Model Grade LT-1 - Metal-Ceramic Thermocouple Protection Tubes
Metal Ceramic, Grade LT-1 - hard, abrasion-resistant and dense - is a slip-cast composite of two compatible high temperature materials, chromium and aluminum oxide. Products made from Grade LT-1 material possess three properties of particular interest: Superior oxidation resistance to 2500ºF. Thermal conductivity comparable to that of stainless steel. Good resistance to wetting by most molten metals.
By the very nature of its constituents, metal-ceramic LT-1 exhibits properties that are not found solely in either a metal or pure ceramic alone.
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 oxidizing atmospheres over 2200ºF, 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-1 has good strength at temperatures where many high-temperature metals melt. Above about 2800ºF, it begins to soften and becomes plastic. LT-1 thermocouple protection tubes have, however, been used successfully for dip immersion at a temperature of 3000ºF. In use or service, care must be taken to avoid conditions*of extreme thermal shock, extreme thermal gradients, mechanical shock, and impact. Although 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 900ºF before immersion in molten metal at 2000ºF 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.
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.
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 LT-1 thermowells over a three-year period to affect thermocouple wires.
- Thermal Conductivity: 17 Btu ft/ft2 h ºF
- Coefficient of Thermal Expansion: 5 x 10-4 in/in/°F
- Density: 5.8 g/cm³
- Flexural Strength: 45000 psi
- Compresive Strength: 110000 psi
- Hardness: 34 Rc
- Chemical Composition: Al2O3-23, Weight Cr-77%
- Molten copper and brass to 2100 ºF intermittent and continuous immersions.
- Corrosive SO2 and SO3 gas (to 2500 ºF) and SO3 and HF gas (to 2000 ºF).
- Open hearth furnace checker chambers to 2460 ºF.
- Steel mill soaking pits to 2500 ºF.
- Pelletizing chamber of Taconite refining operation to 2100 ºF.
- Molten zinc to 1600 ºF.
- Molten lead to 650 ºF.
- Basic steels and slags to 3150 ºF (intermittent) and 2500 ºF (continuous in open hearth and general foundry practices).
- Calcining kilns to 2200 ºF.
- Barium titanate (barium oxide service) to 2200 ºF.
- Magnesium oxide calcining kilns.
- Fluid bed cement process with severe corrosion and temperature to 2400 ºF (fluid method of producing builders cement).
- Gas and ethylene cracking atmosphere.
- Atmosphere directly upon burning sodium (1800 - 2500 ºF).
- 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 aluminum.
- Tin (stannous) chloride (750 ºF).
- Acid slag.
- Carbide slag.
- Copper alloys above 2100 ºF
- Molten glass.
- Boiling sulphuric acid - 10%.
- Carburizing atmospheres.
- Nitriding atmospheres.
- Barium chloride salt bath.
- Sodium Nitrate - nitrate salt bath.