Heat resistant alloy, also known as superalloy, is of great significance for industrial departments and application technology fields under high temperature conditions.
Generally speaking, the higher the melting point of metal materials, the higher the temperature limit that can be used. This is because with the increase of temperature, the mechanical properties of metal materials decrease significantly, and the trend of oxidation corrosion increases accordingly. Therefore, general metal materials can only work at 500 ℃~600 ℃ for a long time. The metal that can work at a high temperature higher than 700 ℃ is generally called heat-resistant alloy. "Heat resistance" means that it can maintain sufficient strength and good oxidation resistance under high temperature.
There are two ways to improve the oxidation resistance of steel: one is to add Cr, Si, Al and other alloying elements into the steel, or to carry out Cr, Si, Al alloying treatment on the surface of the steel. They can quickly form a dense oxide film in the oxidizing atmosphere and firmly attach to the steel surface, thus effectively preventing the oxidation from continuing. The other is to form high melting point oxide, carbide, nitride and other high temperature resistant coatings on the steel surface by various methods.
There are many ways to improve the high temperature strength of steel. From the chemical point of view of structure and property, there are roughly two main methods:
One is to increase the bonding force between atoms in steel at high temperatures. It is pointed out that the bond strength in metals is mainly related to the number of unpaired electrons in atoms. From the periodic table, the metal bond of element VI B is the strongest in the same period. Therefore, adding Cr, Mo, W atoms into the steel has the best effect.
The other is to add elements that can form various carbides or intermetallic compounds to strengthen the steel matrix. Carbides generated from a number of transition metals and carbon atoms belong to interstitial compounds. They add covalent bond components on the basis of metal bonds, so they have great hardness and high melting point. For example, adding W, Mo, V, Nb can generate WC, W2C, MoC, Mo2C, VC, NbC and other carbides, thus increasing the high temperature strength of steel.
In addition to iron based heat-resistant alloys, nickel based, molybdenum based, niobium based and tungsten based heat-resistant alloys can also be prepared by alloy method. They have good mechanical properties and chemical stability at high temperatures. Among them, nickel base alloy is the best super heat resistant metal material, and the matrix in the structure is Ni? Cr? The solid solution of Co and Ni3Al metal compound can be used at 1000 ℃~1100 ℃ after treatment.
