Metal Ceramic LT-1 is a composite material made up of chromium and aluminium oxide. This combination of metal and ceramic exhibits excellent resistance to oxidation above 1200°C and also makes it resistant to wetting by many metals and alloys.
UCAR metal-ceramic LT-l is a combination of a metal matrix, chromium, and a pure ceramic phase, aluminium oxide (alumina). It is composed 65% by volume of metallic phase and 35% by volume of ceramic phase. The material is slip cast, sintered, and then oxidised. Although the exact nature of the bond between the phases is not known, a physical-chemical bond may be formed through the sharing of oxygen by the chromium and the alumina. There is no evidence of wetting or solution.
Slip casting is a process whereby finely divided solid constituents are put into liquid vehicle to form a colloidal suspension called a “slip”. This slip or suspension is poured into a porous plaster mould. The mould absorbs the liquid leaving the solids in the shape of the mould cavity. Axial ID holes are obtained by drain casting which involves draining off the excess “slip” after the wall has built up to the desired thickness.
By the very nature of its constituents, UCAR metal-ceramic LT-l exhibits properties that are not found solely in either a metal or pure ceramic alone.
LT-l has excellent oxidation resistance and also resists wetting by many metals and alloys, as well as basic furnace slays. The chromium-metal phase takes on a very tightly bonded layer of chromium oxide which, together with the naturally inert nature of the alumina, provides this material with its remarkable resistance to oxidising atmospheres over 1200°C, good corrosion resistance, and the ability to resist wetting by molten metals.
High thermal conductivity and the resultant excellent sensitivity to temperature changes accounts in part for its demand in the high-temperature pyrometry field as a thermocouple protection tube
LT-l has good strength at temperatures where many high-temperature metals melt. Above about 1540°C, it begins to soften and becomes plastic. LT-l thermocouple protection tubes have, however, been used successfully for dip immersion at a temperature of 1650°C. In use or service, care must be taken to avoid conditions of extreme thermal shock, extreme thermal gradients, mechanical shock, and impact.
Although LT-l is superior to ceramics in all of these properties, it is less resistant to shock and impact than the metallic alloys. Therefore, a standard thermocouple protection tube should be preheated to about 480°C before immersion in molten metal at 1100°C or higher. Whenever practical the following preheat procedure can also be used: hold the tube immediately above the molten metal for approximately one minute before immersing. In tests conducted this procedure proved to be adequate to prevent thermal shock failure.
UCAR metal-ceramic LT-l exhibits good resistance to wear under conditions of sliding friction as well as resistance to abrasion at high temperatures. The hardness of this material (Rockwell C 37) is more indicative of the crushing strength of the material than its true hardness because the individual particles have a greater hardness than the combined body.
UCAR metal-ceramic LT-l is less porous than most compacts. There is no significant passage of gases through the body at high temperature, except under high vacuum. For the usual industrial application, it is sufficiently impermeable. For example, S02 and S03 gases have not penetrated LT-l thermowells over a three year period to affect thermocouple wires.
In summary, UCAR metal-ceramic LT-l possesses several attractive properties:
- Non-wetted by most molten metals and basic slags.
Good erosion resistance.
- Good abrasion resistance.
- Good oxidation resistance.
- Good thermal conductivity.
- High strength above the temperature at which most material melt or otherwise fail.
- Merchantable by most standard shop practices.
- Molten copper and brass to 1150°C intermittent and continuous immersions.
- Corrosive SO2 and SO3 gas (to 1375oC) and SO3 and HF gas (to 1100°C).
- Open hearth furnace checker chambers to 1350°C.
- Steel mill soaking pits to 1375°C.
- Pelletising charter of Taconite refining operation to 1150°C.
- Molten zinc to 875°C.
- Molten lead to 350°C.
- Basic steels and slags to 1735°C (intermittent) and 1375°C (continuous) in open hearth and general foundry practices.
- Calcining kilns to 1200°C.
- Barium titanate (barium oxide service) to 1200°C.
- Magnesium oxide calcining kilns.
- Fluid bed cement process with severe corrosion and temperature to 1315°C (fluid method of producing builders cement).
- Gas and ethylene cracking atmosphere.
- Atmosphere directly above burning sodium (975-1375°C).
- Oil fired furnace chambers.
- Atmosphere directly above molten glass in an open hearth glass furnace.
- Molten silver solder.
- Molten tin.
- Borax flux.
- Copper matte.
- Boiling sulphuric acid - 97%.
- Blast furnace stove dome and bustle pipes.
- Molten aluminium.
- Tin (stannous) chloride (400°C)
- Acid slag.
- Carbide slag.
- Molten glass.
- Boiling sulphuric acid - 10%.
- Carburising atmospheres.
- Nitriding atmospheres.
- Barium chloride salt bath.
- Sodium Nitrate - nitrate salt bath.