Materials

A wide range of ceramic oxide materials can be used for crucibles, the type being dependent on the metal/alloy, the basicity of the melt and the foundry practice, whether erosion resistance or thermal shock resistance takes prominence.

The most common type, historically, are the aluminosilicate range; these were based on clays but now utilise the purer synthetic materials.  At one end of the range lies fused silica, with an excellent thermal shock resistance, whilst at the other extreme is fused alumina with a very good erosion resistance.

Magnesia generally offers better chemical and erosion resistance, but is better suited to the faster cycling times.  The most resistant material is zirconia, and there are many iterations of this to suit different applications.

 

induction_melting_crucibles

Alumina

ALUMINA A97 is made from fused alumina and which contributes to a good abrasion resistance. It is the most commonly used product in air-melting, and is suitable for steels, nickel alloys and cobalt alloys. It can cope with a wide variety of slags as it is chemically neutral.

ALUMINA 90 is a preferred grade of Alumina when a good thermal shock resistance is needed.

Zirconia enhanced Aluminas improve the properties further. The zirconia alters the wetting characteristics of the surface resulting in notable improvement in erosion resistance.

The vibrocast version used for the larger crucibles and shapes, ALUMINA 90Z, contains just 3.5% zirconia but this is enough to boost the service life by up to 30%

For smaller crucibles up to 80kg capacity, the isostatically pressed ALUMINA AZ80 contains almost 15% zirconia, which improves the performance further such that it effectively becomes a bridge product between Alumina and Zirconia. As such, it is well suited for melting Ni-based superalloys; for steel melting, it can act as a replacement for Chrome-Alumina and so avoid the health & safety and environmental problems with that material.

 

 

 

Properties

Chemical Properties

Product
 SiO2(%)
 Fe2O3(%)
 Al2O3(%)
 ZrO2 (%)
Alumina A97 1.4 0.2 97.9 –
Alumina 90 10.5 0.1 88.5 –
Alumina 90Z 8.5 <0.1 87.5 3.6
Alumina AZ80 8.4 0.1 76.2 14.7

Physical and Thermal Properties

Product AP (%) >BD (g/cc) CTE (x10-6/K) TC (W/m/K)
Alumina A97 22.4 2.95 9.8 3.05
Alumina 90 20.4 2.80 – 3.0
Alumina 90Z 19.5 2.96 – 1.6
Alumina AZ80 21.6 2.96 – –
AP=Apparent Porosity BD=Bulk Density CTE=Coefficient of Thermal Expansion TC=Thermal Conductivity