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:
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.
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Typical Physical Properties