Cobalt-based high-temperature alloys
Cobalt-based high-temperature alloys are austenitic high-temperature alloys containing 40 to 65% cobalt. It has certain high temperature strength, good resistance to thermal corrosion and oxidation resistance under 730~1100 conditions. It is suitable for making guide vanes and nozzle blades of aviation jet engines, industrial gas turbines, naval gas turbines and diesel engine nozzles.
Cobalt-based high-temperature alloy is one of the high-temperature alloys, which is a kind of alloy with cobalt as the main component, containing a considerable amount of nickel, chromium, tungsten and a small amount of molybdenum, niobium, tantalum, titanium, lanthanum and other alloying elements, and occasionally also contains iron. Depending on the composition of the alloy, they can be made into welding wire, powder for hard surface surfacing, thermal spraying, spray welding and other processes, and can also be made into cast and forged parts and powder metallurgy parts.
The thermal stability of carbides in cobalt-based high-temperature alloys is good. When the temperature rises, the growth rate of carbide agglomeration is slower than the growth rate of γ-phase in nickel-based alloy, and the temperature of re-solution in the matrix is also higher (up to 1100℃), so the strength of cobalt-based alloy generally decreases slowly when the temperature rises.
Cobalt-based alloys have excellent resistance to thermal corrosion. It is generally believed that cobalt-based alloys are superior to nickel-based alloys in this respect because the sulfide melting point of cobalt (e.g., Co-Co4S3 eutectic, 877°C) is higher than that of nickel (e.g., Ni-Ni3S2 eutectic, 645°C), and the diffusion rate of sulfur in cobalt is much lower than in nickel. And because most cobalt-based alloys contain higher amounts of chromium than nickel-based alloys, a protective layer of Cr2O3 that resists corrosion by alkali metal sulfides (such as Na2SO4) can be formed on the surface of the alloy. However, the oxidation resistance of cobalt-based high-temperature alloys is usually much lower than that of nickel-based alloys.
Early cobalt-based alloys were produced by non-vacuum smelting and casting processes. Later developed alloys, such as Mar-M509 alloy, are produced by vacuum smelting and vacuum casting because they contain more active elements such as zirconium and boron.
The size and distribution of carbide particles and grain size in cobalt-based high-temperature alloys are sensitive to the casting process. In order to achieve the required lasting strength and thermal fatigue properties of cast cobalt-based alloy parts, the casting process parameters must be controlled. Cobalt-based high-temperature alloys need to be heat treated, mainly to control the precipitation of carbide. For cast cobalt-based high-temperature alloys, firstly, high-temperature solution treatment is carried out, usually at a temperature of about 1150°C, so that all primary carbides, including some MC-type carbides, are dissolved into the solid solution; then aging treatment is carried out at 870-980°C, so that carbides (most commonly M23C6) can precipitate again.
Wear of alloy workpieces is largely influenced by contact or impact stresses on their surfaces. Surface wear under stress depends on the dislocation flow and the interaction characteristics of the contacting surfaces. For cobalt-based high-temperature alloys, this characteristic is related to the matrix having a low layer dislocation energy and the transformation of the matrix organization from face-centered cubic to hexagonal dense crystal structure under the influence of stress or temperature, and the wear resistance of metallic materials with hexagonal dense crystal structure is superior. In addition, the content, morphology and distribution of the second phase of the alloy, such as carbide, also have an effect on the wear resistance. The alloy carbides of chromium, tungsten and molybdenum are distributed in the cobalt-rich matrix as well as some chromium, tungsten and molybdenum atoms are solidly soluble in the matrix, which strengthens the alloy and thus improves the wear resistance. In cast cobalt-based alloys, the carbide particle size is related to the cooling rate, and the carbide particles are finer when cooling is fast. In sand casting, the hardness of the alloy is lower and the carbide particles are coarser. In this state, the abrasive wear resistance of the alloy is significantly better than that of graphite casting (finer carbide particles), while there is no significant difference between the two in terms of adhesive wear resistance, indicating that the coarser carbides are beneficial to improving the resistance to abrasive wear.
Cobalt-based high-temperature alloy is one of the high-temperature alloys, which is a kind of alloy with cobalt as the main component, containing a considerable amount of nickel, chromium, tungsten and a small amount of molybdenum, niobium, tantalum, titanium, lanthanum and other alloying elements, and occasionally also contains iron. Depending on the composition of the alloy, they can be made into welding wire, powder for hard surface surfacing, thermal spraying, spray welding and other processes, and can also be made into cast and forged parts and powder metallurgy parts.
The thermal stability of carbides in cobalt-based high-temperature alloys is good. When the temperature rises, the growth rate of carbide agglomeration is slower than the growth rate of γ-phase in nickel-based alloy, and the temperature of re-solution in the matrix is also higher (up to 1100℃), so the strength of cobalt-based alloy generally decreases slowly when the temperature rises.
Cobalt-based alloys have excellent resistance to thermal corrosion. It is generally believed that cobalt-based alloys are superior to nickel-based alloys in this respect because the sulfide melting point of cobalt (e.g., Co-Co4S3 eutectic, 877°C) is higher than that of nickel (e.g., Ni-Ni3S2 eutectic, 645°C), and the diffusion rate of sulfur in cobalt is much lower than in nickel. And because most cobalt-based alloys contain higher amounts of chromium than nickel-based alloys, a protective layer of Cr2O3 that resists corrosion by alkali metal sulfides (such as Na2SO4) can be formed on the surface of the alloy. However, the oxidation resistance of cobalt-based high-temperature alloys is usually much lower than that of nickel-based alloys.
Early cobalt-based alloys were produced by non-vacuum smelting and casting processes. Later developed alloys, such as Mar-M509 alloy, are produced by vacuum smelting and vacuum casting because they contain more active elements such as zirconium and boron.
The size and distribution of carbide particles and grain size in cobalt-based high-temperature alloys are sensitive to the casting process. In order to achieve the required lasting strength and thermal fatigue properties of cast cobalt-based alloy parts, the casting process parameters must be controlled. Cobalt-based high-temperature alloys need to be heat treated, mainly to control the precipitation of carbide. For cast cobalt-based high-temperature alloys, firstly, high-temperature solution treatment is carried out, usually at a temperature of about 1150°C, so that all primary carbides, including some MC-type carbides, are dissolved into the solid solution; then aging treatment is carried out at 870-980°C, so that carbides (most commonly M23C6) can precipitate again.
Wear of alloy workpieces is largely influenced by contact or impact stresses on their surfaces. Surface wear under stress depends on the dislocation flow and the interaction characteristics of the contacting surfaces. For cobalt-based high-temperature alloys, this characteristic is related to the matrix having a low layer dislocation energy and the transformation of the matrix organization from face-centered cubic to hexagonal dense crystal structure under the influence of stress or temperature, and the wear resistance of metallic materials with hexagonal dense crystal structure is superior. In addition, the content, morphology and distribution of the second phase of the alloy, such as carbide, also have an effect on the wear resistance. The alloy carbides of chromium, tungsten and molybdenum are distributed in the cobalt-rich matrix as well as some chromium, tungsten and molybdenum atoms are solidly soluble in the matrix, which strengthens the alloy and thus improves the wear resistance. In cast cobalt-based alloys, the carbide particle size is related to the cooling rate, and the carbide particles are finer when cooling is fast. In sand casting, the hardness of the alloy is lower and the carbide particles are coarser. In this state, the abrasive wear resistance of the alloy is significantly better than that of graphite casting (finer carbide particles), while there is no significant difference between the two in terms of adhesive wear resistance, indicating that the coarser carbides are beneficial to improving the resistance to abrasive wear.
The difference between Constantan alloy and Mn Cu alloy
Powder metallurgy iron-based materials
Types and uses of iron-based materials
What material is nickel base alloy?
What kind of tools are used for machining nickel-based alloys?
Nickel-based alloy hardness
Precautions for welding nickel-based alloys
Nickel-based alloy applications
Cobalt-based alloys