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

Graphite basedCG-a_shape

CLAY-GRAPHITE and SILICON CARBIDE crucibles are used for non-ferrous alloys, including lower melting point precious metals. With induction furnaces, the choice of which to use is generally made by the coil type and field profile although, as a general rule, silicon carbide crucibles are better suited to low frequency furnaces whilst high frequency furnaces  favour clay-graphite.

CLAY-GRAPHITE crucibles are plastically formed and are supplied with a ceramic bond.  They can be used for alloys being cast at temperatures up to 1400oC.

Characteristics:

  • High refractoriness
  • Good thermal conductivity
  • Very good thermal shock resistance
  • Chemical resistance against fluxes
  • High mechanical strength
  • Good oxidation resistance

SILICON CARBIDE crucibles have a carbon bond, which imparts excellent stability at high temperatures. They are particularly suited to fast thermal cycling regimes in furnaces with high power, even with heavy metal alloys, to temperatures above 1400oC. They are also used in non-ferrous holding furnaces at temperatures up to 1200oC.

Characteristics:

  • Very good thermal conductivity
  • Excellent thermal shock resistance
  • High oxidation resistance for longer life
  • High mechanical strength
  • Good erosion resistance
  • Resistant to chemical attack by fluxes
  • Very low dross adhesion.

They are particularly suited to fast thermal cycling regimes in furnaces with high power, even with heavy metal alloys, to temperatures above 1400oC. They are also used in non-ferrous holding furnaces at temperatures up to 1200oC.

Properties

Chemical Properties

Product
 SiO2(%)
 Al2O3(%)
 SiC(%)
 Si(%)
 C (%)
Clay-Graphite 25-30 10-15 10-25 5-10 30-45
Silicon Carbide
 5-15 5-15 25-40 – 40-50

Physical and Thermal Properties

Product AP (%) >BD (g/cc) CTE (x10-6/K) TC (W/m/K)
Clay-Graphite 27-32 – – 0.24-0.17
Silicon Carbide
 25-29 – – na
AP=Apparent Porosity BD=Bulk Density CTE=Coefficient of Thermal Expansion TC=Thermal Conductivity