Cobalt-based alloys
Cobalt-based alloy, a hard alloy that is resistant to all types of wear and corrosion as well as high temperature oxidation. It is commonly referred to as cobalt-chromium-tungsten (molybdenum) alloy or stearic.
Cobalt-based alloys are a class of alloys that contain cobalt as the main component, with significant amounts of nickel, chromium, tungsten and small amounts of molybdenum, niobium, tantalum, titanium, lanthanum and other alloying elements, as well as occasional 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, but also into cast and forged parts and powder metallurgy parts.
According to the use of classification, cobalt-based alloys can be divided into cobalt-based wear-resistant alloys, cobalt-based high-temperature resistant alloys and cobalt-based wear-resistant and aqueous corrosion-resistant alloys. Under general conditions of use, they are actually both wear and high temperature resistant or wear and corrosion resistant, and some conditions may require high temperature, wear and corrosion resistance at the same time, and the more complex the conditions, the more the advantages of cobalt-based alloys can be reflected.
The general cobalt-based high-temperature alloy lacks the reinforced phase of the common lattice, although the medium-temperature strength is low (only 50-75% of the nickel-based alloy), but in higher than 980 ℃ has a higher strength, good resistance to thermal fatigue, thermal corrosion and wear corrosion resistance, and has good weldability. Suitable for the production of aviation jet engines, industrial gas turbines, naval gas turbine guide vanes and nozzle guide vane and diesel engine nozzles, etc.
Carbide-strengthened phase. The most important carbides in cobalt-based high-temperature alloys are MC, M23C6 and M6C. In cast cobalt-based alloys, M23C6 is precipitated at grain boundaries and between dendrites during slow cooling. In some alloys, the fine M23C6 can form co-crystals with the matrix γ. The MC carbides are too large to have a direct and significant effect on dislocations and thus have little effect on the strengthening of the alloy, while the fine diffuse carbides have a good strengthening effect. Carbides located on grain boundaries (mainly M23C6) can prevent grain boundary slip, thus improving the lasting strength. The microstructure of cobalt-based high-temperature alloy HA-31 (X-40) is a diffuse strengthening phase of (CoCrW)6 C-type carbides.
Topologically dense phases such as sigma phase and Laves, which occur in some cobalt-based alloys, are detrimental and can make the alloy brittle. Cobalt-based alloys are less often strengthened with intermetallic compounds because Co3 (Ti, Al), Co3Ta, etc. are not stable at high temperatures, but in recent years there has been a development of cobalt-based alloys strengthened with intermetallic compounds.
The thermal stability of carbides in cobalt-based 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 alloys, and the temperature of re-solubilization in the matrix is also higher (up to 1100℃), so the strength of cobalt-based alloys 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 can be formed on the alloy surface to resist corrosion by alkali metal sulfides (such as Na2SO4). However, the oxidation resistance of cobalt-based 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 of the high content of active elements such as zirconium and boron.
Wear resistance
The wear of alloyed workpieces is largely influenced by the contact or impact stresses on their surfaces. The surface wear under stress depends on the dislocation flow and the interaction characteristics of the contact surfaces. For cobalt-based 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 overlay alloy contains 25-33% chromium, 3-21% tungsten and 0.7-3.0% carbon. As the carbon content increases, the metallographic organization changes from sub-eutectic austenite + M7C3 eutectic to per-eutectic M7C3 incipient carbide + M7C3 eutectic. The more carbon content, the more incipient M7C3, the greater the macro hardness, and the increased resistance to abrasive wear, but the impact resistance, weldability, and machinability are reduced. The cobalt-based alloy alloyed with chromium and tungsten has good oxidation resistance, corrosion resistance and heat resistance. It can still maintain high hardness and strength at 650℃, which is an important feature that distinguishes this type of alloy from nickel-based and iron-based alloys. Cobalt-based alloys have low surface roughness after machining, high scuff resistance and low friction coefficient, and are also suitable for adhesive wear, especially on sliding and contact valve sealing surfaces. However, in the case of high stress abrasive wear, the wear resistance of cobalt-chromium-tungsten alloy with low carbon content is not as good as that of low carbon steel. Therefore, the selection of expensive cobalt-based alloys must be guided by professionals in order to realize the maximum potential of the material. There are also foreign cobalt-based overlay alloys alloyed with chromium and molybdenum containing Laves phase, such as Co-28Mo-17Cr-3Si and Co-28Mo-8Cr-2Si. Due to the low hardness of Laves compared to carbide, the materials paired with it in the metal friction payment wear less
Cobalt-based alloys are a class of alloys that contain cobalt as the main component, with significant amounts of nickel, chromium, tungsten and small amounts of molybdenum, niobium, tantalum, titanium, lanthanum and other alloying elements, as well as occasional 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, but also into cast and forged parts and powder metallurgy parts.
According to the use of classification, cobalt-based alloys can be divided into cobalt-based wear-resistant alloys, cobalt-based high-temperature resistant alloys and cobalt-based wear-resistant and aqueous corrosion-resistant alloys. Under general conditions of use, they are actually both wear and high temperature resistant or wear and corrosion resistant, and some conditions may require high temperature, wear and corrosion resistance at the same time, and the more complex the conditions, the more the advantages of cobalt-based alloys can be reflected.
The general cobalt-based high-temperature alloy lacks the reinforced phase of the common lattice, although the medium-temperature strength is low (only 50-75% of the nickel-based alloy), but in higher than 980 ℃ has a higher strength, good resistance to thermal fatigue, thermal corrosion and wear corrosion resistance, and has good weldability. Suitable for the production of aviation jet engines, industrial gas turbines, naval gas turbine guide vanes and nozzle guide vane and diesel engine nozzles, etc.
Carbide-strengthened phase. The most important carbides in cobalt-based high-temperature alloys are MC, M23C6 and M6C. In cast cobalt-based alloys, M23C6 is precipitated at grain boundaries and between dendrites during slow cooling. In some alloys, the fine M23C6 can form co-crystals with the matrix γ. The MC carbides are too large to have a direct and significant effect on dislocations and thus have little effect on the strengthening of the alloy, while the fine diffuse carbides have a good strengthening effect. Carbides located on grain boundaries (mainly M23C6) can prevent grain boundary slip, thus improving the lasting strength. The microstructure of cobalt-based high-temperature alloy HA-31 (X-40) is a diffuse strengthening phase of (CoCrW)6 C-type carbides.
Topologically dense phases such as sigma phase and Laves, which occur in some cobalt-based alloys, are detrimental and can make the alloy brittle. Cobalt-based alloys are less often strengthened with intermetallic compounds because Co3 (Ti, Al), Co3Ta, etc. are not stable at high temperatures, but in recent years there has been a development of cobalt-based alloys strengthened with intermetallic compounds.
The thermal stability of carbides in cobalt-based 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 alloys, and the temperature of re-solubilization in the matrix is also higher (up to 1100℃), so the strength of cobalt-based alloys 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 can be formed on the alloy surface to resist corrosion by alkali metal sulfides (such as Na2SO4). However, the oxidation resistance of cobalt-based 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 of the high content of active elements such as zirconium and boron.
Wear resistance
The wear of alloyed workpieces is largely influenced by the contact or impact stresses on their surfaces. The surface wear under stress depends on the dislocation flow and the interaction characteristics of the contact surfaces. For cobalt-based 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 overlay alloy contains 25-33% chromium, 3-21% tungsten and 0.7-3.0% carbon. As the carbon content increases, the metallographic organization changes from sub-eutectic austenite + M7C3 eutectic to per-eutectic M7C3 incipient carbide + M7C3 eutectic. The more carbon content, the more incipient M7C3, the greater the macro hardness, and the increased resistance to abrasive wear, but the impact resistance, weldability, and machinability are reduced. The cobalt-based alloy alloyed with chromium and tungsten has good oxidation resistance, corrosion resistance and heat resistance. It can still maintain high hardness and strength at 650℃, which is an important feature that distinguishes this type of alloy from nickel-based and iron-based alloys. Cobalt-based alloys have low surface roughness after machining, high scuff resistance and low friction coefficient, and are also suitable for adhesive wear, especially on sliding and contact valve sealing surfaces. However, in the case of high stress abrasive wear, the wear resistance of cobalt-chromium-tungsten alloy with low carbon content is not as good as that of low carbon steel. Therefore, the selection of expensive cobalt-based alloys must be guided by professionals in order to realize the maximum potential of the material. There are also foreign cobalt-based overlay alloys alloyed with chromium and molybdenum containing Laves phase, such as Co-28Mo-17Cr-3Si and Co-28Mo-8Cr-2Si. Due to the low hardness of Laves compared to carbide, the materials paired with it in the metal friction payment wear less
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