What are high temp alloys?
High-temperature alloy is an alloy based on iron, nickel and cobalt elements. It is a metal material that can work for a long time at high temperatures above 600℃ and under certain stress; it has good comprehensive properties such as high-temperature strength, oxidation resistance, corrosion resistance, fatigue performance, fracture toughness, etc. High-temperature alloy is a single austenite structure with good stability and reliability at various temperatures. Based on the above performance characteristics and high-content alloy, it is also called "high-temperature alloy". It is widely used in aviation, aerospace, petroleum, chemical industry and other fields, and is an important material for ships.
Classification of high-temperature alloys
Element type, alloy strengthening type, material formation method.
According to the matrix elements, high-temperature alloys are divided into iron-based, nickel-based, and cobalt-based high-temperature alloys. The operating 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 and are widely used in the manufacture of aviation jet engines and the hottest parts of various industrial gas turbines.
Classification by matrix element type
▪ Fe-Ni-Cr/ Fe-Cr-Mn high temperature alloy
Iron-based high temperature alloy can also be called heat-resistant alloy steel, which is formed by adding a small amount of alloying elements such as Ni and Cr to Fe. Heat-resistant alloy steel can be divided into martensitic, austenitic, pearlitic and ferritic heat-resistant steel according to its normalizing requirements.
▪ Nickel-based high temperature alloy
Nickel-based high temperature alloy has a nickel content of 50% or more, and its creep resistance, compressive strength and yield strength can be significantly improved by solid solution and aging treatment. At present, the application range of nickel-based high temperature alloy far exceeds that of iron-based and cobalt-based high temperature alloys. Many turbine engines, even the turbine blades and combustion chambers of turbochargers are made of nickel-based alloys.
▪ Cobalt-based high temperature alloy
The composition of cobalt-based high temperature alloy is about 60%, and the addition of elements such as Cr and Ni can improve its heat resistance. Although this type of high temperature alloy has good heat resistance, the yield of cobalt is low and the processing is difficult. It is usually used to manufacture parts that are exposed to high temperatures (600~1000℃) and extremely complex stress environments for a long time, such as the working blades, turbine disks, combustion chambers and hot end parts of aircraft engines. In order to obtain better heat resistance, elements such as W, Mo, Ti, Al and Co are generally added during the preparation process to ensure its excellent heat resistance and fatigue resistance.
Classification by alloy strengthening type
According to the alloy strengthening type, high temperature alloys can be divided into solid solution strengthening high temperature alloys and aging precipitation strengthening high temperature alloys.
▪ Solid solution strengthening high temperature alloy
Solid solution strengthening high temperature alloy refers to the addition of certain 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. Certain solute atoms can reduce the delamination energy of the alloy system, increase the decomposition tendency of dislocations, and make lateral slip difficult.
▪ Aged precipitation strengthening high temperature alloy
The so-called aging precipitation strengthening refers to a heat treatment process in which the alloy workpiece is subjected to solid solution treatment and then cold plastic deformation to maintain its performance at a higher temperature or room temperature. For example, the maximum yield strength of Inconel 718 alloy at 650℃ is 1 000 MPa, and the alloy can be prepared at 950℃.
Classification by material forming method
According to the material forming method, it is divided into cast high-temperature alloys (including ordinary casting alloys, single crystal alloys, directional alloys, etc.), deformed high-temperature alloys, and powder metallurgy high-temperature alloys (including ordinary powder metallurgy and oxide dispersion strengthened high-temperature alloys).
With its excellent comprehensive performance, high-temperature alloys are widely used in the fields of aerospace and energy, and have become an irreplaceable key material for the hot end components of aviation engines, and its usage accounts for 40%~60% of the total engine. Taking Inconel 718 alloy as an example, it is the most widely used grade, mainly used for main parts such as bolts, compressors, impellers and oil covers of turboshaft engines. In addition, it is also used for casings, rings, afterburners and nozzles. The superheaters and reheaters of high-parameter ultra-supercritical power boilers of coal-fired power plants use high-temperature alloy tubes with good creep resistance and excellent corrosion resistance. Turbine blades and guide vanes of gas-fired power plants, nuclear power heat pipe steam generators, etc.
