Principle of heat resistant castings

- Nov 03, 2019-

Heat resistant steel refers to steel having high temperature oxidation resistance and high temperature strength. High temperature oxidation resistance is an important condition for ensuring long-lasting work of the workpiece at high temperatures. In the oxidizing environment such as high temperature air, the steel chemically reacts with the steel surface to form a plurality of iron oxide layers. The oxide layer is loose, losing the original characteristics of the steel and easily falling off. In order to improve the high temperature oxidation resistance of the steel, an alloying element is added to the steel to change the structure of the oxide. Commonly used alloying elements are chromium, silicon, aluminum, and the like. They react with oxygen to form a dense, stable oxide layer on the surface of the steel, or a passivation layer, Cr2O3, SiO2 or Al2O3, to protect the steel from further oxidation. The addition of chromium, silicon and aluminum is high, and the high temperature oxidation resistance of steel is good. However, if the amount of silicon and aluminum added is too large, the mechanical properties and workability of the steel deteriorate. Therefore, heat-resistant steel uses chromium as the main alloying element and silicon and aluminum as auxiliary elements. In short, the high-temperature oxidation resistance of steel is only related to chemical composition.

High temperature strength refers to the ability of steel to withstand mechanical loads for extended periods of time at elevated temperatures. When the steel is subjected to mechanical load at a high temperature, it is softened, that is, the strength decreases as the temperature increases. The second is creep, that is, under the action of constant stress, the amount of plastic deformation slowly increases with time, and the plastic deformation of steel at high temperature is caused by intragranular slip and grain boundary slip. To increase the high temperature strength of steel, an alloying method is usually employed. It is also the addition of alloying elements to steel to improve the bonding between atoms and to form favorable structures. Adding chromium, molybdenum, tungsten, vanadium, titanium, etc., can strengthen the matrix of the steel, increase the recrystallization temperature, and form strengthening phase carbides or intermetallic compounds such as Cr23C6, VC, TiC, and the like. 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. Austenite is denser than atoms in ferrite, and the bonding between atoms is strong, and atomic diffusion is difficult. Therefore, the high temperature strength of austenite is good. It can be seen that the high temperature strength of heat resistant steel is not only related to chemical composition, but also related to the structure.