Neither black, nor nonferrous metallurgy does without heat treatment of alloys. Such procedure is carried out to change characteristics of material to the required values. Distinguish several types of heat treatment, each of which is applied taking into account properties of concrete alloys.
General information about heat treatment of alloys
In the course of production of metal products, semi-finished products and ready metal alloys details they are subjected to thermal influence. Such processing gives to materials the necessary properties:
- durability;
- corrosion resistance;
- resistance to wear.
On heat treatment in the most general sense understand set of the operated technological processes at which in alloys under the influence of critical temperatures useful physicomechanical and structural changes are observed. The chemical composition of initial substance at similar processing remains invariable.
Products from metals and their alloys which are used in various branches of the national economy have to have certain indicators of resistance to wear and to influence of adverse factors of the environment.
Metal raw materials, including alloys, often need increase in useful operational qualities. It most often can achieve by means of high temperatures. Heat treatment of alloys is capable to make changes to initial structure of substance. At the same time components of alloy are redistributed, will be transformed a form and the sizes of crystals. These changes lead to reduction of internal tension in materials, to improvement of physical and mechanical characteristics of metals.
Main types of heat treatment of alloys
Distinguish three not most difficult technological processes concerning heat treatment of alloys. It is heating of initial raw materials to the required temperature; its keeping in the reached conditions during strictly certain time; fast cooling of alloy.
In traditional modes of production it is applied several types of heat treatment excellent from each other. The algorithm of processes almost everything remains without changes, only separate technological features change.
Depending on a way of commission of heat treatment distinguish its such types:
- thermal (training, holiday, aging, annealing, cryogenic influence);
- thermomechanical (processing combination high temperatures and mechanical impact on material);
- chemical and thermal (here the subsequent enrichment of a surface of alloy with carbon, chrome, nitrogen, etc. is added to thermal influence).
Understand technological process at which alloy is heated to the necessary temperature as annealing then material in the natural way (together with the furnace) cools down. As a result are eliminated heterogeneity of composition of substance, stress in material are removed. The structure of alloy becomes granular. Its hardness goes down; it does further processing of alloy by less labor-consuming.
Distinguish two types of annealing. When annealing the first sort the phase composition of alloy almost does not change. And here annealing of the second sort is followed by phase change of raw materials. This kind of annealing can be:
- full;
- incomplete;
- diffusive;
- isothermal;
Training call technological process which is carried out for achievement by alloy of martensitic transformation. At the same time density of material increases and its plastic properties decrease. During training, metal is heated to critical values of temperature above. Cooling of products is conducted in a special bathtub with special liquid.
Types of training:
- faltering;
- step;
- isothermal;
- training with a self-holiday (at the same time in the middle of a product during cooling leave the heated site).
The final stage of heat treatment is a holiday. It defines final structure of alloy. This process is conducted for reduction of fragility of a product. The principle of a holiday is simple: alloy is heated, without bringing temperature to critical, and after that cool. Distinguish a high, average and low holiday. Each mode is applied taking into account purpose of a product.
Heat treatment of alloys which causes disintegration of alloy after training is called aging. After performance of this technological process, material becomes fluid, limits of its durability and hardness raise. Very often subject to aging aluminum alloys.
Aging can be both artificial, and natural. Natural aging of alloys happens when after training of a product maintain at a usual temperature, without raising it.
Cryogenic processing of alloys
Studying features of the production technology of metals and alloy, researchers noticed that the necessary combination of properties of material can achieve both at temperature increase of processing of products, and at low temperatures.
Heat treatment of alloys at temperatures is called cryogenic processing below zero. Such technological processes are applied in the form of an additional measure in combination with processing by high temperatures. Advantage of cryogenic processing is obvious: it gives the chance to sharply reduce costs on hardening of details. Service life of products increase. Considerably anticorrosive characteristics of alloys improve.
For cryogenic processing of alloys, as a rule, use special cryogenic processors. In them temperature near minus 196 degrees Celsius is set.
Thermomechanical processing
It is rather new way of processing of alloys. In it use of high temperatures is combined with mechanical deformation of material which is given a plastic state.
Types of thermomechanical processing:
- low-temperature;
- high-temperature.
Chemical heat treatment of alloys
Carry the whole group of methods in which are combined thermal and chemical impact on alloy to this kind of heat treatment. Procedure purposes: increase in hardness and resistance to wear, giving to products of fire resistance and resistance to influence of acids.
Main types of chemical heat treatment:
- cementation;
- nitriding;
- cyanation;
- diffusion metal coating.
Cementation is applied when the surface of alloy needs to give special durability. For this purpose metal is saturated with carbon.
When nitriding the surface of alloy is sated in the nitric environment. Such processing increases anticorrosive characteristics of details.
Cyanation assumes simultaneous impact on the surface of alloy both carbon, and nitrogen. Process can be conducted in the liquid or gaseous environment.
One of the most modern methods of processing – diffusion metal coating. Such process consists in saturation of a surface of alloys certain metals (for example, chrome or aluminum). Sometimes instead of metals apply metalloids (pine forest or silicon).
Heat treatment of color alloys
Properties of non-ferrous metals and their alloys significantly differ. Therefore apply various technological processes to their processing.
For example, copper alloys are exposed to annealing of rekristallizatsionny type (it levels the chemical composition).
Brass is subjected to processing by low-temperature annealing as such alloy in the damp environment is quite capable to crack. Bronze is annealed at a temperature up to 550 degrees Celsius. Magnesium is often subjected to artificial aging.
At heat treatment of titanic alloys apply rekristallizatsionny annealing, training and also aging, cementation and nitriding.
The current technologies give the chance to choose a processing method most of which is suitable for concrete alloy. At the same time it is important to consider structural features of material and its chemical composition.