Heat treatment of metal is a change in the internal structure (structures) of metal under the influence of changes in temperature conditions and obtaining the necessary mechanical and physical properties of the metal. The vast majority of heat treatment occurs at critical temperatures, in which there is a structural transformation in alloys.
Therefore, heat treatment of metal is reduced to three successive operations and types:
- heating the metal at a certain speed to a given temperature;
- holding the metal for some time at this temperature;
- cooling at a process speed.
Depends on how you need to change the properties of a particular steel product and use different types of heat treatment, which differ in the maximum heating temperature, exposure time and cooling rate. Heat treatment has found the widest application in mechanical engineering.
Heat treatment of metal, alloy, become
All the properties of any alloy depend on its structure. The main way, which allows you to change this structure and is heat treated. Its foundations were developed by DK Chernov., and later his work was supported by Bochvar AA, G.V. Kurdyumova, Gulyaeva AP.
Heat treatment of metal and alloy is a sequence of operations such as: heating, exposure and cooling, which are performed in a certain sequence and in a certain mode, to change the internal structure of the alloy and obtain the desired properties, while the chemical composition of the metal does not change.
What is the heat treatment of metal and alloy?
- In annealing
- hardening
- vacation
- normalization
Annealing. This is the heating of the metal to a high temperature, and then there is a slow cooling. Annealing is of different types - it all depends on the heating temperature and cooling rate.
Hardening. Heat treatment of steel, alloys, metal, which is based on recrystallization of steel when heated above the critical temperature. After curing the steel at this temperature should be very fast cooling. Such steel is of non-equilibrium structure and therefore after hardening follows - leave.
Vacation. It is carried out after hardening, to reduce or remove residual stress in steels and alloys, increase viscosity, reduce the hardness and brittleness of the metal.
Normalization. It looks like annealing, the only difference is, that the normalization of the metal occurs in air, and annealing - in the furnace.
heating the workpiece
This operation is very responsible. It depends on its proper conduct, first - the quality of the product, and secondly, labor productivity. You need to know, that when heated, the metal changes structure, properties and all the characteristics of the surface layer. As at interaction of steel or an alloy with air there is an oxidation of iron and on a surface scale is formed. The thickness of the scale depends on the chemical composition of the metal, what was the temperature and time of its heating.
Steel begins to oxidize intensely when heated more 900 degrees, then the oxidation increases twice - when heated 1000 degrees C, and at temperature 1200 degrees C - c 5 times.
Oxidation occurs in various steels?
Chromium-nickel steel - it is called heat-resistant back, that it is practically not oxidizable.
Alloy steel - it forms a dense, but a thin layer of scale, which protects against further oxidation and prevents cracking during forging.
Carbon steel - it loses about 2-4 mm of carbon from the surface when heated. This is very bad for metal, because it loses strength, hardness and steel deteriorates in hardening. And especially very detrimental is decarburization for forging of small details with the subsequent hardening. To avoid cracks on high-alloy and high-carbon steel, they should be heated slowly.
Be sure to refer to the "iron-carbon" chart, where the temperature for the beginning and end of forging is determined. It is necessary to do it for that, that the metal at heating did not get coarse-grained structure and its plasticity did not decrease.
But overheating of the workpiece can be corrected by heat treatment, but this requires extra energy and time. If the metal is heated to an even higher temperature, then it will lead to a burn, that will come to that, that the bond between the grains in the metal will be broken and it will be completely destroyed during forging.
overspending
This is the most incorrigible marriage. Be sure to monitor the temperature when heating the metal or alloy, time and end of heating. Dross rise, if the heating time is increased, and cracking may occur with rapid or intense heating.
The overconsumption of the alloy is due to the diffusion of oxygen at the grain boundaries, where oxides are formed immediately, which separate the grains at a high alloy temperature and at the same time the strength drops sharply. And plasticity at this time comes to zero. This marriage is immediately sent to the smelter.
What is the heat treatment of metals and alloys
Heat treatment is divided into:
- thermal;
- thermomechanical;
- chemical-thermal
Heat treatment includes the main types - annealing of the 1st kind, annealing of the 2nd kind, hardening and vacation. Normalization does not apply to all types of steel, it all depends on its degree of doping.
All types of heat treatment have different heating temperatures, the duration of exposure at this temperature and the cooling rate after exposure.
1-This type of annealing is diffuse annealing, annealing to relieve stress.
2-This type of annealing is divided into incomplete, full, isothermal annealing, spheroidization, normalization.
Hardening is used for that, to make the products solid, strong and durable.
Chemical-thermal treatment
This is the heat treatment of steel, which is combined with the saturation of the surface of the product - carbon, nitrogen, aluminum, silicon, chromium, etc., Forming solid substitution solutions with iron. They are longer and more energy intensive, than steel is saturated with iron and carbon, forms with iron solid solutions of introduction.
Chemical - heat treatment at creation on a surface of products of favorable residual compressive stresses increases durability and reliability of a product. It also increases corrosion resistance, hardness.
This treatment is designed to change a certain layer of steel. Such methods include:
- cementation - in this method the upper layer of steel is enriched with carbon. This results in products with combined properties - a soft core and a hard surface layer;
- nitriding is the enrichment of the surface layer with nitrogen, to increase the corrosion resistance and fatigue strength of the product;
- boring is the saturation of the surface layers of steel with boron, with this method, the product increases wear resistance, especially with friction and dry sliding. In addition, when drilling eliminates grip or welding of parts in the cold state. Details after boring become very resistant to acid and alkali;
- alitirovanie is saturation of steel with aluminum. This is done for that, to give steel resistance to aggressive gases - sulfur dioxide, hydrogen sulfide;
- chrome plating - chromium saturation of the surface layer of steel. Chromium plating of low-carbon steels has almost no effect on their characteristics. Chromium plating of steel with a higher chromium content is called solid chrome plating, as chromium carbide is formed on the surface of the parts, which has:
- high hardness
- scale resistance
- corrosion resistance
- increased wear resistance
cryogenic treatment
This is a hardening heat treatment of metals and alloys with cryogenic, very low temperatures - below -153 degrees C. Previously, such heat treatment was called "cold treatment" or "heat treatment of metal at sub-zero temperatures". But these names did not fully reflect the essence of cryogenic treatment.
Its essence is as follows: the workpieces are placed in a cryogenic processor, where they are slowly cooled, and then withstand the details at room temperature -196 degrees C for some time. Then they gradually return to room temperature. When this process is going on, then structural changes occur in the metal. Due to this, wear resistance increases, cyclic strength, corrosion and erosion resistance.
Basic properties, obtained during processing, as cold cooling, stored for the entire life of the workpiece and therefore does not require re-processing.
Of course, cryogenic technology will not replace thermal hardening methods, and when treated with cold will give the material new properties.
Ultra-low temperature machined tools allow businesses to reduce back costs, what:
- the wear resistance of the tool increases, parts and mechanisms;
- the number of marriages decreases;
- reduced costs for repair and replacement of technological equipment and tools.
It was Soviet scientists who fully appreciated the effect of cold treatment on metal and alloys and initiated the use of this method..
Currently, the method of cryogenic treatment of products is widely used in all industries.
Mechanical engineering and metalworking:
- increases the life of equipment and tools to 300%;
- increases the wear resistance of the material;
- increases cyclic strength;
- increases corrosion and erosion resistance;
- removes residual voltage.
Special equipment and transport:
- increases the life of brake discs by 250%;
- increases the efficiency of the brake system;
- increases the cyclic strength of the suspension springs and other elastic elements on 125%;
- increases engine life and power;
- reduces the cost of operating vehicles.
Defense industry:
- increases the survivability of barrels to 200%;
- reduces the effect of heating the barrels on the results of shooting;
- increases the life of components and mechanisms.
Mining and manufacturing industry:
- increases the resistance of the rock-breaking tool to 200%;
- reduces abrasive wear of units and mechanisms;
- increases corrosion and erosion resistance of equipment;
- increases the life of industrial and mining equipment.
Audio equipment and musical instruments:
- reduces signal distortion in conductors;
- improves musical activity, clarity and transparency of sound;
- expands the sound range of musical instruments.
Cryogenic treatment is used in almost all industries, where it is necessary to increase the resource, increase strength and durability, as well as increase productivity.
What does heat treatment require??
Reliability and durability of metal structures, equipment, pipelines depends on the quality of the nodes, details, elements of which they consist. During operation, they are subject to statistics, dynamic and cyclic loads and exposure to aggressive environments. They have to work at low and high temperatures and are in rapid wear.
And therefore operation of any metalware directly depends on wear resistance, strength, thermo- and corrosion resistance of the elements of which they are composed.
In order to improve all these characteristics, it is necessary to choose the right material for the parts, improve their design, eliminate assembly inaccuracies, to improve methods of hot and cold processing.
Materials in a state of delivery seldom meet such high requirements. The main part of the delivered structural elements requires stabilization of operational properties, so that they do not change over time. And to improve the mechanical and physicochemical properties of metallic materials, apply heat treatment. This is a sequence of heating operations, aging and cooling of metals and alloys.
It is carried out to change the structure and properties of metals and alloys in the direction, which was set. Heat treatment is used to change the structure of the phase composition and redistribution of components, size and shape of crystal grains, types of defects, their number and distribution. And all this allows you to easily get the desired property of the material.
Be sure to remember, that the properties of metals and alloys depend not only on not only on the structure, but also from the chemical composition, which is formed during the metallurgical and foundry process.
The task of heat treatment is to eliminate internal stresses in metals and metals, improvement of mechanical and operational properties and other.
Steel is subjected to heat treatment, cast iron, alloy based on non-ferrous metals.
You need to know, that materials with one chemical composition when carrying out different modes of heat treatment can obtain several completely different structures, which will have completely different properties. Alloys of simpler composition can be used to improve the mechanical properties by means of heat treatment. Permissible voltages, reducing the weight of parts and mechanisms, increasing their reliability and durability can also be achieved through heat treatment.
At low costs for heat treatment, the result can significantly affect the complexity and cost of work in adjacent areas of production. Many manufacturers do not heat-treat products, thereby reducing the entire technological process in the manufacture of products. Sometimes this is justified, and sometimes not.
It is always necessary not only to carefully consider the whole process of volumetric and local heat treatment, but also strictly adhere to their regimes, to achieve optimal structures and a high level of physical, mechanical and operational properties in products to ensure their reliable and long-lasting operation.