NiChrome Alloys for Heating Applications

Choosing a Nichrome Heating element

The Nickel-Chrome (NiCr) alloys have been in use back to 1900 and these have been successfully employed in heating element applications. Hence the realistic field experience of equipments and industrial furnaces gives a confidence in the usage of these alloys in the advanced and already established design applications.

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What is a Resistance Heating Alloy?

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

Generally common steels and alloys like stainless steel prevent the electricity flow. This property term is known as resistivity. In North America, the tradition is to use ohms per circular mil ft to describe resistivity and this term has been following in the data. The technically suitable designation would be ohm.cmil/ft or ohm times circular mils per foot. In European countries, common unit of resistivity is ohm mm² per m.

If resistivity solely was considered as the major factor for an electric heating element, the option could be from several alloy materials in a wide array of cost. By its extreme nature, an electric heating element gets hot often red hot and ordinary alloys cannot endure such extent of heat for a long period. They fail and it is called as poor life as a heating element.

The alloy families were prepared traditionally with suitable combination of two certain properties:

1. High electric resistivity
2. Prolonged service life, endurance potential as a heating material

These alloy groups can be categorized in the six major classes. While the whole alloy families are enlisted for comprehensiveness, this article is about Nickel-Chrome (NiCr) alloys. The major grades of these alloys are shown with their composition and resistivity.

American standards for testing and materials

StandardDescription 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 alloys should possess the following attributes:

1. Good high electric resistivity to keep small cross sectional area
2. High strength and ductility at the service temperatures
3. Low temperature coefficient of electric resistance to prevent changes in resistance at the service temperature significantly from that attained to room temperature.
4. Excellent resistance to oxidation in air while moderate procedures
5. Suitable working and potential for being formed into the required shape.

The materials that possess these properties are 80/20 Nichrome, 70/30 Nichrome, 60/15 Nichrome and 35/20 Nichrome. The evaluation of the properties of these alloys in air is made following:

  A grade 80/20 NiCr 70/30 NiCr C Grade 60/15 NiCr D Grade 35/20 NiCr UNS N06003 N06008 N06004 None Highest Service Temperature in Air 1200 °C or 2200 °F 1260 °C or 2300 °F 1150 °C or 2100 °F 1100 °C or 2000 °F Melting point 1400 °C or 2550 °F 1380 °C or 2520 °F 1390 °C or 2530 °F 1390 °C or 2530 °F Specific Gravity 8.41 8.11 8.25 7.95 Density 0.304 lb/in³ 0.293 lb/in³ 0.298 lb/in³ 0.287 lb/in³ Specific Heat .107 Btu/lb/F .110 Btu/lb/F .107 Btu/lb/F .110 Btu/lb/F Tensile Strength 830 MPa or 120 ksi 900 MPa or 130 ksi 760 Mpa or 110 ksi 620 Mpa or 90 ksi Yield Strength, 0.2 % 415 MPa or 60 ksi 485 MPa or 70 ksi 380 Mpa or 55 ksi 345 MPa or 50 ksi Elongation % 240 MPa or 35 ksi 240 MPa or 35 ksi 240 MPa or 35 ksi 240 MPa or 35 ksi Reduction of Area 55 % 55 % 55 % 55 %

The most popular resistance alloy made up of 80% nickel and 20 % chromium is still extensively employed, however a variety of researches have suggested some enhancements in the basic chemistry. The inclusion of nominal magnitudes of iron, manganese and silicon and slight contents of rare earth metals and others are made that enable the alloy to be employed up to 1200 °C or 2192 °F.

70/30 Nickel-Chromium alloy is made to provide an enhanced service life in air up to 1260 °C or 2300 °F. It gives outstanding performance in resisting oxidation in the low oxygen conditions, a mechanism known as green rot due to green shade of oxide.

The Nichrome alloy comprising of 60% Nickel and 16% chromium and remaining iron is normally chosen when the application temperature doesn’t need to be above 1100 °C or 2012 °F like in electric flat irons.

Alloy comprising of 35% nickel, 20% chromium and Rem iron is used in industrially controlled condition furnaces working at temperatures 800 °C to 1000 °C or 1472 °F to 1832 °F. It provides significant contribution in preventing the damage that may take place in above two alloys when the service temperature is same but conditions vary between reducing and oxidizing. Nichrome A or 80/20 is not suggested for employ in the conditions that reduce nickel and oxidize chromium.

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

Resistance wire or strip forms are normally introduced in the annealed form, unless otherwise individually request. These are conveniently formed by coiling or bending in the annealed condition.

The suitable life of a heating element starts with the production of alloy and subsequent outcomes from the suitable care of the alloy - wire, ribbon, strip when it is formed as a heating element and installed in the consumer’s appliance. The nichrome alloys are corrosion resistant similar to stainless steels however these are vigorously damaged in the few conditions therefore precautions are required to keep them clean.

Variety of heating elements

Resistance elements are used in the several manners and applications as stated following:

Wire or ribbon can be exposed or covered. The exposed heater distributes heat more efficiently, permits it to function at the elevated temperature without the need of heavy material. But it is not secured from the external factors like rust and short circuits and may cause potential risks of electric shock for the user.

The concept of mounting wire or strip is of utmost significance. It can be hanged up or implanted. The standard suspension applications can be seen in air heaters in which a heating coil is threaded by an array of doughnut-shaped beads supported through a wire-frame.

The supported materials are commonly employed in furnaces where regular support is offered for the coil to lie on the walls. Generally, such supported kind of heater is made of Iron based alloys (FeCrAl) that have small hot strength. They are slow in thermal response as the supporting material requires to be heated. The major reason for using these alloys is their economical price.

There are a variety of heaters classified as tubular or sheathed heaters in which the wire is inserted in stainless steel or heat resistant material cover. The wire coil coated by magnesium oxide packed in a tube, the coating offers sufficient electrical insulation and heat transfer by conduction to the outside. The heaters vary from the peak grades employed in top and oven operations to cheap small heaters for immersion in a bowl.

How Electric Resistance Alloys Work

An electric resistance alloy generates heat, depending on its composition, it opposes the flow of electricity. The alloy should be able of conducting electricity to an appropriate temperature to perform as a heating material.

Temperature coefficient resistance

The resistance to the current flow, stated in ohms for a specific alloy varies as per the variation in the alloy’s temperature. This variation is stated as percentage change from the actual room temperature resistance. Normally, with increase in temperature, resistance increases, thus a heating element as a wire, has a resistance of 1 ohms at RT (20 °C or 68 °F) may attain resistance up to 1.08 ohms at 650 °C or 1202 °F, hence 8% increase in resistance because of heating. Following diagram describes the standard resistance for the major heating alloys.

With the continuous function of a heating element, its variation in resistance should be considered while choosing an element design. Choose the hot condition then work back to return at the room temperature resistance that an element should be made. Resistance heating wire, ribbon and strip are every time offered with their mention room temperature resistance.

Oxide Production and Service Life

All metals can perform as a heating element, if they do not have sufficiently very high resistance, but their cross section area should be kept very small to make it practical. After choosing an alloy as a heating element, it should have the required potential of producing an adherent oxide layer in hot form in fact while the repeated hot cold cycles.

The oxide layer tends to secure the metal beneath it from the tragic oxidation to the level of failure. It is similar to rust that keeps the under steel secured from the quick corrosion. When the surface layer is removed, it shows beneath a new surface of the steel. It is essential that oxide layer on the heating element remains isolated to protect the underlying element.

Every manufacturer when they develop an alloy, a specimen wire is constructed and evaluated prior the melt is allowed for production. The evaluated is performed by a method stated in ASTM B-76 and shows life stated in hours. Following chart shows the temperatures lives of different Nichrome alloys.

Effect of Processing on Resistivity

Electric resistance is an internal property of every metal, lead by its composition as well as configuration. The resistance can be in influenced by fabricating and processing methods like cold processing and annealing processing, to the extent that they change the physical structure of the material.

Change in resistivity with cooling rate is particularly significant with bright annealed material in which processing includes annealing in a secured media then quick quenching. When the material functions at temperatures above 300 °C or 572 °F, resistivity may be altered from its original value, specifically if the elements are cooled slightly. The following variations may occur:

However, the capability for variation in resistivity of shining annealed wire or ribbon is subjected to the section size. As the light parts cool down more quickly than massive parts, light parts describe more specific influence of cooling rate on electric resistivity. The influence is maximum with Nichrome 80/20 and Nichrome 70/30, and moderate with 60/15 alloy. No considerable size effect has been noticed with 35Ni20Cr alloy.

When precise calibration of heater is essential before installation, an oxidized layer is stated for the wire or strip due to production of oxide, metal is slightly quenched in air condition from annealing temperature. No significant change in electric resistance will occur while application because its initial resistance will be stabilized by the actual annealing process close to the maximum value for alloy.

The basic resistance of annealed wire can be modified by coiling process in developing the heating element as the coiling includes cold processing. The extent of cold processing should be kept identical in the whole coil to maintain the uniform resistance and to produce coils developing uniform stretch properties. The coiling stress should be kept constant and uniform as much as possible while the coiling process without any abrupt jerks on the wire. Consistency of stretch shows the consistency of cold process and diameter across the coiled wire.

Nichrome Alloy Heating Elements

Electric resistance heating element have been used for a prolonged period of time. Therefore many designs are enhanced to provide excellent performance. It is essential to test the whole factors that will give design of heater that will offer a satisfactory functionality at an affordable cost. To perform this task, following factors should be taken into account:

Application: All heating elements are not same. They are categorized as industrial furnaces and equipments. In furnaces such as industrial heaters, cost of heating element is not crucial because of mass production. In appliances, a small mistake may cause early damage which is a critical factor as it may need to recall several devices. 1% defect may be accepted by some companies but having a defective appliance is 100% failure for a customer. The design engineer is always making attempts to prevent any issue overall.

Mechanical Effects: if the heated equipment is to be subjected to serious mechanical shock, the method of installing the elements becomes should be given utmost importance.

Temperature: It is the major factor while choosing an alloy and the size of a heating material. The application of the heating element states the required temperature. It is also essential to differentiate between the ambient temperature and that of resistance wire. In a furnace, they are kept quite close, but at the other extreme in an electric teapot, the water rises to 100 °C or 212 °F while the wire itself may be increased to 1000 °C or 1832 °F. The same is followed in a super heater.

Space Needed: The space introduced for installing the heater is normally controlled. This states that the adequate space may not be practical. For even toasting of bread in a toaster, the material should be kept away from the surface, yet offset space introduced for the equipment should be adequate.

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Atmosphere: It states gases or solids to interact with the heater. The security layer in a furnace or splattering in a broiler are normally determined.

Thermal Cycling: The suitable operating condition for a heating element is to remain at the constant temperature. It is normally impractical. At the elevated service temperature such as 800 °C or 1472 °F and above, lab tests have described the regular energized heater to have long total life. Due to outstanding service life of a non-cycling heater, many tests are designed to cycle at a high rate. The cycle time is determined by the length needed to cycle the device between stabilized test temperature and room temperature.

Safety: Safety is must in the appliances involving high heat or subjected to electrical conductors included. Installing the appliances behind the barriers can cause abrupt increase in temperature than expected.

Power Density: The essential factor to be understood is power density showing a number expressing watts dissipated per unit area and is commonly called as watt loading. For greater loading applications, higher temperatures are required. It is a suitable design concept to select the maximum value as it refers to minimum magnitude of material, providing cost-effective system while offering suitable service life. It is received by a combination of the smallest conductor cross-section and suitable resistivity. In heating coils and furnace ribbons a self heating between loops is allowed by radiation from coil turns.

Nichrome 60 Versus Nichrome 80

As Nichrome 80 was discovered, efforts have been made to decrease the cost of material by decreasing nickel and chromium magnitudes. Several alloys have been tested and many have been failed. In the recent years, enhancements in the alloy melting process and cleaner raw materials have encouraged the manufacturing of Nichrome 60 material with life properties similar to or even better than Nichrome 80 for several temperature limits. Nichrome 80 is preferred when the material has to be exposed to its temperature limit. Although in various applications, Nichrome C can be successfully employed as it gives a chance to cut off the cost.

As heater alloys are drawn, rolled to resistance, it is common for users to ask for an alloy be drawn to obtain the same resistance in ohms per foot similar to Nichrome 80. As Nichrome 60 has higher resistivity, the wire diameter will be nominally larger to accompany this. It refers to the application temperature, that is found by the power density, will be decreased. This temperature reduction is slight but in the correct manner, as life is inversely proportional to temperature.

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

Resistivity Data – Nichrome A and Nichrome C

Diameter (mm) Diameter Tolerance Cross Sectional Area (mm²>) NI80CR20 NI60CR15 Tolerance of Material Resistance (%) Resistance per Metre (20°C Ω/m) Length per Kg (m/kg) Weight per metre (kg/m) Resistance per Metre (20°C Ω/m) Length per Kg (m/kg) Weight per Metre (kg/m) .020 mm ± 0.003 0.000314 mm² 3472 - - 3567 - - ± 15 % .025 mm ± 0.003 0.000491 mm² 2220 - - 2281 - - ± 15 % .028 mm ± 0.003 0.000616 mm² 1770 - - 1818 - - ± 15 % .032 mm ± 0.003 0.000804 mm² 1356 - - 1393 - - ± 14 % .036 mm ± 0.003 0.001018 mm² 1071 - - 1100 - - ± 14 % .040 mm ± 0.004 0.001257 mm² 867 - - 891 - - ± 13 % .045 mm ± 0.004 0.001591 mm² 685 74828 0.00001 704 76649 0.00001 ± 13 % .050 mm ± 0.004 0.001964 mm² 555.1 60617 0.00002 570.3 62092 0.00002 ± 12 % .060 mm ± 0.004 0.002828 mm² 385.5 42097 0.00002 396.0 43122 0.00002 ± 11 % .070 mm ± 0.005 0.003849 mm² 283.2 30930 0.00003 291.0 31683 0.00003 ± 10 % .080 mm ± 0.005 0.005027 mm² 216.9 23682 0.00004 222.8 24259 0.000041 ± 10 % .100 mm ± 0.006 0.007854 mm² 138.8 15158 0.000065 142.6 15527 0.000064 ± 9 % .120 mm ± 0.006 0.01131 mm² 96.38 10526 0.000095 99.03 10788 0.000092 ± 9 % .132 mm ± 0.007 0.01369 mm² 79.62 8697 0.00011 81.81 8907 0.00011 ± 8 % .150 mm ± 0.008 0.01767 mm² 61.68 6738 0.00014 63.38 6901 0.00014 ± 8 % .152 mm ± 0.008 0.01815 mm² 60.05 6557 0.00015 61.70 6720 0.00015 ± 8 % .170 mm ± 0.008 0.02270 mm² 48.02 5243 0.00019 49.34 5373 0.00018 ± 8 % .173 mm ± 0.008 0.02351 mm² 46.37 5062 0.00020 47.64 5186 0.00019 ± 8 % .190 mm ± 0.009 0.02835 mm² 38.44 4198 0.00023 39.50 4301 0.00023 ± 8 % .193 mm ± 0.009 0.02926 mm² 37.24 4069 0.00025 38.27 4168 0.00024 ± 8 % .210 mm ± 0.010 0.03464 mm² 31.47 3437 0.00029 32.34 3521 0.00028 ± 8 % .250 mm ± 0.010 0.04909 mm² 22.21 2425 0.00041 22.82 2484 0.00040 ± 8 % .270 mm ± 0.012 0.05726 mm² 19.04 2079 0.00048 19.56 2129 0.00046 ± 7 % .280 mm ± 0.013 0.06158 mm² 17.70 1933 0.00052 18.19 1980 0.00051 ± 7 % .290 mm ± 0.013 0.06605 mm² 16.50 1802 0.00055 16.96 1846 0.00054 ± 7 % .300 mm ± 0.013 0.07070 mm² 15.41 1684 0.00059 15.84 1724 0.00058 ± 7 % .310 mm ± 0.013 0.07548 mm² 14.44 1577 0.00063 14.84 1615 0.00061 ± 7 % .315 mm ± 0.013 0.07794 mm² 13.98 1527 0.00065 14.37 1564 0.00064 ± 7 % .345 mm ± 0.013 0.09349 mm² 11.66 1273 0.00079 11.98 1304 0.00077 ± 7 % .350 mm ± 0.013 0.09621 mm² 11.33 1237 0.00080 11.64 1267 0.00078 ± 7 % .355 mm ± 0.013 0.09899 mm² 11.01 1203 0.00083 11.31 1232 0.00081 ± 7 % .375 mm ± 0.015 0.11046 mm² 9.87 1078 0.00093 10.14 1104 0.00091 ± 7 % .400 mm ± 0.016 0.125 mm²7 8.674 947 0.00105 8.913 970 0.00103 ± 7 % .450 mm ± 0.016 0.1591 mm² 6.853 748 0.00133 7.042 766 0.00130 ± 7 % .475 mm ± 0.016 0.1772 mm² 6.153 672 0.00148 6.323 688 0.00145 ± 7 % .500 mm ± 0.016 0.1963 mm² 5.551 606 0.00164 5.704 621 0.00161 ± 7 % .560 mm ± 0.016 0.2463 mm² 4.424 483.3 0.00206 4.546 495.0 0.00202 ± 7 % Because of its low cost of manufacture, strength, ductility, resistance to oxidation, stability at high temperatures, and resistance to the flow of electrons, NiChrome is widely used in electric heating elements in applications such as hair dryers and heat guns.

Nickel chrome alloys, commonly referred to as Nichrome, stand out as versatile materials with a unique composition of nickel, chromium, and sometimes iron. The chemical formula for these alloys is NiCr, and in the presence of iron, it becomes NiFeCr. This blog post from William Rowland explores the remarkable properties and diverse applications of nickel chrome alloys, including their significance in various industries.

Composition and Characteristics of Nickel Chrome Alloys

Nichrome alloys are typically crafted with an 80% nickel and 20% chromium ratio, known as Nichrome 80/20. This alloy exhibits a distinctive silver-grey hue and showcases exceptional resistance to electrical flow and heat. Its corrosion resistance, durability, and an impressive melting point of approximately 1,400°C make Nichrome a sought-after material in numerous applications.

Applications in Heating Elements

One of the primary applications of nickel chrome alloys is in the creation of heating elements. Household toasters, for instance, commonly employ thick nichrome wire in the construction of their heating elements, with this wire being wound into coils to achieve a specific electrical resistance. When an electrical current passes through, the nichrome wire heats up, emitting the desired heat for toasting. Notably, the oxidation resistance of nichrome prevents the heating element from succumbing to oxidation, ensuring longevity and reliability.

Pyrotechnics and Explosives

Nichrome's ability to rapidly heat up and glow red-hot when an electric current flows through makes it an ideal choice for applications in pyrotechnics and explosives. Specifically, it serves as an electrical ignition source, allowing for the safe and efficient activation of fireworks and other pyrotechnic displays. Its quick response to low voltage makes nichrome a reliable and remote-controlled ignition option.

Wear-Resistant Coatings

The wear-resistant nature of nichrome extends its utility to the creation of coatings for both decorative and engineering purposes. In aerospace applications, nickel chrome alloys are widely used in aircraft engine turbines due to their robustness, heat resistance, and overall durability. These coatings contribute to the longevity and performance of critical components in demanding environments.

How are Nickel Chrome Alloys Made?

1. Raw Materials Selection

The journey of crafting nickel chrome alloys begins with the careful selection of raw materials. High-purity nickel and chromium are pivotal, with the option to introduce iron for specific alloy compositions, known as NiFeCr. The quality and purity of these elements significantly influence the final characteristics of the alloy.

2. Alloying Process

The alloying process is a critical phase where the chosen elements are blended to form a homogenous mixture. This typically occurs in a controlled environment, often within a furnace capable of reaching high temperatures. The exact ratios of nickel and chromium, or the inclusion of iron, are crucial factors determining the alloy's final properties.

The raw materials, usually in the form of pellets or powders, are meticulously weighed to achieve the desired composition. This precision ensures that the resulting alloy exhibits the specific characteristics required for its intended applications.

3. Melting and Homogenization

Once the raw materials are weighed and positioned in the furnace, the melting process commences. The furnace environment is carefully controlled to prevent impurities and ensure a uniform melt. The high temperatures cause the materials to fuse, forming a molten alloy. This molten mass is subjected to thorough mixing or stirring, promoting homogenization and a consistent distribution of elements.

The alloy's composition is continuously monitored during this stage to guarantee it aligns with the predetermined specifications. The success of this step is crucial in achieving the alloy's desired properties, such as heat resistance, electrical conductivity, and corrosion resistance.

4. Solidification

Following the homogenization process, the molten alloy is allowed to cool and solidify. The method of solidification can vary, with options including casting into moulds or extrusion into desired shapes. The cooling rate and solidification method influence the microstructure of the alloy, impacting its mechanical and thermal properties.

5. Forming and Shaping

Once solidified, the nickel chrome alloy may undergo additional processing to attain specific forms. This can involve rolling, drawing, or other mechanical methods to produce sheets, wires, or custom shapes. The choice of form depends on the intended application; for instance, heating elements may be crafted from coiled wires.

6. Heat Treatment

Heat treatment is a crucial step that further refines the properties of nickel chrome alloys. This process involves subjecting the alloy to controlled heating and cooling cycles to optimise its microstructure. The goal is to enhance mechanical strength, hardness, and other desirable characteristics, ensuring the alloy meets the stringent requirements of various applications.

7. Quality Control

Throughout the entire production process, rigorous quality control measures are implemented. This includes testing the alloy for composition accuracy, mechanical properties, and resistance to environmental factors. Advanced techniques such as spectroscopy, microscopy, and mechanical testing are employed to validate the alloy's conformity to specifications.

8. Final Inspection and Packaging

Before reaching end-users, the manufactured nickel chrome alloy undergoes a final inspection to guarantee it meets the highest quality standards. Once approved, the alloy is carefully packaged to preserve its integrity during transportation and storage.

William Rowland's Expertise

As a leading supplier of nickel products, William Rowland plays a crucial role in providing high-quality nickel chrome alloys to various industries. Our offerings include nickel chromium in both lumps and powder form, available in a range of sizes. The company's commitment to delivering high-purity nickel and nickel powders underscores its dedication to meeting the evolving needs of industries that rely on nickel chrome alloys.

Nickel chrome alloys, particularly Nichrome, are indispensable materials with a wide array of applications. From heating elements to pyrotechnics and aerospace engineering, these alloys exhibit remarkable properties that contribute to their versatility and reliability. With William Rowland's expertise in the supply of nickel products, industries can confidently integrate nickel chrome alloys into their processes, benefiting from the unique advantages these alloys offer.

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