Cobalt-chromium-tungsten alloy (CoCrW)
Cobalt-chromium-tungsten alloy (CoCrW) is one of the (Stellite) alloys. Stalli alloy is a kind of cemented carbide which can resist various types of wear and corrosion and oxidation at high temperature. Commonly referred to as cobalt-based alloys. Stalli alloy was Co-Cr binary alloy at first, and later developed into Co-Cr-W ternary composition. Cobalt-chromium-tungsten alloys contain a considerable amount of chromium, tungsten and a small amount of nickel, molybdenum, silicon, carbon, niobium, tantalum and other alloy elements, and occasionally also contain iron. According to the composition of alloys, they can be made into welding wire, powder can be used for hard surface surfacing, thermal spraying, spray welding, etc., also can be made into castings, forgings and powder metallurgy parts. The main carbides in cobalt-chromium-tungsten alloy are MC, M23C6 and M6C. In cast cobalt-chromium-tungsten alloy, M23C6 precipitates at grain boundary and interdendrite during slow cooling. In some alloys the fine M23C6 can form eutectic with the matrix γ. The MC carbide particles are too large to have a significant effect on the dislocation so the strengthening effect on the alloy is not obvious. However, the fine dispersed carbides have a good strengthening effect. The carbide on the grain boundary (mainly M23C6) can prevent the grain boundary slip and thus improve the rupture strength. The microstructure of Co-Cr-W superalloy HA-31 (X _ (?) _ (40) is dispersed strengthening phase (CoCrW) ~ (6 C) carbides). In some Co-Cr-W alloys, the topological dense phase, such as Sigma phase and Laves phase, is harmful and will make the alloy brittle. Cobalt-chromium-tungsten alloys are rarely strengthened by intermetallic compounds, because Co3 (Ti,Al) and Co3Ta are not stable enough at high temperature, but non-cobalt-chromium-tungsten alloys strengthened by intermetallic compounds are also developed. The thermal stability of carbides in Co-Cr-W alloy is better. When the temperature rises, the growth rate of carbides is slower than that of γ phase in Ni-based alloy, and the temperature of re-dissolving in matrix is also higher (up to 1100 ℃), so when the temperature rises, the growth rate of γ phase in Ni-based alloy is higher than that in Ni-based alloy. The strength drop of Co-Cr-W alloy is generally slow. Cobalt-chromium-tungsten alloys have good thermal corrosion resistance. It is generally believed that cobalt-chromium-tungsten alloys are superior to nickel-based alloys in this respect because of the sulphide melting point of cobalt (such as Co-Co4S3 eutectic). The sulphide melting point of nickel is higher than that of nickel (such as Ni-Ni3S2 eutectic 645 ℃), and the diffusivity of sulfur in cobalt is much lower than that in nickel. Because the chromium content of most cobalt-chromium-tungsten alloys is higher than that of nickel-based alloys, Cr2O3 protective layers of alkali metal sulfate (such as Na2SO4 corrosion resistance) can be formed on the surface of alloys. However, the oxidation resistance of cobalt-chromium-tungsten alloys is usually much lower than that of nickel-based alloys. Cobalt-chromium-tungsten alloy is widely used in locomotive diesel engine, nuclear power plant valve, marine diesel engine and all kinds of aircraft. Early Co-Cr-W alloys were produced by non-vacuum smelting and casting processes. Later developed alloys, such as Mar-M509 alloys, contain more active elements, such as zirconium, boron, etc., which are produced by vacuum smelting and vacuum casting. The common cobalt-chromium-tungsten alloy lacks coherent strengthening phase. Although the medium-temperature strength is low (only 50% of that of nickel-based alloy), it has high strength, good thermal fatigue resistance, thermal corrosion resistance and wear resistance when it is higher than 980 ℃. And has better weldability. Suitable for aero-jet engine, industrial gas turbine, ship gas turbine guide blade and nozzle guide vane, diesel engine nozzles and so on.
