Stellite is a hard alloy that is resistant to all types of wear and corrosion and high temperature oxidation. Known as the cobalt-based alloy, stellite was invented in 1907 by American Elwood Hayness. Stellite alloys contain cobalt as a main component and contain a considerable amount of nickel, chromium, tungsten and a small amount of alloying elements such as molybdenum, niobium, tantalum, titanium and niobium, and occasionally iron. Depending on the composition of the alloy, they can be made into welding wire. The powder can be used for hard surface surfacing, thermal spraying, spray welding, etc., and can also be used for casting and forging parts and powder metallurgy parts.
According to the classification of use, stellite alloy can be divided into stellite wear-resistant alloy, stellite high-temperature alloy and stellite wear-resistant and aqueous corrosion alloy. Under normal working conditions, in fact, it is both wear-resistant, high-temperature resistant, wear-resistant and corrosion-resistant. Some working conditions may also require high temperature resistance, wear resistance and corrosion resistance, and the more complicated it is. Under the circumstance, the more the advantages of the stellite alloy can be reflected.
Typical grades for Stellite are: Stellite 1, Stellite 4, Stellite 6, Stellite 12, Stellite 20, Stellite 31, Stellite 100, and the like. In China, the research on the stellite superalloy is mainly deep and thorough. Unlike other superalloys, the stellite superalloy is not reinforced by an ordered precipitate phase that is firmly bonded to the matrix, but consists of an austenitic fcc matrix that has been solid solution strengthened and a small amount of carbide distributed in the matrix. Casting stellite superalloys rely heavily on carbide strengthening. The pure cobalt crystal is a close packed hexagonal (hcp) crystal structure below 417 ° C and is converted to fcc at a higher temperature. In order to avoid this transition in the use of the stellite superalloy, virtually all of the stellite alloy is alloyed with nickel to stabilize the structure from room temperature to melting point. Stellite has a flat fracture stress-temperature relationship, but exhibits excellent hot corrosion resistance at temperatures above 1000 °C, which may be due to the higher chromium content of the alloy. a feature.
In the late 1930s, cobalt-based superalloys began to be developed due to the need for turbochargers for piston aeroengines. In 1942, the United States first succeeded in making turbocharger blades with the dental metal material Vitallium (Co-27Cr-5Mo-0.5Ti). This alloy gradually precipitates out of the carbide phase and becomes brittle during use. Therefore, the carbon content of the alloy was reduced to 0.3% while 2.6% of nickel was added to increase the solubility of the carbide forming element in the matrix, thus developing into the HA-21 alloy. In the late 1940s, the X-40 and HA-21 produced aerospace jet engines and turbochargers for casting turbine blades and guide vanes, operating at temperatures up to 850-870 °C. S-816, used in 1953 as a forged turbine blade, is an alloy that is solid solution strengthened with a variety of refractory elements. From the late 1950s to the late 1960s, four types of cast stellite alloys were widely used in the United States: WI-52, X-45, Mar-M509 and FSX-414. The deformed stellite alloy is mostly sheet, such as L-605 used to make combustion chambers and conduits. HA-188, which appeared in 1966, improved its antioxidant properties due to its inclusion of antimony. The Soviet Union used to make guide vanes, the stellite alloy K4, which is equivalent to HA-21. The development of stellite alloys should take into account the resources of cobalt. Cobalt is an important strategic resource, and most countries in the world lack cobalt, which limits the development of stellite.
Generally, cobalt-based superalloys lack a coherent strengthening phase. Although the medium temperature strength is low (only 50-75% of nickel-based alloys), it has higher strength, good thermal fatigue resistance and hot corrosion resistance above 980 °C. And abrasion resistance, and has good weldability. Suitable for the production of air jet engines, industrial gas turbines, guide vanes and nozzle guide vanes for marine gas turbines, and diesel engine nozzles.
Carbide-fortified phase The most important carbide in cobalt-based superalloys is MC. M23C6 and M6C are found in cast stellite alloys. M23C6 precipitates between grain boundaries and dendrites when slowly cooled. In some alloys, fine M23C6 can form a co-crystal with the matrix γ. The MC carbide particles are too large to directly affect the dislocations directly, so the strengthening effect on the alloy is not obvious, and the finely dispersed carbides have a good strengthening effect. The carbides located on the grain boundaries (mainly M23C6) can prevent the grain boundary from slipping and improve the permanent strength. The microstructure of the cobalt-based superalloy HA-31 (X-40) is the dispersed strengthening phase (CoCrW)6. Type C carbide.
Topological close-packed phases, such as sigma phase and Laves, which are present in certain stellite alloys, are detrimental and can cause the alloy to become brittle. Stellite alloys are less strongly reinforced with intermetallic compounds because Co3 (Ti, Al), Co3Ta, etc. are not sufficiently stable at high temperatures, but stellite alloys which have been strengthened with intermetallic compounds have also developed in recent years.
The thermal stability of carbides in the stellite alloy is better. When the temperature rises, the carbide growth rate is slower than that of the γ phase in the nickel-based alloy, and the temperature of the redissolved matrix is also higher (up to 1100 ° C). Therefore, when the temperature rises, the temperature is too high. The strength of the vertical alloy is generally slower.
Stellite alloy has good resistance to hot corrosion. It is generally believed that the reason why stellite is superior to nickel-based alloy in this respect is that the melting point of cobalt sulfide (such as Co-Co4S3 eutectic, 877 °C) is better than nickel. The melting point of the sulfide (such as Ni-Ni3S2 eutectic 645 ° C) is high, and the diffusion rate of sulfur in cobalt is much lower than in nickel. Moreover, since most of the stellite alloys have a higher chromium content than the nickel-based alloys, an alkali metal sulphate (such as a Cr2O3 protective layer etched by Na2SO4) can be formed on the surface of the alloy. However, the resistance of the stellite alloy is usually much lower than that of the nickel-based alloy.
Early stellite alloys were produced using non-vacuum smelting and casting processes. Later developed alloys, such as Mar-M509 alloy, were produced by vacuum smelting and vacuum casting because they contained more active elements such as zirconium and boron.
The size and distribution of carbide particles in the stellite alloy and the grain size are sensitive to the casting process. In order to achieve the required permanent strength and thermal fatigue properties of the cast stellite components, the casting process parameters must be controlled. The stellite alloy needs to be heat treated, mainly to control the precipitation of carbides. For the cast stellite alloy, firstly, the solution is treated at a high temperature, and the temperature is usually about 1150 ° C, so that all the primary carbides, including some MC type carbides, are dissolved in the solid solution; then the aging treatment is performed at 870-980 ° C. To re-precipitate the carbides (most commonly M23C6).
Surfacing of stellite alloy Sitali surfacing alloy contains 25-33% chromium, 3-21% tungsten and 0.7-3.0% carbon. With the increase of carbon content, the metallographic structure changed from hypoeutectic austenite + M7C3 eutectic to hypereutectic M7C3 nascent carbide + M7C3 eutectic. The more carbonaceous, the more the primary M7C3, the greater the macro hardness, the higher the abrasion resistance, but the impact resistance, weldability and machining performance will decrease. Stellite alloyed with chromium and tungsten has excellent oxidation resistance, corrosion resistance and heat resistance. Maintaining high hardness and strength at 650 ° C is an important feature distinguishing such alloys from nickel-based and iron-based alloys. After processing, the stellite alloy has low surface roughness, high scratch resistance and low friction coefficient, and is also suitable for adhesive wear, especially on sliding and contact valve sealing surfaces. However, in the case of high-stress abrasive wear, the low-carbon cobalt-chromium-tungsten alloy is not as wear-resistant as low-carbon steel. Therefore, the selection of expensive stellite alloy must be guided by professionals to maximize the potential of the material. .
There are also Sitaili surfacing alloys containing Laves phase, such as Co-28Mo-17Cr-3Si and Co-28Mo-8Cr-2Si, which are alloyed with chromium and molybdenum. Since Laves has a lower hardness than carbides, the material paired with the metal friction is less worn.
The wear of the alloy workpiece is largely affected by the contact or impact stress of the surface. Surface wear depends on the interaction of dislocation flow and contact surfaces under stress. For stellite alloys, this feature has a lower stacking fault energy with the matrix and the matrix structure is transformed from a face-centered cubic to a hexagonal close-packed crystal structure under the influence of stress or temperature, and has a hexagonal close-packed crystal structure. Metal materials, wear resistance is superior. In addition, the content, morphology and distribution of the second phase of the alloy, such as carbides, also have an effect on wear resistance. Since the alloy carbides of chromium, tungsten and molybdenum are distributed in the cobalt-rich matrix and some of the chromium, tungsten and molybdenum atoms are solid-solubilized in the matrix, the alloy is strengthened to improve wear resistance. In cast stellite alloys, the carbide particle size is related to the cooling rate, and the carbide particles are relatively fine when cooled. 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 (the carbide particles are fine), and the adhesive wear resistance is both There is no significant difference, indicating that coarse carbides contribute to improved abrasive wear resistance
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