Heat-resistant steel refers to steel with high-temperature oxidation resistance and high-temperature strength. High-temperature oxidation resistance is an important condition for ensuring the long-term operation of workpieces at high temperatures. In oxidizing environments such as high-temperature air, oxygen reacts chemically with the steel surface to form various iron oxide layers. This oxide layer is very porous, loses the original properties of steel, and is easily peeled off. To improve the high-temperature oxidation resistance of steel, alloying elements are added to the steel, thereby changing the structure of the oxides. Commonly used alloying elements include chromium, silicon, and aluminum. They react with oxygen to form a dense and stable oxide layer, or passivation layer such as Cr2O3, SiO2, or Al2O3, on the steel surface to protect the steel from further oxidation. Higher amounts of chromium, silicon, and aluminum result in better high-temperature oxidation resistance, but excessive amounts of silicon and aluminum deteriorate the mechanical properties and processability of the steel. 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 its chemical composition.
High-temperature strength refers to the ability of steel to withstand mechanical loads for extended periods at high temperatures. Steel experiences two main types of mechanical loads at high temperatures: softening (strength decreases with increasing temperature) and creep (slowly increasing plastic deformation over time under constant stress). Plastic deformation in steel at high temperatures is caused by intragranular slip and grain boundary slip. Alloying is commonly used to improve the high-temperature strength of steel. This involves adding alloying elements to enhance interatomic bonding and create favorable microstructures. Adding chromium, molybdenum, tungsten, vanadium, and titanium strengthens the steel matrix, increases the recrystallization temperature, and forms reinforcing carbides or intermetallic compounds such as Cr23C6, VC, and TiC. These reinforcing phases are stable at high temperatures, do not dissolve, do not aggregate, and maintain their hardness. Adding nickel primarily aims to obtain austenite. Austenite has a denser atomic arrangement than ferrite, resulting in stronger interatomic bonding and less atomic diffusion. Therefore, austenite exhibits better high-temperature strength. It is evident that the high-temperature strength of heat-resistant steel is related not only to its chemical composition but also to its microstructure.