Stellite Cobalt-base Alloy is a kind of cemented carbide which can resist various types of wear and corrosion as well as high temperature oxidation. That is commonly known as cobalt chromium tungsten (molybdenum) alloy or Stellite alloy (Stellite alloy was invented by Elwood Hayness in 1907). Cobalt-based alloys consist mainly of cobalt, containing a considerable amount of nickel, chromium, tungsten, a small amount of molybdenum, niobium, tantalum, titanium and lanthanum, and occasionally iron alloys. Depending on the composition of the alloys, they can be made into welding wires, powder for hard surfacing, thermal spraying, spraying welding and other processes, as well as casting and forging and powder metallurgy parts. According to the usage, drill-based alloys can be divided into drill-based wear-resistant alloys, drill-based superalloy, drill-based wear-resistant alloys and water solution corrosion alloys. Usually, it is both wear-resistant and high temperature-resistant or wear-resistant and corrosion-resistant. In some cases, the workpiece may be required to bear high temperature-resistant, wear-resistant and corrosion-resistant at the same time. The more complex the working condition, the better the drill-based alloy will be. Cobalt based alloys are generally divided into two categories: cobalt chromium tungsten and cobalt chromium molybdenum. Cobalt chromium tungsten focuses on high temperature wear resistance; cobalt chromium molybdenum focuses on high-temperature corrosion resistance.
Designation and Application for Stellite Cobalt-base Alloy By EB
●EB Stellite 1 Cobalt-base Alloy are mainly used in valve seat and bearing seat,which hardness is HRC48-53.
●The hardness for EB Stellite 3 Cobalt-base Alloy By EB is HRC48-53. S3 parts are mainly used in the working condition without thermal and mechanical impact, such as pump bushing, rotary seal ring, wear-resistant gasket, bearing bushing, homogenizer parts, tinplate roll, plastic extrusion gasket.
●The hardness for EB Stellite 4 is HRC43-47. S4 alloy parts are used for mechanical impact heat, zinc plating equipment for hot extrusion and copper-aluminium alloy dies, bearing battery dies.
●The hardness for EB Stellite 6Z is greater than or equal to HRC37 for steam and chemical seats
●The hardness for EB Stellite 12Z is HRC45-50 for hot extrusion dies, galvanizing equipment bearings.
●The hardness for EB Stellite 20Z is HRC52-58 for non-thermal and mechanical impact occasions, such as pump bushes, rotary sealing rings, wear-resistant gaskets,bearing bushes, homogenizer parts, tinplate rolls, plastic extrusion liners.
●The hardness for EB Stellite 21Z is less than or equal to HRC34.S21 alloy is mainly used for steam turbine blades, brass forging dies, hot extrusion dies.
●The hardness for EB Stellite 31Z is less than or equal to HRC34 for steam turbine blades, bushes, hot extrusion dies.
●The hardness for EB Stellite 33Z is greater than or equal to HRC52 for high temperature corrosion resistant parts,such as food industry.
●The hardness for EB Stellite T800Z is greater than or equal to HRC50 for high temperature, strong corrosion bearing bushes, gate valves, gates and plastic extruder liners.
●The hardness for EB T400Z is between HRC52-55 for engine valve seat rings.
Stellite Cobalt-base Alloy Electrode Characteristics
Generally, Stellite Cobalt-base Alloy superalloys lack coherent strengthening phases. Although their strength at medium temperature is low (only 50-75% of that of nickel-based alloys), they have high strength, good thermal fatigue resistance, thermal corrosion resistance and wear resistance, and good weldability when they are higher than 980 C. It is suitable for making guide vanes and nozzle guide vanes of aero-jet engines, industrial gas turbines, naval gas turbines and diesel engine nozzles.
Carbide strengthening phase: The main carbides in cobalt-based superalloys are MC in cast cobalt-based alloys. M23C6 precipitates at grain boundaries and between dendrites during slow cooling. In some alloys, fine MC can form a eutectic with the substrate gamma. MC carbide particles are too large to directly affect dislocation, so the strengthening effect of the alloy is not obvious, while fine dispersed carbides have a good strengthening effect. Carbides (mainly MC) located at grain boundaries can prevent grain boundary slip and improve the rupture strength. The microstructure of Co-based superalloy HA-31 (X-40) is dispersed reinforced phase (CoCrW) C-type carbide.
Topological dense-packed phases, such as Sigma and Laves, occur in some cobalt-based alloys, which are harmful and brittle. Cobalt-based alloys are hardly strengthened by intermetallic compounds because Co (Ti, Al) and CoTa are not stable at high temperatures, but cobalt-based alloys strengthened by intermetallic compounds have also developed in recent years.
The thermal stability of carbides in cobalt based alloys is better. When the temperature rises, the growth rate of carbide aggregation is slower than that of gamma phase in nickel-based alloys, and the temperature of re-dissolving in the matrix is higher (up to 1100 C). Therefore, the strength of cobalt-based alloys decreases slowly when the temperature rises.
Cobalt-based alloys have good thermal corrosion resistance. It is generally believed that the reason why cobalt-based alloys are superior to nickel-based alloys in this respect is that the melting point of cobalt sulfide (such as Co-CoS eutectic, 877 C) is higher than that of nickel sulfide (such as Ni-NiS eutectic 645 C), and the sulfur diffusivity in cobalt is much lower than that in nickel. Moreover, because most cobalt-based alloys contain more chromium than nickel-based alloys, alkali metal sulfates (such as CrO protective coating for NaSO corrosion) can be formed on the surface of the alloys. However, the oxidation resistance of cobalt based alloys is usually much lower than that of Ni based alloys. The early cobalt based alloys were produced by 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.
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