Nichrome is peculiar for its high resistance. It determines the application of products made of this alloy. In particular, nichrome coil heaters are most widely used in electric resistance furnaces.
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The ways of manufacturing these products, grades and application recommendations are specified in the corresponding sections of this page.
Grades
Coils are usually made of NiCr 80 20 (also called Ni80Cr20, Cr20Ni80 or Nikrothal® 80) and NiCr 60 15 (also called Ni60Cr15, Cr15Ni60 and Nikrothal® 60) nichrome grades. This material is also called NiCr alloy, nickel-chromium alloy or chromium-nickel alloy. The first one contains ~80% Ni and ~20% Cr, the second one - ~60% Ni and ~15% Cr. The differences in nickel mass % strongly impacts the operational and economical alloy properties. NiCr 80 20 has a higher maximum working temperature and cost than NiCr 60 15. Also the chemical composition of the considered materials includes iron (Fe) and manganese (Mn).
The chemical composition of precision alloys (including NiCr 80 20 and NiCr 60 15 nichromes) is regulated by GOST 10994-74.
Estimation
Nichrome coils are mainly used as heaters in electric resistance furnaces. Such application comprises requirements that shall be strictly followed during designing the electric furnace and producing its heaters.
Heater estimationis an important component of resistance furnace design. In general, it includes the following:
heater location on the working chamber walls (side walls only, on hearth and roof only or on all chamber walls) and heater system;
heater material (usually NiCr 80 20 and NiCr 60 15 nichrome grades are applied, but also CrAl 23 5 and CrAl 27 7 fecral so as refractory metals - tungsten, molybdenum, tantalum, niobium - can be used);
heater size and weight (the detailed estimation of these values is described in “Heaters. Estimation techniques and examples”; also a special calculator can be used);
location of heaters inside the furnace chamber in compliance with their sizes in item 3.
Heaters placement option
Item 4 deals with the estimation of nichrome coil made of wire with diameter and length determined in item 3. The main task is to place the heater of estimated length in the confined space of the furnace. Nichrome wire is wounded in coil form or zigzagged for placing more material (by length) into the working chamber. Usually coil form is used.
Nichrome coil in electric furnace
Usually nichrome wire with Ø 3-7 mm is used for making heaters of industrial furnaces. However, for furnaces with working temperature 1000 °C and higher wire with Ø<5 mm shall be used (see “Heaters. Estimation techniques and examples”). The ratio of the coil lead and diameter to the wire diameter is determined in such a way that makes placing the heaters in the furnace easier, provides their rigidness, prevents the local overheating of the coil wind to the utmost while allowing transmitting the heat from them to the products.
The higher diameter and smaller lead the coil has, the easier it is to place the heaters in the furnace. But coils with higher diameters are less rigid and their winds are more likely to lie on each other. On the other side, higher winding frequency increases the screening action of the wind facing the product on other parts, so this worsens its surface use and leads to local overheating.
In practice, accurate recommended ratios of the wire diameter (d) and lead (t) to the coil diameter (D) for wire with Ø 3-7 mm are t ≥ 2d and D = (7-10)d for nichrome and D = (4÷6)d for less strong iron-chromium-aluminum alloys such as fecral etc.
Nichrome coil size
For thinner wires the ratio of D to d and t is usually higher. Coil heaters on ceramic tubes are very wide-spread. In terms of irradiation and power distribution among the furnace walls such heaters are similar to freely radiant nichrome winds and, vice versa, they are much more efficient than coils in slots and shelves. Wire coil heaters on ceramic tubes have a universal design, both in terms of materials applied and heater location in the furnace chamber. The ratio of the coil inner diameter to the tube outer diameter for such heaters can be set approximately to 1.1-1.2, distance between the tube axes - 1.5-2D.
Production
Nichrome coil is wound from cold-drawn wire without heating. If the heater is made of fecral wire, it is wound with heating due to passage of the current through the blank. A transformer with 5-10 V voltage tap is applied for this. The voltage is supplied to the lathe bed and toolholder through the slideway with a hole for the wire. The slideway is isolated from the lathe bed with textolite gaskets.
The coil winding speed that allows heating the corresponding wire section to 200-300 °C is set (temperature about 400-500 °C crisps alloys). The blank is heated on the section between the toolholder and coil mandrel assembly only. Jumps shall be bowed in a special appliance avoiding sharp blows and bends not to break the blank.
Prices
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Calculate the nichrome products weight
