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Topics Covered
Background
Slip Casting UCAR Metal-Ceramic LT-1 Sheaths
Physical Appearancof Metal-Ceramic LT-1 Sheaths
Properties of UCAR Metal-Ceramic LT-1 Sheaths
Standard Metal-Ceramic LT-1 Sheaths
Oxidation Resistance and Wetting by Molten Metals
Thermal Properties
Impact and Thermal Shock Resistance
Wear Resistance
Porosity and Permeability
Summary of Attractive Properties
Background
UCAR metal-ceramic LT-1 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 UCAR Metal-Ceramic LT-1 Sheaths
Slip casting is a process whereby finely divided solid constituents are put into liquid vehicle to form a collodial 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.
Physical Appearance of Metal-Ceramic LT-1 Sheaths
In general, UCAR metal-ceramic LT-1 has a dull grey colour in the as-produced condition, and a metallic appearance when either ground or machined. After the oxidising heat-treatment, the surface colour is generally a very dark green to black.
Properties of UCAR Metal-Ceramic LT-1 Sheaths
Standard Metal-Ceramic LT-1 Sheaths
By the very nature of its constituents, UCAR metal-ceramic LT-1 exhibits properties that are not found solely in either a metal or pure ceramic alone.
Oxidation Resistance and Wetting by Molten Metals
UCAR metal-ceramic LT-1 has excellent oxidation resistance and also resists wetting by many metals and alloys, as well as basic furnace slags. 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.
Thermal Properties
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 UCAR metal-ceramic LT-1 has good strength at temperatures where many high-temperature metals melt. Above about 1540°C, it begins to soften and becomes plastic. UCAR metal-ceramic LT-1 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.
Impact and Thermal Shock Resistance
Although UCAR metal-ceramic LT-1 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.
Wear Resistance
UCAR metal-ceramic LT-1 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.
Porosity and Permeability
UCAR metal-ceramic LT-1 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, SO2 and SO3 gases have not penetrated UCAR metal-ceramic LT-1 thermowells over a three year period to affect thermocouple wires.
Summary of Attractive Properties
In summary, UCAR metal-ceramic LT-1 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.
Source: Pyrosales
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