What is a Metal-ceramic LT-l protection sheath?

METAL-CERAMIC LT-1

Pyrosales is the only Australian distributor of this Metal -ceramic PyroSheath LT-1® is a widely used protection sheath, also referred to ‘metal ceramic’ or ‘cermet’. It is made out of chromium and alumina. Due to the materials of this sheath as well as the manufacturing process, the LT-1 presents itself as a sturdy product which is ideal for high temperature furnaces.

General Information

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.

Physical Appearance

In general, metal-ceramic LT-l 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 dark green to black.

Properties

By the very nature of its constituents, 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 slags. The chromium-metal phase takes on a 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 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. This procedure proved to be adequate to prevent thermal shock failure in tests conducted. 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. 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, 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 materials melt or otherwise fail.
• Marketable by most standard shop practices.

Applications

The following is a list of recommended and non-recommended applications for LT-l Metal Ceramic.

Recommended 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 generalfoundry 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 ofproducing 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

Non –Recommended Applications

• Molten aluminium.
• Cryolite.
• Tin (stannous) chloride (400°C)
• Acid slag.
• Carbide slag.
• Molten glass.
• Boiling sulphuric acid – 10%.
• Carburising atmospheres.
• Nitrating atmospheres.
• Barium chloride salt bath.
• Sodium Nitrate – nitrate salt bath.

Thermocouple protection tubes, 7/8″ O.D. x 5/8″ I.D. up to 48″ long with ±1/16″ overall tolerances, are in stock for immediate shipment. Solid rods and ‘other type’ cast tubes are made-to-order items. The following designated sizes are typical capabilities.

Diameter Maximum Length

Solid Rods: 1/8″ 8″

1/4″ 15″

1″ 24″

2″ 12″

Hollow Tubes: Under 1″ O.D. 48″

1″ – 2″ O.D.

Wall Thickness

7/32” maximum

1/8” minimum

3/16” preferred

Rod Tolerance: All dimensions ±1/16″

Curvature: 1.3% measured chord to arc.

Hollow Tube Tolerance: All Dimensions ±1/16″.

Straightness: 2” long probe with diameter 1/8″ smaller than tube I.D. will pass freely

Miscellaneous

Rods and tubes in lengths greater than standard are available by coupling sections together. Also, the possibility of a redesign of a part, or shape, in order to make it amenable to the process should not be overlooked if the product potential can offset development work required.

This information is not to be taken as a warranty or representation for which we assume legal responsibility, nor as permission or recommendation to practice any patented invention without a license. It is offered solely for your consideration, investigation, and verification.