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Properties of inclusions
i) Thermal expansion
An inclusion is a mismatch with the steel matrix. There are inclusions like MnS,CaS etc. which have thermal expansion greater than steel matrix. On heating steel with these types of inclusions voids or parting of the matrix can occur. The void can act as cracks.
On the other hand  and  , etc inclusions have thermal expansion smaller than steel matrix. On heating steels with these type of inclusions internal stresses of thermal origin can develop.
Density and melting point
ii) Density and melting point |
Composition of inclusions |
Melting point(oC) |
Density at 20oC(g/cm3) |
Ferrous oxides (FeO) |
1369 |
5.8 |
Manganous oxides (MnO) |
1785 |
5.5 |
Silica |
1710 |
2.2-2.6 |
Alumina (Al2 O3) |
2050 |
4.0 |
Chrome oxide Cr2 |
2280 |
5.0 |
Titanium oxide, TiO2 |
1825 |
4.2 |
Zirconium oxide, ZrO2 |
2700 |
5.75 |
Iron silicate, (FeO)2SiO2 |
1205 |
4.35 |
Iron sulphide, FeS |
988 |
4.6 |
Manganese sulphide, MnS |
1620 |
4.04 |
Magnesia, MgO |
2800 |
3.58 |
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Inclusions like are solid at steelmaking temperature.
iii) Plastic deformability
The plastic deformability of an inclusion will govern any change in its shape under the action of external forces and will determine the amplitude of stress concentration. Brittle inclusions are dangerous as they may crack and cause fracture of the component under the application of external force. The majority of inclusions belong to pseudo-ternary system:
 etc. Sulphide inclusions are mainly  . Other elements like Ti, Zr, rare earths, Nb, V etc. usually appear as solid solutions in existing inclusion phases. The following classification of inclusions according to kieslling is useful to the metallographers to determine type of inclusions: According to Kieslling
- Calcium aluminates and
 inclusions in steel are undeformable at temperatures of interest in steelmaking.
- Spinel type double oxides
 (where A is Ca, Fe(l), Mg and Mn,and B is Al,(Cr etc) are deformable at temperatures greater than  .
- Silicates are deformable at higher temperature range. The extent of deformation depends on their chemical compositions. Silicates are not deformable at room temperature.
 and  are plastic at room temperature but gradually lose plasticity above  .
- which is highly deformable at
 temperature but becomes slightly less deformable above .
- Pure silica is not deformable up to
 .
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