Stellite properties- High temperature resistance & corrosion resistance (Tech Center of Shanghai HY Industry Co., Ltd)
General cobalt-based high-temperature alloy lacks the strengthening phase of the common lattice, although the medium-temperature strength is low (only 50-75% of the 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 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 strengthening phase in Stellite properties: The main carbides in cobalt-based high-temperature alloys are MC, M23C6 and M6C. In cast Stellite alloys, M23, C6 are precipitated between grain boundaries and dendrites during slow cooling. In some alloys, the fine M23,C6 can form co-crystals with the matrix γ. The MC carbide particles are too large to have a direct and significant effect on dislocations and thus have little strengthening effect on the alloy, while the fine diffuse carbides have a good strengthening effect. Carbides located on grain boundaries (mainly M23, C6) 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 phases and Laves, which occur in some stellite alloys, are detrimental and can make the alloy brittle. The use of intermetallic compounds for strengthening is rare in Stellite alloys because Co3 (Ti, Al), Co3Ta, etc. are not stable at high temperatures, but in recent years, the use of intermetallic compounds for strengthening has been developed.
The thermal stability of carbides in stellite 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-solution in the matrix is also higher (up to 1100°C), so the strength of stellite alloys generally decreases more slowly when the temperature rises.
It is generally believed that stellite 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 stellite alloys contain higher amounts of chromium than nickel-based alloys, they can form a protective layer of Cr2O3 on the alloy surface that resists corrosion by alkali metal sulfides (such as Na2SO4). However, the oxidation resistance of stellite alloys is usually much lower than that of nickel-based alloys. Early stellite alloys were produced by non-vacuum smelting and casting processes. Later developed alloys, such as Mar-M509 alloy, can effectively avoid the problem by vacuum smelting and vacuum casting production because of the high content of active elements such as zirconium and boron.
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