Metal ceramic LT-1, available from Pyrosales , is a composite material made up of chromium and aluminium oxide. This combination of metal and ceramic makes it resistant to oxidation above 1200°C and also makes it resistant to wetting by metals and alloys.
The UCAR metal-ceramic LT-l material is a combination of a metal matrix, chromium and a pure ceramic phase, aluminium oxide (alumina). It is composed 65% by volume of the metallic phase and 35% by volume of the 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 vehicles 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.
The UCAR metal-ceramic LT-l material has better oxidation resistance and also resists wetting by metals and alloys, as well as basic furnace slays. The chromium-metal phase takes on a tightly bonded layer of chromium oxide which together with the inert nature of the alumina, provides this material with resistance to oxidising atmospheres over 1200°C, better corrosion resistance and the ability to resist wetting by molten metals.
High thermal conductivity and the resultant sensitivity to temperature changes accounts in part for its demand in the high-temperature pyrometry field as a thermocouple protection tube.
The UCAR metal-ceramic LT-l material has better strength at temperatures, where high-temperature metals melt. Above about 1540°C, it begins to soften and becomes plastic. LT-l thermocouple protection tubes have been used 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.
The UCAR metal-ceramic LT-l material is less resistant to shock and impact. Therefore, a standard thermocouple protection tube should be preheated to about 480°C before immersion in molten metal at 1100°C or higher. The following preheat procedure can also be used. The users need to hold the tube immediately above the molten metal for one minute before immersing. In tests conducted, this procedure proved to be adequate to prevent thermal shock failure.
The UCAR metal-ceramic LT-l material exhibits 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 indicative of the crushing strength of the material because the individual particles have a higher hardness than the combined body.
The UCAR metal-ceramic LT-l material is less porous. 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.
The UCAR metal-ceramic LT-l material has the following properties:
- Non-wetted by molten metals and basic slags
- Better erosion resistance
- Better abrasion resistance
- Better oxidation resistance
- Better thermal conductivity
- High strength above the temperature
- Merchantable by most standard shop practices
The UCAR metal-ceramic LT-l material is suitable for the following applications:
- Molten copper and brass to 1150°C intermittent and continuous immersions
- Corrosive SO2 and SO3 gas (to 1375°C) 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° to1375°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
The UCAR metal-ceramic LT-l material is not suitable for the following applications:
- Molten aluminium
- Cryolite
- 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