stellite material (marketing department of Shanghai HY Industry Co., Ltd)
Stellite material can be classified according to its application into Stellite wear resistant alloys, Stellite high temperature resistant alloys and Stellite wear and aqueous corrosion resistant alloys. Under general conditions of use, they are actually both wear and high temperature resistant or wear and corrosion resistant, and some conditions may require high temperature, wear and corrosion resistance at the same time.
Typical grades of Stellite alloys
There are: Stellite1, Stellite4, Stellite6, Stellite8, Stellite12, Stellite20, Stellite21, Stellite31, Stellite100 and so on. In China, Stellite high-temperature alloys are mainly studied in depth and thoroughly. Unlike other high-temperature alloys, Stellite high-temperature alloys are not strengthened by the ordered precipitation phase firmly bonded to the matrix, but by the austenite fcc matrix that has been solid solution strengthened and a small amount of carbide distributed in the matrix. Cast Stellite high temperature alloys, however, rely heavily on carbide strengthening. Pure cobalt crystals have a dense hexagonal (hcp) crystal structure below 417°C, which transforms to fcc at higher temperatures. relationship, but show superior resistance to thermal corrosion above 1000°C compared to other high temperatures, probably due to the high chromium content of the alloy, a characteristic of this type of alloy.
Development of Stellite alloys
The development of cobalt-based high-temperature alloys began in the late 1930s due to the need for turbochargers for piston engines, and in 1942 the United States first succeeded in making turbocharger blades from the dental metal Vitallium (Co-27Cr-5Mo-0.5Ti). During use the alloy became brittle due to the precipitation of carbide phases. In the late 1940s, X-40 and HA-21 were used to make cast turbine blades and guide vanes for aero-jet engines and turbochargers at operating temperatures up to 850-870°C. In 1953, the S-21 alloy was developed for use in forging turbine blades. S-816, used for forging turbine blades, is an alloy strengthened by solid solution with a variety of refractory elements. From the late 1950s to the late 1960s, four cast Stellite alloys were used extensively in the USA: WI-52, X-45, Mar-M509 and FSX-414. deformed Stellite alloys, mostly in sheet form, such as L-605, were used to make combustion chambers and ducts, and HA-188, which appeared in 1966, had improved oxidation resistance due to the presence of lanthanum. The development of Stellite alloys should take into account the availability of cobalt. Cobalt is an important strategic resource and most countries in the world are short of cobalt, so the development of Stellite alloys is limited.
stellite material alloy performance characteristics
General cobalt-based alloy lacks the reinforcing 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 resistance to thermal fatigue, thermal corrosion and wear corrosion resistance, and has good weldability when it is higher than 980℃. 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.
Main carbides in cobalt-based high-temperature alloys are MC, M23C6 and M6C in cast Stellite alloys, M23C6 precipitated at grain boundaries and between dendrites during slow cooling. In some alloys, fine M23C6 can form co-crystals with the matrix γ. MC carbides are too large to have a significant direct effect on dislocations and therefore do not have a significant strengthening effect on the alloy, whereas fine diffuse carbides have a good strengthening effect. Carbides located on grain boundaries (mainly M23C6) 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.
Stellite alloys are less often strengthened with intermetallic compounds because Co3 (Ti, Al), Co3Ta etc. are not stable at high temperatures, but in recent years the use of intermetallic compounds for strengthening Stellite alloys has also been developed. However, the use of intermetallic compounds for strengthening has also been developed in recent years.
Thermal stability of carbides in Stellite alloys is good. When the temperature rises, the rate of growth of carbide aggregation is slower than that of the γ-phase in nickel-based alloys, and the temperature of re-solution in the matrix is higher (up to 1100°C), so the strength of Stellite alloys generally decreases more slowly when the temperature rises.
Stellite material has very good resistance to thermal corrosion, and it is generally accepted that Stellite alloys are superior to nickel-based alloys in this respect because the sulphide 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 sulphur in cobalt is much lower than in nickel. And because most Stellite alloys contain higher levels of chromium than nickel-based alloys, a protective layer of Cr2O3 can be formed on the alloy surface to resist corrosion by alkali metal sulphides (e.g. Na2SO4). However, Stellite alloys are generally much less resistant to oxidation than nickel-based alloys
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