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Everything About Heating Element Alloys

Wire Heating Element Materials

Introduction

Around the year 1900, W.C. Heraeus, a German-based firm developed the first commercial platinum furnace. In the year 1902, the company marketed a platinum ribbon-bound furnace that could reach a temperature of 1500ºC in 5 minutes, operate at 1500ºC for several hours and could attain a temperature of 1700ºC for brief time periods. For the past nearly 200 years, since that furnace was first developed, resistive electric furnaces have seen numerous advancements in insulation, controls and heating materials.

Understanding Heating Elements

The heaters for the industrial environment are usually power by an electrical source. All electric heaters with heater elements made out of specially designed electric heating rods. The typical heating elementsare made of steel or stainless steel. They are used in heating water or similar liquid medium for a general purpose and usually corrosion free. Other corrosion resistant materials used are a composition of alloy such as copper or titanium. They are most resistant to high temperature and withstand the highly corrosive medium. Recently, specially manufactured alloy such as Inconel® and Incoloy® were introduced for more advanced application.

The heating elements are a vital part of the industrial heater with various added benefits contingent to a particular application. The selection of heating elements depends largely on the type and nature of medium it is used for. In addition to the medium, the type of heaters it will be installed also has a bearing on what kind of alloy it must be constructed of. These elements are factory configured into any shape and size. They operate in an extremely high temperature as some heating elements are required to operate well above the process temperatures of 1,600⁰F.The material for heating elements vary depending on its application. The immersion heaters often require the material that are highly resistant to the breakdown in an extreme temperature and to stay immersed without giving in to an erosion factor. Having considered these conditions, stainless steel is an ideal choice for heating up water and other similar chemicals. The stainless steel is made of steel alloy with at least 10.5%, more likely 13% to 26%, of chromium content by mass called FeCrAl alloys. The biggest advantage of stainless steel as compared to the regular carbon steel is obviously the resistance to oxidization. However, stainless steel is not full-proof to erosion by no means. There are certain external environments such as low oxygen, high salinity or poor circulation under which stainless steel becomes vulnerable to a passive film of chromium oxides.

The use of exotic heating element alloys further enhances the ability of the heating elements to withstand its inherent erosive nature. Copper, for example, does not react to water so that it could avoid the normal oxidization. However, it ultimately reacts to the atmospheric oxygen over the prolonged use and forms a layer of copper oxide as opposed to iron oxide. The use of titanium relieves much concern for corrosion as one of its properties is a strong corrosion-resistance. An added benefit of titanium is the nature of its feather-weight as compared to other metals.

Heating Element Material Overview

There are a wide range of materials offered that can be utilised as heating elements for electric resistance furnaces. The materials include ceramic metal-based materials, metallic alloys and carbon or graphite materials. Certain cermets can be obtained in a wire form, a cermet being a robust alloy of a heat-resistant compound. This article focuses on traditional wire metallic alloys, the commonly present ones include tungsten, iron-chrome-aluminum, nickel-chrome, molybdenum, tungsten, platinum, tantalum and platinum-rhodium alloys. These alloys can be grouped into two classes, one that are workable when oxygen is present and the other that should be provided adequate protection from oxygen. The class of alloys that need to be protected from oxygen include tantalum, tungsten and molybdenum.

When temperatures increase, the atmosphere plays an important role as materials react in different ways to different compounds. It is possible that a system that functions perfectly at a particular temperature in air may fail rapidly if used at an identical temperature but a different atmosphere. The element's service life is also an essential service parameter. It is important to find out whether you need the element to work for some weeks, a few months or years. For any particular element, the higher the working temperature, the shorter its lifespan. In order to have a long life, the heating element must have a minimal head temperature with reference to the temperature of the furnace. This is possible when the wait loading is considerably lowered. It is essential to note that when the element loading is reduced, more elements need to be added to satisfy the heat load requirements of the furnace.

Types of Materials Used as Heating Elements

The different materials discussed below include the following:

Iron-Chrome-Aluminum Alloys

The most convenient and economic alloy to be used here is iron-chrome aluminum alloys. These also have a minimal operating temperature in an oxidizing atmosphere. nickel based Metallic alloy materials are quite rugged when subjected to thermal or mechanical shock. Their resistance does not change with respect to service life and element temperature. Using these two factors one can obtain a product that can be easily controlled, providing a convenient and cost-effective power supply. In doing so, the overall project capital costs are considerably reduced proving that this group of materials is really worth using. A large number of these alloys are offered in strip, wire, tube and rod forms. Standard element configurations include coils in ceramic tubes or grooves, a free radiating design termed as an ROB or a sinuous loop element or as a section of a packaged system in which the alloy is either implanted or mounted on a ceramic or insulation panel on floors, roofs or walls of the furnace.

Nickel-Chrome Alloys

Nickel-chrome alloys are probably the oldest electrical heating materials and are used widely even now. They exhibit properties of ductility, hot strength and form stability. The three commonly used compositions utilized in heat applications include the following:

Another alloy has been recently introduced that has a mix of 70% nickel and 30% chromium called NiCr 70:30 alloy. Among these alloys, NiCr 70:30 the 70/30 material has the highest maximum element temperature of 1250°C in air, and a maximum chamber temperature of 1150°C. The main reason for its introduction was to resist "Green rot". Green rot may be defined as an intergranular oxidation of chromium that occurs in other ASTM grades when utilized in either endothermic or exothermic atmospheres in a temperature range of 1500 to 1800°F.

Iron-Chrome-Aluminum Alloys

Iron-chrome-aluminum alloys have a standard mix of 72.5% iron, 22% chrome and 5.5% aluminum. The higher grades obtained from conventional melt technologies have temperatures restricted to 1300ºC for the chamber and 1400°C for the element. Several other grades are also offered where the amount of aluminum has been reduced and the balance comprises iron. The operating temperature and resistance are high and the density is low when compared to nickel-chrome alloys. This ensures a cost-effective, long-lasting heating element. Some disadvantages include low hot strength, lower ductility and embrittlement on use.

Iron-Chrome-Aluminum PM Grades

Recently, iron-chrome-aluminum alloys are used with powder metal (PM) technology in their manufacturing process. First a high-grade iron-chrome-aluminum alloy obtained from conventional melt technology is crushed into a powder form and then subjected to compression to form a billet. The billet is formed by a hot isostatic press or, sometimes, a cold isostatic pressing operation. From this billet the final strip, wire or tube product is obtained. Although the process is expensive and complicated, the hot strength and end-use temperature show a drastic increase.

A Detaild Analysis of Heating Element Materials

Many heating equipment's or appliances such as electric furnace, electric oven, electric heaters etc. utilise electrical energy to produce heat. In these equipment or appliances, heating elements are used to convert the electrical energy into the form of heat. The working of heating elements is based on heating effect of electric current. When a current is passed through a resistance, it produces the heat. To produce the heat, the electric energy consumed by resistance is given by,

E = I²Rt Joules

Where,
'I' is the current through the resistance (in A)
'R' is resistance of element (in Ω)
't' is the time (in seconds)

The performance and life of heating element depend on properties of the material used for heating element. The required properties in material used for heating elements-

  1. High melting point.
  2. Free from oxidation in open atmosphere.
  3. High tensile strength.
  4. Sufficient ductility to draw the metal or alloy in the form of wire.
  5. High resistivity.
  6. Low temperature coefficient of resistance.

Following material are used for manufacturing heating element-

  1. Nichrome
  2. Fecral
  3. Cupronickel
  4. Platinum

Nichrome

Composition of Nichrome

Ni = 80% + Cr = 20%

Properties of Nichrome

  1. Resistivity : 40 µΩ-cm
  2. Temperature coefficient of resistance: 0.0004 /°C
  3. Melting point: 1400 °C
  4. Specific gravity: 8.4 gm/cm³
  5. High resistance to oxidation

Use of Nichrome

Used in making heating elements for electric heaters and furnaces.

Note

Nichrome is best suitable and ideal material for making heating element. It has comparatively high resistance. When the heating element is heated first time, chromium of the alloy react with oxygen of atmosphere and form a layer of chromium oxide on outer surface of heating element. This layer of chromium oxide works as a protective layer for element and protect the material beneath this layers against oxidation, preventing the element wire from breaking and burning out. Heating elements made of Nichrome can be used for continuous operation at a temperature up to 1200 °C.

Fecral

"Kantahl" is the commercial trademark name for Fecral alloys made by composing Iron-Chromium-Aluminum (Fe-Cr-Al). These alloys are used in wide range resistance and heating applications.

Composition of Fecral

Fe = (62.5 - 76)% + Cr = (20 - 30)% + Al = (4 - 7.5)%

Properties of Fecral

  1. Resistivity at 20°C: 145 µΩ-cm
  2. Temperature coefficient of resistance at 20°C: 0.000001 /°C
  3. Melting point: 1500°C
  4. Specific gravity: 7.10 gm /cm³
  5. High resistance to oxidation

Use of Fecral

Used in making heating elements for electric heaters and furnaces.

Note

When the element made of FeCrAl is heated first time, the aluminum of alloy react with oxygen of atmosphere and form a layer of aluminum oxides over heating element. This layer of aluminum oxides, is an electrical insulator but has good thermal conductivity. This electrical insulating layer of aluminum makes the heating element shock proof. Heating elements made of Kanthal can be used for continuous operation at a temperature up to 1400°C. Therefore, it is very much suitable for making heating elements for Electric Furnaces used for heat treatment in ceramics, steels, glass and electronic industries.

Cupronickel

Cupronickel is also called as copper-nickel. It an alloy made by composing copper, nickel and strengthening elements such as iron and manganese.

Composition of Cupronickel

Cu = 66% + Ni = 30% + Fe = 2% + Mn = 2%

Properties of Cupronickel

  1. Resistivity at 20°C: 50 µΩ-cm
  2. Temperature coefficient of resistance at 20-500°C : 0.00006 /°C
  3. Melting point: 1280°C
  4. Specific gravity: 8.86gm /cm³
  5. High resistance to oxidation

Use of Cupronickel

Used in making heating elements for electric heaters and furnaces, for making coins.

Note

Cupronickel is having high electrical resistance, high ductility and good corrosion resistance. Heating elements made of "Cupronickel" can be used for continuous operation at a temperature up to 600°C.

Platinum

Platinum is a chemical element. It is having the chemical symbol Pt and atomic no. 78. Platinum is least reactive metal. It has remarkable resistance to corrosion, even at high temperature. Therefore it is considered as noble metal.

Properties of Platinum

  1. Resistivity at 20°C: 10.50 µΩ-cm
  2. Temperature coefficient of resistance at 20°C: 0.00393 /°C
  3. Melting point: 1768.30°C
  4. Specific gravity: 21.45gm /cm³
  5. High resistance to oxidation
  6. High ductility
  7. Highly malleable
  8. Good mechanical strength
  9. Good stability with temperature and mechanical stress

Use of Platinum

  1. Platinum is an incredible material with high resistivity and melting point. It is very much suitable for electrical heating elements, rheostats. But due to very high cost, its use in electrical engineering is limited to laboratory furnaces with a working temperature of 1300°C, rheostats, and resistance thermometers.
  2. Platinum is a precious metal, it is very popular for making jewelry.
  3. In medical platinum is used in chemotherapy for treatment of certain types of cancers.

Conclusions

The wide variety of materials that can be utilized as heating elements for electrical resistance furnaces have been discussed in detail. The materials include ceramic metal-based materials, nickel based metallic alloys, platinum and carbon or graphite materials.