Properties of metals and alloys: mechanical, physical, chemical

10/27/2021 Author: VT-METALL

From this material you will learn:

• Concept of metals and alloys
• Definition and types of technological properties of metals and alloys
• Changing technological properties using steel as an example
• Technological testing of metals and alloys

The technological properties of metals and alloys determine the suitability of the material for a particular type of processing and, in general, the possibility of its use in a particular production cycle. Adding third-party elements to a metal or alloy directly affects their main characteristics. To determine the technological properties, it is necessary to carry out tests.

In our article we will tell you what these properties are, how impurities manifest themselves, and also give an example of production tests that reveal the suitability of a material for use in production.

Determination of metals and alloys.

The basic chemical elements are divided into metals and non-metals, but a clear boundary cannot be drawn between them. A metal can be described as a chemical element that has a metallic luster and which, during electrolysis, carries a positive charge that is released at the cathode.

An alloy is a homogeneous metallic material, but it is not the only chemical element. An alloy forms a compound or mixture of two or more metals. In some cases, it may be composed of one or more metals and non-metals. For example, an alloy of iron and carbon forms steel.

When working with metals and alloys, it is very important to correctly determine the type of welding or machining, on which the quality and success of the final result directly depends. And in order to make the right choice, you need to know the basic properties of metals and alloys, among which 4 large groups can be distinguished.

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Properties of metals and alloys: mechanical, physical, chemical

Mechanical properties

The main mechanical properties include: - strength - ductility - hardness

Strength is the ability of a material to resist destruction under loads. Plasticity is the ability of a material to change its shape and size under the influence of external forces. Hardness is the ability of a material to resist the penetration of another body into it.

Physical properties

Physical properties include: - color - density - melting point - thermal conductivity - electrical conductivity - magnetic properties

Color is the ability of metals to reflect radiation of a certain wavelength. For example, copper is pinkish-red in color, while aluminum is silvery-white.

The density of a metal is determined by the ratio of mass to unit volume. Based on their density, metals are divided into light (less than 4500 kg/m3) and heavy.

Melting point is the temperature at which a metal changes from solid to liquid. Based on the melting point, they distinguish between refractory (tungsten - 3416 °C, tantalum - 2950 °C, etc.) and low-melting (tin - 232 °C, lead - 327 °C). In SI units, the melting point is expressed in degrees Kelvin (K).

Thermal conductivity is the ability of metals to transfer heat from more heated areas of the body to less heated ones. Silver, copper, and aluminum have high thermal conductivity. In SI units, thermal conductivity has the dimension W/(m K).

The ability of metals to conduct electric current is assessed by two opposing characteristics - electrical conductivity and electrical resistance. Electrical conductivity is measured in SI units in siemens (Sm). Electrical resistance is expressed in ohms (Ohms). Good electrical conductivity is necessary, for example, for current-carrying wires (they are made of copper, aluminum). In the manufacture of electric heating devices and furnaces, alloys with high electrical resistance (from nichrome, constantan, manganin) are required. As the temperature of a metal increases, its electrical conductivity decreases, and as it decreases, it increases. Magnetic properties are expressed in the ability of metals to be magnetized. Iron, nickel, cobalt and their alloys, which are called ferromagnetic, have high magnetic properties. Materials with magnetic properties are used in electrical equipment and for the manufacture of magnets.

Chemical properties

Chemical properties characterize the ability of metals and alloys to resist oxidation or combine with various substances: atmospheric oxygen, acid solutions, alkali solutions, etc.

Chemical properties include: - corrosion resistance - heat resistance

Corrosion resistance is the ability of metals to resist chemical destruction under the influence of an external aggressive environment on their surface (corrosion occurs when it enters into chemical interaction with other elements).

Heat resistance - the ability of metals to resist oxidation at high temperatures.
Chemical properties are taken into account primarily for products or parts operating in chemically aggressive environments: - containers for transporting chemical reagents - chemical pipelines - instruments and instruments in the chemical industry

Physical properties.

This group of properties are related to the atomic structure and density of the material and their measurement does not cause residual deformation of the body.

• Color - this feature can be used to judge some other properties. For example, most metals change color when heated. A similar situation is observed during oxidation.
• Thermal conductivity is the ability of a material to conduct or transfer heat.
• Electrical conductivity is similar to the previous property, but instead of heat, electricity acts.
• Magnetic susceptibility - whether a material conducts a magnetic field when it is magnetized.
• Melting point is an indicator of the temperature at which a substance turns into a liquid state from a solid. For pure metals it is constant, but for alloys it is a temperature range.
• Density is the amount of substance contained in one unit of volume.
• Specific heat capacity is the amount of heat required to raise the temperature of 1g of a substance by 1°C.

Classification and types of metals

There are pure, single-component structures and alloys. The most classic example is the different types of steel. They differ according to GOST in accordance with the addition of alloying additives. The higher the carbon content, the stronger the material. There is also a generally accepted distinction; below we present the subtypes.

Black

They are mined from metal ore. In production they occupy 90% of all raw materials. Usually these are cast iron and steel. To change the characteristics, more or less amount of carbon and alloying additives are added: copper, silicon, chromium, nickel. One of the very popular subspecies is stainless steel, which is distinguished by its shiny surface and unique properties - lightness, high strength and resistance to humidity and temperature changes.

What applies to non-ferrous metals

The second name is non-iron, that is, alloys do not contain iron, but consist of more expensive materials. Substances have different colors and have unique qualities:

• durability;
• long-term preservation of properties;
• the formation of an oxide film that prevents corrosion.

Thanks to this, certain varieties can be used in medicine, jewelry, the chemical industry, and in the manufacture of electrical wires. Non-ferrous metals include aluminum, zinc, tin, lead, nickel, chromium, silver, gold and others.

Copper and its alloys are popular metals

Copper ore was one of the first to be processed by man because it is subjected to the cold method of forging and stamping. Pliability has led to demand everywhere. Oxygen in the composition leads to a red tint. But decreasing the valency in various compounds will lead to yellow, green, blue color. Excellent thermal conductivity is considered an attractive quality - second only to silver, which is why it is used for wires. Connections can be:

• solid - in combination with iron, arsenic, zinc, phosphorus;
• with poor solubility with bismuth, lead;
• fragile - with sulfur or oxygen.

Metals include aluminum and alloys

Al was discovered in 1825 and is distinguished by its ease and simplicity in metalworking. It is made from bauxite, and the reserves of this rock are practically inexhaustible. Next, the element is combined in various proportions with copper, manganese, magnesium, zinc, and silicon. Less often with titanium, lithium, beryllium. Features depending on additives:

• good weldability;
• corrosion resistance;
• high fatigue strength;
• plastic.

It is used for the manufacture of jewelry, cutlery, as well as for glass melting, in the food and military industries, for the creation of rockets and for the production of hydrogen and heat in aluminum energy.

All about the metals magnesium, titanium and their alloys

Mg is the lightest substance of this group. It does not have strength, but it has advantages, for example, plasticity, chemical activity. Due to its high structural ability, it is added to compositions to increase weldability and ease of metalworking with a cutting knife. It must be taken into account that magnesium is very susceptible to rust. Titanium has similar qualities - lightness, ductility, silver color. But the anti-corrosion film appears upon first contact with oxygen. Distinctive features are low thermal conductivity, electrical conductivity, and lack of magnetism. Metal containing titanium is a substance used in the aviation, chemical, and shipbuilding industries.

Anti-friction alloys

A characteristic feature of this group is its ease of use under mechanical stress. They create virtually no friction and also reduce it in other composites. Very often they act as a solid lubricant for components, for example, for bearings. The composition usually includes fluoroplastic, brass, bronze, iron graphite and babbit.

Soft

These are those whose metal bonds are weakened. For this reason, they have a lower melting and boiling point and simply become deformed. Sometimes you can make a dent with one finger press, or leave a scratch with your fingernail. These include: copper, silver, gold, bronze, lead, aluminum, cesium, sodium, potassium, rubidium and others. One of the softest is mercury; it is found in nature in a liquid state.

What does hard metal mean?

In nature, such ore is extremely rare. The rock is found in fallen meteorites. One of the most popular is chrome. It is refractory and can be easily processed into metal. Another element is tungsten. It melts very poorly, but when properly processed is used in lighting applications due to its heat resistance and flexibility.

Metal materials in the energy sector

We would not have such a developed electrical network and a lot of devices that consume electricity if a number of substances were not distinguished by the presence of free electrons, positive ions and high conductivity. Wires are made from lead, copper and aluminum. Silver would be great, but its rarity affects the cost, so it is rarely used.

Features of Ferrous Secondary Metals

This is waste that is generated as a result of one of the metalworking stages - forging, cutting. These could be scraps or shavings. They are sent to steel-smelting furnaces, but before that they must pass inspections in accordance with GOST. Scrap is called ferrous metal, it is distinguished into steel and cast iron according to price. Its use is in great demand instead of ore processing.

Alkaline earth alloys

These are solid substances that have high chemical activity. They are very rarely found in their pure form, but are used in compounds. Their importance cannot be overestimated from the point of view of human and animal anatomy. Magnesium and calcium are essential microelements.

Alkali metal concept

They are able to dissolve in water, forming an alkali. Due to its increased chemical activity (reaction occurs with violent action, ignition, release of gas, smoke) it is almost never found in nature. After all, at the external level there is only one electron, which is easily given to any substance. Hydroxides are very important in industry.

General characteristics of materials from the d- and f-families

These are transition elements that can be both oxidizing and reducing agents. Properties depend on the environment in which they are located. But there are also common ones:

• there are many electrons in the outer level;
• several oxidation states;
• increased valence;
• strength;
• ductility;
• ductility.

What are the side subgroups of metals in the periodic system?

In fact, these are varieties of the previous category - transitional elements. This is a line from scandium to zinc. They are often smelted and have virtually the same characteristics as the above materials from the d- and f-families.

Chemical properties.

This type includes those properties that determine their relationship to the chemical influences of such environments as water, air, acids, alkalis and others.

• Solubility is the ability of a material to dissolve in any solvent (usually strong acids and caustic alkalis).
• Oxidability is the ability of a metal to form oxides (combine with oxygen).
• Corrosion resistance - the extent to which materials can resist destruction during chemical exposure to the environment.

Mechanical properties.

When measuring mechanical properties, the body is usually subject to destruction or irreversible deformation.

• Strength - the material is able to resist destruction when external forces act on it.
• Elasticity - the material is able to return to its original shape when the external load has ended.
• Plasticity - the material is able to change its size and shape when the external load has ended. The new dimensions and shape are preserved, the material is not destroyed.
• Viscosity - a material is able to resist when subjected to sharply increasing loads.
• Hardness – the material does not allow penetration of another material that is harder.
• Wear resistance - the material is able to maintain its surface unchanged if it is exposed to friction.
• Fragility - the material is capable of breaking under the influence of external force (there is no plastic deformation).

Basic properties of metals and their definition

Properties of metals

All properties of metals and alloys can be divided into four groups: physical, chemical, technological and mechanical.

The physical properties of metals and alloys include color, density, fusibility, electrical and thermal conductivity, heat capacity, magnetic and other properties.

The density of metals is of great importance for the choice of materials when designing machines and devices. The use of light metals and alloys (aluminum, magnesium, titanium, beryllium) reduces the total weight of the apparatus and structure, which is especially important in such industries as the aircraft industry, rocketry, automobile and tractor engineering.

The fusibility of metals is determined by their melting point. Low-melting metals and alloys are used for casting typographic matrices, making bearings, etc.

Metals with high electrical conductivity (copper, aluminum) are used to construct power lines; alloys with high electrical resistance for incandescent lamps and heating devices, etc. Metals and alloys with magnetic properties are used in the manufacture of dynamos, electric motors, transformers; for the manufacture of communication devices: telephone, telegraph and other types of devices and machines.

Metals and alloys with high thermal conductivity heat up evenly during hot working and are easy to weld and solder. They are widely used for the manufacture of heat exchangers.

The coefficient of linear expansion is the amount by which the linear dimensions of a body change with temperature changes. This coefficient must be taken into account when designing equipment. Parts made of metals with different coefficients of linear expansion and connected to each other may collapse or bend when heated. Alloys with a very low coefficient of linear expansion are used in the manufacture of precision instruments.

The chemical properties of metals and alloys include their oxidability, solubility, and corrosion resistance.

Technological ones include fluidity, malleability, weldability, machinability with cutting tools. The technological properties of metals and alloys are of exceptional importance when performing certain operations in production and, in particular, when choosing techniques and methods for producing machine parts.

Malleability is the ability of metals and alloys to be processed by pressure. This property of metals and alloys is associated with their plastic deformation, especially when heated. The most important types of processing such as rolling, pressing, drawing, forging and stamping are associated with malleability.

Fluidity is the ability of metals and alloys to easily flow and completely fill the casting mold. Copper, even when the melt is overheated, is thick and does not spread, so it is impossible to prepare products from it by casting, while its alloys (bronze and brass) and many other metals (cast iron, steel, magnesium and aluminum alloys) are quite fluid.

Weldability is the ability of metals and alloys to form strong permanent connections between parts made from them. Welding is used to produce welded structures instead of cast, riveted ones, restore broken parts, correct casting defects, etc. Welded structures are lighter, stronger and cheaper than riveted ones.

Metals and alloys are processed by cutting. This property is widely used in technology, despite the large waste (chips) of metals. This is explained by the fact that it is much more rational to obtain the desired shape, exact dimensions and surface finish of the part by cutting processing compared to other methods.

The mechanical properties of metals and alloys characterize their strength, plastic and viscous states.

Strength is the ability of a metal or alloy to resist applied external forces without breaking. During design, this property is taken into account, since it is used to determine the permissible stresses and calculate the equipment. The stronger the metal or alloy, the smaller the size of the part, its weight and the less metal consumption for its manufacture.

Toughness is the ability of a metal to resist impact loads.

Plasticity is the property of a metal to deform without destruction by the application of external forces. Plasticity and viscosity do not affect the mass of manufactured products, but with low plasticity and viscosity, the product with high strength properties becomes brittle and will be destroyed under accidental overloads (when shock loads are applied). As the strength properties of the product decrease, its plasticity and viscosity increase. Therefore, with a certain strength, the product must have the necessary (minimum) plasticity and viscosity. The mechanical properties of metals and alloys are influenced by the chemical composition, structure and factors associated with the operation of the products.

Elasticity is the property of a metal to restore its shape after the action of applied external forces.

Hardness is the ability of one body to resist the penetration of another, harder body.

Thus, metals (alloys) used as structural ones must have certain properties, and the choice of metal or alloy for the manufacture of a product depends on the entire complex of these properties.

Determination of metal properties

Plastic deformation and recrystallization . When a load is applied to a metal or alloy, it undergoes deformation, which can be elastic, that is, disappearing after the load is removed, and remaining plastic or residually plastic or residual. Plastic deformation is associated with a shift of part of the crystallites relative to another part along slip planes. Elastic deformation is directly proportional to stress.

Let us consider the behavior of a metal sample when testing it for tension. For these tests, round or flat (for sheet materials) samples are prepared from the metal being tested (Fig. 6.4) of standard sizes: the diameter of the working part is 20 mm, the working length of the sample is 200 mm. Samples of other sizes can be used for testing. Tests are carried out on various types of tensile testing machines. In Fig. Figure 13 shows a general view and kinematic diagram of an IM-4R type tensile testing machine.

The machine consists of loading and force-measuring mechanisms. The test sample is secured with the heads in the clamps, after which the working mechanisms of the machine are activated and the pen automatically writes down on paper a diagram in load-strain coordinates, where the load P is plotted along the ordinate axis, and the elongation of the sample Δl is plotted along the abscissa axis. Consider the tensile diagram of mild steel (Fig. 6.5), obtained with a gradual increase in tensile force until the test sample breaks.

Knowing the applied load (force) P and the cross-sectional area F0, it is possible to determine the stress o at any point in the diagram, i.e. σ = P/F0. Analysis of the diagram shows that section OA is a straight line segment and characterizes the rigidity of the metal. The elongation of the sample to point A is strictly proportional to the force. Such a change in the deformation (elongation) of the metal due to the applied force is called the law of proportionality or Hooke’s law, and the maximum stress that the sample can withstand without deviating from this law is the limit of proportionality σPTs, which is determined by the equation σPTs = RPTs/F0.

Point A corresponds to the moment of appearance of plastic deformation and with further loading of the sample to point B, the law of proportionality is violated and a curved section appears on the diagram. The resulting deformations of the sample from point A to B are elastic if they completely disappear when the load is removed. The conditional stress corresponding to the appearance of the first signs of deformation remaining after removing the load of the sample, 0.05% (or even less) of the original length of the sample, is called the elastic limit σUP:

σUP = RUP/F0.

For steel, the elastic limit is assumed to be equal to the proportionality limit, as can be seen from the diagram (points A and B are close to each other).

At point C, the sample begins to elongate without increasing the load. The minimum stress at which such elongation of the sample occurs is called the physical yield strength:

σT = RT/F0.

Most metals and, in particular, medium and high carbon steels do not exhibit a horizontal section on the tensile diagram, and therefore for such materials the proof strength σ0.2 is determined:

σ0.2 = Р0.2/F0.

where 0.2 is the residual elongation equal to 0.2% of the original length of the sample; P0.2 is the load corresponding to the conditional yield strength.

The maximum load that the sample can withstand without destruction is applied at point B. The stress corresponding to the highest load preceding the destruction of the sample is called tensile strength or temporary tensile strength σB:

σВ=РВ/F0.

The true tensile strength SK is determined by the equation

SK = RK/FK.

where Pk is the load at the moment of sample rupture; FC is the cross-sectional area of ​​the sample in the neck after rupture.

The diagram shows that up to point D, elongation Δ1 of the sample (and, accordingly, a narrowing of its cross-section) occurs uniformly along the entire length of its working part, but upon reaching point D, the elongation is concentrated at the point of least resistance and further elongation Δ12 of the sample occurs at the expense of that place where the neck is formed, and its rupture under load Pk.

The plasticity of metals is characterized by relative elongation δ and narrowing of the cross-sectional area ψ, which are expressed as a percentage and determined by the equation

And

where lK is the length of the sample after rupture, mm; 10 — estimated length of the sample, mm; Fк is the cross-sectional area of ​​the sample after rupture, mm; F0 is the initial cross-sectional area, mm.

For a number of metals, the relative elongation and relative contraction are close to zero. Such metals are brittle, whereas ductile metals have high characteristics (tens of percent).

Plastic deformation of metals and alloys is taken into account when choosing a material for the manufacture of products and, in particular, when obtaining wire, operations, bending, drawing, upsetting, stamping, etc. Plastic deformation ensures the structural strength of metal structures, apparatus and other products. If the metal is not capable of plastic deformation, then it is prone to so-called brittle fractures, i.e. to fractures that occur at low stresses.

As a result of plastic deformation, hardening of the metal occurs, which is called cold hardening or hardening, while the ductility of the metal sharply decreases. With large deformation as a result of sliding processes, the grains change their shape, stretching in the direction of the acting forces, grains are crushed, the crystal lattice is distorted, the metal structure is in an unstable, stressed state. To remove cold hardening, products are heated at a certain temperature, and metal atoms acquire the ability to move, which leads to the elimination of crystal lattice distortions, the formation of new crystallization centers and the growth of crystallites. At the same time, the strength of the metal decreases, and the plastic properties increase. This process is called recrystallization.

Between the absolute recrystallization temperature Track and the absolute melting temperature Tmel there is a simple relationship Track = a*Tmp, where the coefficient a is determined by the degree of purity of the metal.

For chemically pure metals a is equal to 0.1 to 0.2, for metals of technical purity 0.3 - 0.4, and for alloys a can reach a value equal to 0.8. Thus, when heating a product that is in the hardening stage above the Track, its plasticity is restored. If plastic deformation of the metal is carried out at a temperature above Trek, then the hardening (hardening) achieved during the deformation process is eliminated during recrystallization, which occurs at these temperatures. This type of processing is called hot forming, in contrast to cold forming, when pressure treatment is carried out at a temperature below Trek.

For some metals, the Trek is below room temperature (for lead 30°C, tin - 70°C), therefore, during plastic deformation at room temperature (+20°C), the resulting hardening will spontaneously be eliminated after processing. Therefore, the deformation of metals such as lead and tin at room temperature will constitute hot working for them. For a metal such as tungsten (recrystallization temperature 1200 C), pressure treatment even at 1000 - 1100 ° C. is cold plastic deformation.

Therefore, cold and hot forming should be distinguished depending on the ratio of the deformation temperature to the recrystallization temperature.

Determination of hardness. When determining hardness, the devices used are simple in design and easy to use; there is no need to make test samples (product blanks can be tested). To determine hardness (indentation, scratching, elastic recoil, magnetic method), the most widely used methods are based on the ability of a body (metal) to resist the penetration of another harder body into it.

According to the Brinell method, hardness is determined by pressing a hardened steel ball of a certain diameter (10; 5; 2.5 mm) into the product (Fig. 6.6). The Brinell hardness number HB is characterized by the ratio of the load acting on the ball to the imprint surface: where P is the load on the ball, N (kgf); F—imprint surface, mm. 2, D—diameter of the pressed ball, mm; d—imprint diameter, mm.

Lever and hydraulic presses are used to determine hardness. The sample, mounted on the table with a screw, is pressed against the ball so as to compress the spring. Then, using an electric motor, an eccentric is driven, when rotated, the connecting rod is lowered, and the weights create pressure through a system of levers. The eccentric, rotating, lifts the connecting rod and thus removes the pressure of the loads from the sample. When the connecting rod is in the upper position, the electric motor is automatically switched off. Then the sample is released, the diameter of the indent is determined using a special magnifying glass, from which the hardness is calculated using the given formula, which takes a lot of time. In practice, a special table is used, in which each indent diameter corresponds to a hardness number HB. The diameter of the ball and the load are set depending on the metal being tested, its hardness and thickness. When testing steel and cast iron P=30D2 (for example, D=10 mm, P=30000 N (3000 kgf)]; when testing copper P=10D2 (for example, D =10 mm, P=10,000 N (1000 kgf)) ; when testing aluminum P = 2.5D2 (for example, D = 10 mm, P = 2500 N (250 kgf)]. Using the Brinell method, metals with a hardness higher than HB 450 cannot be tested, since the ball will be deformed and the result will be incorrect.

For many materials, having determined the hardness of NV, you can find the tensile strength σB = KNV, where K is a value depending on the material, for example, for mild steel K = O.34, cast steel K = O.3 - 0.4, copper and its alloys K = 0.55, etc. According to the Rockwell method, hardness is determined by pressing a steel ball with a diameter of 1.59 mm into the product with a hardness of the metal being determined not more than 2200 MN/m2 (220 kgf/m2) [load 1000 N (100 kgf)] or a diamond cone with an angle of 120° when testing harder materials (load 1500 N (150 kgf) and when testing superhard alloys (load 600 N (60 kgf)]. The ball or cone is pressed into the sample on the device under the influence of two loads: preliminary P, always equal to 100 N (10 kgf), and main P when pressing the ball with a force of 900 N (90 kgf) (scale B), a force of 1400 N (140 kgf) (scale C) and 500 N (50 kgf ) (scale A).The total load P (Fig. 6.7) is the sum of these loads: P=P0+P1.

Depending on whether a steel ball or a diamond cone is used, and the loads at which the test is carried out (i.e. on which scale - B, C or A), the hardness number is designated HRB, HRC, HRA.

Hardness determination is carried out using a Rockwell device (Fig. 6.8). The sample installed on table 2 is brought into contact with a steel ball or diamond cone 3 by rotating the flywheel 1. Then the rotation of the flywheel is continued until the small hand 4 on the dial reaches the red dot, and the large hand of the dial is in the vertical position. position This creates a preload of 100 N (10 kgf). Next, the main load is applied to the sample using handle 5. At the end of the determination (the indentation lasts 5 - 6 s), the main load is removed by turning handle 5 back and the hardness is determined according to the readings of the large arrow of the dial, which is a conditional value characterizing the difference in the depths of the prints. To convert Rockwell hardness values ​​to Brinell hardness values, use a conversion table.

The diamond pyramid indentation test (Vickers method) is used to determine the hardness of thin parts and thin surface layers that have high hardness. When testing c, the metal is pressed into a tetrahedral pyramid (with an apex angle of 136°) under a load of 50 N (5 kgf) to 1000 N (100 kgf). The size of the diagonal of the print is determined using a microscope mounted on the device, and according to the readings obtained, the hardness number is determined, denoted by the formula HV=P/F,

where P is the load on the pyramid, N (kgf); F—imprint area, mm2.

Usually, special tables are used to determine the hardness number based on the diagonal size of the indentation.

Definition of endurance. Endurance (fatigue) is the ability of a metal to resist destruction (fatigue) from periodically repeated forces. Under varying influence of forces (variable deformations and stresses), microcracks and cracks can form, which are concentrated in places of high stress. But metal destruction will not occur if the voltage is less than a certain value. The greatest stress, which under variable action of forces does not cause the formation of cracks, is called the endurance limit. It is determined using special machines, bending tests during rotation, tension, compression, and torsion. The endurance limit for structural steel is usually 10 million cycles, and for non-ferrous metals 20 million. cycles.

In addition to the above methods, tests for shear, torsion, compression, bending, weldability, bending, upset, extrusion, compression, etc. are used.

Basic information about alloys

General concept of alloys

Due to the lack of technically useful properties in most pure metals, their alloys are most widely used in technology. An alloy is a substance containing two or more components. An alloy consisting of metallic elements and having metallic properties is called a metal alloy.

Alloys are currently produced in several ways, for example, by the interaction of elements in a liquid state (alloying); sintering and diffusion in the solid state; deposition of several elements on the cathode during electrolysis of aqueous solutions. According to the number of components contained in the alloy, they are divided into double, triple, etc. Substances included in the alloy, during solidification, can be in the form of individual particles, grains of both components (mechanical mixture), or in the form of formed chemical compounds ( chemical compound), or components mutually dissolving in each other (solid solutions).

Alloys of the mechanical mixture type are formed from substances that do not dissolve and do not interact chemically with each other in the solid state to form compounds. Such alloys consist of a mixture of crystallites of substances that retain their crystal lattices. The properties of the alloy will be determined by the ratio of the components included in its composition. The more components an alloy contains, the closer its properties are to those of the pure component.

Alloys of the type of chemical compound are formed by the interaction of the components that make up the alloy with each other, and the content of the components must be strictly defined. They have a lattice that is different from the crystal lattices of the components, therefore they also have other mechanical, physical and chemical properties.

Solid solution alloys come in three types: substitutional solid solutions, interstitial solid solutions, and subtractive solid solutions.

Substitutional solid solutions are formed in cases when atoms of the solute replace solvent atoms in the crystal lattice (Fig. 6.9, a). This is possible in the case when the components have the same lattice, the sizes of their atoms should differ slightly from each other (no more than 15%).

The sizes of the atoms of the solute influence the lattice parameters, increasing it if the diameter of the atom is larger or decreasing it if it is smaller. The atoms of the solute can occupy a strictly defined position in the crystal lattice of the solvent (ordered solid solutions) or be arranged in a random order (disordered solid solutions).

a B C)

Rice. 6.9 Scheme of distribution of atoms in the lattices of solid solutions

The formation of interstitial solid solutions (Fig. 6.9, b) occurs when atoms of a dissolved element are dissolved in the crystal lattice of a solvent, that is, when atoms of a dissolved element are introduced into the lattice of a solvent in the spaces between solvent atoms. This is possible only in the case when the atoms of the dissolved element are small in size, that is, when the ratio of the diameter of the atom of the dissolved element to the diameter of the solvent atom is less than 0.59. As a rule, interstitial solid solutions are formed with nonmetals, and the crystal lattice parameters always increase.

Subtraction solid solutions (Fig. 6.9, c) can only form in alloys containing chemical compounds when excess atoms of one of the components occupy a strictly defined position in the crystal lattice, and the places that should be removed by atoms of another component remain partially free, for example in lattices of TiC, WC carbides (places belonging to carbon remain free). Subtraction solutions are often found in semiconductor compounds.

In addition to these types of alloys, metals form electronic compounds and interstitial phases. Electronic compounds are characterized by a certain ratio of the number of valence electrons to the total number of atoms in a chemical compound, for example, in the CuZn3 compound. The indicated ratio will be 1/4. Each such ratio corresponds to a specific crystal lattice, for example, the ratio 3/2 is the lattice of a face-centered cube, 21/3 is a complex cubic lattice; 7/4 - hexagonal close-packed lattice alloys of copper with zinc, copper with tin, copper with silicon, tear with aluminum, etc. contain electronic compounds. Interstitial phases can form atoms of iron, chromium, tungsten, molybdenum with elements having a small atomic diameter, for example hydrogen, carbon, nitrogen, boron. They have a crystal lattice that differs from the lattices of both phases. Implementation phases can be of three types: MeX (WC, VC, TiN, etc.); Ме2Х (W2С, Fe2N, etc.); Me4Х (Fe4N, etc.).

The state of the alloy depending on concentration and temperature is depicted graphically. This image of the state of the alloy is called a phase diagram. Since the phase diagram shows the stable state of the system (a set of phases that are in equilibrium), it is an equilibrium diagram of the phases existing under given conditions. The state of the alloy shown in the diagram refers to equilibrium conditions without taking into account overheating or undercooling, which in reality cannot occur. Therefore, the phase diagrams under consideration represent a theoretical case. A mathematical description of the general laws of existence of stable phases that meet equilibrium conditions was given by Gibbs, called the phase rule, which establishes a quantitative relationship between the degree of freedom of the system C, the number of phases F and components K.

Components K are substances that make up a system and are capable of moving from one phase to another, i.e. if we have a chemically pure element, then it is a one-component system. A single-component system is also a chemical compound that does not decompose into its component parts in the temperature range of the system being studied.

Phase F is a homogeneous part of a system with the same state of aggregation and composition, separated from other phases of the system by an interface, the transition of which sharply changes the chemical composition or structure of the substance. For example, molten metal is a single-phase system, and a mechanical mixture is a two-phase system, etc.

Iron-carbon alloys

Iron-carbon alloys include steels, cast irons and ferroalloys, which occupy first place in scale of production, and in terms of variety of applications they are the most widely used in technology. Pure iron has two allotropic modifications - α and β (see allotropic transformations of iron). Pure iron with many elements forms both chemical compounds and solid solutions. For example, with carbon it forms the chemical compound cementite Fe3C, which contains 6.67% C. Cementite has high hardness (HB 800) and very low ductility.

Iron dissolves carbon to form solid solutions. Depending on the modification of iron, the solubility of carbon is different. Thus, in γ-iron, the maximum solubility of carbon at 1147 ° C is 2.14%, and the minimum solubility at 727 ° C is 0.8%. The solid solution of carbon in γ-Fe is called austenite after the English scientist R. Austen. Austenite is plastic, its hardness HB is 160-200. In α - Fe, the solubility of carbon is much lower. Thus, the maximum solubility of carbon at 727°C is only 0.02°/o, and the minimum is 0.006% at room temperature. A solid solution of carbon in α -Fe is called ferrite (from the Latin word ferrum - iron). Ferrite has low hardness (HB 80) and high ductility.

Iron-cementite phase diagram

Iron-carbon alloys above 6.67% C are not used as structural materials, so we will only consider the state diagram of iron-cementite alloys (Fig. 6.10).

Rice. 6.10 State diagram of the Fe-Fe3C system

To simplify the iron-cementite phase diagram, we will not consider structural changes associated with the existence of δ-iron in the temperature range above 14010C.

On the diagram, point A corresponds to the melting point of pure iron at 1539 ° C, at point D the melting point of cementite is about 16000 C. The area above the ACD line (liquidus line) is characterized by the liquid state of the alloy. Complete solidification of the alloys occurs along the solidus line AESB. Below the AC line to the AEC line, austenite crystals (A) stand out from the liquid melt (L). In the ACE region, the alloy consists of a liquid solution (L) and austenite (A). Below the CD line to the CB line, crystals of cementite, called primary (CI), fall out of the liquid melt, and in the DCB region there is a mixture of liquid solution (G) and cementite (CI). When the carbon content in the alloy is 4.3% and 11470 C, austenite and cementite simultaneously crystallize at point C, forming a mechanical mixture (eutectic) called ledeburite (L). All alloys containing from 2.14 to 6.67% carbon - cast iron - contain ledeburite. Alloys lying to the left of point E belong to the group of steels.

Transformations in the solid state in alloys of the Fe-Fe3C system are associated with the transition of iron from one modification to another and a change in the solubility of carbon in iron as a result of this transition.

In the region of the AESG diagram is austenite. When the temperature decreases below the GS line (critical point A3), ferrite (F) is released from austenite, and below the ES line, cementite, called secondary (CII), is released. In the region of the GSP diagram, the alloys consist of ferrite and austenite, and in the region below the SE line (critical point Ast) - of cementite and austenite. Alloys containing from 2.14 to 4.3% carbon at temperatures from 1147 to 7270 C consist of austenite, cementite and ledeburite, and those containing more than 4.3% carbon - from cementite and ledeburite.

At 727° C (PSK line, critical point A1), austenite decomposes to form a mechanical mixture of ferrite and cementite, which is called pearlite (P). Below 7270 C, iron-carbon alloys have the following structures.

Steels containing from 0.02 to 0.8% C ferrite + pearlite are hypoeutectoid steels; 0.8% C perlite - eutectoid steel; from 0.8 to 2.14% cementite + pearlite—hypereutectoid steels.

White cast irons containing from 2.14 to 4.3% C perlite + secondary cementite + ledeburite are hypoeutectic cast irons; 4.3% C ledeburite—eutectic cast iron; from 4.3 to 6.67% C primary cementite + ledeburite - hypereutectic cast iron.

The PQ line shows that with decreasing temperature, the solubility of carbon in ferrite decreases from 0.02% at 727°C to 0.006% at room temperature. When cooled below 727° C, excess carbon is released from ferrite in the form of cementite, called tertiary. In low-carbon steels, under conditions of slow cooling, tertiary cementite is released along the grain boundaries of ferrite, which reduces the plastic properties, especially the ability for cold stamping of sheet steel.

Alloys containing <= 0.02% C are called technical iron.

In practice, the iron-carbon phase diagram is used not only to obtain specified (initial or final) structures in the alloy, but also to determine the thermal regime during heat treatment, the heating temperature of the metal during pressure treatment (rolling, coking, stamping), as well as to determine temperature limits for these operations.

Effect of carbon, permanent impurities and alloying elements on steel

Carbon has a major influence on the properties of steel. With increasing carbon content in steel, hardness and strength increase, ductility and toughness decrease.

In addition to iron and carbon, carbon steel also contains permanent impurities of elements such as sulfur, phosphorus, silicon, and manganese.

Sulfur and phosphorus are harmful impurities. Their permissible content, depending on the quality of the steel, should be no more than 0.05% each. Sulfur does not dissolve in iron, but forms iron sulfide with it, which with iron forms the Fe-FeS eutectic with mp. 988° C. When steel hardens, this eutectic is located around the grains and forms a low-melting shell, and during hot processing (forging, rolling, etc.) such shells melt, the connection between the grains is lost, and cracks form. This phenomenon is called red brittleness of steel. Phosphorus, dissolving in ferrite, sharply reduces its plasticity, causes intracrystalline segregation, promotes grain growth, which leads to brittleness of products at ordinary temperatures (cold brittleness). Silicon (up to 0.5%) and manganese (up to 0.8%) have virtually no effect on the properties of steel.

Alloyed steels are steels that contain specially introduced alloying elements, such as nickel, chromium, molybdenum, titanium, vanadium, tungsten, etc., or which contain an increased (more than 0.5-1.0%) amount silicon or manganese. The introduction of alloying elements into steel increases the mechanical properties or gives them special properties, such as heat resistance, heat resistance, acid resistance, etc.

Alloying elements, when introduced into steel, can form solid solutions with iron, dissolve in cementite [for example, (Fe, Cr)3C] or form independent chemical compounds with carbon (special carbides), for example VC, Cr7C3, etc.

Alloy steels, according to their structure under equilibrium conditions, are divided into the following classes: pearlitic, ledeburite, ferritic, austenitic, and according to the structure obtained after cooling in air, into pearlitic, martensitic, austenitic, carbide.

Technological properties.

This type of properties determines how suitable a particular type of processing is for a metal or alloy.

• Cutting – a metal or alloy is subjected to processing by a cutting tool during machining.
• Malleability - the material is able to take a different shape if it is subjected to pressure. There is no destruction of the material.
• Weldability – the metal is suitable for welding, forming permanent joints without cracks or other defects.
• Fluidity - molten metals calmly take exactly the shape into which they are poured.
• Shrinkage is the opposite process of thermal expansion. Represents a decrease in the volume of a material as it cools.

• Liquation - a material from a liquid state, when the temperature decreases, turns into a solid state and, as a result, breaks down into individual compounds with different melting points.

Comparison of properties

The second part of the elements in the periodic table is characterized by a variety of characteristics, so it is almost impossible to provide a complete summary table. We offer a table that shows 4 distinctive features:

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