Heat resistance and heat resistance are very important characteristics. Some engineering products operate in very difficult conditions at elevated temperatures. Conventional structural steels, when heated, stepwise change their mechanical and physical properties, begin to actively oxidize and form scale, which is completely unacceptable and poses a threat of failure of the entire unit, and possibly a serious accident. To work at elevated temperatures, material engineers using metallurgists created a number of special steels and alloys. This article gives a brief description of them.
Heat resistant steel
Many people identify the concept of heat resistance with a concept such as heat resistance. In no case should this be done. Heat resistance is also called red breaking. And this concept means the ability of a metal (or alloy) to maintain high mechanical properties when working at elevated temperatures. That is, such a metal, even when heated to a red glow (it is characteristic for temperatures above 550 ° C), will not creep and retain sufficient rigidity.
In simple terms, heat resistance is the ability of a material to remain operational when heated to high temperatures. Conventional structural steels, even with slight heating, become ductile, which excludes the possibility of their use for the manufacture of products operating at high temperatures.
Different grades of metals and alloys have different heat resistance. This indicator depends on the chemical composition of the material. Heat resistance tests can be carried out for a long time. But most often, samples heated in a furnace to a certain temperature are tested in tension for a short period of time.
Heat resistant steel
Heat resistance, in contrast to heat resistance, is the ability of materials to withstand the development of corrosion processes when working at high temperatures. Conventional steels, if subjected to heat (with the exception of heat treatment in a protective atmosphere or in vacuum), begin to oxidize. In addition, with prolonged heating, carbon on the surface of the product begins to burn out. As a result, the surface is depleted of carbon, which leads to a sharp change in the mechanical properties (primarily hardness) on the surface. Wear resistance drops. The development of such a negative phenomenon as badass. This group of steels can operate at temperatures around 550 ° C.
In order to increase the heat resistance of steel, its melt is alloyed with silicon, aluminum and chromium. Sometimes it is enough to increase the heat resistance of the surface of the part. In this case, they resort to siliconization or alitization (saturation of the surface layer with silicon or aluminum atoms, respectively) in a powder medium.
High melting point materials
When used in conditions of especially high temperatures, the considered materials cannot be used, since at a temperature in the region of 2000 ° melting begins to occur (a liquid phase is released). For these purposes, refractory metals are used: tungsten, niobium, vanadium, zirconium and so on. These materials are quite expensive, but engineers have not yet found a worthy alternative for them.
Characterization of alloys based on chromium and nickel
Alloys with high heat resistance are very popular in power engineering (steam turbine blades, parts of aircraft engines, and so on). Moreover, the need for such materials is constantly growing. Moreover, production requires scientists to obtain more and more advanced materials that can maintain their performance at very high temperatures. Therefore, work is constantly ongoing to increase the heat resistance indicators. Nickel, more precisely alloying with this element of steel, contributes to this.
All heat-resistant steels are alloyed with nickel (at least 65%). Mandatory there is chrome. The content of this element should not be less than 14%. Otherwise, the metal surface will be intensely oxidized.
Steel, additionally alloyed with aluminum, vanadium and other refractory elements. Aluminum, for example, even at room temperature is covered with a thin oxide film, which prevents the penetration of corrosion deep into the metal. That is, no scale is formed.