Cerium

Chemical element from the group of rare earths.

SymbolCe
Atomic number58
Atomic weight140.13
Specific weight at 20 °C6.9 g/cm3
Melting point775 °C
Boiling point2900 °C
Melting heat63 kJ/kg
Specific thermal capacity at 20 °C0.20 kJ/(kg·K)
Thermal conductivity11 kJ/(m·K)
Therm. coefficient for linear expansion Coefficient for linear expansion8.5·10-6/K


Cerium in steel

Cerium displays strong affinity to oxygen, hydrogen, nitrogen, and sulfur. Therefore, it is very difficult to introduce cerium into steel without losses or segregations. Silicates present in the steel are readily broken down by cerium to form CeO oxides and CeSi silicides. Cerium is the main component of what is referred to as rare earths. They can be used in steel production either as mischmetal, which is obtained through alumino-thermal reduction of rare earth oxides, or as intimate mixture of such oxides with an appropriate reducing agent.

In the same way as manganese, cerium can combine with sulfur and prevent occurrence of iron sulfide. Part of this sulfur can then be removed but cerium-treated steel always contains some quantity of cerium sulfide and oxide compounds of cerium. It is possible for part of the cerium present in steel being available in the form of carbide.

Cerium and other rare earth elements, such as lanthanum and Neodymium function as nucleants for the primary grain if introduced in high-alloy steels (s. Balance of nuclei, Nucleation conditions) and as such they prevent formation of transcrystalline solidification structures which otherwise usually occurs in the blocks.

Killed, unalloyed steels with low carbon content only have a low high-temperature tensile strength and thus have a strong tendency for crack formation during rolling. This cracking tendency can be reduced by the addition of rare earths. Addition of onyl 1 kg of rare earth oxides per ton of steel is sufficient to significantly increase the output in the rolling mill.

Cerium-containing fluorides of rare earths are added to unkilled steel with higher carbon contents to facilitate degassing. When using such additives, it is possible to produce unkilled steel with up to 0.35 % C and 1 % Mn, even in shapes of large blocks. Cerium-containing fluorides of rare earths are also believed to improve the distribution of sulfur in blocks of unkilled steel.

Additives in the amount of approx. 1 to 2 kg of rare earth oxides per ton of steel are added to low-alloy nickel-chromium-molybdenum structural steels to prevent formation of dendritic structures and thus improve deformability and output. Moreover, more favorable low temperature impact properties can be achieved.

Cerium in cast iron

Cerium is a main constituent of what is known as mischmetal and in this form it is normally also used for production of nodular graphite cast iron and vermicular graphite cast iron. In particular in production of nodular graphite cast iron, the addition of 0.01 % to 0,06 % of cerium to the molten bath largely eliminates the effect of detrimental elements, which may otherwise significantly contribute to graphite degeneration. Therefore, a great number of commercially available master alloys contain 0.6 to 0.1 % cerium for prophylactic purposes.

Other rare earths:

Neodymium

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