Share this page by mail What type of crucible can be used to melt non-ferrous metals and special alloys?
To melt non-ferrous metals and special alloys, Final Advanced Materials recommends the use of graphite or ceramic crucibles.
Graphite and vitreous carbon are recommended for foundry applications due to their excellent thermal conductivity (between 90 and 180 W.m-1.K-1) and their low expansion (between 2 and 5 10-6.K-1). However, these types of crucibles can react with certain oxides and must be used in an inert atmosphere or vacuum.
Alumina (Al2O3) is chemically stable and suitable for less reactive alloys.
Zirconia (ZrO2) has excellent chemical inertia and is suitable for special alloys and reactive materials.
Silicon carbide (SiC) has great thermal conductivity (125 W.m-1.K-1) and low expansion (4.5 10-6.K-1). It is suitable for aluminium, copper and intensive production.
Magnesia (MgO) is resistant to basic environments and nickel-rich alloys.
To help you choose the crucible best suited for your needs Final Advanced Materials considers two criteria: the maximum attainable heating temperature and the material which is to be melted.
Alumina (Al2O3) is the best choice in terms of value for money. It is chemically very inert, and can be used up to 1,700°C.
Zirconia (ZrO2) is more resistant to thermal shock but chemically less stable. Depending on the type of zirconia, it can be used up to 1,800°C.
Magnesia (MgO) is even more inert, and is resistant up to 2,000°C, but it also has the lowest thermal shock resistance.
Graphite, and vitreous carbon, can be used up to 3,000°C in an inert atmosphere or vacuum. However, when used in air these materials oxidise, and can be used only up to approximately 450-500°C.
Final Advanced Materials recommends that you always work in an argon or nitrogen atmosphere, or a vacuum, to optimise use of these materials. In an oxidising atmosphere their use is very limited.
Silicon carbide (SiC) crucibles have limited oxidisation resistance in an oxygen-rich atmosphere. Silicon carbide starts forming a protective layer of silica (SiO2) at 800-1,000°C. At 1,200°C or above, in air, oxidation becomes more substantial, which can embrittle the crucible over the long term. In these cases, Final Advanced Materials recommends working in an inert atmosphere to prevent these problems from appearing, and to make the material remain stable up to 1,600-1,800°C.
Regarding chemical corrosion, silicon carbide (SiC) has excellent chemical resistance to non-ferrous metals. However, it is sensitive to strong acids and powerful oxidisers at high temperature.
To prevent thermal shock when using ceramic crucibles Final Advanced Materials recommends the following practices:
• control the temperature variations gradually, and limit mechanical stresses
• never heat crucibles, in particular MgO crucibles, at full power from the start (200°C/h)
• prevent thermal gradients, contact with cold surfaces and localised hot spots
• use an inert atmosphere for silicon carbide, graphite or vitreous carbon