Heat treatment of alloys. Types of heat treatment

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2018-04-01 21:52:18

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Heat treatment of alloys is an integral part of the production process of ferrous and nonferrous metallurgy. As a result of this procedure, the metals can change their characteristics to the desired values. In this article we consider the main types of heat treatment used in modern industry.

Entity heat treatment

In the process of production semi-finished products, metal parts undergo heat treatment to impart desired properties (strength, resistance to corrosion and wear, etc.). Heat treatment of alloys – is a set of artificially created processes, which in alloys under high temperatures, there are structural and physico-mechanical changes, but keeps the chemical composition of the substance.

Purpose of heat treatment

Metal products which are in daily use in all sectors of the economy must meet high standards of durability. Metal, as a raw material, needs to gain the desired performance properties, which can be achieved by exposure to high temperatures. Heat treatment of alloys in high temperatures changes the original structure of the substance, reallocates its components, converts the size and shape of the crystals. All this leads to minimize the internal stress of the metal and thus increases its physico-mechanical properties.

Heat treatments

Heat treatment of metal alloys can be reduced to three simple processes: heating of raw material (semi-finished) to the desired temperature, maintaining it at the desired conditions necessary time and rapid cooling. Modern production uses several types of heat treatment, differing in some technological features, but the algorithm process in General everywhere is the same.

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According to the method of making heat treatment is of the following types:

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

Annealing

Annealing-manufacturing process by which metals and alloys are heated to a defined temperature, and then with the oven, which was the procedure very slowly by natural cooling. As a result of annealing resolve the heterogeneity of the chemical composition of the substance, to remove internal stress, to achieve the grain structure and improve it as such, but also to reduce the hardness of the alloy to facilitate its further processing. There are two types of annealing: annealing the first and second kind.

heat treatment of alloys

The Annealing of the first kind involves heat treatment, which resulted in the change of the phase state of the alloy is negligible or non-existent. It also has its varieties: homogenized – the annealing temperature is 1100-1200 , in such circumstances, the alloys aged for 8-15 hours, recrystallization (at t 100-200 ) annealing is used for riveted steel that is deformed as cold.

The Annealing of the second kind leads to significant phase change alloy. He also has several varieties:

  • Full annealing-heating the alloy above the critical temperature of 30-50 mark, characteristic for the substance, and cooling at a specified rate (200 /hour – carbon steel, 100 /hour 50 /hour – low alloy and high alloy steels, respectively).
  • Partial – heat to a critical point and slow cooling.
  • Diffusion-annealing temperature 1100-1200.
  • Isothermal-heating is the same as in full annealing, but this was followed by rapid cooling to a temperature slightly below the critical and allowed to cool in air.
  • Normalized – full annealing followed by cooling of the metal in the air, not in a furnace.

Hardening

Hardening – manipulation with the alloy, the purpose of which is to achieve a martensitic transformation metal that provides decreasing of plasticity of the product and increase its strength. Tempering, like annealing, involves heating metal in a furnace above the critical temperature to the quenching temperature, the difference consists in the greater speed of cooling, which occurs in the bath with the liquid. Depending on the metal, and even its shape use different types of quenching:

  • Hardened in the same environment, i.e. in a tub of liquid (water – for large parts, oil – for fine details).
  • Intermittent annealing – is cooled in two successive stages: first in the liquid (sharper cooler) to a temperature of approximately 300 , then air-or in any other bath oil.
  • Step – to achieve a product temperature annealing, it is cooled for some time in a molten salt with subsequent cooling in air.
  • Insulated – technology is very similar to speed hardening, characterized only by the exposure time of the product when the temperature of the martensitic transformation.
  • Hardened with szamotulska differs from other types by the fact that a heated metal is cooled, not fully,leaving in the middle of a part of the warm area. As a result of this manipulation, the product acquires the properties of increased strength on the surface and high toughness in the middle. This combination is extremely necessary for impact tools (hammers, chisels, etc.)

types of heat treatment

Holidays

Vacation – this is the final stage of heat treatment of alloys which determines the final structure of the metal. The main purpose of the holiday is to reduce the fragility of metal products. The principle consists in heating the part to temperature below the critical, and cooling. Since the heat treatment and the cooling rate of the metal products for various purposes may vary, there are three types of leave:

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

metals and alloys

Aging

Aging – heat treatment of alloys, determining processes of decomposition of the supersaturated metal after quenching. The result of aging is the increased limits of hardness, strength and durability of the finished product. Aging affects not only cast iron but nonferrous metals, including easily deformable aluminum alloys. If the metal product is subjected to hardening to withstand at normal temperature, there occur the processes leading to spontaneous increase of strength and decrease of plasticity. This is called natural aging of the metal. If the same manipulation to be done at elevated temperatures is called artificial aging.

heat treatment

Cryogenic processing

Changes in the structure of alloys, and hence their properties can be achieved not only high but also extremely low temperatures. Heat treatment of the alloys at t below zero received the name of cryogenic. This technology is widely used in different industries as a complement to heat treatments with high temperatures, as it reduces the costs of thermal processes of hardening of products.

heat treatment of alloys

Cryogenic treatment of alloys is carried out at -196 t in special cryogenic processor. This technology allows to significantly increase the service life of the machined part and anticorrosive properties, and also eliminate the need for repeated treatments.

Thermomechanical treatment

A New method of processing the alloy combines treatment of metals at high temperatures with mechanical deformation of the products that are in plastic condition. Thermomechanical treatment (TMT) by way of Commission can be of three types:

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

hardening release annealing

Chemical-heat treatment

To Change the structure and properties of alloys possible with the help of chemical-thermal treatment, which combines thermal and chemical effects on the metals. The ultimate goal of this procedure besides giving increased strength, hardness, wear resistance of the product is and giving details of the acid resistance and fire resistance. This group includes the following types of heat treatment:

  • Cementation is carried out to give the surface extra strength. The procedure involves saturation of the metal with carbon. Cementation can be accomplished in two ways: solid and gas carburizing. In the first case the processed material together with the coal and its activator are placed in a furnace and heated to a certain temperature, followed by aging it in the environment and cooling. In the case of gas carburizing, the product is heated in a furnace to 900 under a continuous stream of carbonaceous gas.
  • Nitriding – chemical-thermal processing of metal products by saturation of their surfaces in nitrogen environments. The result of this procedure is to increase the tensile strength of the part and increase its corrosion resistance.
  • Cyanide – the saturation of the metal and simultaneously nitrogen, and carbon. Medium can be liquid (molten carbon - and nitrogen-containing salts) and gaseous.
  • Diffusion metallization is a modern method of making metal products, heat resistance, acid resistance and abrasion resistance. The surface of such alloys saturated with various metals (aluminium, chromium) and metalloids (silicon, boron).

Features of heat treatment of cast iron

Cast alloys of cast iron subject to heat treatment on a slightly different technology than the alloysnon-ferrous metals. Cast iron (gray, ductile, alloy) undergoes the following types of heat treatment: annealing (at t 500-650 ­), normalizing, quenching (continuous, isothermal surface), vacation, nitriding (grey cast iron), alifirova (pearlitic cast iron), chrome plating. All these treatments significantly improve the properties of the final products of cast iron: increase service life, eliminate the likelihood of cracking when you use the product, increase the strength and heat resistance cast iron.

annealing temperature

Heat treatment of non ferrous alloys

Non-ferrous metals and alloys have different from each other properties, so are treated by different methods. Thus, copper alloys for the alignment of the chemical composition are subjected to recrystallization annealing. For brass provided by the technology of low-temperature annealing (200-300 ), since this alloy is prone in a wet environment to spontaneous cracking. Bronze is subjected to homogenization and annealing at t up to 550 . Magnesium annealed, tempered, and subjected to artificial aging (natural aging for tempered magnesium does not occur). Aluminium, like magnesium, is subjected to the three methods of heat treatment: annealing, quenching and ageing, after which wrought aluminum alloys significantly increases their strength. Machining of titanium alloys include: recrystallization annealing, tempering, aging, nitriding, and carburizing.

Summary

Heat treatment of metals and alloys is a major technological process, as in black and nonferrous metallurgy. Modern technologies have many methods of heat treatment, allowing to achieve the desired properties of each of the treated alloys. For each metal characterized by its own critical temperature, this means that heat treatment needs to be based on structural and physico-chemical characteristics of the substance. Ultimately, this will allow not only to achieve the desired results, but also largely to streamline production processes.


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Alin Trodden - author of the article, editor
"Hi, I'm Alin Trodden. I write texts, read books, and look for impressions. And I'm not bad at telling you about it. I am always happy to participate in interesting projects."

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