High temperature oxidation resistance
The high-temperature oxidation resistance of metal refers to the oxidation resistance of steel at high temperatures. Oxidation is a typical chemical corrosion. The chemical reaction between metal and oxygen is oxidative corrosion. The corrosion product (oxide film) adheres to the metal surface in an oxidizing atmosphere such as high-temperature air and combustion gas. With the development of oxidation, the thickness of the oxide film continues to increase. Whether metal oxidation continues to oxidize after reaching a certain level directly depends on the performance of the oxide film on the metal surface. If a dense and stable oxide film is formed, and the oxide film has high binding force and high strength with the base metal, it can prevent oxygen atoms from diffusing into the metal and reduce the oxidation rate. Otherwise, oxidation will be accelerated, causing the metal surface to peel off, leading to early failure of the parts.
The composition of the oxide layer on the steel surface is related to temperature. The oxide layer is composed of dense Fe2O3+Fe3O4, which can effectively prevent oxygen diffusion below 570°C. When heated to above 570°C, the oxide film is composed of FeO+Fe2O3+Fe3O4 from the inside to the outside. FeO is loose and porous, accounting for approximately 90% of the entire oxide film thickness. Metal atoms and oxygen atoms easily diffuse in the FeO layer and accelerate oxidation. High temperature FeO greatly reduces the oxidation resistance of steel. The higher the temperature, the faster the atoms diffuse and the faster the oxidation rate.
The main method to improve the oxidation resistance of steel is to add alloying elements such as chromium, silicon and aluminum. Therefore, when steel contacts oxygen at high temperatures, it forms a dense high-melting point oxidation film of Cr2O3, silica and alumina, which covers The surface of the steel is tight, preventing it from further oxidation.
High temperature strength
The high temperature strength of metal refers to the ability of metal materials to resist mechanical load at high temperatures, that is, the ability of metal materials to resist plastic deformation and damage at high temperatures. The mechanical properties of metals at high temperatures are very different from those at room temperature. When the working temperature is greater than the recrystallization temperature, the metal undergoes plastic deformation and work hardening. In addition to being affected by external forces, recrystallization and softening will also occur.
The mechanical properties of metals at high temperatures are related to temperature, time and structure. Creep often occurs at high temperatures, 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 over time. The higher the creep resistance of a metal, the higher its high temperature strength.
The high temperature strength of metal is generally expressed by creep limit and durability strength. The creep limit is the stress value when the residual deformation of a 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 within a certain period of time under constant temperature conditions. At high temperatures, the grain boundary strength of metal materials is lower than that of internal grains, so adding alloying elements increases the recrystallization temperature and forms stable special carbides. Using coarse-grained materials to reduce grain boundaries can effectively improve the high-temperature strength of steel.
Mo is a key alloy element in creep-resistant ferritic steel, which has an operating temperature of up to 530°C. Solid solution molybdenum can effectively reduce the creep rate of steel. Molybdenum can slow down the agglomeration and coarsening of carbides at high temperatures. Vanadium, titanium, and niobium are strong carbide-forming elements that can form fine dispersed carbides to improve the high-temperature strength of steel. Titanium, niobium and carbon compounds can also prevent intergranular corrosion of austenitic steels at high temperatures or after welding.
