The steel with good oxidation resistance and high temperature strength under high temperature condition is called heat-resistant steel, which is often used in manufacturing boilers, steam turbines, power machinery, industrial furnaces and parts for aviation, petrochemical and other high temperature fields. Today we are going to learn what makes heat-resistant steel so special?
High temperature oxidation resistance
The high temperature oxidation resistance of metal refers to the resistance of steel to oxidation under high temperature. Oxidation is a typical chemical corrosion, metal and oxygen contact chemical reaction is oxidation corrosion, corrosion products (oxidation film) adhere to the surface of the metal in high temperature air, combustion gas and other oxidizing atmosphere. With the development of oxidation, the thickness of the oxide film continues to increase. Whether the metal continues to oxidize after oxidation reaches a certain degree directly depends on the performance of the oxide film on the metal surface. If a dense and stable oxide film is formed with a high binding force and a high strength of the oxide film with the matrix metal, it can prevent the diffusion of oxygen atoms into the metal and reduce the oxidation speed. Otherwise, it will accelerate the oxidation and make the metal surface peel and fall off, leading to the early failure of parts.
The composition of oxide layer on steel surface is related to temperature. Oxide layer is composed of dense Fe2O3+Fe3O4, which can effectively prevent oxygen diffusion below 570℃. When heated more than 570℃, the oxide film is composed of FeO+Fe2O3+Fe3O4 (from the inside out). FeO is loose and porous, which accounts for about 90% of the thickness of the entire oxide film. Metal atoms and oxygen atoms are easy to spread through the FeO layer to accelerate oxidation. High temperature FeO greatly reduces the oxidation resistance of steel. The higher the temperature, the faster the atom diffusion and oxidation speed.
The main method to improve the oxidation resistance of steel is the addition of alloy elements, such as Cr, Si and Al, so that when the steel is in contact with oxygen at high temperature, it forms dense high-melting oxidation films Cr2O3, SiO2 and Al2O3 first, which cover the surface of the steel closely and prevent its further oxidation.
High temperature strength
The high temperature strength of metal refers to the resistance of metal materials to mechanical loading at high temperature, that is the resistance of metal materials to plastic deformation and damage at high temperature. The mechanical properties of metal at high temperature are greatly different from that at room temperature. When the working temperature is greater than the recrystallization temperature, the metal produces plastic deformation and work hardening. Besides being affected by external forces, produced recrystallization and softening.
The mechanical properties of metals at high temperature are related to temperature, time and microstructure. Creep often occurs at high temperature, that is, when the working temperature is greater than the recrystallization temperature, the working stress exceeds the elastic limit at that temperature, and the metal slowly deforms along with time. The higher the resistance of metal to creep, the higher its high temperature strength.
The high temperature strength of metals is generally expressed by creep limit and endurance strength. Creep limit is the stress value when the residual deformation of metal reaches a certain value after a period of time at a certain temperature. Durable strength refers to the stress value of a metal material at a constant temperature for a certain period of time. At high temperature, the grain boundary strength of metal materials is lower than that of the inside grain, so adding alloy elements to increase the recrystallization temperature, forming stable special carbide, and using coarse crystal materials to reduce grain boundary can effectively improve the high temperature strength of steel.
Mo is a key alloy element in creep resistant ferrite steel with an operating temperature of up to 530°C.Molybdenum in solid solution can reduce creep rate of steel very effectively. Molybdenum can slow down the agglomeration and coarsening of carbide at high temperature. Vanadium, titanium and niobium are strong carbide forming elements, which can form fine dispersed carbide and improve the high-temperature strength of steel. Titanium, niobium and carbon compounds can also prevent intercrystalline corrosion of austenitic steel at high temperature or after welding.