Cobalt-based alloy is a kind of hard alloy that can withstand various types of wear, corrosion and high temperature oxidation. It is commonly referred to as cobalt-chromium-tungsten (molybdenum) alloy or Stellite alloy (Stellite alloy was invented by American Elwood Hayness in 1907). Cobalt-based alloys are based on cobalt as the main component, containing a considerable amount of nickel, chromium, tungsten and a small amount of alloying elements such as molybdenum, niobium, tantalum, titanium, lanthanum, and occasionally iron. According to the different components in the alloy, they can be made into welding wire, powder used for hard-surface welding, thermal spraying, spray welding and other processes, and can also be made into castings and forgings and powder metallurgy parts.
Classification of cobalt based alloy:
Classified by use, cobalt-based alloys can be divided into cobalt-based wear-resistant alloys, cobalt-based high-temperature alloys, and cobalt-based wear-resistant and aqueous solution corrosion alloys. In general operating conditions, in fact, they are both wear-resistant, high-temperature resistant or wear-resistant and corrosion-resistant. Some operating conditions may also require high-temperature, wear-resistant and corrosion-resistant at the same time, and the more complex the work is Under these circumstances, the more it can reflect the advantages of cobalt-based alloys.
Typical grades and organization of cobalt based alloy:
In China, the research on cobalt-based high-temperature alloys is relatively in-depth and thorough (the typical research and promotion units in China are the Central Iron and Steel Research Institute and Shanghai HY Industry Co., Ltd, etc.). The typical grades of cobalt-based high temperature alloys are: Hayness188, Haynes25 (L-605), Alloy S-816, UMCo-50, MP-159, FSX-414, X-40, Stellite6B, etc. Chinese grades are: GH5188 (GH188 ), GH159, GH605, K640, DZ40M and so on.
Different from other superalloys, cobalt-based superalloys are not strengthened by an ordered precipitation phase firmly bonded to the matrix, but are composed of an austenite fcc matrix that has been solid solution strengthened and a small amount of carbides distributed in the matrix. Casting cobalt-based superalloys relies heavily on carbide strengthening. Pure cobalt crystals have a hexagonal close packed (hcp) crystal structure below 417°C, which transforms to fcc at higher temperatures. In order to avoid this transformation during use of cobalt-based superalloys, practically all cobalt-based alloys are alloyed with nickel in order to stabilize the structure from room temperature to melting point temperature. Cobalt-based alloys have a flat fracture stress-temperature relationship, but they show better thermal corrosion resistance than other high temperatures above 1000°C. This may be due to the higher chromium content of the alloy, which is the most important part of this type of alloy. A feature.
Development process of cobalt-based superalloys In the late 1930s, due to the need for turbochargers for piston aero-engines, the development of cobalt-based superalloys began. In 1942, the United States first succeeded in manufacturing turbocharger blades with dental metal material Vitallium (Co-27Cr-5Mo-0.5Ti). During use, this alloy continuously precipitates carbide phases and becomes brittle. Therefore, the carbon content of the alloy is reduced to 0.3%, and 2.6% of nickel is added to improve the solubility of the carbide forming elements in the matrix, thus developing into the HA-21 alloy. At the end of the 1940s, X-40 and HA-21 produced cast turbine blades and guide vanes for aviation jet engines and turbochargers, and their operating temperature could reach 850-870°C. S-816, which appeared in 1953 for forging turbine blades, is a solid solution strengthened alloy with a variety of refractory elements.
From the late 1950s to the late 1960s, four cast cobalt-based alloys were widely used in the United States: WI-52, X-45, Mar-M509 and FSX-414. Most of the deformed cobalt-based alloys are plates, such as L-605 used to make combustion chambers and ducts. HA-188, which appeared in 1966, has improved oxidation resistance due to its lanthanum content. The cobalt-based alloy ∏K4 used in the Soviet Union to make guide vanes is equivalent to HA-21. The development of cobalt-based alloys should consider cobalt resources. Cobalt is an important strategic resource. Most countries in the world lack cobalt, which limits the development of cobalt-based alloys.
High temperature and corrosion resistance of cobalt based alloy:
Generally, cobalt-based superalloys lack coherent strengthening phases. Although the strength at medium temperature is low (only 50-75% of nickel-based alloys), they have higher strength, good thermal fatigue resistance, and thermal corrosion resistance above 980°C. And abrasion resistance, and has better weldability. It is suitable for making guide vanes and nozzle guide vanes for aviation jet engines, industrial gas turbines, naval gas turbines, and diesel engine nozzles.
Carbide strengthening phase The most important carbides in cobalt-based superalloys are MC, M23C6 and M6C. In cast cobalt-based alloys, M23C6 is precipitated between grain boundaries and dendrites during slow cooling. In some alloys, the fine M23C6 can form a eutectic with the matrix γ. MC carbide particles are too large to directly have a significant effect on dislocations, so the strengthening effect on the alloy is not obvious, while fine and dispersed carbides have a good strengthening effect. The carbides located on the grain boundary (mainly M23C6) can prevent the grain boundary slip, thereby improving the endurance strength. The microstructure of the cobalt-based superalloy HA-31 (X-40) is a dispersed strengthening phase (CoCrW)6 C-type carbide.
Topological close packed phases that appear in some cobalt-based alloys, such as sigma phase and Laves, are harmful and make the alloy brittle. Cobalt-based alloys seldom use intermetallic compounds for strengthening, because Co3 (Ti, Al), Co3Ta, etc. are not stable at high temperatures, but in recent years, cobalt-based alloys that use intermetallic compounds for strengthening have also been developed.
Thermal stability of carbides in cobalt-based alloys is better. When the temperature rises, the growth rate of carbide accumulation is slower than the growth rate of the γ phase in the nickel-based alloy, and the temperature of re-dissolving into the matrix is also higher (up to 1100°C). Therefore, when the temperature rises, the cobalt-based alloy The strength of the alloy generally decreases slowly.
Cobalt-based alloys have good thermal corrosion resistance. It is generally believed that the reason why cobalt-based alloys are superior to nickel-based alloys in this respect is that the melting point of cobalt sulfide (such as Co-Co4S3 eutectic, 877℃) is higher than that of nickel. The melting point of the substance (such as Ni-Ni3S2 eutectic at 645°C) is high, and the diffusion rate of sulfur in cobalt is much lower than that in nickel. And because most cobalt-based alloys have higher chromium content than nickel-based alloys, it can form a protective layer of corrosion-resistant Cr2O3 on the surface of the alloys. However, the oxidation resistance of cobalt-based alloys is generally much lower than that of nickel-based alloys. Early cobalt-based alloys were produced by non-vacuum smelting and casting processes. The alloys developed later, such as Mar-M509 alloy, are produced by vacuum smelting and vacuum casting because they contain more active elements such as zirconium and boron.
Cobalt based alloy wear resistance:
Wear of alloy workpieces is largely affected by the contact stress or impact stress on the surface. Surface wear under stress depends on the interaction characteristics of dislocation flow and contact surface. For cobalt-based alloys, this feature is related to the lower stacking fault energy of the matrix and the transformation of the matrix structure from face-centered cubic to hexagonal close-packed crystal structure under the effect of stress or temperature. Metals with hexagonal close-packed crystal structure Material, abrasion resistance is better. In addition, the content, morphology and distribution of the second phase of the alloy, such as carbides, also have an impact on the wear resistance.
Because the alloy carbides of chromium, tungsten and molybdenum are distributed in the cobalt-rich matrix and part of the chromium, tungsten and molybdenum atoms are solid-dissolved in the matrix, the alloy is strengthened, thereby improving wear resistance. In cast cobalt-based alloys, the size of carbide particles is related to the cooling rate. Faster cooling means finer carbide particles. In sand casting, the hardness of the alloy is lower, and the carbide particles are also coarser. In this state, the abrasive wear resistance of the alloy is significantly better than that of graphite casting (fine carbide particles), while the adhesive wear resistance of both There is no significant difference, indicating that coarse carbides are beneficial to improve the ability of abrasive wear resistance.
Size and distribution of carbide particles and grain size in cobalt-based alloys are very sensitive to the casting process. In order to achieve the required endurance strength and thermal fatigue properties of cast cobalt-based alloy parts, the casting process parameters must be controlled. Cobalt-based alloys need heat treatment, mainly to control the precipitation of carbides. For cast cobalt-based alloys, first carry out high-temperature solid solution treatment, usually at a temperature of about 1150°C, so that all primary carbides, including some MC-type carbides, are dissolved into solid solution; then, aging treatment is carried out at 870-980°C. Make carbides (the most common M23C6) precipitate again.
Cobalt based alloy surfacing:
Cobalt-based surfacing alloy contains 25-33% chromium, 3-21% tungsten, and 0.7-3.0% carbon. As the carbon content increases, the metallographic structure changes from hypoeutectic austenite + M7C3 eutectic to hypereutectic M7C3 primary carbide + M7C3 eutectic. The more carbon content, the more nascent M7C3, the greater the macroscopic hardness, and the improved abrasive wear resistance, but the impact resistance, weldability, and machining performance will all decrease.
Co-based alloys alloyed with chromium and tungsten have 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. After machining, the cobalt-based alloy has low surface roughness, high scratch resistance and low friction coefficient. It is also suitable for adhesive wear, especially on sliding and contacting valve sealing surfaces. However, the wear resistance of low-carbon cobalt-chromium-tungsten alloys is not as good as that of low-carbon steel during high-stress abrasive wear. Therefore, the selection of expensive cobalt-based alloys must be guided by professionals in order to maximize the potential of the material. There are also cobalt-based surfacing alloys containing Laves phases alloyed with chromium and molybdenum, such as Co-28Mo-17Cr-3Si and Co-28Mo-8Cr-2Si. Due to the lower hardness of Laves compared to carbides, the mating materials wear less in the metal friction pair.
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