature (Fig. 16).

Diffusion metallization. The process of saturating the surface layer of parts by diffusion at high temperature

Rice. 16. Carbon (C) and nitrogen (N) content during cyanidation process

different metals is called diffusion metallization. It can be carried out in solid, liquid and gas carburizers (metallizers).

Solid metallizers are powder mixtures consisting of ferroalloys: ferrochrome, metallic chromium, ammonium chloride, etc.

Liquid metallizers are usually molten metal, such as zinc, aluminum, etc.

Gas metallizers are volatile chlorides of metals: aluminum, chromium, silicon, titanium, etc.

Depending on the used diffusion metal of the parts, the following types of diffusion metallization are distinguished: aluminizing (saturation with aluminum), chrome plating, titanium plating, tungsten plating, sulfation (saturation with sulfur), boriding, etc.

Aluminizing carried out at a temperature of 700 ... 1,100 °C. In the surface layer in the structure of α-iron, aluminum dissolves, a dense film of aluminum oxide is formed on the surface, which has high corrosion resistance in the atmosphere and sea water, as well as high scale resistance at temperatures of 800 ... 850 ° C, hardness 500 HV. Aluminizing is applied to parts operating at elevated temperatures: engine valves, covers for thermocouples, etc. Aluminizing is carried out using the following methods: in powder mixtures, in molten aluminum, by electrolysis, in aerosols with aluminum and gas spraying. Chrome plating subject to parts operating in aggressive environments: parts of steam installations, steam-water devices, parts and assemblies operating in gas environments at high temperatures. Chrome plating is carried out in powder mixtures, vacuum, molten chromium, a gas environment and ceramic masses. The surface, saturated with chromium to a depth of 0.15 mm, is resistant to scale in a gas environment up to a temperature of 800 ° C, in fresh and sea water and in weak acids. Any steel can be chrome-plated. The hardness of the chrome layer in the surface reaches 1,200 ... 1,300 HV. To increase hardness and toughness, after chrome plating, parts are normalized.

One of the technological processes of hardening treatment is thermomechanical treatment (TMT).

Thermo-mechanical treatment refers to combined methods of changing the structure and properties of materials.

Thermo-mechanical processing combines plastic deformation and heat treatment (hardening of pre-deformed steel in the austenitic state).

The advantage of thermomechanical treatment is that with a significant increase in strength, the ductility characteristics decrease slightly, and the impact strength is 1.5...2 times higher compared to the impact strength for the same steel after quenching with low tempering.

Depending on the temperature at which deformation is carried out, a distinction is made between high-temperature thermomechanical treatment (HTMT) and low-temperature thermomechanical treatment (LTMT).

The essence of high-temperature thermomechanical treatment is to heat steel to the temperature of the austenitic state (above A 3 ). At this temperature, the steel is deformed, which leads to hardening of austenite. Steel with this state of austenite is subjected to hardening (Fig. 16.1 a).

High-temperature thermomechanical processing virtually eliminates the development of temper embrittlement in the dangerous temperature range, weakens irreversible temper embrittlement and dramatically increases toughness at room temperature. The temperature threshold for cold brittleness decreases. High-temperature thermomechanical treatment increases resistance to brittle fracture and reduces sensitivity to cracking during heat treatment.

Rice. 16.1. Scheme of thermomechanical treatment modes of steel: a – high-temperature thermomechanical treatment (HTMT); b – low-temperature thermomechanical treatment (LTMT).

High-temperature thermomechanical processing can be effectively used for carbon, alloy, structural, spring and tool steels.

Subsequent tempering at a temperature of 100...200 o C is carried out to maintain high strength values.

Low-temperature thermomechanical processing (ausforming).

The steel is heated to an austenitic state. Then it is kept at a high temperature, cooled to a temperature above the temperature of the onset of martensitic transformation (400...600 o C), but below the recrystallization temperature, and at this temperature pressure treatment and quenching are carried out (Fig. 16.1 b).

Low-temperature thermomechanical treatment, although it gives higher strengthening, does not reduce the tendency of steel to temper brittleness. In addition, it requires high degrees of deformation (75...95%), so powerful equipment is required.

Low-temperature thermomechanical processing is applied to martensite-hardened medium-carbon alloy steels that have the secondary stability of austenite.

The increase in strength during thermomechanical treatment is explained by the fact that as a result of deformation of austenite, its grains (blocks) are crushed. The dimensions of the blocks are reduced by two to four times compared to conventional hardening. The dislocation density also increases. With subsequent quenching of such austenite, smaller martensite plates are formed and stresses are reduced.

Mechanical properties after different types of TMT for engineering steels on average have the following characteristics (see Table 16.1):

Table 16.1. Mechanical properties of steels after TMT

Test

In 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 treatment

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 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 and cutting; in others, it is the final operation that provides the necessary set of indicators of the 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 (improved workability), 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 within wide limits. The possibility of significantly increasing the mechanical properties after heat treatment compared to the initial state makes it possible to increase the permissible stresses, reduce the size and weight of machines and mechanisms, and increase the reliability and service life of products. Improving properties as a result of heat treatment allows the use of alloys of simpler compositions, and therefore cheaper. Alloys also acquire some new properties, and therefore their scope of application expands.

Purpose and types of heat treatment

Thermal (thermal) treatment refers to processes whose essence is the heating and cooling of products according to 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 and improve the strength characteristics of metals and alloys. Heat treatment is divided into thermal, thermomechanical and chemical-thermal. Heat treatment is only thermal exposure, thermomechanical is a combination of thermal exposure and plastic deformation, chemical-thermal is a combination of thermal and chemical exposure. 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 consists of heating the metal to certain temperatures, holding it and then very slowly cooling it along with the furnace. Used to improve metal cutting, reduce hardness, obtain a grain structure, and also to relieve stress, eliminates partially (or completely) all kinds of inhomogeneities that were introduced into the metal during previous operations (machining, pressure treatment, casting, welding), improves the structure of steel.

Annealing of the first kind. This is annealing in which no phase transformations occur, and if they occur, they do not affect the final results intended for its intended purpose. The following types of annealing of the first kind are distinguished: homogenization and recrystallization.

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

Recrystallization- This is the annealing of hardened steel at a temperature exceeding the temperature at which recrystallization begins, 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 in hour for high alloy steels. The structure of steel after annealing is equilibrium and stable.

Partial annealing produced by heating steel to one of the temperatures located in the transformation range, holding and slow cooling. Partial annealing is used to reduce internal stresses, reduce 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 occur more fully.

Isothermal annealing is as follows: the steel is heated and then quickly cooled (usually by transferring it to another furnace) to a temperature below the critical temperature by 50–100ºС. Mainly used for alloy steels. Economically beneficial, since the duration of conventional annealing is (13 – 15) hours, and isothermal annealing (4 – 6) hours

Spheroidizing annealing (on granular pearlite) consists of heating steel above the critical temperature by 20 - 30 ° C, holding it at this temperature and slowly cooling.

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

Normalization– consists of heating the metal to a temperature (30–50) ºС above the critical point and subsequent cooling in air. The purpose of normalization varies depending on the composition of the steel. Instead of annealing, low-carbon steels are normalized. For medium-carbon steels, normalization is used instead of hardening 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 alloy steels. Normalization, compared to annealing, is a more economical operation, since it does not require cooling with the furnace.

Hardening

Hardening– this is heating to the optimal temperature, holding and subsequent rapid cooling in order to obtain a non-equilibrium structure.

As a result of hardening, the strength and hardness of the steel increases 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 ensures the formation of a structure - martensite or martensite and retained austenite.

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

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

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

Step hardening. A part heated to the quenching temperature is cooled in molten salts; after holding for the time necessary to equalize the temperature over the entire cross-section, the part is cooled in air, which helps reduce quenching stresses.

Isothermal hardening just like the stepped one, it is produced in two cooling environments. The temperature of the hot medium (salt, nitrate or alkaline baths) is different: it depends on the chemical composition of the steel, but is always 20–100 °C above 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, steel acquires high strength properties, that is, a combination of high toughness and strength.

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

Vacation

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

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

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

Average holiday 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 remains almost unchanged; low tempering is used for carbon and alloy tool steels, which require high hardness and wear resistance.

Tempering control is carried out by tarnish colors appearing on the surface of the part.

Aging

Aging is a process of changing the properties of alloys without noticeable changes in the microstructure. Two types of aging are known: thermal and deformation.

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

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

If the process occurs at elevated temperatures, then aging is called artificial.

Deformation (mechanical) aging occurs after cold plastic deformation.

Cold treatment

A new type 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 hardening is a heat treatment process that involves heating the surface layer of steel to a temperature above critical and subsequent cooling in order to obtain a martensite structure in the surface layer.

The following types are distinguished: induction hardening; hardening in an electrolyte, hardening when heated with high frequency currents (HF), hardening with gas-flame heating.

Induction hardening is based on a physical phenomenon, the essence of which is 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 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 quenching by movement (used for long parts).

Gas flame hardening. The process of gas-flame hardening consists of quickly heating the surface of a part with an acetylene-oxygen, gas-oxygen or oxygen-kerosene flame to the hardening temperature, followed by cooling with water or emulsion.

Quenching in electrolyte. The process of hardening in an electrolyte is as follows: the part to be hardened is lowered into a bath of 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 quenching tank.

Thermo-mechanical treatment

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

Low temperature thermomechanical treatment (L.T.M.O.) is based on step 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.) 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 20ºC. Next, the usual heat treatment is carried out: hardening and tempering.

Heat treatment of alloys is an integral part of the production process of ferrous and non-ferrous metallurgy. As a result of this procedure, metals are able to change their characteristics to the required values. In this article we will look at the main types of heat treatment used in modern industry.

The essence of heat treatment

During the production process, semi-finished products and metal parts are subjected to heat treatment to give them the desired properties (strength, resistance to corrosion and wear, etc.). Heat treatment of alloys is a set of artificially created processes during which structural and physical-mechanical changes occur in alloys under the influence of high temperatures, but the chemical composition of the substance is preserved.

Purpose of heat treatment

Metal products that are used daily in any sector of the national economy must meet high wear resistance requirements. Metal, as a raw material, needs to enhance the necessary performance properties, which can be achieved by exposing it to high temperatures. Thermal high temperatures change the original structure of a substance, redistribute its constituent components, and transform the size and shape of crystals. All this leads to minimizing the internal stress of the metal and thus increases its physical and mechanical properties.

Types of heat treatment

Heat treatment of metal alloys comes down to three simple processes: heating the raw material (semi-finished product) to the required temperature, maintaining it in the specified conditions for the required time and rapid cooling. In modern production, several types of heat treatment are used, differing in some technological features, but the process algorithm generally remains the same everywhere.

Depending on the method of implementation, heat treatment can be of the following types:

• Thermal (hardening, tempering, annealing, aging, cryogenic treatment).
• Thermo-mechanical involves processing at high temperatures in combination with mechanical stress on the alloy.
• Chemical-thermal involves heat treatment of metal with subsequent enrichment of the surface of the product with chemical elements (carbon, nitrogen, chromium, etc.).

Annealing

Annealing is a production process in which metals and alloys are heated to a given temperature, and then, together with the furnace in which the procedure took place, they cool very slowly naturally. As a result of annealing, it is possible to eliminate inhomogeneities in the chemical composition of the substance, relieve internal stress, achieve a grain structure and improve it as such, as well as reduce the hardness of the alloy to facilitate its further processing. There are two types of the first and second kind.

Annealing of the first kind involves heat treatment, as a result of which changes in the phase state of the alloy are insignificant or absent altogether. It also has its own varieties: homogenized - the annealing temperature is 1100-1200, under such conditions the alloys are kept for 8-15 hours, recrystallization (at t 100-200) annealing is used for riveted steel, that is, deformed when it is already cold.

Second-order annealing leads to significant phase changes in the alloy. It also has several varieties:

• Full annealing is heating the alloy 30-50 above the critical temperature characteristic of a given substance and cooling at a specified rate (200 / hour - carbon steels, 100 / hour and 50 / hour - low-alloy and high-alloy steels, respectively).
• Incomplete - heating to a critical point and slow cooling.
• Diffusion - annealing temperature 1100-1200.
• Isothermal - heating occurs in the same way as during full annealing, but after this, rapid cooling is carried out to a temperature slightly below critical and left to cool in air.
• Normalized - complete annealing followed by cooling of the metal in air rather than in a furnace.

Hardening

Hardening is a manipulation with an alloy, the purpose of which is to achieve a martensitic transformation of the metal, which reduces the ductility of the product and increases its strength. Hardening, as well as annealing, involves heating the metal in a furnace above a critical temperature to the hardening temperature; the difference is a higher cooling rate, which occurs in a bath of liquid. Depending on the metal and even its shape, different types of hardening are used:

• Quenching in one environment, that is, in one bath with liquid (water for large parts, oil for small parts).
• Intermittent quenching - cooling takes place in two successive stages: first in a liquid (a sharper coolant) to a temperature of approximately 300, then in air or in another bath of oil.
• Stepped - when the product reaches the hardening temperature, it is cooled for some time in molten salts, followed by cooling in air.
• Isothermal - the technology is very similar to step hardening, differing only in the exposure time of the product at the martensitic transformation temperature.
• Quenching with self-tempering differs from other types in that the heated metal is not completely cooled, leaving a warm area in the middle of the part. As a result of this manipulation, the product acquires properties of increased strength on the surface and high viscosity in the middle. This combination is extremely necessary for percussion instruments (hammers, chisels, etc.)

Vacation

Tempering is the final stage of heat treatment of alloys, determining the final structure of the metal. The main purpose of tempering is to reduce the fragility of the metal product. The principle is to heat the part to a temperature below critical and cool it. Since heat treatment modes and cooling rates of metal products for various purposes may differ, there are three types of tempering:

• High - heating temperature from 350-600 to a value below critical. This procedure is most often used for metal structures.
• Medium - heat treatment at t 350-500, common for spring products and leaf springs.
• Low - the heating temperature of the product is not higher than 250, which allows you to achieve high strength and wear resistance of parts.

Aging

Aging is a heat treatment of alloys that causes the decomposition of supersaturated metal after hardening. The result of aging is an increase in the limits of hardness, fluidity and strength of the finished product. Not only cast iron, but also easily deformable aluminum alloys undergo aging. If a metal product subjected to hardening is kept at normal temperature, processes occur in it that lead to a spontaneous increase in strength and a decrease in ductility. This is called natural. If the same manipulation is done under conditions of elevated temperature, it will be called artificial aging.

Cryogenic treatment

Changes in the structure of alloys, and therefore their properties, can be achieved not only at high, but also at extremely low temperatures. The heat treatment of alloys at temperatures below zero is called cryogenic. This technology is widely used in a variety of sectors of the national economy as a complement to high-temperature heat treatments, since it can significantly reduce the costs of thermal hardening of products.

Cryogenic processing of alloys is carried out at t -196 in a special cryogenic processor. This technology can significantly increase the service life of the treated part and anti-corrosion properties, as well as eliminate the need for repeated treatments.

Thermo-mechanical treatment

A new method of processing alloys combines the processing of metals at high temperatures with mechanical deformation of products in a plastic state. Thermomechanical treatment (TMT) can be of three types according to the method of implementation:

• Low-temperature TMT consists of two stages: plastic deformation followed by hardening and tempering of the part. The main difference from other types of TMT is the heating temperature to the austenitic state of the alloy.
• High-temperature TMT involves heating the alloy to a martensitic state in combination with plastic deformation.
• Preliminary deformation is carried out at t 20 followed by hardening and tempering of the metal.

Chemical-thermal treatment

It is also possible to change the structure and properties of alloys using chemical-thermal treatment, which combines thermal and chemical effects on metals. The ultimate goal of this procedure, in addition to imparting increased strength, hardness, and wear resistance to the product, is also to give the part acid resistance and fire resistance. This group includes the following types of heat treatment:

• Cementation is carried out to give the surface of the product additional strength. The essence of the procedure is to saturate the metal with carbon. Cementation can be performed in two ways: solid and gas carburization. In the first case, the material being processed, together with coal and its activator, is placed in a furnace and heated to a certain temperature, followed by keeping it in this environment and cooling. In the case of gas carburization, the product is heated in a furnace to 900 under a continuous stream of carbon-containing gas.
• Nitriding is a chemical-thermal treatment of metal products by saturating their surface in nitrogen environments. The result of this procedure is an increase in the tensile strength of the part and an increase in its corrosion resistance.
• Cyanidation is the saturation of a metal with both nitrogen and carbon. The medium can be liquid (molten carbon- and nitrogen-containing salts) and gaseous.
• Diffusion metallization is a modern method of imparting heat resistance, acid resistance and wear resistance to metal products. The surface of such alloys is saturated with various metals (aluminum, chromium) and metalloids (silicon, boron).

Features of heat treatment of cast iron

Cast iron alloys are subjected to heat treatment using a slightly different technology than non-ferrous metal alloys. Cast iron (gray, high-strength, alloyed) undergoes the following types of heat treatment: annealing (at t 500-650 -), normalization, hardening (continuous, isothermal, surface), tempering, nitriding (gray cast iron), aluminizing (pearlitic cast iron), chrome plating. As a result, all these procedures significantly improve the properties of the final cast iron products: they increase the service life, eliminate the possibility of cracks during use of the product, and increase the strength and heat resistance of cast iron.

Heat treatment of non-ferrous alloys

Non-ferrous metals and alloys have different properties and therefore are processed using different methods. Thus, copper alloys undergo recrystallization annealing to equalize the chemical composition. For brass, low-temperature annealing technology (200-300) is provided, since this alloy is prone to spontaneous cracking in a humid environment. Bronze is subjected to homogenization and annealing at temperatures up to 550. Magnesium is annealed, hardened and subjected to artificial aging (natural aging does not occur for hardened magnesium). Aluminum, like magnesium, is subjected to three heat treatment methods: annealing, hardening and aging, after which the deformed material significantly increases its strength. Processing of titanium alloys includes: hardening, aging, nitriding and carburizing.

Summary

Heat treatment of metals and alloys is the main technological process in both ferrous and non-ferrous metallurgy. Modern technologies have a variety of heat treatment methods that make it possible to achieve the desired properties of each type of processed alloys. Each metal has its own critical temperature, which means that heat treatment must be carried out taking into account the structural and physicochemical characteristics of the substance. Ultimately, this will allow not only to achieve the desired results, but also to significantly streamline production processes.