Niobium

High-melting heavy metal of glossy light gray color. Niobium is only attacked by concentrated sulfuric acid and is otherwise insoluble in acids.

SymbolNb
Atomic number41
Atomic weight92.91
Specific thermal weight at 20°C7.6 g/cm3
Melting point2468° C
Boiling point4400° C
Melting heat298 kJ/kg
Specific thermal capacity at 20°C0.27 kJ/(kg · K)
Thermal conductivity52 kJ/(m · K)
Therm. Coefficient for linear expansion7.1 10-6/K

Niobium in steel
γ iron may probably contain up to 2% Nb in solid solution at 1220°C, at 990°C this percentage decreases to approximately 1%, at room temperature, the solubility is slightly below 0.35%.

Niobium has a strong affinity to carbon and thus forms extraordinarily stable carbides. If added in excess to the amount required for carbide formation, niobium forms a high-melting (1660°C) intermetallic compound Fe3Nb2, which passes into solution to different extents in the various iron phases. For example, the solubility in δ ferrite, which is resistant to high temperatures, is higher than in the α phase, which occurs at lower temperatures. As a result of this, iron-rich iron-niobium alloys, which were subjected to solution annealing at high temperatures and then quenched, experience a hardening caused by precipitation when being tempered to moderately increased temperatures.

The formation of the low-solubility intermetallic compound Fe3Nb2 and the solid-solution formation of niobium and ferritelead to an increase of the tensile strength of niobium-containing steels with low carbon contents. Such steels are fine-grained and have a good impact strength at low temperatures.

A major advantage when using niobium as a grain refinement agent is that no unwanted oxidic inclusions in the steel occur, due to niobium being a weak oxide former. Fine-grained niobium-containing steels are used for carburization purposes. Strong grain growth occurs only at high temperatures so that the fine grain structure persists over wide hardening range.

Niobium in cast iron
Usually, niobium is not added to cast iron; however, there can be traces of this element if cupola furnaces are charged with niobium-containing scrap steel. In rare cases, it is used in nodular graphite cast iron to increase wear resistance and in copper-nickelcasting materials to achieve good welding properties. Cobaltcasting alloys can also contain niobium.

Another name for niobium is columbium (chemical symbol Cb).

Additional references:
Niobium carbide
J-Alloy

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