The basic structure of a graphite refractory crucible is natural graphite, clay and silicon carbide.
GRAPHITE - is basis of heat conduction, electrical conduction, it gives necessary thermal stability to crucible and is for the most part stable to chemical action. The only solvent for graphite is melted iron. That's why graphite crucibles are not intended for melting of cast iron. One more “disadvantage” of graphite is its corrosion at oxygen's access, beginning from 600°C. Therefore crucibles are covered with glaze, which defends graphite from corrosion for the most part.CLAY - is high-refractory bond for graphite articles.
SILICON CARBIDE - is additional heat conductor which also increases mechanical strength and durability of crucibles essentially.
Marking of crucibles includes:
Depending on composition and assignment the following types of products are distinguished
• On clay bond:
А - intended for aluminum melting and its alloys in the induction furnace. Mark of the material “A” intended for melting of non-ferrous metals and its alloys in the resistance furnace. The melting temperature should not exceed 1500°C.
К - for melting of non-ferrous metals with melting temperature up to 1600°C in induction, gas or liquid furnaces; for distribution of non-ferrous metals in resistance furnaces..
С - non-ferrous metal casting with working temperature up to 1600°C.
Т - for melting of precious metals with melting temperature up to 1600°C in induction and muffle furnaces.
D - for melting of precious metals with melting temperature up to 1250°C heating at the expense of passing a current through a crucible.
Е - for foundry accessories.
Н - for non-ferrous metals testing.
• On carbon bond:
Х - for melting of non-ferrous metals with melting temperature up to 1500°C in resistant and gas or liquid furnaces. They don't demand preliminary drying.
The crucibles are protected from oxidation and erosion with special glazes
L - for operations with metals with melting temperature lower than 1000°C.
Т - for working in conditions of temperature over 1000° C.
As further details the following signs are accepted
"1" - application of glazes on outer surface only.
"2" - crucibles are intended for resistance furnaces.
Examples of notation conventions
ВА 750L2 ТУ..... that is crucible form B type A, with capacity 750 kg on copper, for melting of non-ferrous metals with melting temperature up to 1000°C (L) in resistance furnaces (2).
ZК 175Т ТУ..... that is cylinder crucible form Z type K, with capacity 175 kg on copper, for melting of non-ferrous metals with melting temperature up to 1000°C (T) in induction furnaces.