Heat-resistant steel

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 the lasting work of the piece at high temperature. Steel parts in the high temperature air and other oxidation environment, oxygen and steel surface chemical reaction to form 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, alloy elements are added to the steel to change the structure of the oxide. Commonly used alloy elements are chromium, silicon, aluminum and so on. They react with oxygen to form a dense, stable oxide layer, or passivation layer Cr2O3, SiO2, or Al2O3, to protect the steel from further oxidation. The amount of chromium, silicon and aluminum is added, and the high temperature oxidation resistance of steel is good, but if the amount of silicon and aluminum is added too much, the mechanical properties and process of steel will become worse. Therefore, the heat-resistant steel with chromium as the main alloy element, silicon, aluminum as the auxiliary element, in short, the high temperature oxidation resistance of steel is only related to the chemical composition.
High temperature strength refers to the ability of the steel to withstand the mechanical load for a long time at high temperature. Steel under high temperature mechanical load is softening, 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 is slowly increasing with the extension of time, and the plastic deformation of steel at high temperature is caused by the crystal slip and the grain boundary slip. Alloying method is usually used to improve the high temperature strength of steel. It is also to add alloy elements to the steel to improve the bonding force between the atoms and form a favorable organization. Adding chromium, molybdenum, tungsten, vanadium, titanium, etc., can strengthen the matrix of steel, improve the recrystallization temperature, but also can form enhanced phase carbide or intermetallic compounds, such as Cr23C6, VC, TiC, etc. These intensified phases are stable at high temperature, not soluble, not aggregate aggregate, and maintain their hardness. Nickel is added, mainly to get the austenite. Austenite is more closely arranged than the atoms in ferrite, and the interatomic binding force is strong, so the atomic diffusion is difficult. So the high temperature intensity 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 tissue.