Choose the best alloy for your heating element

Choose the best alloy for your heating element

Nickel Alloys for Heating

NiChrome Alloys for Heating Applications

Nickel-Chrome (NiCr) alloys have been in use since 1900 and have been successfully employed in heating element applications. Due to extensive field experience, these alloys instill confidence in their use for advanced and established design applications.

Choosing a Nichrome Heating Element

Nickel-Chrome (NiCr) alloys have been utilized since 1900, successfully serving in heating element applications. Their longstanding field experience in equipment and industrial furnaces ensures their reliability in advanced designs.

What is a Resistance Heating Alloy?

The selection of electric heating materials depends on inherent resistance to current flow to produce heat. Copper wire doesn’t produce sufficient heat when conducting electricity. Hence, an alloy used as wire, rod, strip, or ribbon for an electric heating element must oppose the flow of electricity.

Generally, common steels and alloys like stainless steel prevent the flow of electricity, a property known as resistivity. In North America, resistivity is traditionally expressed as ohms per circular mil foot, while in Europe, it is described using ohm mm² per meter.

If resistivity were the sole criterion, we could choose from various alloys over a wide cost range. However, electric heating elements often get extremely hot, and ordinary alloys cannot endure such heat for long. Their failure, known as poor life, makes them unsuitable as heating elements.

Suitable alloy families combine two properties:

• High electric resistivity
• Prolonged service life and endurance

Six major classes of these alloy groups are prepared traditionally. This article focuses on Nickel-Chrome (NiCr) alloys. Their major grades, with compositions and resistivity, are listed below.

American Standards for Testing and Materials

• B63: Resistivity of metallic conducting resistance and contact metals
• B70: Test method for variable resistance with temperature of electric heating elements
• B76: Accelerated service test of nichrome and Nickel-chrome-iron alloys for electric heating purposes
• B78: Increased life test for FeCrAl electric heating alloys
• B344: Specification for drawn or rolled nickel-chromium and nickel-chromium-iron alloys for heating applications
• B603: Specification for drawn or rolled FeCrAl alloys

Characteristics of Resistance Heating Alloys

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

• High electric resistivity, maintaining a small cross-sectional area
• High strength and ductility at service temperatures
• Low temperature coefficient of electric resistance to prevent significant resistance changes at service temperature versus room temperature
• Excellent resistance to oxidation in moderate procedures in the air
• Workable and formable into the required shape

80/20 Nichrome, 70/30 Nichrome, 60/15 Nichrome, and 35/20 Nichrome alloys possess these properties. Their evaluation in air follows:

The 80/20 Nickel-Chrome alloy is extensively employed, but research has suggested enhancements to its basic chemistry, such as adding small amounts of iron, manganese, silicon, and rare earth metals. This allows the alloy to be employed up to 1200 °C or 2192 °F.

The 70/30 Nickel-Chromium alloy provides enhanced service life in air up to 1260 °C or 2300 °F. Its outstanding performance in low oxygen conditions is due to a mechanism known as green rot.

The 60/16 Nichrome alloy, primarily used when application temperatures do not exceed 1100 °C or 2012 °F (e.g., in electric flat irons), consists of 60% Nickel and 16% chromium, with the remaining composition being iron.

The 35% Nickel, 20% chromium, and balance iron alloy is typically used in industrially controlled furnaces operating between 800 °C and 1000 °C or 1472 °F and 1832 °F. It prevents damage that may occur in 80/20 or 70/30 alloys under varying conditions. Nichrome A (80/20) is not suggested for use in conditions involving nickel reduction and chromium oxidation.

All heating alloys mentioned above have a great service life when adequately designed in the appropriate wire size and coil specification.

Resistance wire or strip forms are typically introduced in annealed form unless otherwise requested. These can be conveniently coiled or bent in the annealed condition.

A heating element's suitable life begins with alloy production, followed by proper care as it forms a heating element and is installed in the consumer's appliance. While Nichols are corrosion-resistant like stainless steels, they can be damaged under certain conditions, necessitating precautions to keep them clean.

Variety of Heating Elements

Resistance elements come in various forms and applications:

• Wire or ribbon can be exposed or covered. Exposed heaters distribute heat efficiently and allow operation at elevated temperatures without heavy material, though they are vulnerable to external factors like rust and short circuits, posing electric shock risks to users.
• Mounting wire or strip holds significance; it can be hung or implanted. Common suspension applications include air heaters, where heating coils thread through bead arrays supported by wire frames.
• Supported materials, often used in furnaces, provide regular support for coils resting on walls. These are typically iron-based alloys (FeCrAl) with low hot strength, resulting in slower thermal response, but they are economical.
• Heaters classified as tubular or sheathed involve wire insertion into stainless steel or heat-resistant covers. Wire coils coated with magnesium oxide packed in tubes offer electrical insulation and heat transfer by conduction. These vary from high-grade heaters for ovens to inexpensive small immersion heaters.

How Electric Resistance Alloys Work

An electric resistance alloy generates heat by opposing electricity flow. The alloy must conduct electricity to an appropriate temperature to function as a heating material.

Temperature Coefficient Resistance

The resistance of an alloy, stated in ohms, varies with temperature changes. This variation is expressed as a percentage change from room temperature resistance. Typically, as temperature increases, so does resistance. A heating element with 1 ohm resistance at RT (20 °C or 68 °F) might reach 1.08 ohms at 650 °C or 1202 °F (an 8% increase). The diagram below describes the standard resistance for major heating alloys.

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Considering a heating element’s continuous function, its variation in resistance should be factored into design choices. Design for the hot condition, then determine room temperature resistance accordingly. Resistance heating wire, ribbon, and strip are always supplied with their specified room temperature resistance.

Oxide Production and Service Life

All metals can serve as heating elements if they offer sufficiently high resistance, albeit requiring a small cross-sectional area for practicality. The chosen alloy should produce an adherent oxide layer even during hot-cold cycles.

This oxide layer protects the metal beneath it from catastrophic oxidation leading to failure. It's akin to rust protecting underlying steel from rapid corrosion. Maintaining an intact oxide layer is essential to preserve the heating element.

Manufacturers test alloy specimens before production. Evaluations follow ASTM B-76 methods, with life expressed in hours. Temperature-life data for different Nichrome alloys is summarized in the following chart:

Effect of Processing on Resistivity

Electric resistance is an internal metal property affected by composition and configuration. Fabricating and processing methods like cold processing and annealing can influence resistance by altering the material's physical structure.

Changes in resistivity due to cooling rates are significant, particularly in bright annealed material processed with secured media annealing followed by quick quenching. Operating above 300 °C or 572 °F may alter resistivity from its original value, especially if elements cool slowly. The following variations might occur:

• Variations depend on section size; light parts cool quickly, showing significant resistance changes in Nichrome 80/20 and 70/30, moderate in 60/15, and negligible in 35Ni20Cr.
• Precise heater calibration requires an oxidized wire or strip layer due to oxide production. Annealing stabilizes initial resistance close to the maximum value for the alloy, preventing significant changes during application.
• Coiling processes, including cold working, modify annealed wire resistance. Uniform cold processing across the coil ensures consistent resistance and stretch properties. Maintaining consistent coiling stress prevents abrupt jerks and 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, allowing enhanced designs for excellent performance. To achieve satisfactory functionality at an affordable cost, various factors must be considered:

Application

All heating elements aren't the same. Categorized as industrial furnaces and appliances, cost considerations differ. Industrial heaters' element cost is minor, while in appliances, early damage is critical. Some companies may accept 1% defects, but for customers, a defective appliance is a 100% failure. Design engineers strive to prevent issues.

Mechanical Effects

Installing elements in equipment subjected to mechanical shocks requires utmost care in installation methods.

Temperature

Temperature is crucial in selecting an alloy and heating material size. The application defines required temperatures, distinguishing ambient and resistance wire temperatures.

Space Needed

Installation space, typically constrained, determines practicality. Even toasting in a toaster requires elements to be appropriately distanced despite space restrictions.

Atmosphere

Gases or solids interacting with the heater, such as furnace's security layer or broiler's splattering, must be determined.

Thermal Cycling

Ideal operating conditions would maintain constant temperatures, but elevated service temperatures (e.g., 800 °C or 1472 °F and above) benefit from non-cycling heater longevity. Tests simulate high-rate cycling to emulate practical conditions.

Safety

Safety in appliances involving high heat or electrical conductors is paramount. Unforeseen temperature rises require careful installation to prevent hazards.

Power Density

Expressing watts dissipated per unit area (watt loading), power density determines application temperature needs. Optimal design maximizes value with minimal material, ensuring cost-effective systems and suitable service life through a combination of conductor cross-section and resistivity. Self-heating between coil loops is considered in heating coils and furnace ribbons.

Nichrome 60 Versus Nichrome 80

Efforts to reduce Nichrome 80 costs led to reduced nickel and chromium contents, but many tested alloys failed. Recent advancements in alloy melting and cleaner raw materials enabled Nichrome 60 manufacturing with life properties comparable or superior to Nichrome 80 at various temperatures. While Nichrome 80 remains preferred for exposure to its limit, Nichrome C offers cost-cutting opportunities for many applications.

As heater alloys are drawn or rolled to resistance, users often request matching resistance in ohms per foot to Nichrome 80. Nichrome 60’s higher resistivity requires a nominally larger wire diameter, which slightly decreases application temperature, benefiting lifespan.

Nichrome 60 is not used in industrial furnaces due to the overall furnace setup cost versus heating element cost; thus, Nichrome 80, 70/30 grades, or 35/20 grades are used.

Resistivity Data – Nichrome A and Nichrome C

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