Cementation of metal - types of cementation of steel and nuances of the process
Cementation, carried out in various environments and exclusively under the influence of high temperatures, which is a very common method of chemical and thermal treatment of metal, have been successfully used for more than a dozen years.
Preparation of parts for cementation
The essence of the cementation process
The meaning of any methods of chemical and thermal treatment of metals, which includes cementation of steel, is, that the product is heated to a high temperature in a special environment (liquid, solid or gaseous). Such influence leads to, that the chemical composition of the metal changes - the surface of the processed product is saturated with carbon, as a result, it becomes more solid and wear-resistant. What is important, the core of the processed parts remains viscous.
It is possible to achieve the desired effect after such exposure to the metal only in that case, if low-carbon steels are subjected to processing, which contain no more carbon 0,2%. In order to perform cementation, the product is heated to temperature 850-950 degrees Celsius, and the composition of the medium is selected as follows, so that it emits activated carbon when heated.
If the cementation began to be carried out in a qualified manner, it is possible not only to change the chemical composition of a metal product, but also transform its microstructure and even phase composition. As a result, it is possible to significantly strengthen the surface layer of the part, give it characteristics, similar to the properties of hardened steel. In order to achieve such results, it is necessary to correctly select the parameters of the chemical and thermal treatment of the metal - the heating temperature and the time of exposure of the processed product in a special environment.
Equipment for vacuum cementation of steel
This technological operation takes quite a long time, since the process of saturation of the surface layer of steel with carbon is very slow (0,1 mm for 60 minutes). Given that fact, that the reinforced surface layer for most products should be at least 0,8 mm, can be calculated, that it will be necessary to spend no less on metal cementation 8 hours. The main types of media for cementation of metal (or, as they are properly called, carburetors) is:
- gaseous media;
- electrolyte solutions;
- pasty media;
- boiling layer;
- solid media.
The most common are gaseous and solid carburetors.
Dependence of the thickness of the cemented layer on the processing time and temperature
Carrying out cementation of steel in a solid medium
Most often, a mixture is used for metal cementation in a solid environment, consisting of sodium carbon dioxide, barium or calcium and birch or oak charcoal (70-90%). Before that, all the components of such a mixture are ground into fractions 3-10 mm and sifted, which is necessary to remove very small particles and dust.
After, how the components of the mixture for chemical and thermal treatment of metal are prepared, they can be mixed in several ways.
- Components of the mixture (salt and coal) thoroughly mixed in a dry state. If you ignore this requirement, then after the end of the cementation process, spots may form on the surface of the product.
- Salt is dissolved in water and charcoal is poured with the resulting solution, after which it is dried until the moisture content is no more 7%.
It should be noted, that the second method is better, because it allows you to get a mixture with a more uniform composition.
Charcoal carburetor
As in production, and at home cementation of steel products is carried out in boxes, which filled carburetor. To improve the quality of the surface layer of the treated metal, and also reduce time, which is spent on heating the boxes, it is best to make them as close as possible to the sizes and shapes of the parts.
Optimum conditions for cementation of steel can be created, rejecting the leakage of gases, formed in the carburetor during heating. That's what boxes are for, which must have tightly closing lids, carefully coated with refractory clay before placing in the furnace.
Naturally, it is advisable to use specially made boxes only in industrial conditions. Boxes of standard sizes and shapes are used for cementation of metal at home (square, rectangular, round), selecting them depending on the number of processed parts and the internal dimensions of the furnace.
The optimal material for such boxes is heat-resistant steel, but containers made of low-carbon steel can also be used. The technological process of cementation of metal products is as follows.
A visual representation of the change in structure after cementation
- Підготовлені для обробки деталі укладають в ящики, пересипаючи шарами карбюризатора.
- Наповнені ящики, обмазані вогнетривкою глиною, поміщають у попередньо прогріту піч.
- The so-called through heating of boxes with parts is performed, at which they are heated to a temperature 700-800 degrees Celsius. About that, that the boxes have warmed up well, judged by the color of the floor slab: it should not have dark spots in the places of contact with the container.
- The temperature in the oven is raised to 900-950 degrees Celsius. It is at such values that steel is cemented.
High temperature and special environment, in which the metal is located, contribute to that, that active carbon atoms diffuse into the crystal lattice of steel. It should be noted, that it is possible to carry out cementation of steel at home, but often does not allow to achieve the desired effect. This is due to the fact, that the cementation process requires a long exposure of the part at a high temperature. Usually, it is difficult to provide at home.
Cementation of parts in a gas environment
The authors of this technology are S. Illinsky, N. Minkevich and St. Prosvirin, which under the leadership of P. Anosov used it for the first time at the plant in. in Zlatoust. The essence of this technology is, that processed metal parts are heated in an environment of carbon gases, which can be of artificial or natural origin. Most often, gas is used, which is formed during the decomposition of oil refining products. This gas is obtained in the following way:
- heat a steel container and add kerosene to it, which, upon evaporation, decomposes into a mixture of gases;
- composition of some part (60%) the obtained gas is modified (cracking).
The resulting mixture is used for chemical and thermal treatment of steel.
The process of cementation of steel
If cementation was carried out using only pyrolysis gas, without adding cracked, then the depth of the carbonized layer will be insufficient. in addition, in this case, a large layer of soot will settle on the surface of the processed part, the removal of which can take a lot of time and effort.
Furnaces, which are used to perform gas cementation of metal, must be hermetically sealed. Two main types of such furnaces are used at modern production enterprises: methodical and stationary. The process of cementation in a gas environment is as follows. The processed parts are placed in the furnace, the temperature at which it is brought to 950 degrees Celsius. Gas is supplied to the heated furnace and parts are kept in it for a certain time.
In comparison with cementation of steel using a solid carburizer, this technology has a number of significant advantages:
- providing better conditions for service personnel;
- high speed of achieving the required effect due to the fact, that parts in a gas environment can withstand a shorter amount of time (in addition, it does not take time to prepare a solid carburetor).
Cementation of steel at home
In what other environments can cementation of steel be carried out
Separate varieties of carbon, low-carbon and alloy steels, in particular 15, 20, 20HGNR, 20X, 20H2N4A, 18X2H4VA, 20G, 12ХН3А and others., can undergo cementation in other environments.
Electrolytic solution
In such an environment, only parts can be carburized, що відрізняються невеликими розмірами. Даний метод ґрунтується на анодному ефект, завдяки якому і відбувається насичення поверхні металу вуглецем, що містяться в розчині електроліту. In order for the solution to contain a sufficient amount of active carbon, glycerin is added to it, acetone, sucrose and other substances. Before, how to place a steel part in a solution, it is heated to temperature 450-1050 degrees Celsius (depending on the processed metal and the dimensions of the part). Electric current with voltage is used to heat the solution 150-300 St..
Boiling layer
Cementation of steel according to this technology is carried out in the environment of a hot gas flow, which is formed when methane and endogas pass through a layer of heated crushed (0,05-0.2 mm) corundum.
Pasty mixtures
Special pastes are used for carburizing the metal surface using this technology, which consist of yellow salt, wood dust and soot. Before processing, the part is coated with this paste and dried, and only then heated to temperature 910-1050 degrees Celsius using high frequency currents.
Regardless of the technology, cementation of steel was performed, after it ends, it is recommended to take a metal vacation.
