stellitealloy (marketing department of Shanghai HY Industry Co., Ltd)
Stellitealloy is a cemented carbide resistant to various types of wear and corrosion and high temperature oxidation, commonly known as cobalt-based alloys. Stellitealloy was invented by American Elwood Hayness in 1907. Stellitealloy is a type of alloy with 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 for hard surfacing, thermal spraying, spray welding and other processes, and can also be made into castings and forgings and powder metallurgy parts.
Stellitealloy classification and main grades
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Classified by usage, Stellite alloy can be divided into Stellite wear-resistant alloy, Stellite high-temperature alloy and Stellite wear-resistant and aqueous solution corrosion alloy. In general working conditions, it is actually both wear-resistant and high-temperature resistant or wear-resistant and corrosion-resistant. Under such circumstances, the advantages of Stellitealloy can be better reflected.
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Typical grades of Stellitealloy are: Stellite1, Stellite4, Stellite6, Stellite8, Stellite12, Stellite20, Stellite21, Stellite31, Stellite100, etc. In our country, the research on Stellite superalloy is relatively in-depth and thorough (the typical research and promotion units in China are General Iron and Steel Research Institute and Beijing Rongpin Technology Co., Ltd.). Unlike other high-temperature alloys, Stellite superalloys are not strengthened by ordered precipitates firmly combined with the matrix, but are composed of austenite fcc matrix that has been solid-solution strengthened and a small amount of carbides distributed in the matrix. Cast Stellite superalloys rely heavily on carbide strengthening. Pure cobalt crystals have a hexagonal close-packed (hcp) crystal structure below 417 °C and transform into fcc at higher temperatures. In order to avoid this transformation of Stellite superalloys when they are used, practically all Stellite alloys are alloyed with nickel to stabilize the structure in the temperature range from room temperature to melting point. Stellitealloy has a flat fracture stress-temperature relationship, but above 1000°C shows superior hot corrosion resistance than other high temperatures, probably due to the alloy’s higher chromium content, a characteristic of this class of alloys .
Development of Stellitealloy
In the late 1930s, due to the needs of turbochargers for piston aeroengines, cobalt-based superalloys began to be developed. In 1942, the United States first used the dental metal material Vitallium (Co-27Cr-5Mo-0.5Ti) to make turbocharger blades successfully. During use, this alloy continuously precipitates carbide phases and becomes brittle. Therefore, the carbon content of the alloy was reduced to 0.3%, and 2.6% nickel was added at the same time to improve the solubility of carbide-forming elements in the matrix, thus developing into HA-21 alloy. In the late 1940s, X-40 and HA-21 were used to make aerojet engines and turbochargers to cast turbine blades and guide vanes, and their working temperature could reach 850-870°C. S-816, which appeared in 1953 for forging turbine blades, is an alloy strengthened by solid solution with various refractory elements. From the late 50’s to the late 60’s there were 4 cast Stellitealloys widely used in the US: WI-52, X-45, Mar-M509 and FSX-414. The deformed Stellitealloy is mostly sheet material, such as L-605, which is used to make combustion chambers and ducts. HA-188, which appeared in 1966, has improved antioxidant properties due to its lanthanum content. Stellitalloy ∏K4 used to make guide vanes in the Soviet Union is equivalent to HA-21. The development of Stellitealloy should consider the resources of cobalt. Cobalt is an important strategic resource. Most countries in the world lack cobalt, so that the development of Stellitealloy is limited.
Stellitalloy performance characteristics
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Generally, cobalt-based superalloys lack coherent strengthening phases. Although the medium-temperature strength is low (only 50-75% of nickel-based alloys), they have high strength, good thermal fatigue resistance, and thermal corrosion resistance when they are higher than 980°C. And corrosion resistance, and has good weldability. It is suitable for making guide vanes and nozzle guide vanes of aviation jet engines, industrial gas turbines, ship gas turbines and diesel engine nozzles.
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Carbide strengthening phase The most important carbides in cobalt-based superalloys are MC, M23C6 and M6C. In casting Stellitealloy, M23C6 is precipitated at grain boundaries and between dendrites during slow cooling. In some alloys, fine M23C6 can form eutectic with matrix γ. MC carbide particles are too large to directly have a significant impact on dislocations, so the strengthening effect on the alloy is not obvious, while fine and dispersed carbides have a good strengthening effect. The carbides (mainly M23C6) located on the grain boundaries can prevent the grain boundary slippage, thereby improving the durable strength. The microstructure of the cobalt-based superalloy HA-31 (X-40) is a dispersed strengthening phase (CoCrW)6 C-type carbide.
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Topological close-packed phases such as sigma phase and Laves that appear in some stellite alloys are harmful and will make the alloy brittle. Stellitealloy rarely uses intermetallic compounds for strengthening, because Co3 (Ti, Al), Co3Ta, etc. are not stable at high temperatures, but in recent years, Stellitealloy using intermetallic compounds for strengthening has also developed.
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Thermal stability of carbides in Stellitealloy is better. When the temperature rises, the growth rate of carbide agglomeration is slower than that of the γ-phase growth rate in nickel-based alloys, and the temperature for re-dissolving in the matrix is also higher (up to 1100°C). Therefore, when the temperature rises, Stellitealloy’s Intensity decline is generally slow.
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Stellitealloy has good thermal corrosion resistance. It is generally believed that the reason why Stellitealloy is superior to nickel-based alloys in this regard is that the melting point of cobalt sulfide (such as Co-Co4S3 eutectic, 877 ° C) is higher than the melting point of nickel sulfide (such as The Ni-Ni3S2 eutectic (645°C) is high, and the diffusivity of sulfur in cobalt is much lower than in nickel. And because most Stellitealloys contain more chromium than nickel-based alloys, they can form a protective layer against alkali metal sulfates (such as Cr2O3 corrosion by Na2SO4) on the surface of the alloy. However, Stellitealloy’s oxidation resistance is usually much lower than that of nickel-based alloys
Stellitealloy’s means of production
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Early Stellitealloys were produced using 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.
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The size and distribution of carbide particles and grain size in Stellitealloy are very sensitive to the casting process. In order to achieve the required permanent strength and thermal fatigue properties of cast Stellitealloy components, casting process parameters must be controlled. Stellitealloy needs heat treatment, mainly to control the precipitation of carbides. For casting Stellitealloy, high temperature solution treatment is firstly carried out, the temperature is usually about 1150 ° C, so that all primary carbides, including some MC carbides, are dissolved into solid solution; The substance (the most common is M23C6) re-precipitates.
