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Cobalt-based alloy
Nov 16, 2017

Cobalt-based alloy

Cobalt based alloy are hardmetals that are resistant to all types of wear and corrosion and high temperature oxidation. Commonly known as cobalt chromium tungsten (molybdenum) alloy or Stellite alloy (Stellite alloy invented by the American Elwood Hayness in 1907). Cobalt-based alloy are based on cobalt as a major component, containing a considerable amount of nickel, chromium, tungsten and a small amount of molybdenum, niobium, tantalum, titanium, lanthanum and other alloying elements, and occasionally also contain a class of iron alloy. Depending on the composition of the alloy, they can be made into wire, powder for hard surfacing, thermal spraying, spray welding and other processes, can also be made into castings and forgings and powder metallurgy.

Basic Information

Alloy   materialCobalt   chromium tungsten (molybdenum)
Use Hard surfacing, thermal spraying, spray welding
MaterialSpecial
TemperatureHigh temperature and pressure

Classification

According to the use of the use of classification, cobalt-based alloys can be divided into cobalt-based wear-resistant alloys, cobalt-based high temperature alloys and cobalt-based abrasion and corrosion of aqueous alloys. Under normal operating conditions, both are actually wear-resistant high temperature or wear-resistant corrosion-resistant conditions, and some conditions may also require high temperature wear-resistant corrosion-resistant, and more in this complex work Under the circumstances, the more able to reflect the advantages of cobalt-based alloys.

Grade organization

In China, the research on cobalt-base superalloy is more thorough and thorough. (The typical research and promotion units in China are Steel Research Institute and Beijing Rongpin Science and Technology Co., Ltd., etc.). The typical grades of Co-based high temperature resistant alloys include Haynes 188, Haynes 25 (L-605), Alloy S-816, UMCo-50, MP- 159, FSX-414, X-40 and Stellite 6B. The Chinese grades are: GH5188 ), GH159, GH605, K640, DZ40M and the like. Unlike other superalloys, Co-based superalloys are not strengthened by an ordered precipitated phase that is firmly bonded to the matrix but rather consist of a matrix of austenitic fcc that has been solution-strengthened and a small amount of carbides distributed in the matrix. Casting cobalt-based superalloy is to a large extent rely on carbide strengthening. Pure cobalt crystals at 417 ° C are densely packed hexagonal (hcp) crystalline structures that are converted to fcc at higher temperatures. In order to avoid this transition during use of the cobalt-based superalloy, virtually all cobalt-based alloys are alloyed with nickel to stabilize the tissue from room temperature to the melting temperature. Cobalt-based alloys have a flat fracture stress-temperature relationship, but exhibit superior resistance to hot corrosion at temperatures above 1000 ° C compared to other high temperatures, probably because of the higher chromium content of the alloys A feature.

Development path

Cobalt-based superalloy development In the late 1930s, Co-based superalloys were started due to the turbocharger requirements for piston-aero engines. In 1942, the United States first made turbocharger blades with the dental metal Vitallium (Co-27Cr-5Mo-0.5Ti). In the course of this alloy carbide phase precipitation continued to become brittle. Therefore, the alloy carbon content reduced to 0.3%, while adding 2.6% nickel, in order to improve the solubility of carbide forming elements in the matrix, thus evolving into a HA-21 alloy. In the late 1940s, the X-40 and HA-21 were used to make aircraft jet and turbocharger cast turbine blades and guide vanes that operated at temperatures of 850 to 870 ° C. The S-816, which was used as a forged turbine blade in 1953, is an alloy solid-solution strengthened with a variety of refractory elements. From the late 1950s to the late 1960s, four types of cast cobalt-based alloys were widely used in the United States: WI-52, X-45, Mar-M509 and FSX-414. Deformed cobalt-based alloys are mostly plates, such as L-605 for the production of combustion chambers and ducts. HA-188, which appeared in 1966, has improved oxidation resistance due to lanthanum contained therein. The Soviet Union used to make guide vane cobalt-based alloy ΠK4, equivalent to HA-21. The development of cobalt-based alloys should consider the resources of cobalt. Cobalt is an important strategic resource. Cobalt-based alloys are limited in the development of cobalt-deficient cobalt in most countries in the world.

Performance

The general cobalt-base superalloy lacks a coherent reinforcing phase. Although the mid-temperature strength is low (only 50-75% of the nickel-based alloy), it has higher strength at temperatures above 980 ° C, good thermal fatigue resistance, hot corrosion resistance And abrasion resistance, and has good weldability. Suitable for making aviation jet engines, industrial gas turbines, guide vanes and nozzle guide vanes for marine gas turbines, and diesel engine nozzles.

Carbide-enhanced phase The most important carbides in the cobalt-based superalloy are MC, M23C6 and M6C in the cast cobalt-based alloy, and M23C6 precipitates between the grain boundaries and the dendrite during slow cooling. In some alloys, the fine M23C6 can form a co-crystal with the matrix γ. MC carbide particles are too large, can not have a direct impact on the dislocation, so the strengthening effect of the alloy is not obvious, while fine dispersed carbide has a good strengthening effect. The carbides (mainly M23C6) located on the grain boundaries can prevent the grain boundary from sliding and thus improve the long-term strength. The strengthening phase of the cobalt-based superalloy HA-31 (X-40) with dispersed microstructure is (CoCrW) 6 C-type carbide.

Topcoats that appear in some cobalt-based alloys, such as Sigma Phase and Laves, can be detrimental and can make the alloy brittle. Cobalt-based alloys are less strengthened with intermetallics because Co3 (Ti, Al), Co3Ta, etc. are not stable enough at high temperatures, but cobalt based alloys reinforced with intermetallics have also been developed in recent years.

Cobalt-based alloy carbide thermal stability is better. When the temperature rises, the carbide grows faster than the γ-phase in the nickel-based alloy, and re-dissolves in the matrix to a higher temperature (up to 1100 ° C). Therefore, when the temperature rises, the cobalt-based The strength of the alloy decreased generally slower.

Cobalt-based alloys have a very good resistance to hot corrosion. Cobalt-based alloys are generally considered superior to nickel-based alloys in this respect because the melting point of cobalt sulfides (eg, Co-Co4S3 eutectic, 877 ° C) The melting point of the material (such as Ni-Ni3S2 eutectic 645 ° C) is high and the diffusion of sulfur in cobalt is much lower than in nickel. And since most cobalt-based alloys contain a higher amount of chromium than nickel-based alloys, alkali metal sulfates (such as Na2SO4-attacked Cr2O3) can be formed on the alloy surface. However, cobalt-based alloys generally have much lower oxidation resistance than nickel-based alloys. Early cobalt-based alloys were produced using non-vacuum smelting and casting processes. Later developed alloy, such as the Mar-M509 alloy, due to containing more active elements zirconium, boron, etc., with vacuum smelting and vacuum casting production.

Durable

The wear of alloy workpieces is greatly influenced by the contact stresses or impact stresses on their surfaces. The surface wear under stress is dependent on the dislocation flow and the interaction of the contact surfaces. For cobalt-based alloys, this feature has a lower stacking fault with the matrix and the matrix structure is transformed from face-centered cubic to hexagonal close-packed crystal structure under the influence of stress or temperature. Metals with hexagonal close-packed crystal structure Material, wear resistance is better. In addition, the second phase of the alloy, such as carbide content, morphology and distribution also have an impact on the wear resistance. Due to chromium, tungsten and molybdenum alloy carbide distribution in the cobalt-rich matrix and some chromium, tungsten and molybdenum atoms solid solution in the matrix, so that the alloy is strengthened, thereby improving the wear resistance. In the casting of cobalt-based alloys, carbide particle size and cooling rate, rapid cooling carbide particles are relatively small. In the case of sand casting, the hardness of the alloy is lower and the carbide particles are coarser. In this state, the abrasion wear resistance of the alloy is obviously superior to that of the graphite type casting (fine carbide particles), and both the adhesive wear and abrasion resistance There is no significant difference, indicating that the coarse carbides help to improve the abrasion resistance of abrasives.

Heat treatment

The size and distribution of carbide particles in the Co-based alloy and the grain size are sensitive to the casting process and the casting process parameters have to be controlled in order to achieve the desired long-term strength and thermal fatigue properties of the cast Co-based alloy part. Cobalt-based alloys need to be heat treated, mainly to control the precipitation of carbides. For the cast cobalt-based alloy, the first high-temperature solution treatment, the temperature is usually about 1150 , so that all the primary carbides, including some MC-type carbide dissolved in solid solution; and then 870-980 aging treatment, Carbide (the most common is M23C6) re-precipitation.

Surfacing

Cobalt-based surfacing alloy containing chromium 25-33%, containing 3-21% tungsten, carbon 0.7-3.0%. , With the increase of carbon content, the microstructure changes from hypoeutectic austenite + M7C3 eutectic to hypereutectic M7C3 primary carbide + M7C3 eutectic. The more carbon, the more primary M7C3, macroscopically increased hardness, abrasive wear resistance increased, but the impact resistance, weldability, machining performance will decline. Cobalt-based alloys alloyed with chromium and tungsten have good oxidation, corrosion and heat resistance. At 650 can still maintain a high hardness and strength, which is different from this type of alloy nickel and iron-based alloy important features. Cobalt-based alloy machining low surface roughness, high abrasion resistance and low coefficient of friction, but also for adhesive wear, especially in sliding and contact the valve sealing surface. However, when high-stress abrasive wear, low carbon-containing cobalt-chromium tungsten alloy wear resistance is not as good as low-carbon steel, therefore, expensive cobalt-based alloy selection, must have the guidance of professionals in order to play the maximum potential of the material. There are foreign countries with chromium, molybdenum alloy containing Laves phase cobalt-based surfacing alloys, such as Co-28Mo-17Cr-3Si and Co-28Mo-8Cr-2Si. Due to the low hardness of Laves compared to carbides, the material to which it is paired during metal friction wear is less abrasive.


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