Cementation of steel
Cementation of metal and steel in Dnipro
Among all the materials used in various industries, few can be used as extensively as steel. This is primarily due to the fact that steel can be produced in a huge number of grades, differing in its chemical composition and method of production, as well as in its physical and chemical properties.
An additional advantage of steel is the possibility of improving individual parameters at various stages of processing, which makes it possible to achieve the required properties at the optimum moment of the technological process.
This is possible through the use of thermal and thermochemical treatment methods. Particularly common are techniques that allow the introduction of additional elements into the metal structure, for example by carburising (metal carburising) or nitriding of steels.
How to improve the strength characteristics of steel?
Steel is very often used for the production of products that must have special properties in terms of resistance to various external factors associated with the conditions in which it will be used. In particular, this applies to elements of various machines and devices operating with high mechanical and thermal loads.
These would be, for example:
- parts and assemblies for internal combustion engines;
- parts for transmission systems, including all kinds of gears, shafts, levers or bushings;
- housing parts and so on.
The resulting friction or significant impacts in such cases make it necessary to fulfil the requirements for individual elements, especially for their surfaces, which are subjected to the direct impact of high forces.
One way to improve the mechanical and thermal strength of the metal surface is to use various heat treatment methods by applying high temperatures, which will change the internal structure of the material, transforming it into improved properties, such as hardness or abrasion resistance.
Annealing and solid solution treatment are also performed as part of heat treatment. In the first case, it is a matter of relaxing the material and unifying its structure, while in the second case, used for various alloys, it is a matter of uniform distribution of individual elements throughout the structure.
Heat treatment in the form of quenching and tempering can be successfully applied to the entire element or only to its outer layer. However, even more far-reaching effects can be obtained by first saturating the steel with selected elements, e.g. carbon. For this purpose, metal carburising (carburising) is carried out before hardening and tempering.
Steel carburising process
The process of carburising or carburising steel is the introduction of additional carbon atoms into the steel structure. Because of the crystalline structure of the material, to make this possible, it is necessary to heat its surface to a temperature that will allow the incorporation of carbon into their structure.
Cementation of steel is accompanied by a change in the chemical composition of the alloy surface, which also requires ensuring an appropriate concentration of carbon atoms in the vicinity of the improved surface.
Carbon atoms are introduced into the steel structure by diffusion. It allows the element to adsorb in the environment until the concentration level is equalised and then diffuse further into the interior. The free carbon atoms can be delivered to the surface of the carburised metal either in gaseous form and in the corresponding liquid solution or in the solid state.
After activation of the metal by thermal stress, carbon diffuses through the free spaces of the crystal lattice and occupies the next free nodes. The diffusion rate is related to both the heating temperature of the carburised substrate and the presence of free carbon atoms.
Metal cementation – techniques
Carburising can be carried out in different centres. The presence of a solid is utilised, e.g. in the form of charcoal powder with the addition of activators without access to air. In the case of liquids, selected salts such as chlorides or alkali metal carbonates are used.
A frequently used method which gives excellent control over the composition of the carburising medium and hence the amount of carbon available is carburising in gas. Typically methane or carbon monoxide gas is used. Vacuum carburising of steel 20 (or any other grade) at significantly reduced pressure or ionisation with plasma formation is also possible.
Cementation is a process that requires heating the material above the temperature of austenite formation, i.e. to 850-950°C. Steels with lower carbon content require higher temperatures.
Cementation of parts – advantages
The process of carburising gears and other metal products significantly increases the surface hardness of the metal and at the same time improves its abrasion resistance, although it still requires additional hardening treatments. Depending on the alloy used and the procedures applied, the hardness obtained can range from 55 to 62 HRC.
The advantage of carburising is the simultaneous retention of ductility and strength in the core, i.e. outside the carburising zone. In practice, the thickness of the carburised layer is about 0.5 to 2 mm, and the carbon content in this zone is about 1 to 1.1 per cent.