Improved Nickel Chromium Wire Heating

Improvement of Resistance heating wire and resistance heating strip in iron-chromium-aluminium (FeCrAl) alloys and nickel-chromium (NiCr) alloys for the manufacturing of electric heating elements. Using FeCrAl alloys instead of NiCr alloys result in both weight-saving and longer element life.

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Resistance Heating Wire and Strip

FeCrAl Resistance Heating Alloys

Resistance heating alloys based on iron-chromium-aluminium for maximum element temperature of °C ( °F). Fecral resistance heating alloys are characterized by high resistivity and capabiltity to withstand high surface load.

NiCr resistance heating alloys

Nickel-chromium-based resistance heating alloys suitable for element temperatures up to °C (°F). Nickel Chromium resistance heating alloys are characterized by very good mechanical properties in the hot state as well as good oxdiation and corrosion properties.

Product forms and size ranges

Resistance heating alloys are available in the following product forms and sizes:

• Round wire: 0.10-12 mm (0.-0.472 inch)
• Ribbon (flat wire): thickness: 0.023-0.8 mm (0.-0.031 inch)
  Width: 0.038-4 mm (0.-0.157 inch)
  Width/thickess ratio max 40, depending on alloy and tolerance
• Strip: thickness 0.10-5 mm (0.-0. inch), width 5-200 mm (0.-7.874 inch)

Improvement of Heating Properties of NiCr

In the nickel chromium alloys the chromium is easily soluble in the nickel. The solubility is highest at 47% concentration at the eutectic temperature and lowers at 30% at the normal room temperature. The group of industrial nickel chromium alloys is based on firm solution of nickel chromium metals. This nickel chromium alloyoffers large resistance to oxidation conditions at the high temperature as well as it as appreciable wear resistance.

With the inclusion of little concentration of chromium in the Nickel the alloy's potency to oxidation is improved. The reason is that the dispersion rate of oxygen has been improved.This process alters after the addition grades increased by 7% chromium and improves to increased level by 30%. Beyond this grade, minor changes occur.

The oxidation resistance of nickel chromium heating wire to the production of extensively adherent secured level. The adherent and coherent level can further be enhanced with the inclusion of little concentrations of other materials like zirconium, silicon, cerium, calcium and more. The level produced is the combination of nickel and chrome oxides. These add up to produce nickel chromite that possesses spinal shape.

Significant improvement in the resistance of nickel chromium heating wire is noticed with the chromium addition such that 20% chromium is said to be the suitable material amount for resistance in the electrical equipments. Such combination provides excellent electrical features with fine potency and ductility that the material suitable for drawing.

The little upgradation for this composition can be made to improve the wire for certain operations. With the inclusion of suitable reactive alloy metals an alteration in the properties is certain. The performance conditions of nickel chromium alloy wire are extremely influenced by its composition.

However the concentration alternations have minor impact on mechanical features, large concentration of reactive metals causes to avoid the flaking of scale while periodic heating and cooling.

The nickel chromium wire with binary expression of 90/10 is used for heating operations and it has highest performance temperature of degree Celsius. Moreover nickel chromium heating wire is also used in thermocouples.The combination of nickel and chromium in 90:10 ratio is more preferred for thermocoupling as compare to 95:5 nickel chromium addition.

Alloy Electrical Resistance Heating Elements

The article relates to a special nickel-chromium heat resistant alloy particularly to electrical resistance heating elements made of such alloys and having improved service life when subjected in use to elevated temperatures, especially under conditions involving repeated heating and cooling.

Nickel-chromium alloys used for electrical resistance heating elements contain small amounts of both calcium and other rare earth metals for the purpose of increasing the service life and these alloys contain small amounts of other elements like silicon. In making the alloys, the cerium is commonly used as Mischmetall, and the term cerium is used herein to mean not only cerium itself but also any other rare earth metals present. In practice the silicon contents of nickel-chromium alloys containing both calcium and cerium have hitherto been very low of the order of 0.5%. While such nickel-chromium alloys have been found beneficial when employed as electrical resistance heating elements, demands by industry for heating elements with greatly improved service lives have placed further burdens on such alloys with the result that the problem of providing improved alloys to meet the needs of industry has been greatly accentuated. Although many attempts were made to meet the needs and demands of industry, none were entirely successful when carried into practice commercially on an industrial scale.

By employing higher and critical silicon content in conjunction with a very low and critical cerium content, it is possible to considerably increase the service life of an electrical resistance element made of such an alloy. To be forgeable, the alloy must also contain a critical amount of calcium. The objective is to provide a special heat resistant nickel-chromium alloy having improved service life at elevated temperatures. And also provide special electrical resistance heating elements characterized by improved performance at elevated service temperatures, especially under conditions involving repeated heating and cooling.

A critical amount of silicon proves improvement on the average life in hours of a nickel-chromium alloy containing a critical amount of cerium and tested in accordance with designation B 76-39 of the American Society for Testing Materials (ASTM). The critical effect of cerium on the average life in hours of a nickel-chromium alloy containing a critical amount of silicon is likewise tested in accordance with the aforementioned ASTM designation B 7 6-39.

Nickel-chromium alloys of for electrical resistance heating elements contain from 10 to 2.5%, chromiumr from 0.0.05 to 0.051% calcium. 0.01 to 0.1% cerium and 1.15 to 2% sililcon and the balance (except for impurities) being of nickel. Preferably these elements are present in newer ranges, namely from 15 to. 2.5% Chromium, to. 0.03% calcium, from 0.025 to 0.06% cerium and from 1.4 to 1.6% silicon. Although these heating elements are concerned with nickel-chromium alloys, as distinguished from nickel-chromium-iron alloys, iron is present as an impurity in the raw materials and as a consequence the alloys of the heating element may contain upto 2% iron. Moreover some of the nickel in an amount up to 15% of the total alloy may be replaced by cobalt. Nickel Chromium alloys contain various other elements without detriment, namely up to 1% aluminum, upto 0.3% carbon, upto 0.16% copper and upto .3% manganese. The impurities present may includey traces of various other elements like titanium.

In evaluating heat resistant alloys of the type containing nickel and 20% chromium for use as electrical resistance heating elements, an accelerated life test is employed in accordance. with the American Society for Testing Materials designation B I6-39. In this test, the alloy specimen in the form of a wire measuring about 12 inches long and having a diameter corresponding to not larger than No. 20 American Wire Gauge (AWG) nor smaller than No. 22 AWG i. e., within the range of about 0.025 inch to 0.032 inch, is subjected to intermittent heating and cooling under prescribed conditions at a temperature of about °C., the heating being accomplished by passing electric current through the wire. The service lives of electrical resistance wires of a number of alloys have been measured in accordance with the aforementioned ASTM test. The percentage compositions of some of the alloys tested and the results obtained. The first alloy was a typical nickel chromium alloy. The second alloy had a silicon content which, though much higher than usual, was still below the preferred range. The third alloy has silicon and cerium contents both within the preferred ranges. The results of tests conducted on the alloy provided indicated markedly improved service lives are obtained when the alloy contains about 0.03% to 0.05% cerium, particularly when the alloy contains 1.4% to 1.6% silicon.

The way in which the service life varies with the silicon content shows the average lives obtained with electrical resistance wires of alloys containing about 0.010% calcium, 0.04% cerium, 0,2% aluminum, 20% chromium and 0.4% iron and of varying silicon contents. It is seen that as the silicon content rises above the normal low figure, there is no appreciable increase in the service life until it is about 1.0%. At this figure the service life begins to increase at a rate which itself rapidly increases. Between 1.15 and 1.4%, the increase is most striking, the service life rising to at least three times the value at 0.6% silicon. When the silicon content exceeds 1.4%, the rate remains approximately constant up to 2%.

The critical nature of the cerium content shows the average lives obtained with electrical resistance wires of alloys containing about 0.010% calcium, 0.2% aluminum, 1.5% silicon, 20% chromium and 0.4% iron, and of varying cerium contents. It is seen that in the narrow range of 0.03 to 0.05% cerium, the best lives are obtained. Generally, alloys provided as above exhibit service lives at °C., as determined by the ASTM designation B 76-39 of the order of about 400 hours and higher, while the best commercially available alloys of the 80-20 nickelchromium type of alloy which do not show good lives of this order when tested in this manner. The term service life in this specification refers in all cases to the total life to burn-out of the wire, since this occurs before the resistance of the wire has increased by 10 percent.

So, an electric resistance heating element made of a nickel-chromium alloy consisting essentially of 15% t0 25% chromium, 0.01% to 0.03% calcium, 1.4% to 1.6% silicon, 0.03% to 0.05% cerium, upto 15% cobalt, upto 2% iron, upto 3% manganese, upto 1% aluminum, upto 0.16% copper, upto 0.3% carbon, and the balance consisting essentially of nickel, the electric resistance heating element being characterized by markedly improved service life of at least 400 hours when subjected to intermittent heating and cooling at an elevated temperature of °F in accordance with ASTM designation B 76-39.

And a heat resistant nickel-chromium alloy adapted for the manufacture of electrical resistance heating elements and consisting essentially of 15% to 25% chromium, 0.01% to 0.03% calcium, 1.4% to 1.6% silicon, 0.03% to 0.05% cerium, upto 15% cobalt, upto 2% iron, upto 3% manganese, upto 1% aluminum, upto 0.16% copper, upto 0.3% carbon, and the balance consisting essentially of nickel, characterized by improved service life of at least 400 hours when subjected as an electrical resistance wire to intermittent heating and cooling at an elevated temperature of °F in accordance with ASTM designation B 76-39.

Nickel-chromium heating element alloy having improved operating life

The "80/20" nickel-chromium alloy in wire or strip form is used extensively as the heating element in resistance heating applications. An accepted means for evaluating the performance of a heating element is by ASTM life test B76-65. In this test, a constant temperature of °F on a 0. inch diameter wire, maintained by resistance heating, is applied at "2 minute on - 2 minute off" intervals until failure by burnout occurs. This life test may be significantly accelerated by raising the wire being tested to a temperature of °F, while keeping all other test conditions the same. In addition, carrying out the test as a constant temperature test, by changing the power supplied to the sample during the test, is a more severe test than a constant voltage test or constant current test which have been used in the past. In a constant voltage test, the input voltage is maintained constant throughout the test. Because of high temperature oxidation, the effective diameter of the wire decreases, causing an increase in resistance. This in turn cause a decrease in electrical current flowing through the wire, because of the constant voltage. The net result is a decrease in power supplied to the wire, and a significant decrease in test temperature. Therefore, the test temperature toward the end of a constant voltage life test could be 100°F lower than the initial temperature. On this basis, the constant temperature test is much more severe than the constant voltage test and results from these tests should not be directly compared without an understanding of the boundary condition of these two tests. The average life to failure at °F of a commercial 80/20 nickel-chromium alloy produced is 197 hours.

The beneficial effect of zirconium upon operating life of the 80/20 nickel-chromium alloy heating elements is known. The addition of calcium and zirconium to such an alloy increases its operating life. The addition of aluminum with calcium and zirconium to nickel-chromium-iron alloys also does the same. Subsequently the addition of calcium, aluminum and rare earths to improve life of nickel-chromium-iron alloys over lives obtainable for such alloys containing calcium, aluminum and zirconium. Zirconium has also been added to nickel-chromium-iron alloys of the superalloy type, high temperature resistant and corrosion resistant

However, the addition of zirconium to nickel-chromium alloys for the purpose of extending life of heating elements of these alloys has several attendant disadvantages, including a detrimental effect upon workability of the alloys at addition levels approaching 0.2 weight percent, loss of zirconium during charging into the alloy melt, and variations of such charge losses from melt to melt. All of these factors have led to difficulty and expense in producing heating element alloys of predictably long operating lives by the addition of zirconium.

It is felt that significant increases in the operating life of an 80/20 nickel-chromium alloy without attendant processing difficulties would enable longer life of heating elements incorporating these alloys, or alternatively enable smaller size heating elememts without a corresponding reduction in operating life, and that accordingly such increases in operating life would be an advancement in the art.

The addition of from about 0.1 to 0.75 weight percent of hafnium to a nickel-chromium heating element alloy having a nominal base composition of 20 weight percent chromium, 1.4 weight percent silicon, trace amounts of Ca, Al and B, up to 0.5 weight percent total, balance essentially nickel, significantly increases operating life of the alloy as a heating element over alloys which do not contain hafnium. For example, the average operating life at °F of the nominal base composition alloy plus about 0.17 to 0.58 weight percent hafnium is about 250 hours, more than 100 hours greater than the average life of the nominal base composition alloy containing neither hafnium nor zirconium. The alloys thus made would find use in resistance heating applications where longer operating lives or smaller sizes of heating elements are desired. It appears that an increase in operating life of from about 20-25 percent over that of the common alloy could be achieved by the use of hafnium additions in the amounts specified to the 80/20 nickel-chromium based alloy.

Thus, a nickel-chromium heating element alloy consisting essentially of a base composition in weight percent within the range of: 18 to 22 percent chromium, 1.0 to 1.6 weight percent silicon, balance essentially nickel, characterized in that the alloy contains from about 0.1 to 0.75 weight percent hafnium, whereby the operating life of the alloy as a resistance heating element is improved.

Elements are manufactured using the heavy copper pipe and quality chrome-plated superior Kanthal and Nichrome wire, which increases their efficiency. Nichrome wire is an alloy made from nickel and chromium and copper nickel improves on the melting point of bronze and can endure high heat without softening.

Device that converts electricity into heat

A heating element is a device used for conversion of electric energy into heat, consisting of a heating resistor and accessories.[1] Heat is generated by the passage of electric current through a resistor through a process known as Joule Heating. Heating elements are used in household appliances, industrial equipment, and scientific instruments enabling them to perform tasks such as cooking, warming, or maintaining specific temperatures higher than the ambient.

Heating elements may be used to transfer heat via conduction, convection, or radiation. They are different than devices that generate heat from electrical energy via the Peltier effect, and have no dependence on the direction of electrical current.

Principles of operation

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Resistance & resistivity

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A piece of resistive material with electrical contacts on both ends

Materials used in heating elements have a relatively high electrical resistivity, which is a measure of the material's ability to resist electric current. The electrical resistance that some amount of element material will have is defined by Pouillet's law as R = ρ &#; A {\displaystyle R=\rho {\frac {\ell }{A}}} where

• R {\displaystyle R}

• ρ {\displaystyle \rho }

• &#; {\displaystyle \ell }

  length of the specimen

• A {\displaystyle A}

  cross-sectional area of the specimen

The resistance per wire length (Ω/m) of a heating element material is defined in ASTM and DIN standards.[2]:&#;2&#;[3][4] In ASTM, wires greater than 0.127 mm in diameter are specified to be held within a tolerance of ±5% Ω/m and for thinner wires ±8% Ω/m.

Power density

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Heating element performance is often quantified by characterizing the power density of the element. Power density is defined as the output power, P, from a heating element divided by the heated surface area, A, of the element.[5] In mathematical terms it is given as:

Φ = P / A {\displaystyle \Phi =P/A}

Power density is a measure of heat flux (denoted Φ) and is most often expressed in watts per square millimeter or watts per square inch.

Heating elements with low power density tend to be more expensive but have longer life than heating elements with high power density.[6]

In the United States, power density is often referred to as 'watt density.' It is also sometimes referred to as 'wire surface load.'

Components

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Resistance heater

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Wire

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A coiled heating element from an electric toaster

Resistance wires are very long and slender resistors that have a circular cross-section. Like conductive wire, the diameter of resistance wire is often measured with a gauge system, such as American Wire Gauge (AWG).[7]

Ribbon

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Resistance ribbon heating elements are made by flattening round resistance wire, giving them a rectangular cross-section with rounded corners.[8]:&#;54&#; Generally ribbon widths are between 0.3 and 4 mm. If a ribbon is wider than that, it is cut out from a broader strip and may instead be called resistance strip. Compared to wire, ribbon can be bent with a tighter radius and can produce heat faster and at a lower cost due to its higher surface area to volume ratio. On the other hand, ribbon life is often shorter than wire life and the price per unit mass of ribbon is generally higher.[8]:&#;55&#; In many applications, resistance ribbon is wound around a mica card or on one of its sides.[8]:&#;57&#;

Coil

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Resistance coil is a resistance wire that has a coiled shape.[8]:&#;100&#; Coils are wound very tightly and then relax to up to 10 times their original length in use. Coils are classified by their diameter and the pitch, or number of coils per unit length.

Insulator

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Heating element insulators serve to electrically and thermally insulate the resistance heater from the environment and foreign objects.[9] Generally for elements that operate higher than 600 °C, ceramic insulators are used.[8]:&#;137&#; Aluminum oxide, silicon dioxide, and magnesium oxide are compounds commonly used in ceramic heating element insulators. For lower temperatures a wider range of materials are used.

Leads

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Electrical leads serve to connect a heating element to a power source. They generally are made of conductive materials such as copper that do not have as high of a resistance to oxidation as the active resistance material.[8]:&#;131&#;132&#;

Terminals

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Heating element terminals serve to isolate the active resistance material from the leads. Terminals are designed to have a lower resistance than the active material by having with a lower resistivity and/or a larger diameter. They may also have a lower oxidation resistance than the active material.[8]:&#;131&#;132&#;

Types

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Heating elements are generally classified in one of three frameworks: suspended, embedded, or supported.[8]:&#;164&#;166&#;

• In a suspended design, a resistance heater is attached at two or more points to normally either a ceramic or mica insulator. Suspended resistance heaters can transfer heat via convection and radiation, but not conduction as they are surrounded by air.
• In an embedded heating element, the resistance heater is encased in the insulator. In this framework the heater can only transfer heat via conduction to the insulator.
• Supported heating elements are a combination of the suspended and embedded frameworks. In these assemblies, the resistance heater can transfer heat via conduction, convection, or radiation.

Tubes (Calrods®)

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Tubular electric heater.

1. Resistance heating element
2. Electrical insulator
3. Metal casing

Tubular oven heating element

Tubular or sheathed elements (also referred to by their brand name, Calrods®[10]) normally comprise a fine coil of resistance wire surrounded by an electrical insulator and a metallic tube-shaped sheath or casing. Insulation is typically a magnesium oxide powder and the sheath is normally constructed of a copper or steel alloy. To keep moisture out of the hygroscopic insulator, the ends are equipped with beads of insulating material such as ceramic or silicone rubber, or a combination of both. The tube is drawn through a die to compress the powder and maximize heat transmission. These can be a straight rod (as in toaster ovens) or bent to a shape to span an area to be heated (such as in electric stoves, ovens, and coffee makers).

Screen-printed elements

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Are you interested in learning more about Nickel Base Alloy? Contact us today to secure an expert consultation!

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Screen-printed metal&#;ceramic tracks deposited on ceramic-insulated metal (generally steel) plates have found widespread application as elements in kettles and other domestic appliances since the mid-s.

Radiative elements

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Radiative heating elements (heat lamps) are high-powered incandescent lamps that run at less than maximum power to radiate mostly infrared instead of visible light. These are usually found in radiant space heaters and food warmers, taking either a long, tubular form or an R40 reflector-lamp form. The reflector lamp style is often tinted red to minimize the visible light produced; the tubular form comes in different formats:

• Gold-coated &#; Made famous by the patented Phillips Helen lamp. A gold dichroic film is deposited on the inside that reduces the visible light and allows most of the short and medium wave infrared through. Mainly for heating people. A number of manufacturers now manufacture these lamps and they improve constantly.
• Ruby-coated &#; Same function as the gold-coated lamps, but at a fraction of the cost. The visible glare is much higher than the gold variant.
• Clear &#; No coating and mainly used in production processes.

Removable ceramic core elements

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Removable ceramic core elements use a coiled resistance heating alloy wire threaded through one or more cylindrical ceramic segments to make a required length (related to output), with or without a center rod. Inserted into a metal sheath or tube sealed at one end, this type of element allows replacement or repair without breaking into the process involved, usually fluid heating under pressure.

Etched foil elements

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Etched foil elements are generally made from the same alloys as resistance wire elements, but are produced with a subtractive photo-etching process that starts with a continuous sheet of metal foil and ends with a complex resistance pattern. These elements are commonly found in precision heating applications like medical diagnostics and aerospace.

Polymer PTC heating elements

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A flexible PTC heater made of conductive rubber

Resistive heaters can be made of conducting PTC rubber materials where the resistivity increases exponentially with increasing temperature.[11] Such a heater will produce high power when it is cold, and rapidly heat itself to a constant temperature. Due to the exponentially increasing resistivity, the heater can never heat itself to warmer than this temperature. Above this temperature, the rubber acts as an electrical insulator. The temperature can be chosen during the production of the rubber. Typical temperatures are between 0 and 80 °C (32 and 176 °F).

It is a point-wise self-regulating and self-limiting heater. Self-regulating means that every point of the heater independently keeps a constant temperature without the need of regulating electronics. Self-limiting means that the heater can never exceed a certain temperature in any point and requires no overheat protection.

Thick-film heaters

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A thick-film heater printed on a mica sheet Thick-film heaters printed on a metal substrate

Thick-film heaters are a type of resistive heater that can be printed on a thin substrate. Thick-film heaters exhibit various advantages over the conventional metal-sheathed resistance elements. In general, thick-film elements are characterized by their low-profile form factor, improved temperature uniformity, quick thermal response due to low thermal mass, high energy density, and wide range of voltage compatibility. Typically, thick-film heaters are printed on flat substrates, as well as on tubes in different heater patterns. These heaters can attain power densities of as high as 100 W/cm2 depending on the heat transfer conditions.[12] The thick-film heater patterns are highly customizable based on the sheet resistance of the printed resistor paste.

These heaters can be printed on a variety of substrates including metal, ceramic, glass, and polymer using metal- or alloy-loaded thick-film pastes.[12] The most common substrates used to print thick-film heaters are aluminum -T6, stainless steel, and muscovite or phlogopite mica sheets. The applications and operational characteristics of these heaters vary widely based on the chosen substrate materials. This is primarily attributed to the thermal characteristics of the substrates.

There are several conventional applications of thick-film heaters. They can be used in griddles, waffle irons, stove-top electric heating, humidifiers, tea kettles, heat sealing devices, water heaters, clothes irons and steamers, hair straighteners, boilers, heated beds of 3D printers, thermal print heads, glue guns, laboratory heating equipment, clothes dryers, baseboard heaters, warming trays, heat exchangers, deicing and defogging devices for car windshields, side mirrors, refrigerator defrosting, etc.[13]

For most applications, the thermal performance and temperature distribution are the two key design parameters. In order to maintain a uniform temperature distribution across a substrate, the circuit design can be optimized by changing the localized power density of the resistor circuit. An optimized heater design helps to control the heating power and modulate the local temperatures across the heater substrate. In cases where there is a requirement of two or more heating zones with different power densities over a relatively small area, a thick-film heater can be designed to achieve a zonal heating pattern on a single substrate.

Thick-film heaters can largely be characterized under two subcategories &#; negative-temperature-coefficient (NTC) and positive-temperature-coefficient (PTC) materials &#; based on the effect of temperature changes on the element's resistance. NTC-type heaters are characterized by a decrease in resistance as the heater temperature increases and thus have a higher power at higher temperatures for a given input voltage. PTC heaters behave in an opposite manner with an increase of resistance and decreasing heater power at elevated temperatures. This characteristic of PTC heaters makes them self-regulating, as their power stabilizes at fixed temperatures. On the other hand, NTC-type heaters generally require a thermostat or a thermocouple in order to control the heater runaway. These heaters are used in applications which require a quick ramp-up of heater temperature to a predetermined set-point as they are usually faster-acting than PTC-type heaters.

Liquid

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An electrode boiler uses electricity flowing through streams of water to create steam. Operating voltages are typically between 240 and 600 volts, single or three-phase AC.[14]

Laser heaters

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Laser heaters are heating elements used for achieving very high temperatures.[15]

Materials

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Materials used in heating elements are selected for a variety of mechanical, thermal, and electrical properties.[9] Due to the wide range of operating temperatures that these elements withstand, temperature dependencies of material properties are a common consideration.

Metal alloys

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Resistance heating alloys are metals that can be used for electrical heating purposes above 600 °C in air. They can be distinguished from resistance alloys which are used primarily for resistors operating below 600 °C.[8]

While the majority of atoms in these alloys correspond to the ones listed in their name, they also consist of trace elements. Trace elements play an important role in resistance alloys, as they have a substantial influence on mechanical properties such as work-ability, form stability, and oxidation life.[8] Some of these trace elements may be present in the basic raw materials, while others may be added deliberately to improve the performance of the material. The terms contaminates and enhancements are used to classify trace elements.[9] Contaminates typically have undesirable effects such as decreased life and limited temperature range. Enhancements are intentionally added by the manufacturer and may provide improvements such as increased oxide layer adhesion, greater ability to hold shape, or longer life at higher temperatures.

The most common alloys used in heating elements include:

Ni-Cr(Fe) alloys (AKA nichrome, Chromel)

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Ni-Cr(Fe) resistance heating alloys, also known as nichrome or Chromel, are described by both ASTM and DIN standards.[2][4] These standards specify the relative percentages of nickel and chromium that should be present in an alloy. In ASTM three alloys that are specified contain, amongst other trace elements:

• 80% Ni, 20% Cr
• 60% Ni, 16% Cr
• 35% Ni, 20% Cr

Nichrome 80/20 is one of the most commonly used resistance heating alloys because it has relatively high resistance and forms an adherent layer of chromium oxide when it is heated for the first time. Material beneath this layer will not oxidize, preventing the wire from breaking or burning out.

Fe-Cr-Al alloys (AKA Kanthal®)

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Fe-Cr-Al resistance heating alloys, also known as Kanthal®, are described by an ASTM standard.[3] Manufacturers may opt to use this class of alloys as opposed to Ni-Cr(Fe) alloys to avoid the typically relatively higher cost of nickel as a raw material compared to aluminum. The tradeoff is that Fe-Cr-Al alloys are more brittle and less ductile than Ni-Cr(Fe) ones, making them more delicate and prone to failure.[16]

On the other hand, the aluminum oxide layer that forms on the surface of Fe-Cr-Al alloys is more thermodynamically stable than the chromium oxide layer that tends to form on Ni-Cr(Fe), making Fe-Cr-Al better at resisting corrosion.[16] However, humidity may be more detrimental to the wire life of Fe-Cr-Al than Ni-Cr(Fe).[8]

Fe-Cr-Al alloys, like stainless steels, tend to undergo embrittlement at room temperature after being heated in the temperature range of 400 to 575 °C for an extended duration.[17]

Other alloys

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Ceramics & semiconductors

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Applications

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Toaster with red hot heating elements

Heating elements find application in a wide range of domestic, commercial, and industrial settings:

• Home Appliances: Common household appliances such as ovens, toasters, electric stoves, water heaters, and space heaters rely on heating elements to generate the necessary heat for their functions.
• Industrial Processes: In industries, heating elements are integral to processes such as metal smelting, plastic molding, and chemical reactions that require controlled temperatures.
• Scientific Instruments: Laboratories use heating elements in various equipment, including incubators, furnaces, and analytical instruments.
• Automotive Industry: Heating elements are utilized in vehicles for applications like heated seats, rear window defrosters, and engine block heaters.

Life cycle

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The life of a heating element specifies how long it is expected to last in an application. Generally heating elements in a domestic appliance will be rated for between 500 and hours of use, depending on the type of product and how it is used.[8]:&#;164&#;

A thinner wire or ribbon will always have a shorter life than a thicker one at the same temperature.[8]:&#;58&#;

Standardized life tests for resistance heating materials are described by ASTM International. Accelerated life tests for Ni-Cr(Fe) alloys[22] and Fe-Cr-Al alloys[23] intended for electrical heating are used to measure the cyclic oxidation resistance of materials.

Packaging

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Resistance wire and ribbon are most often shipped wound around spools.[8]:&#;58&#;59&#; Generally the thinner the wire, the smaller the spool. In some cases pail packs or rings may be used instead of spools.

Safety

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General safety requirements for heating elements used in household appliances are defined by the International Electrotechnical Commission (IEC).[24] The standard specifies limits for parameters such as insulation strength, creepage distance, and leakage current. It also provides tolerances on the rating of a heating element.

See also

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References

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