The vast majority of commercially available Cobalt Allo […]
The vast majority of commercially available Cobalt Alloy is air or argon melted because they lack the highly reactive elements aluminum and titanium, and the presence of these requires more complex and expensive vacuum melting techniques. The addition of silicon and manganese is used to improve the fusibility of the alloy in terms of fluidity, melt deoxidation practices and sulfur control. Vacuum smelting is required to control the lower alloying levels of the contemporary carbides such as MM-509 to form the active carbides of zirconium, hafnium and titanium. Tensile and fracture properties improvements of the more traditional alloys, such as X-40, are also due to vacuum melting caused by lower clearance levels and "cleaner" materials.
For example, air-melted alloys typically have 400 ppm of oxygen and 700 ppm of nitrogen, whereas vacuum-melted alloys contain less than 100 ppm of these elements. Although chemical analysis showed a slight decrease in the sulfur and phosphorous content of the ESR material, there was no significant change in the alloy microstructure or non-metallic inclusions.
Cobalt alloys for commercial use as corrosion-resistant alloys are also used in aerospace for coating of turbine engine components. They are reinforced with a uniform non-stick CoAl precipitate, producing properties similar to carbide-reinforced alloys. CoAl exceeds about 1400 ° F (760 ° C), however, the addition of tungsten to refractory elements in alloy AR215 and tantalum to S-57 stabilizes the precipitate to higher service temperatures.
Addition of nitrogen to some air-melted casting alloys, either as a deliberate or unintentional addition, also has positive but weaker strengthening similar to the formation of carbon by forming nitrides and carbonitrides. In general, these are not as thermodynamically stable as carbides and suffer degenerative reactions during use.
Over the past two decades, the oxidation resistance of cobalt alloys has been significantly improved, especially under thermal cycling conditions and is particularly effective in stabilizing Cr2O3 oxides and minimizing the formation of CoCr204 spinel and COO.