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High temperature alloy refers to a type of metal material based on iron, nickel and cobalt, which can work for a long time at high temperature above 600℃ and under certain stress; and has high high temperature strength, good oxidation resistance and corrosion resistance, good fatigue performance, fracture toughness and other comprehensive properties. High temperature alloy is a single austenite structure, with good organizational stability and reliability at various temperatures.
Based on the above performance characteristics, and the high degree of alloying of high temperature alloy, it is also called "super alloy", which is an important material widely used in aviation, aerospace, petroleum, chemical industry, and ships. According to the matrix elements, high temperature alloys are divided into iron-based, nickel-based, cobalt-based and other high temperature alloys. The use temperature of iron-based high temperature alloys can generally only reach 750~780℃. For heat-resistant parts used at higher temperatures, nickel-based and refractory metal-based alloys are used. Nickel-based high temperature alloys occupy a particularly important position in the entire field of high temperature alloys. It is widely used to manufacture the hottest parts of aviation jet engines and various industrial gas turbines.
1. High temperature resistance and corrosion resistance
The high temperature resistance and corrosion resistance of high temperature alloys mainly depend on their chemical composition and organizational structure. Taking GH4169 nickel-based deformed high temperature alloy as an example, it can be seen that the GH4169 alloy has a high niobium content. The degree of niobium segregation in the alloy is directly related to the metallurgical process. The GH4169 matrix is a Ni-Gr solid solution. The Ni mass fraction is more than 50%, which can withstand a high temperature of about 1,000°C. Similar to the American grade Inconel718, the alloy consists of γ matrix phase, δ phase, carbide and strengthening phases γ' and γ″ phases. The chemical elements and matrix structure of GH4169 alloy show its strong mechanical properties. The yield strength and tensile strength are several times better than 45 steel, and the plasticity is also better than 45 Good steel. Stable lattice structure and a large number of strengthening factors construct its excellent mechanical properties.
2. High processing difficulty
Due to its complex and harsh working environment, the integrity of the processing surface of high-temperature alloys plays a very important role in the performance of their performance. However, high-temperature alloys are typical difficult-to-process materials. Their micro-strengthening items have high hardness, severe work hardening, high shear stress resistance and low thermal conductivity. The cutting force and cutting temperature in the cutting area are high. During the processing, problems such as low processing surface quality and very serious tool breakage often occur. Under general cutting conditions, the surface of high-temperature alloys will produce problems such as excessive hardening layer, residual stress, white layer, black layer, and grain deformation layer.
Main classification:
The traditional classification of high-temperature alloy materials can be carried out according to the following 3 methods: classification by matrix element type, alloy strengthening type, and material forming method.
1. According to the type of matrix element
(1) Iron-based high-temperature alloy
Iron-based high-temperature alloy can also be called heat-resistant alloy steel. Its matrix is Fe element, with a small amount of Ni and Cr added. According to the normalizing requirements, heat-resistant alloy steel can be divided into martensite, austenite, pearlite, ferrite heat-resistant steel, etc.
(2) Nickel-based high-temperature alloy
Nickel-based high-temperature alloy contains more than half of nickel and is suitable for working conditions above 1,000°C. The solid solution and aging processing process can greatly improve the creep resistance and compressive yield strength. Analyzing the high-temperature alloys used in high-temperature environments, the scope of use of nickel-based high-temperature alloys far exceeds that of iron-based and cobalt-based high-temperature alloys. Many turbine blades and combustion chambers of turbine engines, and even turbochargers, also use nickel-based alloys as preparation materials.
(3) Cobalt-based high-temperature alloy
Cobalt-based high-temperature alloys are based on cobalt, with a cobalt content of about 60%. At the same time, elements such as Cr and Ni need to be added to improve the heat resistance of the high-temperature alloy. Although this high-temperature alloy has good heat resistance and is difficult to process, it is not used in large quantities. Usually used in high temperature conditions (600 to 1000°C) and high-temperature parts that are subjected to extreme complex stress for a long time, such as the working blades, turbine disks, hot end parts of combustion chambers and aerospace engines of aircraft engines. In order to obtain better heat resistance, elements such as W, MO, Ti, Al and Co should be added during preparation under general conditions to ensure its superior heat resistance and fatigue resistance.
2. Alloy strengthening type
According to the alloy strengthening type, high-temperature alloys can be divided into solid solution strengthening type high-temperature alloys and aging precipitation strengthening alloys.
(1) Solid solution strengthening type
The so-called solid solution strengthening type is to add some alloying elements to iron, nickel or cobalt-based high-temperature alloys to form a single-phase austenite structure. The solute atoms distort the solid solution matrix lattice, increase the slip resistance in the solid solution and strengthen it. Some solute atoms can reduce the stacking fault energy of the alloy system, increase the tendency of dislocation decomposition, make cross slip difficult, strengthen the alloy, and achieve the purpose of strengthening the high-temperature alloy.
(2) Aging precipitation strengthening
The so-called aging precipitation strengthening is a heat treatment process in which the alloy workpiece is subjected to solid solution treatment, cold plastic deformation, and then placed at a higher temperature or kept at room temperature to maintain its performance. For example: GH4169 alloy, the maximum yield strength at 650℃ is 1000 MPa, and the alloy temperature for making blades can reach 950℃.
3. Material forming method
The material forming method is divided into: casting high temperature alloy (including ordinary casting alloy, single crystal alloy, directional alloy, etc.), deformation high temperature alloy, powder metallurgy high temperature alloy (including ordinary powder metallurgy and oxide dispersion strengthening high temperature alloy).
(1) Casting high temperature alloy
The alloy material that directly prepares parts by casting method is called casting high temperature alloy. According to the alloy matrix composition, it can be divided into three types: iron-based casting high temperature alloy, nickel-based casting high temperature alloy and cobalt-based casting high temperature alloy. According to the crystallization method, it can be divided into four types: polycrystalline casting high temperature alloy, directional solidification casting high temperature alloy, directional eutectic casting high temperature alloy and single crystal casting high temperature alloy.
(2) Deformed high temperature alloy
It is still the most used material in aviation engines. Taking GH4169 alloy as an example, it is a major variety with the most applications at home and abroad. It is mainly used as the main parts of bolts, compressors and wheels, and oil slingers of turboshaft engines.
(3) New high-temperature alloys
Including powder high-temperature alloys, titanium-aluminum intermetallic compounds, oxide dispersion-strengthened high-temperature alloys, corrosion-resistant high-temperature alloys, powder metallurgy and nanomaterials and other sub-product areas.
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