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2019-10-12

Heat resistant alloy

Heat resistant alloy alloys, also known as high temperature alloys, are of great significance for the industrial sector and application technology under high temperature conditions.

In general, the higher the melting point of a metal material, the higher the temperature limit at which it can be used. This is because as the temperature increases, the mechanical properties of the metal material decrease remarkably, and the tendency of oxidative corrosion increases accordingly. Therefore, the general metal materials can only work for a long time at 500 ° C to 600 ° C. Metals that can operate at temperatures higher than 700 °C are commonly referred to as heat resistant alloys. “Heat resistant” means that it maintains sufficient strength and good oxidation resistance at high temperatures.

There are two ways to improve the oxidation resistance of steel: one is to add alloying elements such as Cr, Si, and Al to steel, or to alloy Cr, Si, and Al on the surface of steel. They quickly form a dense oxide film in an oxidizing atmosphere and are firmly attached to the surface of the steel, thereby effectively preventing the oxidation from continuing. The second is to form high temperature melting oxide, carbide, nitride and other high temperature resistant coatings on the steel surface by various methods.

There are many ways to increase the high temperature strength of steel. From the chemical point of view of structure and nature, there are roughly two main methods:

One is to increase the bonding force between atoms in steel at high temperatures. Studies have shown that the binding force in a metal, that is, the strength of a metal bond, is mainly related to the number of unpaired electrons in the atom. From the periodic table, the metal key of the VIB element is the strongest in the same cycle. Therefore, it is most effective to add atoms such as Cr, Mo, and W to steel.

The second is to add elements that can form various carbides or intermetallic compounds to strengthen the steel matrix. Carbides formed from a number of transition metals and carbon atoms are interstitial compounds which, on the basis of metal bonds, add components of covalent bonds and are therefore extremely hard and have a high melting point. For example, the addition of W, Mo, V, Nb can generate carbides such as WC, W2C, MoC, Mo2C, VC, NbC, etc., thereby increasing the high temperature strength of steel.

The alloy method can be used to obtain nickel-based, molybdenum-based, sulfhydryl-based and tungsten-based heat-resistant alloys, which have good mechanical properties and chemical stability at high temperatures. Among them, nickel-based alloy is the best super-heat-resistant metal material. The matrix in the structure is Ni?Cr?Co solid solution and Ni3Al metal compound. After treatment, its use temperature can reach 1 000 °C ~ 1 100 °C.

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