What is Cobalt Based Alloys?

What is Castings Produced of Cobalt Based Alloys?

Cobalt-based alloy is a hard alloy that can withstand various types of wear, corrosion and high temperature oxidation. Cobalt-based alloys are based on cobalt as the main component, containing a considerable amount of nickel, alloying chemical elements such as chromium, tungsten and a small amount of alloying elements such as molybdenum, niobium, tantalum, titanium, lanthanum, and occasionally iron. According to the different composition of the alloy, the cobalt-based alloy can be made into welding wire, and the powder can be used for hard-surface welding, thermal spraying, spray welding and other processes, and it can also be made into castings, forgings and powder metallurgy parts. Classified by the end usage, cobalt-based alloys can be divided into cobalt-based wear-resistant alloys, cobalt-based high-temperature alloys and cobalt-based solution corrosion-resistant alloys. In general operating conditions, they are both wear-resistant and high-temperature resistant or wear-resistant and corrosion-resistant. Some operating conditions may also require high temperature, wear and corrosion resistance at the same time. The more complex the working conditions, the more obvious the advantages of cobalt-based alloys.

Properties of Cobalt-Based Alloys

The main carbides in cobalt-based superalloys are MC, M23C6 and M6C. In cast cobalt-based alloys, M23C6 is precipitated between grain boundaries and dendrites during slow cooling. In some alloys, the fine M23C6 can form a eutectic with the matrix γ. MC carbide particles are too large to directly have a significant effect on dislocations, so the strengthening effect on the alloy is not obvious, while finely dispersed carbides have a good strengthening effect. The carbides located on the grain boundary (mainly M23C6) can prevent the grain boundary slip, thereby improving the endurance strength. The microstructure of the cobalt-based superalloy HA-31 (X-40) is a dispersed strengthening phase (CoCrW)6 C-type carbide. The topological close packed phases that appear in some cobalt-based alloys, such as sigma phase is harmful and make the alloy brittle.

The thermal stability of carbides in cobalt-based alloys is good. When the temperature rises, the growth rate of carbide accumulation is slower than the growth rate of the γ phase in the nickel-based alloy, and the temperature of re-dissolving into the matrix is also higher (up to 1100°C). Therefore, when the temperature rises, the cobalt-based alloy The strength of the alloy generally decreases slowly. Cobalt-based alloys have good thermal corrosion resistance. 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-Co4S3 eutectic, 877℃) is higher than that of nickel ( For example, Ni-Ni3S2 eutectic (645°C) is high, and the diffusion rate of sulfur in cobalt is much lower than that in nickel. And because most cobalt-based alloys have higher chromium content than nickel-based alloys, they can form a protective layer of alkali metal sulfate (such as a Cr2O3 protective layer that is corroded by Na2SO4) on the surface of the alloy. However, the oxidation resistance of cobalt-based alloys is generally much lower than that of nickel-based alloys.

Different from other superalloys, cobalt-based superalloys are not strengthened by an ordered precipitation phase firmly bonded to the matrix, but are composed of an austenite fcc matrix that has been solid solution strengthened and a small amount of carbides distributed in the matrix. Casting cobalt-based superalloys relies heavily on carbide strengthening. Pure cobalt crystals have a hexagonal close packed (hcp) crystal structure below 417°C, which transforms to fcc at higher temperatures. In order to avoid this transformation during use of cobalt-based superalloys, practically all cobalt-based alloys are alloyed with nickel in order to stabilize the structure from room temperature to melting point temperature. Cobalt-based alloys have a flat fracture stress-temperature relationship, but show superior thermal corrosion resistance at temperatures above 1000°C than other high temperatures.

Heat Hreatment of Cobalt-based Alloys

The size and distribution of carbide particles and grain size in cobalt-based alloys are very sensitive to the casting process. In order to achieve the required endurance strength and thermal fatigue properties of cast cobalt-based alloy casting parts, the casting process parameters must be controlled. Cobalt-based alloys need heat treatment, mainly to control the precipitation of carbides. For cast cobalt-based alloys, first carry out high-temperature solid solution treatment, usually at a temperature of about 1150°C, so that all primary carbides, including some MC-type carbides, are dissolved into solid solution; then, aging treatment is carried out at 870-980°C. Make carbides precipitate again.

Common Grades of Cobalt-based Alloys

The typical grades of common cobalt-based high temperature alloys are: 2.4778 (according to DIN EN 10295)Hayness 188, Haynes 25 (L-605), Alloy S-816, UMCo-50, MP-159, FSX-414, X-40, Stellite 6B, Grade 31, etc. , Chinese brands are: GH5188 (GH188), GH159, GH605, K640, DZ40M and so on.

Applications of Cobalt-based Alloy Castings

Generally, cobalt-based superalloys lack coherent strengthening phases. Although the strength at medium temperature is low (only 50-75% of nickel-based alloys), they have higher strength, good thermal fatigue resistance, abrasion resistance, better weldabilityand thermal corrosion resistance above temperature of 980°C. Therefore, cobalt-based alloy castings are mainly suitable for making guide vanes and nozzle guide vanes for aviation jet engines, industrial gas turbines, naval gas turbines, and diesel engine nozzles, etc.

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