Heat conductor alloy

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Heat conductor alloys are alloys made from two or more metals that have a relatively high specific electrical resistance and a low tendency to oxidize.

Their job is to convert electrical power into heat .

Alloying makes use of the positive properties of the individual metals for adaptation to the intended use as a heating resistor .

Function and application forms

The flow of electrical current turns a heating conductor into a heating element as a result of its electrical resistance , which generates heat inside.

The current heat is proportional to the square of the current intensity and proportional to the square of the electrical voltage applied to a given heating element .

Heating coils are heating conductors that are bent into a helix and used in heating elements (hotplates, room heaters, boilers). Heating conductors are often laid or embedded in protective ceramic molded parts, sand or quartz glass tubes . Also mica serves as a holder.

Requirements and examples

Heating conductor materials are predominantly alloys. For particularly high temperatures, however, silicon carbide , molybdenum disilicide , platinum and - in the absence of oxygen - graphite and tungsten are also used. Pure tungsten wire is used there to heat the cathodes of electron tubes.

The material must not melt when it is heated and must be corrosion-resistant and scale-resistant even if it is heated and cooled frequently. The aim is also high strength at the operating temperature and a low tendency to recrystallize.

In order to be able to use short, thick heating conductors (strength, corrosion), a high specific resistance is advantageous.

In the following tables some physical properties of metals, alloys and other heating conductor materials used as heating conductors are listed under their trade names.

material Specific resistance
ρ (20 ° C) in 		Electrical conductivity
ϰ in
Pure copper (for comparison) 0.01720 60.00
nickel 14.60
chrome 06.70
Nickelin 0.4 / 0000.33000 03.0 / 02.50
Manganin 0.43000
Constantan 0.49000 02.04
30Mn 70Cu 1.00000 01.00
Nichrome (80% Ni , 20% Cr ) 1.1000 0.90000
Ferrochronin (NiCr15Fe) 1.030000
Sicromal (X10CrAlSi13) 0.75000
Kanthal® A-1 (FeCrAl alloy) 1.45000 00.69
manganese 1.44 00.69
graphite 13.8 ... 40 00.025 ... 0.073

The evaluation of the figures shows that the addition of nickel and zinc by alloying increases the specific electrical resistance to pure copper by a factor of 28 (nickel-lin). If the zinc content is replaced by increasing the nickel and only 1% manganese (constantan), the specific resistance even increases to 30 times that of copper.

The list also shows that the presence of iron, chromium and aluminum (see Kanth al ) in the alloys significantly increases the resistance to those based on copper-nickel. Nickel with 0.4 contrasts with the alloy Kanthal® A-1 (FeCr22Al6) with 1.45  .

The development of alloys suitable for heating conductors goes back to the beginning of the 20th century and even further. After the invention of the light bulb by Thomas Alva Edison , the search was on for materials that were more durable than a carbon thread and could withstand very high temperatures. A solution was offered by alloys made from osmium and iridium , later from osmium and tungsten (brand name Osram). Abundantly available tungsten (made of wolframite, scheelite / tungsten) with its melting point F = 3380 ° C replaced osmium and iridium (brand name Tungsram).

Nickel-copper alloys

The first resistance wires were made of nickel lin , an alloy of copper, nickel and manganese with a high specific resistance and a very low coefficient of thermal expansion. The "Isabellin" named after the market leader is typical of the genre. This also applies to copper-manganese alloys with additions not only of aluminum, but also other elements, which are collectively referred to as Heusler's alloys . Related to them are very corrosion-resistant alloys with a high nickel content, such as the standardized constantan with 56% copper and 44% nickel, max. 1% manganese. Two-component systems made of nickel and <20% chromium are also efficient heating conductor alloys.

Nickel-copper alloys as resistance material are standardized in DIN 17471. CuNi44 is used for heating resistors in wire form. As wrought material, the alloys are standardized under DIN 17664. The application range is between 500 and 600 ° C, they melt at 1230–1290 ° C, so they cannot be used in iron metallurgy, even in some heavy metal areas.

Chrome steels

Ferritic chromium steels to which up to 5% aluminum is alloyed to form a corrosion-inhibiting oxide layer. They have a high melting point with the addition of aluminum reduced from 5% to 2.5–3%, its effect supported by up to 0.3% yttrium, hafnium and zirconium.

Alloys are e.g. B. CrAl 25 5, i.e. 5% and 70% iron, and CrAl 20 5 with 75% iron. The increasing iron content causes austenitic and ferritic microstructure states with correspondingly higher temperature resistance and service life. All aluminum-containing alloys are characterized by a temperature-resistant protective layer made of aluminum alpha oxide ( corundum ) that forms during use .

Nickel-chromium alloys

Chromine can be found in the literature as early as 1955 as a non-standardized alloy "for the production of electrical heating resistors". It contains 83–84% nickel, the rest chromium. Chromel A and Chromel B are closely related . Chromel C , on the other hand, is a three-component alloy with 25% iron, 11% chromium, the remainder nickel. All are used for heating resistors. Chromel P contains 10% chromium, the remainder nickel and is used together with Alumel wire as a thermocouple (type K) up to max. 1100 ° C (temporarily 1300 ° C) used.

The heating conductor alloys based on nickel and chromium, standardized in accordance with DIN 17470, are both two-component and three-component alloys with iron as a determining factor. The standard includes NiCr 80 20, NiCr 60 15 with 25% iron, NiCr 30 20 with 50% iron, and CrNi 25 20 with 55% iron.

Kanthal

Kanthal® is a trademark of the Sandvik Group for various electrical heating products.

Kanthal was originally a heat conductor alloy with a defined composition. An alloy of iron, chromium and aluminum with a resistance of up to 1400 ° C was developed by the company of the same name as early as 1931. Later, the Kanthal brand was also used for copper and nickel-based heating conductor alloys and also for solid heating elements made of silicon carbide (also known as Silit, Carborundum).

Application examples

To generate heat from electricity, the heating conductors are usually kept insulated and often additionally protected by a metal tube.

Appliances such as electric cookers , hot plates , washing machines and dishwashers , water heater , hair dryer , water heater , water heater , heaters , soldering irons , or immersion heaters , etc. contain heating conductors made of alloys.

In metallurgy, heatable core molds ( hotbox process for hardening synthetic resin-bonded core sands) are heated with heating conductors; further examples are resistance-heated crucible melting furnaces or furnaces for electrical melting, keeping melts warm and the tempering of pressed and rolled bars for processing into semi-finished products .

literature

  • Paul Krais: Materials . Volume 2, Verlag J. Barth, Leipzig 1921, DNB 368670562 .
  • Foundry Lexicon. 17th edition. Verlag Schiele & Schön, Berlin 1997, ISBN 3-7949-0606-3 .
  • Lexicon of metal technology. Verlag A. Hartleben, Vienna / Pest / Leipzig, no year.
  • Copper-nickel alloys, properties, processing, application. (= DKI information print. No. 014). Publisher DKI, Berlin 1992.
  • The new Brockhaus. Verlag FA Brockhaus, Wiesbaden 1974, ISBN 3-7653-0025-X .
  • Sandvik AB , “materials technology” website (page on Kanthal).

Individual evidence

  1. ↑ The New Brockhaus. Volume 2, Brockhausverlag, Wiesbaden 1974, ISBN 3-7653-0025-X .
  2. ↑ J. D'Ans, E. Lax (Ed.): Pocket book for chemists and physicists. 2nd Edition. Springer Verlag Berlin 1949, DNB 450093328 ; Foundry calendar 1993. Foundry publishing house, Düsseldorf, ISBN 3-87260-111-3 .
  3. ↑ http ://woite-ed Nahrungsmittel.info/24816de.html data sheet material no .: 2.4816 from M. Woite GmbH, accessed on May 11, 2019.
  4. ↑ http://www.metalcor.de/datenblatt/51/ data sheet material 1.4724 Fa, Metalcor, accessed on May 11, 2019.
  5. ↑ https://www.kanthal.com/en/products/material-datasheets/wire/resistance-heating-wire-and-resistance-wire/kanthal-a-1/ data sheet from Sandvik, accessed on May 11th 2019.
  6. ↑ Graphite has a strongly negative temperature coefficient of the specific electrical resistance
  7. ↑ Franz Pawlek: metallurgy. Verlag Walter de Gruyter, 2011, ISBN 978-3-11-007458-1 , p. 341.
  8. ↑ https://www.calculand.com/ Einheit-umrechner/stoffe-liste.php?gruppe=Specific+Widerstand&einheit=1e-3--m%E2%84%A6m Norbert Schneider (online offer): value for arc coal , accessed on May 11, 2019.
  9. ^ Isabellenhütte Heusler GmbH
  10. ↑ see details DKI - information print 014 with tables no. 6–8 on copper-nickel resistance alloys and graphics on electrical resistance, coefficient of linear expansion, thermal conductivity and other information on properties.
  11. ↑ Trade press release from Thyssen Krupp on ALUCHROM ECO on March 29, 2004.
  12. ^ Foundry Lexicon. 17th edition. Verlag Schiele & Schön, Berlin 1997, ISBN 3-7949-0606-3 .
  13. ↑ Look for - Nature. VEB Bibliographisches Institut, Leipzig 1956, DNB 454364024 .
  14. ↑ Handbook Kanthal heating conductor alloys . Catalog 1-A-2-2 01.97 3000, 1997, p. 6.