The influence of temperature on the plasticity of metal.

T Thermal treatment refers to processes associated with heating and cooling that cause changes in the internal structure of the alloy, and, in connection with this, changes in physical, mechanical and other properties.

Semi-finished products (blanks, forgings, stampings, etc.) are subjected to heat treatment to improve the structure, reduce hardness, improve workability, and finally manufactured parts and tools to give them the required properties.

As a result of heat treatment, the properties of alloys can vary within very wide limits. For example, you can obtain any steel hardness from 150 to 250 HB (initial state) to 600-650 HB (after hardening). The possibility of significantly increasing mechanical properties using heat treatment compared to the initial state allows you to increase the permissible stresses, as well as reduce the size and weight of the part.

The founder of the theory of heat treatment is the outstanding Russian scientist D.K. Chernov, who is in the middle of X I In the 10th century, observing the change in the color of the heat of steel when it was heated and cooled and recording the temperature “by eye”, he discovered critical points (Chernov points).

Soviet scientists have achieved great success in improving already known and developing new technological processes for the heat treatment of steel.

In the development of the doctrine of heat treatment, in the creation of progressive methods of heat treatment technology, Soviet science and practice occupy a leading place.

The main types of heat treatment of steel are annealing, normalizing, hardening and tempering.

Annealing steel.

The purpose of annealing is to reduce hardness, refine grains (recrystallization), improve machinability, increase ductility and viscosity, relieve internal stresses, eliminate or reduce structural heterogeneity, and prepare for subsequent heat treatment.

The following factors influence the annealing result:

1) heating rate;

2) heating (annealing) temperature;

3) duration of exposure at heating (annealing) temperature;

4) cooling rate.

Heating rate . The permissible heating rate depends on the chemical composition of the steel. The more carbon and special impurities there are in steel, the less thermally conductive it is and, therefore, the slower it should be heated.

Heating temperature . The heating temperature is set depending on the carbon content and special elements.

Full annealing

Complete annealing is characterized by heating 20-30 degrees above the temperature of the transformation range and slow cooling to a temperature below the transformation range (usually up to 400 - 500 0 C). Hypoeutectoid and eutectoid steels are subjected to complete annealing. For hypereutectoid steels, incomplete annealing is appropriate and practically applicable. Full annealing is used to recrystallize the structure in hot-worked steels and shaped castings.

Annealing hot-worked steel reduces strength and increases ductility.

If the original structure is difficult to correct and full annealing is not able to improve the structure of the steel, then double annealing is used. The first high annealing is carried out at an elevated temperature of 950-1000° C.

Partial annealing is used primarily for hypereutectoid steel. Partial annealing of hypoeutectoid steels is used for forgings that have been hot worked correctly to obtain a satisfactory microstructure. In this case, the purpose of incomplete annealing is to recrystallize pearlite and relieve internal stresses before machining. The heating temperature during incomplete annealing of hypoeutectoid steels is 770 - 800 o C.

Isothermal annealing

During isothermal annealing, austenite transforms into a ferrite-cementite mixture not when cooled in a certain temperature range, as happens with conventional full annealing, but during exposure at a constant temperature. For isothermal annealing, the steel is heated to the optimum temperature and, after holding, quickly cooled to a temperature slightly below the critical point (650-700 0 C). The steel is kept at this temperature until the austenite completely decomposes, and then cooled in air. The advantage of isothermal annealing compared to conventional annealing is a significant reduction in annealing time and obtaining a more uniform structure.

The isothermal holding temperature significantly affects the resulting structure and properties. With decreasing temperature, i.e. With an increase in the degree of overcooling of austenite, the cementite grains are crushed, and more dispersed pearlite is obtained.

Almost isothermal annealing is carried out in two furnaces: in one furnace the parts are heated, then they are transferred to another furnace, which has a slightly lower temperature.

Low temperature annealing.

Low-temperature annealing (high tempering) is used mainly for alloy steels (chromium, chromium-nickel, etc.) to relieve internal stresses and reduce hardness. There is no phase recrystallization during this type of annealing. Complete relief of internal stresses is achieved when heated to 600 0 C, therefore low-temperature annealing can be carried out in the temperature range from 600 0 C. The higher the heating temperature, the shorter the holding time for relieving internal stresses. Cooling after heating must be slow enough so that internal stress does not arise again.

Diffusion annealing (homogenization)

This annealing is characterized by heating to a temperature significantly higher than the temperatures of the transformation range (180 - 300 ° C) followed by slow cooling.

Such annealing is used to level out the chemical heterogeneity of solid solution grains by diffusion, i.e. reducing microsegregation in large shaped steel castings and ingots, mainly alloy steel.

Diffusion annealing, due to its purpose of making steel homogeneous (homogeneous), is otherwise called homogenization.

Since the rate of diffusion increases with increasing temperature, and the amount of diffused substance becomes greater the longer the exposure, high temperature and long exposure are necessary for vigorous diffusion to occur.

In practice, the ingots are heated to 1100 - 1150 ° C, kept at this temperature for 12-15 hours, and then slowly cooled to 250-200 ° C. The diffusion annealing process lasts about 80-100 hours.

As a result of high-temperature long-term annealing, grain growth occurs. This microstructure deficiency is eliminated by subjecting the ingots to hot machining, as a result of which the coarse-grained structure of the cast steel is completely destroyed; Therefore, after homogenization, the ingots are not annealed to improve the structure.

Only in cases where, after homogenization, ingots are obtained with increased hardness (for example, ingots of high-alloy steels), additional low-temperature annealing is carried out at 650-680 ° C.

NORMALIZATION OF STEEL

Normalization is the heating of steel to a temperature 30-50 degrees above the upper critical points, holding at this temperature and cooling in still air. When low-carbon steels are heated to normalization temperatures, the same processes occur as during annealing, i.e. grinding grains. In addition, due to cooling faster than during annealing and the resulting supercooling, the structure of pearlite is finer (dispersed), and the amount of eutectoid (or rather, quasi-eutectoid) is greater than during slow cooling (during annealing).

Compared with the annealing structure, the normalization structure is smaller and the mechanical properties are higher (increased strength and hardness); this is ensured by accelerated cooling (in air) compared to slow cooling (together with a furnace) during annealing.

If, upon cooling in air, martensite is not formed (in some high-alloy steels), but martensite, a structure characteristic of hardened steel, then such heat treatment is not called normalization, but air hardening.

HARDENING OF STEEL

Hardening is the heating of steel above the critical point followed by rapid cooling. Typically, heating is carried out 30-50 degrees above the GSK line on the iron-cementite diagram.

The purpose of hardening is to obtain high hardness or increased strength. The result of hardening, like annealing, is influenced by four main factors - heating rate, heating temperature, holding time and cooling rate.

The main and decisive factor is the cooling rate - the hardness and physical and mechanical properties of steel are related to the cooling rate.

TEMPERING HARDENED STEEL

Tempering is the heating of hardened steel to a temperature below the critical point (727 0 C) followed by cooling. The purpose of tempering is to partially or completely eliminate internal stresses, reduce hardness and increase viscosity. Tempering is applied to hardened steel with a structure of tetragonal martensite and retained austenite.

This is the process of obtaining workpieces or parts under the force of a tool on the original workpiece from the original material. The basis of all pressure processing processes is the ability of metals and their alloys to plastically deform under the influence of external forces without breaking. Plastic forming is a low-waste technology; high productivity, low cost, and high product quality have led to the widespread use of these processes. Plastic deformation is a change in the shape and size of a body under the influence of stress. Metals are polycrystalline. The shape of the change in metal during plastic deformation occurs as a result of plastic deformation of each grain. Before deformation, the shape of the grains was round. During the process of deformation, the grains are stretched in the direction of the acting forces, forming a fibrous, layered structure; this orientation of the grains is called the deformation texture. The greater the degree of deformation, the greater the degree of texture; the nature of the structure depends on the nature of the material and the water deformation. The formation of texture contributes to the appearance of heterogeneity in metallic and physical properties. With an increase in the degree of deformation, the strength characteristics: hardness and strength increase, and the plastic properties deteriorate; the phenomenon of hardening of a deformed substance is called hardening. The state of the cold-worked metal is not stable, therefore, when such a metal is heated, recrystallization processes occur in it, causing the return of all properties to the properties of the metal before deformation. Recrystallization is the formation of new grains. At the same time, hardness increases and density decreases. If you heat a metal, the metal will be restored to its reverse state. The temperature at which the recrystallization process begins is called the recrystallization temperature threshold. There are hot and cold deformations. Cold deformation at temperatures below the recrystallization temperature is accompanied by work hardening. With incomplete cold deformation, recrystallization does not occur. Plasticity increases compared to cold deformation. Used for cold forming at high speeds. Incomplete hot deformation: recrystallization occurs incompletely. The result is heterogeneity of the structure, which can lead to destruction. Such deformation is most likely at a temperature not significantly higher than the temperature at which recrystallization begins. This temperature should be avoided during pressure treatment. Hot deformation is called if it is carried out at a temperature above the recrystallization temperature to obtain a completely recrystallized structure; hot plastic deformation improves the properties of the metal, the density of the metal increases, shrinkage and gas cavities are welded.

30) Metal forming, classification of types. The main methods of pressure treatment: 1) Rolling - compression of metal with rotating rolls. They make: sheets, rails, pipes 2) drawing - pulling the workpiece through the tool hole to make wire rods 3) pressing - squeezing metal out of the tool cavity 4) forging - successive deformation of the metal under hammer blows. Receive: shafts, gears with a large diameter 5) stamping - the process of deforming metal in the cavity of the die. Heating the metal before pressure treatment. The main purpose of heating is to increase the ductility of the metal being processed, and reducing its resistance to deformation from heating depends on the quality of products, equipment productivity and production costs. The main requirements for heating are uniform heating of the workpiece in the shortest time with the least loss of metal due to waste. And to save fuel consumption, non-compliance with the set heating mode can lead to defects (cracks, overheating, burnout, oxidation, decarbonization). Selecting heating mode. Heating temperature, heating speed and heating time). Depends on the properties of the steel, the shape and size of the workpiece, and the direction of heat transfer. The heating temperature range in which hot forming is recommended is called the forging temperature range. When the ductility of a metal is greatest, it is determined by the difference between the initial forging temperature (below the melting temperature) and the final temperature (above the recrystallization temperature). This range depends on the chemical composition and the starting metal. To increase the plastic properties of the metal, it is advantageous to heat it as high as possible. Forging should be completed at the lowest temperature at which the deformation is still hot and cold hardening does not appear. The heating rate of the metal depends on the thermal conductivity of the mold and the size of the workpiece, the temperature of the furnace, and the location of the workpiece in the furnace. The heating time of the workpiece depends on the temperature in the furnace, the chemical composition of the cross-section of the workpieces and their location in the furnace. Furnaces (fuel oil gas, melting) and electric (contact and induction. When heating, non-oxidizing heating methods are used: 1) heating in baths with a molten mixture of salts is used to a limited extent for heating small workpieces to a temperature not exceeding 1050 degrees 2) heating in molten glass melt up to 1300 degrees 3) heating in furnaces filled with protective gas.

Stress state diagram. The stressed state is characterized by a pattern of principal stresses in a small volume isolated in the deformable body. With all the variety of pressure treatment conditions, the following patterns of principal stresses (normally directed stresses acting in mutually perpendicular planes on which tangential stresses are zero) can arise in different parts of a deformable body (Fig. 17.2): four volumetric (A), three flat(6) and two linear(V). For each type of pressure treatment, one of the presented schemes is predominant.

Pressing, rolling, hot stamping, forging are characterized by all-round uneven compression. This loading scheme is the most favorable from the point of view of achieving the maximum degree of plastic deformation.

During sheet stamping and drawing, a scheme of double-sided compression with tension is implemented.

Depending on the acting forces and the ratio of their magnitudes, the body experiences deformation. The set of deformations that occur in different directions in space is usually called deformed state.

The diagram of the main deformations can give an idea of ​​the nature of the change in the structure of the source material, the direction of elongation of grain boundaries and grains. The structure takes on a line-by-line character. The grain boundaries, impurities and non-metallic inclusions contained within them are pulled out, forming fibers (see Fig. 17.1). These changes in the deformed metal can be detected visually after etching, since they have macroscopic dimensions.

After pressure treatment, the metal acquires a pronounced anisotropy of properties. At the same time, the strength characteristics are

Rice. 17.2.

A - volumetric; b - flat; V - linear temporary resistance, yield strength in different directions - change less than plastic - relative elongation, impact strength and even wear resistance.

All of the listed characteristics are greater in the direction of the fibers than across them. It is advisable to take into account the resulting anisotropy of properties when designing loaded parts obtained by plastic deformation. In some cases, taking these features into account can significantly increase the durability of parts, as well as reduce their weight.

Influence of chemical and phase compositions. Different metals and their alloys have different ductility indices and resist plastic deformation to the same extent. However, pure metals always have greater plasticity than their solid solutions, and single-phase structures are more plastic than two-phase ones, especially if these phases differ in their mechanical characteristics. The same applies to the presence of sparingly soluble chemical compounds in metals.

Any chemical inhomogeneities, segregations, and dissolved gases significantly reduce the ability of the metal to undergo plastic deformation, especially at high temperatures.

In relation to iron-carbon alloys, the harmful effects of even small amounts of sulfur and phosphorus should be especially emphasized.

Effect of temperature. At low temperatures, the plasticity of the metal decreases due to a decrease in the thermal mobility of atoms. With increasing temperature, plasticity increases, and resistance to deformation decreases (Fig. 17.3). The curves of changes in ductility and strength are not always monotonic; As a rule, in the temperature range of phase transformations, a slight increase in the strength and decrease in the plastic properties of metals can occur. Almost all metals and alloys in the temperature range close to the temperature of

Rice. 173. The influence of the heating temperature of steel on its plastic properties (e) and resistance to plastic deformation (a b) of the lidus reveals a sharp drop in plastic properties - the so-called temperature range of brittleness (TIB). In this range, plastic properties are close to zero values. This is explained by the fact that at these temperatures the grain boundaries and the intercrystalline layers located there, including fusible impurities, soften or melt, and even a slight deformation leads to their destruction. The purer the metal, the shorter the temperature range of the brittle state and the closer it is to the equilibrium solidus temperature.

Effect of strain rate. The rate of deformation of a material during pressure treatment is largely determined by the speed of movement of the deforming tool, although it is not identical to it. It would be more correct to take the deformation rate to be the value of the relative change in the size of a body per unit time in the direction of the acting force, i.e.

where a cf is the average speed of the tool during deformation;h c p - average deformation value.

Typically, the average strain rate for various pressure treatment processes (Table 17.1) varies within the range of KG 12 - 10-V 1.

The influence of strain rate on the plasticity of a metal is ambiguous. When processing by pressure in a hot state, an increase in the deformation rate reduces the ductility of the metal. This is especially true when processing magnesium and copper alloys and high-alloy steels. The negative effect of increasing strain rate when processing aluminum alloys, low-alloy and carbon steels is less noticeable.

When processing by pressure in a cold state, an increase in the deformation rate above certain values ​​leads to an increase in

Table 17.1

Average strain rates for various types of forming equipment

a change in the temperature of the metal being processed due to the release of significant frictional heat on the sliding planes, which does not have time to spread into space. An increase in temperature leads to softening and an increase in plastic properties. This effect can be very significant. For example, during pressure treatment using explosive devices, it is possible to obtain very significant plastic deformations in cold metal.

Test questions and assignments

• 1. What is the mechanism of plastic deformation?
• 2. How does the presence of dislocations affect the resistance to plastic deformation?
• 3. Compare the properties of cast metal and metal subjected to plastic deformation.
• 4. Under what loading scheme can the maximum value of plastic deformation be obtained?
• 5. In what temperature range is the temperature range of brittleness located, and what explains the decrease in the plastic properties of the metal in this range?

• 1. Raw materials for metallurgy: ore, fluxes, refractories, fuel; ways to increase the combustion temperature of metallurgical fuel. Give definitions and examples of chemical formulas.
• 2. The essence of slagging processes; the role of slags and fluxes in metallurgy (using the example of blast furnace smelting).
• 3. Redox reactions in metallurgy (using the example of iron and steel production).
• 4. The essence of the blast furnace process; starting materials for producing cast iron, blast furnace products, assessment of the efficiency of a blast furnace. Scheme and principle of operation of a blast furnace.
• 5. Steel. The essence of the process of producing steel by direct reduction of iron from ore. Give examples of reduction chemical reactions in the direct reduction of iron from ore.
• 6. The essence of the process of converting cast iron into steel. Comparative characteristics of the main methods of steel production: in converters, in open hearths, in electric furnaces.
• 7.Oxygen-converter method of steel production: raw materials, technology, technical and economic indicators. Oxygen converter diagram.
• 8. Open hearth method for producing steel: raw materials, technology, technical and economic indicators. Scheme of an open hearth furnace.
• 9. Melting steel in electric furnaces: the essence of the process, starting materials, advantages, scope of use. Diagram of an electric furnace for steel smelting.
• 11. Steel casting, casting into molds, continuous casting, structure of steel ingot. Schemes of casting into a mold, scheme of continuous casting of steel, diagrams of calm and boiling steel ingots.
• 12. Classification of castings and casting methods according to production scale and technological characteristics (examples of casting in one-time and permanent molds).
• 13. Casting properties of alloys: fluidity, shrinkage, wettability, gas absorption, chemical reactivity, segregation. Comparison of casting properties of steel and cast iron.
• 14. Basic casting alloys: cast iron, silumin, bronze, steel; the connection between their casting properties and manufacturing technology and the quality of foundry products.
• 15. Sand casting: mold design, casting equipment, molding materials, scope. Advantages and disadvantages of sand casting.
• 16. Casting in shell molds: source materials, shell manufacturing technology, scope of the method. Scheme for obtaining a casting. Advantages and disadvantages of shell casting.
• 18. Chill casting: requirements for the chill mold and castings, lined chill molds; area of ​​use of the process. Schematic diagram of the chill mold. Advantages and disadvantages of the press.
• 19. Injection molding: the essence of the process, area of ​​​​use. Schematic diagram of an injection mold. Advantages and disadvantages of the process.
• 20. Centrifugal casting: essence of the process, area of ​​use, advantages and disadvantages. Schematic diagram of centrifugal casting.
• 21. Characteristics of the main methods for obtaining mechanical engineering profiles; their comparative characteristics (rolling, pressing, drawing). Schematic diagrams of these processes.
• 22. The concept of hot and cold metal forming. Hardening and recrystallization. Changes in mechanical properties during cold hardening and subsequent heating.
• 23. Plasticity of metals, influence on plasticity of chemical composition, heating temperature, stress state diagrams, strain rate.
• 24.Basic laws of pressure treatment: constancy of the volume of least resistance, similarity; using them in practice.
• 26. Metal rolling
• 27. Forging. Area of ​​use
• Question 29.
• Question 30.
• 33. Argon arc welding: schematic diagrams and varieties, area of ​​use.
• 34. Automatic and mechanized submerged arc welding: Principles, welding materials, process advantages and applications.
• 36. Metallurgical processes during welding: dissociation of substances, saturation of the metal o, n, h, processes of deoxidation, slagging, refining of the weld metal.
• 37. Welding materials.
• 38. Thermal processes
• 39. contact welding
• 40. The essence of the process and materials for soldering
• 45. Cutting forces
• 49) The main structural parts of metal-cutting tools. The main surfaces and edges of a turning tool.
• 50. Determination of turning tool angles in a static coordinate system, their purpose and influence on the cutting process.
• 51. Tool materials: tool steels, hard alloys, cutting ceramics, superhard tool materials. Their purpose and designation.
• Tool steels
• Metal-ceramic hard alloys
• Coated carbide grades
• Durability of metal-cutting tools
• Permissible metal cutting speed
• 55. General structure of the main components of universal metal-cutting machines: load-bearing systems, motion drives, working parts and auxiliary systems. Main components
• MS carrier systems
• Main motion drives (PGD)
• Actuators
• Assistance systems
• 57. Kinematic character of machine drives
• 61. Cutting mode parameters on lathes and the sequence of determining their rational combination.
• 65. Drilling. Main types of drilling machines and their purpose. Cutting mode parameters when drilling (V, s, t, to) and the sequence of their rational combination.

• Plastic- the ability of a metal to take a new shape under load without collapsing.

  The ductility of metals is also determined by tensile testing. This property is revealed in the fact that, under the influence of a load, samples of different metals elongate to varying degrees, and their cross-section decreases. The more the sample is able to elongate and its cross section to narrow, the more ductile the sample metal is.

  In the conditions of metal forming, plasticity is influenced by many factors: the composition and structure of the metal being deformed, the nature of the stress state during deformation, unevenness of deformation, deformation rate, deformation temperature, etc. By changing certain factors, plasticity can be changed.

  1.Composition and structure of the metal. Plasticity is directly dependent on the chemical composition of the material. With increasing carbon content in steel, ductility decreases. The elements that make up the alloy as impurities have a great influence. Tin, antimony, lead, sulfur do not dissolve in the metal and, located along the grain boundaries, weaken the bonds between them. The melting point of these elements is low; when heated under hot deformation, they melt, which leads to a loss of ductility.

  2.The influence of temperature is ambiguous. Low-carbon and medium-carbon steels become more ductile with increasing temperature (1). High alloy steels have greater cold ductility (2). For ball bearing steels, ductility is almost independent of temperature (3) . Certain alloys may have a range of increased ductility (4). Industrial iron in the range 800...1000 0 C is characterized by a decrease in plastic properties (5). At temperatures close to the melting point, ductility decreases sharply due to possible overheating and burnout.

  3.Nature of the stressed state. The same material exhibits different plasticity when the stress state pattern changes. The all-round compression scheme is the most favorable for the manifestation of plastic properties, since in this case intergranular deformation is hampered and all deformation occurs due to intragranular deformation. The appearance of tensile stresses in the circuit reduces ductility. The lowest plasticity is observed in the all-round tension scheme.

  4. Strain rate. As the strain rate increases under hot deformation conditions, the ductility decreases. The existing unevenness of deformation causes additional stresses, which are relieved only if the rate of softening processes is not less than the rate of deformation.

  Plasticity depends on the structural state of the metal, especially during hot deformation. Heterogeneity of the microstructure reduces plasticity. Single-phase alloys, other things being equal, are always more ductile than two-phase alloys. The phases have unequal mechanical properties, and the deformation is uneven. Fine-grained metals are more ductile than coarse-grained ones. The metal of ingots is less ductile than the metal of a rolled or forged billet, since the cast structure has a sharp heterogeneity of grains, inclusions and other defects.

Superplasticity is not a property of any special alloys and, with appropriate preparation of the structure and under certain deformation conditions, manifests itself in a large number of alloys processed by pressure.

There are many known alloys based on magnesium, aluminum, copper, titanium and iron, the deformation of which is possible in superplasticity regimes.

Superplasticity can occur only under the condition that no local deformation is formed during deformation (stretching the sample).

When deformation is localized in the sample, local thinning of the neck occurs and it is destroyed relatively quickly.

For ideally viscous (Newtonian) solids, m = 1 and elongation should not be accompanied by necking. In the case of ordinary plastic deformation t< 0,2, а в условиях сверхпластической деформации т >0.3 (usually 0.4-0.7).

When necking begins during superplastic deformation, e increases in this section of the sample and, due to the high value of m, the flow resistance a increases, due to which necking stops. This process is continuously repeated, resulting in the formation of a so-called running neck (eroded necks) as it moves along the length of the sample without producing localized compression. With such quasi-uniform deformation, very large elongations are achieved when the sample is stretched.

Superplastic deformation process

Structural superplastic deformation occurs mainly due to grain boundary sliding, although intragranular dislocation sliding also exists to a certain extent.

The problem of creating an industrial structural superplastic material is, first of all, obtaining ultrafine equiaxed grains and preserving them during superplastic deformation.

Stabilization of grain sizes is achieved: 1) by using two-phase alloys with a volumetric phase ratio of 1: 1; in this case, the maximum development of the interphase surface takes place, which ensures mutual inhibition of the growth of phase grains; 2) the use of dispersed precipitates, which are a barrier to the movement of grain boundaries. Currently, zinc-aluminum is more often used for processing in a state of superplasticity.
high alloy TsA22 (22% Al), titanium alloys, two-phase alloys of copper and zinc (brass), aluminum alloy consisting of an a-solution and dispersed particles of Al 3 Zr, and some others.

The phenomenon of superplasticity in industry is used in volumetric isothermal stamping and pneumatic molding. Superplasticity allows the stamping process to produce parts of complex shapes in one operation, increase the metal utilization rate, and reduce the labor intensity and cost of manufacturing products. The disadvantage is the need to heat the dies to processing temperature and the low deformation rate.