Cobalt undergoes isomeric deformation at 417°C to form a solid solution with a face-centered cubic structure. It has good thermal shock resistance, low creep rate, high fracture toughness, and satisfactory high-temperature performance. Chromium is added to make the cobalt matrix obtain good oxidation resistance. Most cobalt-based alloys are developed on the basis of cobalt-chromium alloys. In the cobalt-chromium alloy matrix, a suitable amount of tungsten is melted into the cobalt matrix to further strengthen the cobalt matrix. The solubility of carbon in cobalt is very small. The added carbon combines with excess tungsten to form carbides such as WC, W2C, and M6C (CO3W3C, CO2W4C). These hard phases are dispersed in the cobalt-chromium alloy matrix to produce precipitation strengthening. , So that Cobalt-chromium-tungsten alloy can still maintain excellent high temperature strength and high temperature hardness at a high temperature of 800 ℃, so the alloy has excellent comprehensive properties such as high temperature wear resistance, high temperature oxidation resistance and thermal fatigue. Cobalt has a close-packed hexagonal structure below 417°C, has a low coefficient of friction, and has good wear resistance. At 417°C, the crystal form transition from close-packed hexagonal crystals to face-centered cubic crystals occurs, resulting in volume stress.
Among this series of alloys, Stellite 6 alloy is the most representative. This alloy has comprehensive properties of wear resistance, corrosion resistance and heat resistance in a wide temperature range, especially with excellent self-engagement anti-adhesive wear properties. This alloy has a low carbon and tungsten content, and the hard carbide phase accounts for a small proportion of the structure (10% to 20%). The hardness and impact toughness have a good match. It is also used as a cutting tool material.
Stellite 1 alloy contains 2.4% carbon, and the carbide content in the phase structure is more than 30%, so it has excellent wear resistance and erosion resistance.
Stellite 12 alloy has a higher tungsten content, so its matrix hardness is higher, the red hardness is better, and the wear resistance of abrasive particles is also stronger.
The chemical composition and physical properties of commonly used cobalt-based alloy powders are shown in Table 1 and Table 2, respectively. Adhesive wear performance between metals, wear resistance of wear-resistant materials, and corrosion resistance in different corrosive media are shown in Table 3 and Table 4, respectively.
Table 1 Commonly used cobalt-based alloy powder grades and chemical composition
Table 2 Commonly used cobalt-based alloy powder grades and physical properties
|Grade||Melting point/ºC||High temp.hardness（DPH）|
Table 3 Cobalt-based alloy resistance to metal-to-metal adhesion wear and wear resistance of wear-resistant materials
|Grade||Adhesive wear resistance ①||Wear resistance of wear-resistant materials②|
|40.9③||68.2③||95.5③||136.3③||Oxygen-acetylene gas welding||Powder surfacing|
Adhesive wear test conditions are: stellite alloy oxygen-acetylene gas welding sample and low alloy cast steel (SAE46202 steel, 63Rc) grinding wheel, the grinding wheel speed is 80r/min, different loads are applied to the pattern, and the average of the pattern is measured Volume loss (mm3);
Abrasive wear test equipment is a rubber wheel wear tester. Abrasive particles: quartz sand, particle size Φ229mm. Rubber wheel speed: 200r/min, pressure load: 13kg;
Pressure load applied to the wear pattern, in kg.
Table 4 Corrosion resistance in different corrosive media
|30% acetic acid
|80% formic acid
Optional spraying process methods include supersonic plasma spraying, ordinary plasma spraying, flame spraying, plasma spray welding and so on. When plasma spray welding is used, it is generally necessary to preheat the base material to above 500°C to prevent cracking of the cobalt-based welding layer.
Cobalt-chromium-tungsten-carbon alloy coating can be used as a coating or spray welding layer that is resistant to high-temperature metal adhesion, wear-resistant particle wear, corrosion resistance, fretting resistance, high temperature erosion resistance and wear resistance.