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Cobalt-chromium-tungsten alloy (CoCrW) is one of Stellite alloys. Stellite alloy is a hard alloy that can withstand various types of wear, corrosion and high-temperature oxidation. It is commonly referred to as a cobalt-based alloy. Stellite was originally a binary cobalt-chromium alloy, and later developed into a ternary composition of cobalt-chromium-tungsten. Cobalt-chromium-tungsten alloy is a kind of alloy with cobalt as the main component, containing a considerable amount of chromium, tungsten and a small amount of nickel, molybdenum, silicon, carbon, niobium, tantalum and other alloying elements, and occasionally also containing iron. According to the different components in the alloy, they can be made into welding wire, powder used for hard surface welding, thermal spraying, spray welding and other processes, and can also be made into castings and forgings and powder metallurgy parts.

The Basic Features Of CoCrW

The Description Of CoCrW

The typical grades of cobalt-chromium tungsten alloys are: Hayness188, Haynes25(L-605), Alloy S-816, MP-159, FSX-414, X-40, Stellite6B, etc. Chinese brands are: GH5188(GH188), GH605, K640 , DZ40M and so on. Unlike other high-temperature alloys, cobalt-chromium-tungsten alloys 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. However, the casting of cobalt-chromium-tungsten superalloys relies to a large extent 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 of cobalt-chromium-tungsten superalloys in use, in fact all cobalt-chromium-tungsten alloys are alloyed with nickel in order to stabilize the structure from room temperature to melting point temperature. The cobalt-chromium-tungsten alloy has a flat fracture stress-temperature relationship, but it shows better thermal corrosion resistance than other high temperatures above 1000°C. This may be due to the higher chromium content of the alloy. This is this type of alloy. A feature of.

The Characteristic Of CoCrW

Carbide strengthening phase The most important carbides in cobalt-chromium-tungsten alloys are MC, M23C6 and M6C. In cast cobalt-chromium-tungsten 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. Carbides located on the grain boundary (mainly M23C6) can prevent the grain boundary slip, thereby improving the endurance strength. The microstructure of the cobalt-chromium-tungsten superalloy HA-31 (X-40) is a dispersed strengthening phase (CoCrW) 6 Type C carbide.
The topological close-packed phases such as sigma phase and Laves that appear in some cobalt-chromium-tungsten alloys are harmful and make the alloy brittle. Cobalt-chromium-tungsten alloys rarely use intermetallic compounds for strengthening, because Co3 (Ti, Al), Co3Ta, etc. are not stable at high temperatures, but non-cobalt-chromium-tungsten alloys that use intermetallic compounds for strengthening have also been developed.

The thermal stability of carbides in cobalt-chromium-tungsten alloys is better. 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-chromium The strength of tungsten alloys generally declines slowly.

Cobalt-chromium-tungsten alloy has good thermal corrosion resistance. It is generally believed that the reason why cobalt-chromium-tungsten alloy is better than nickel-based alloy in this respect is that the melting point of cobalt sulfide (such as Co-Co4S3 eutectic, 877℃) is higher than that of nickel. The melting point of sulfide (such as 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-chromium-tungsten alloys have higher chromium content than nickel-based alloys, a Cr2O3 protective layer that resists alkali metal sulfates (such as Na2SO4 corrosion) can be formed on the surface of the alloy. However, the oxidation resistance of cobalt-chromium-tungsten alloys is generally higher than that of nickel-based alloys. Much lower.

The Use Of CoCrW

Cobalt chromium tungsten alloy is widely used in locomotive diesel engines, nuclear power plant valves, marine diesel engines and various aircraft.

Early cobalt-chromium-tungsten alloys were produced by non-vacuum smelting and casting processes. The alloys developed later, such as Mar-M509 alloy, are produced by vacuum smelting and vacuum casting because they contain more active elements such as zirconium and boron.

Generally, cobalt-chromium-tungsten alloy lacks coherent strengthening phase. Although the strength is low at medium temperature (only 50-75% of nickel-based alloy), it has higher strength, good thermal fatigue resistance and thermal corrosion resistance above 980℃. And abrasion resistance, and has better weldability. It is suitable for making guide vanes and nozzle guide vanes for aviation jet engines, industrial gas turbines, naval gas turbines, and diesel engine nozzles.

Production Process

The Heat treatment Of CoCrW

The size and distribution of carbide particles in the cobalt-chromium-tungsten alloy and the grain size are very sensitive to the casting process. In order to make the cast cobalt-chromium-tungsten alloy parts reach the required endurance strength and thermal fatigue performance, the casting process parameters must be controlled. Cobalt-chromium-tungsten alloy requires heat treatment, mainly to control the precipitation of carbides. For cast cobalt-chromium-tungsten alloys, first perform high-temperature solid solution treatment, usually around 1150℃, so that all primary carbides, including some MC-type carbides, are dissolved into solid solution; then aging treatment is carried out at 870-980℃ , So that carbides (the most common is M23C6) precipitation again.

The Surfacing Of CoCrW

The chromium-tungsten surfacing alloy contains 25-33% chromium, 3-21% tungsten, and 0.7-3.0% carbon. As the carbon content increases, the metallographic structure changes from hypoeutectic austenite + M7C3 eutectic to hypereutectic M7C3 primary carbide + M7C3 eutectic. The more carbon content, the more nascent M7C3, the greater the macroscopic hardness, and the improved abrasive wear resistance, but the impact resistance, weldability, and machining performance will all decrease. Cobalt-chromium-tungsten alloy alloyed with chromium and tungsten has good oxidation resistance, corrosion resistance and heat resistance. It can still maintain high hardness and strength at 650℃, which is an important feature that distinguishes this type of alloy from nickel-based and iron-based alloys. Cobalt-chromium-tungsten alloy machining has low surface roughness, high scratch resistance and low friction coefficient. It is also suitable for adhesive wear, especially on sliding and contacting valve sealing surfaces. However, in high-stress abrasive wear, the wear resistance of low-carbon cobalt-chromium-tungsten alloy is not as good as that of low-carbon steel. Therefore, the selection of expensive cobalt-chromium-tungsten alloy must be guided by professionals in order to maximize the potential of the material. .

The Abrasion Resistance Of CoCrW

The wear of alloy workpieces is largely affected by the contact stress or impact stress on the surface. Surface wear under stress depends on the interaction characteristics of dislocation flow and contact surface. For cobalt-chromium-tungsten alloys, this feature is related to the low stacking fault energy of the matrix and the transformation of the matrix structure from face-centered cubic to hexagonal close-packed crystal structure under the effect of stress or temperature. Metal materials have better wear resistance. In addition, the content, morphology and distribution of the second phase of the alloy, such as carbides, also have an impact on the wear resistance. Because the alloy carbides of chromium, tungsten and molybdenum are distributed in the cobalt-rich matrix and part of the chromium, tungsten and molybdenum atoms are solid-dissolved in the matrix, the alloy is strengthened, thereby improving wear resistance. In the cast cobalt-chromium-tungsten alloy, the size of the carbide particles is related to the cooling rate, and the faster the cooling is, the smaller the carbide particles are. In sand casting, the hardness of the alloy is lower, and the carbide particles are also coarser. In this state, the abrasive wear resistance of the alloy is significantly better than that of graphite casting (fine carbide particles), while the adhesive wear resistance of both There is no significant difference, indicating that coarse carbides are beneficial to improve the ability of abrasive wear resistance.

The Ingredient Of CoCrW

The basic composition of cobalt-chromium-tungsten alloy is: Co: 50%~58%, Cr: 28%~30%, W: 4%~6%, Ni: 2%~4% and other alloys. The melting point is 1470℃ .
It can be seen from the alloy composition that the cobalt content and tungsten content of the alloy are very high, so that the material has excellent high temperature performance and poor thermal conductivity. It is these characteristics that make the cobalt-chromium-tungsten alloy sparks dark red during grinding, and the number of sparks is very small. The metal that separates from the surface of the workpiece is very easy to block the grinding wheel, which makes the grinding conditions deteriorate rapidly, and at the same time generates a large amount of grinding heat. It spreads rapidly, causing low efficiency and burns on the surface of the workpiece.

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