High electrical resistance alloys: features and applications

Alloys with elevated electrical resistance are used to manufacture heating elements of industrial equipment and household appliances, temperature sensors, resistors and strain gauges, critical elements of vacuum devices, and connectors in electrical products. They are made on the basis of nickel (Ni), chromium (Cr), molybdenum (Mo), aluminum (Al), and iron (Fe). In production they take the form of cold-rolled strip, band, bar, or wire.

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Classification of alloys with high electrical resistance

Alloys with elevated electrical resistance are divided into groups according to purpose and composition:

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• alloys for heating elements based on iron, chromium, and aluminum — retain high heat resistance in vacuum, hydrogen, sulfur- and carbon-containing environments; under continuous high-temperature exposure they tend to deform;
• materials for heaters based on nickel-chromium — have high resistance to oxidation and deformation at elevated temperatures in vacuum, hydrogen, nitrogen, and ammonia; in sulfur and sulfur compounds metal heat resistance decreases;
• alloys for manufacturing precision resistors — characterized by dependence of conductor resistance on temperature change; composition may include tungsten, manganese, chromium, nickel, aluminum, copper, beryllium, molybdenum, vanadium.

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Regardless of composition base, alloys for heaters do not oxidize and retain high heat resistance in atmospheres with excess oxygen.

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Steel grades and production features

The following steel grades are used to manufacture furnace electric heaters and heating elements of household appliances:

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• Kh15Yu5, Kh23Yu5, Kh23Yu5T, and Kh27Yu5T — precision alloys based on chromium-aluminum. Per GOST 10994-74 “Precision alloys. Grades,” fresh charge materials must be used in their production. In the steels listed, except Kh23Yu5T, no more than 0.1% zirconium is allowed.
• KhN60YuZ, KhN70Yu, Kh15N60-N, Kh15N60YuZA, Kh20N80-N, and KhN20YuS — nickel-chromium compounds. Per GOST 10994-74, steels Kh15N60-N and Kh20N80-N must be made in induction or plasma furnaces (if the latter has a ceramic crucible).

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In steels grade Kh20N80 residual calcium, magnesium, barium, and REE (rare-earth elements) are allowed. In Kh20N80-VI the presence of zirconium and REE is considered a reject.

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Properties of alloys with high electrical resistance

One requirement for alloys with elevated electrical resistance is high ductility. This makes it possible to manufacture wire with cross-section from 0.02 mm and strip from 0.01 mm thick.

In addition, all precision steels of this class except those intended for resistors have minimal variation of electrical resistance along the entire length of the heating element and a high maximum working temperature (depending on steel grade it may reach +1,100°C, +1,200°C, +1,350°C, and +1,400°C). Alloys for resistors retain their electrical properties when heated to +200°C…+600°C.

Heating heat-resistant alloys causes formation of a protective film on element surfaces that in turn prevents metal oxidation on contact with oxygen. But if air access to the metal surface is excluded, product service life can be increased many times. That is why in tubular heaters (TENs) wire from a precision alloy is placed in a housing of oxidation-resistant metal, and the space between them is filled with dielectric powder.