Titanium

 

Titanium is a high-melting light metal of grey color which features low density, high strength and good corrosion resistance.

SymbolTi
Atomic number22
Atomic weight47.90
Specific thermal weight at 20°C4.51 g/cm3
Melting point1600 °C
Boiling point3260 °C
Melting heat393 kJ/kg
Specific thermal capacity at 20° C0.61 kJ/(kg · K)
Thermal conductivity22 kJ/(m · K)
Therm. coefficient for linear expansion Coefficient for linear expansion           8.4 10-6/K


It is derived from its oxidic ores (rutile TiO2 and ilmenite FeTiO3) by chlorination, the reduction of the generated titanium chloride with magnesium and the vacuum distillation of magnesium residues, and it is molten in the electric arc furnace in the presence of shielding gas or vacuum.

Titanium alloys as well as molybdenum, chromium and/or aluminum feature very high strength at low weight.

Titanium content in steel
ɣironmay only contain up to 0.75% of Ti in solid solutions, whereas α-iron may contain up to approximately 6% of Ti. Titanium is an extraordinary carbide former; therefore, subject to an adequate amount of carbon, all titanium-containing steels contain carbide TiC (see also Titanium carbide). With certain accompanying elements in the steel such as oxygen and nitrogen, titanium forms very stable compounds thus being used among others as deoxidizer and denitration agent.

Compared to all other titanium compounds in steel, titanium oxide is the most durable one. The element usually reacts under oxidizing conditions at 1600 °C forming Ti02. If, however, the oxygen concentration is too low as in killed steel, or if the amount of titanium added fordeoxidationis very high, the formation of the oxide Ti203 is also possible.
In steels with a higher carbon content and if there are larger amounts of titanium, there might also be more complex compounds like titanium cyan nitride and titanium carbon nitride TiCN. These kinds of inclusions are not desired because they impair the machining properties and contribute to the generation ofinternal stresses.

Titanium is often used as deoxidizer (in smaller amounts) so that there are contents of around 0.025% in steel. If the titanium content is larger, the element was presumably added for alloying purposes.

Unkilled steel is occasionally partly deoxidized in the ladle by adding ferrotitanium with a high or medium carbon content; aluminum is also added to the ingot mold in order to restrict gas formation. Adding titanium to unkilled steel improves the surface quality of the ingots since segregations and cavities close to the surface are largely avoided.

Titanium in cast iron
Pig iron and cast iron commonly contain titanium which becomes visible in the micrograph as reddish inclusions with angular shape. These inclusions consist of titanium nitride, titanium carbide or a mixture of both.

Adding titanium to cast iron increases the formation of fine graphite during undercooling. It is assumed that this phenomenon is primarily due to the titanium-sulfur compound. An even finer graphite formation is achieved after flushing the melt with carbon dioxide or argon since hydrogen is released during this process. This treatment also improves the macro structure. The formation of graphite, however, implies the formation of a basic ferritestructure which may have lower strength. Titanium dissolved in the matrix, however, generally improves the strength of cast iron.

Cast iron types with fine-lamellar graphite have a dense structure and therefore are less prone to growth and have improved corrosion resistance in acid solutions. In order to keep growth tendencies as low as possible, titanium is occasionally added to heat-resistantcast iron with a high silicon content. In cast iron types highly alloyed with chromium, eutecticcarbide can be refined by adding titanium. This treatment also improves mechanical properties.

The finely distributed titanium carbides and nitrides in the soft matrix ensure the good wear resistance of cast iron without impairing the machining properties.

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