Nickel Alloys for Heating

NiChrome Alloys for Heating Applications

Choosing a Nichrome Heating Element

The Nickel-Chrome (NiCr) alloys have been used since 1900 and have been successfully employed in heating element applications. The extensive field experience from equipment and industrial furnaces provides confidence in using these alloys for both advanced and established design applications.

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What is a Resistance Heating Alloy?

The selection of electric heating materials depends on their inherent resistance to current flow, which produces heat. Copper wire doesn’t produce sufficient heat when conducting electricity. Therefore, for an alloy in forms such as wire, rod, strip, or ribbon to be treated as an electric heating element, it should resist the flow of electricity.

Common steels and alloys like stainless steel prevent the flow of electricity, a property termed as resistivity. In North America, resistivity is traditionally expressed in ohms per circular mil foot (ohm.cmil/ft), while in Europe, the common unit of resistivity is ohm mm² per meter.

If resistivity alone was considered the major factor for an electric heating element, various alloy materials could be options. Due to the extreme nature of their work, electric heating elements often get red hot. Ordinary alloys cannot endure such high heat for a long period and fail, resulting in a poor life as heating elements.

Suitable alloy families have been traditionally prepared with a combination of two properties:

1. High electric resistivity
2. Prolonged service life with high endurance as a heating material

These alloy groups can be categorized into six major classes. While this article focuses on Nickel-Chrome (NiCr) alloys, the major grades of these alloys, along with their composition and resistivity, are also listed.

American Standards for Testing and Materials

Standard descriptions for testing and materials include:

Characteristics of Resistance Heating Alloys

For a metal or alloy to become a significant electric heating element, it should possess the following attributes:

1. Good high electric resistivity to maintain a small cross-sectional area
2. High strength and ductility at service temperatures
3. Low temperature coefficient of electric resistance to prevent significant changes in resistance across service temperatures
4. Excellent resistance to oxidation in air under moderate conditions
5. Suitability for being formed into the required shape

The materials that possess these properties include 80/20 Nichrome, 70/30 Nichrome, 60/15 Nichrome, and 35/20 Nichrome. The evaluation of these alloys in air is as follows:

The most popular resistance alloy, made up of 80% nickel and 20% chromium, is still widely employed, though research has suggested some enhancements in the basic chemistry. Adding nominal amounts of iron, manganese, silicon, rare earth metals, and others allows the alloy to be used up to 1200 °C or 2192 °F.

The 70/30 Nickel-Chromium alloy is made to provide an enhanced service life in air up to 1260 °C or 2300 °F. It offers outstanding oxidation resistance, particularly in low-oxygen conditions, a mechanism known as green rot.

Nichrome alloys comprising 60% Nickel and 16% chromium, with the remainder being iron, are typically chosen for applications where the temperature does not need to exceed 1100 °C or 2012 °F, such as in electric flat irons.

Alloys containing 35% nickel, 20% chromium, and the remainder iron are used in industrially controlled condition furnaces operating at temperatures between 800 °C and 1000 °C or 1472 °F and 1832 °F. These alloys prevent damage that may occur in other alloys when service conditions vary between reducing and oxidizing environments. Nichrome A or 80/20 is not recommended for conditions that reduce nickel and oxidize chromium.

All the heating alloys mentioned in the above table have great service life as heating materials when designed adequately in suitable wire size and coil specifications.

Resistance wire or strip forms are normally provided in the annealed form unless otherwise requested. These can easily be formed by coiling or bending in the annealed condition.

The suitable life of a heating element begins with the production of the alloy and is significantly influenced by the proper care of the alloy - wire, ribbon, or strip - when it is formed into a heating element and installed in the consumer’s appliance. Nichrome alloys are corrosion-resistant similar to stainless steel, but precautions are required to keep them clean in certain conditions.

Variety of Heating Elements

Resistance elements are used in several manners and applications:

Wire or ribbon can be exposed or covered. The exposed heater distributes heat more efficiently and permits it to function at elevated temperatures without heavy material. However, it is not shielded from external factors like rust and short circuits, posing potential risks of electric shock.

The concept of mounting wire or strip is crucial. It can be hung or embedded. Standard suspension applications can be seen in air heaters, where a heating coil is threaded through doughnut-shaped beads supported on a wire frame.

Supported materials are often used in furnaces where regular support is offered for the coil to lie on the walls. Such supported heaters are generally made of iron-based alloys (FeCrAl) with low hot strength. They are slow in thermal response as the supporting material also needs to be heated. The main reason for using these alloys is their economical price.

Various heaters are classified as tubular or sheathed heaters, where the wire is inserted into a stainless steel or heat-resistant material cover. The wire coil is coated with magnesium oxide packed in a tube, providing electrical insulation and heat transfer via conduction.

How Electric Resistance Alloys Work

An electric resistance alloy generates heat by opposing the flow of electricity, depending on its composition. For it to function as a heating material, the alloy must conduct electricity up to an appropriate temperature.

Temperature Coefficient Resistance

The resistance to current flow in ohms for a specific alloy varies with the alloy’s temperature. This variation is expressed as a percentage change from the initial room temperature resistance. Generally, as the temperature increases, the resistance also increases. For example, a heating element with an initial resistance of 1 ohm at room temperature might reach 1.08 ohms at 650 °C or 1202 °F, an 8% increase due to heating.

When designing a heating element, consider its variation in resistance under continuous operation. Start with the hot condition and work back to determine the room temperature resistance that the element should have.

Oxide Production and Service Life

All metals can serve as heating elements if they have sufficiently high resistance, but their cross-sectional area should be kept very small to be practical. After selecting an alloy as a heating element, it should be capable of producing an adherent oxide layer even under repeated hot-cold cycles.

The oxide layer helps protect the metal beneath from oxidation leading to failure. This is similar to rust protecting steel from rapid corrosion. It is crucial that the oxide layer on the heating element remains intact to protect the underlying element.

Before allowing production, manufacturers test a specimen wire using the ASTM B-76 method to evaluate its life in hours. The following chart shows the temperature lives of different Nichrome alloys.

Effect of Processing on Resistivity

Electric resistance is intrinsic to every metal, influenced by its composition and configuration. Fabricating and processing methods such as cold processing and annealing can change the physical structure of the material, affecting its resistance.

Change in resistivity with cooling rate is particularly significant with bright annealed material, involving annealing in a protective medium followed by rapid quenching. When the material operates at temperatures above 300 °C or 572 °F, resistivity may alter from its original value, especially if the elements cool slowly.

The variation in resistivity of bright annealed wire or ribbon depends on section size. Light parts cool faster than heavy parts, showing a more specific influence of cooling rate on electric resistivity. The effect is most significant with Nichrome 80/20 and Nichrome 70/30 and moderate with 60/15 alloy. No noticeable size effect has been noted with 35Ni20Cr alloy.

When precise calibration of the heater is essential, an oxidized layer is stated for the wire or strip due to oxide production. The metal is slightly quenched in the air from annealing temperature. No significant change in electric resistance occurs during application because its initial resistance gets stabilized by the annealing process.

The basic resistance of annealed wire can be modified by the coiling process, which involves cold processing. The extent of cold processing should be uniform across the coil to maintain consistent resistance and stretch properties. Consistency in coiling stress ensures uniform cold processing and diameter across the coiled wire.

Nichrome Alloy Heating Elements

Electric resistance heating elements have been used for a long time, with many designs optimized for excellent performance. To provide satisfactory functionality at an affordable cost, various factors should be considered:

Application: All heating elements are not the same. They are categorized into industrial furnaces and equipment. In furnaces, the cost of heating elements is less crucial because of mass production. However, in appliances, even a small mistake can cause early damage, making defect-free performance critical.

Mechanical Effects: If the heated equipment is subject to serious mechanical shock, the method of installing the elements should be of utmost importance.

Temperature: Temperature is a major factor in choosing an alloy and the size of the heating material. Differentiating between the ambient temperature and that of the resistance wire is crucial for applications.

Space Needed: The space available for installing the heater is often constrained, influencing the design of the heating element.

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Atmosphere: This includes gases or solids that may interact with the heater. The security layer and splattering in a broiler are determined factors.

Thermal Cycling: Ideally, a heating element should remain at a constant temperature, but this is often impractical. Lab tests show that regular energized heaters have a long total life at high service temperatures (800 °C or 1472 °F and above).

Safety: Safety is paramount in applications involving high heat or electrical conductors. Improper installation can cause unexpected temperature increases, posing risks.

Power Density: Power density, expressed as watts per unit area, is essential for designing heating elements. For higher loading applications, optimal design involves a combination of the smallest conductor cross-section and suitable resistivity.

Nichrome 60 Versus Nichrome 80

Since the discovery of Nichrome 80, efforts have been made to reduce material costs by decreasing the amounts of nickel and chromium. Recent advancements in alloy melting processes and cleaner raw materials have enabled the production of Nichrome 60 with similar or even better life properties than Nichrome 80 for several temperature ranges. Nichrome 80 is preferred for applications exposed to high temperatures, while Nichrome 60 can be used successfully in various applications, offering cost benefits.

Users often request alloys to be drawn to obtain the same resistance in ohms per foot similar to Nichrome 80. Due to Nichrome 60's higher resistivity, its wire diameter is nominally larger, leading to a slight temperature reduction in applications. However, this temperature reduction is beneficial, as life expectancy is inversely proportional to temperature.

Nichrome 60 is not employed in industrial furnaces because the overall furnace setup cost outweighs the cost of heating elements; hence, Nichrome 80, 70/30 grade, or 35/20 grades are used in furnaces.

Resistivity Data – Nichrome A and Nichrome C

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