Heat-resistant steel product introduction
Heat-resistant steel refers to the steel with high temperature oxidation resistance and high temperature strength. High temperature oxidation resistance is an important condition to ensure that the workpiece can work long at high temperature. Steel in high temperature air and other oxidizing environment, oxygen and steel surface chemical reaction to produce a variety of iron oxide layer, the oxide layer is very loose, lost the original characteristics of steel, easy to fall off. In order to improve the high temperature oxidation resistance of the steel, alloying elements are added to the steel to change the structure of the oxide. Common alloying elements are chromium, silicon, aluminum and so on. They react with oxygen to form a dense and stable oxide layer, or passivation layer Cr2O3, SiO2 or Al2O3, on the surface of the steel to protect the steel from further oxidation. When the amount of chromium, silicon and aluminum is added, the high temperature oxidation resistance of steel is good, but if the amount of silicon and aluminum is too much, the mechanical properties and processability of steel become worse. Therefore, the heat-resistant steel with chromium as the main alloying element, with silicon and aluminum as auxiliary elements, in short, the high temperature oxidation resistance of steel is only related to the chemical composition.
High temperature strength refers to the ability of steel to withstand mechanical loads at high temperatures for a long time. Steel under high temperature mechanical load is softened, that is, the strength decreases with the increase of temperature. The second is creep, that is, under the action of constant stress, the amount of plastic deformation increases slowly with the extension of time, and the plastic deformation of steel at high temperature is caused by intragranular slip and grain boundary slip. Alloying method is usually used to improve the high temperature strength of steel. It is also the addition of alloying elements to steel to improve the binding force between atoms and form a favorable organization. Adding chromium, molybdenum, tungsten, vanadium, titanium, etc., can strengthen the matrix of steel, increase the recrystallization temperature, and also form a strengthened phase carbide or intermetallic compound, such as Cr23C6, VC, TiC, etc. These strengthening phases are stable at high temperatures, do not dissolve, do not aggregate and grow, and maintain their hardness. Nickel is added mainly to obtain austenite. Compared with ferrite, the atoms of austenite are closely arranged, the binding force between atoms is stronger, and the atomic diffusion is more difficult. So the high temperature strength of austenite is better. It can be seen that the high temperature strength of heat-resistant steel is not only related to the chemical composition, but also related to the organization.
