Cobalt

High-melting, steel gray lustre heavy metal that provides similar properties as nickel in many aspects.

SymbolCo
Atomic number27
Atomic weight58.94
Specific weight at 20 °C8.7 g/cm3
Melting point1492 °C
Boiling point2900 °C
Melting heat268 kJ/kg
Specific thermal capacity at 20 °C0.43 kJ/(kg·K)
Thermal conductivity100 kJ/(m·K)
Therm. coefficient for linear expansion Coefficient for linear expansion12·10-6/K


Materials for permanent magnets, high-speed steels and hot-work steels are alloyed using cobalt. Moreover, cobalt is an alloying element in martensite hardenable die steels, heating conductor materials and creep-resistant high-temperature materials. Cobalt is the main component for special cobalt alloys, that are partly used for investment casting products.

Cobalt in steel

Cobalt is available in two modification variants, one of which is the hexagonal close sphere packing and is resistant below 450 °C. The other, the cubic face-centered type, exists above this temperature. γ iron and cobalt are soluble within each other under any conditions and in any ratios; solubility of cobalt in α iron is up to 75 %.

Addition of cobalt increases the solidus temperature of steel, lowers the A point and extends the region of the γ phase. Cobalt increases the resistance of austenite in high-alloy steels during quenching from high temperatures. In contrast to most other alloying elements, cobalt reduces the hardnesspenetration depth (s.a. Hardness penetration capacity) during quenching. For this reason, steels with poor hardnesspenetration capacities contain up to 1 % Co.

Cobalt hardens ferrite and significantly adds to its hot hardness. It imparts corrosion and wear resistance to steel and leads to increase of tensile strength and yield strength but reduces toughness.

Cobalt displays a tendency towards carbide formation, similar to iron (s. Carbide, Carbide former) and is predomiantly present in steel in the form of non-resistant carbide Co3C. Cobalt-containing steel is often stabilized by addition of chromium, which forms complex carbides. Stabilization prevents decarburization during hot forming and eliminates problems during machining of steel with more than 5 % Co.

Cobalt is an important constituent in tungsten-containing tool steel since it imparts the required hot hardness to those kinds of steels and thus increases their cutting performance at high temperatures. Two frequently used types of tungsten-containing high-speed steels contain between 5 and 10 %, sometimes even 15 % Co. These kinds of steels are used for production of tools for machining of hard materials. Since cobalt increases the melt temperature and therefore allows for utilization of higher solution heat treatment temperatures, a large proportion of carbides is dissolved during annealing and diffused during tempering.

In tungsten-containing tool steel, addition of cobalt also promotes martensite formation.

Cobalt in cast iron

Cobalt is generally not added to cast iron. If added in small amounts, it only has little impact. Cobalt is assumed to have a slightly graphitizing effect, similar to that of nickel (s. Graphitization).

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