Nichrome

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Nichrome is a non-magnetic alloy of nickel, chromium, and often iron, usually used as a resistance wire.

History

Patented in 1905, it is the oldest documented form of resistance heating alloy. A common alloy is 80% nickel and 20% chromium, by mass, but there are many others to accommodate various applications. It is silvery-grey in colour, is corrosion-resistant, and has a high melting point of about 1,400 °C (2,550 °F). Due to its resistance to oxidation and stability at high temperatures, it is widely used in electric heating elements, such as in appliances and tools. Typically, nichrome is wound in coils to a certain electrical resistance, and current is passed through to produce heat.

Uses

Nichrome is used in the explosives and fireworks industry as a bridgewire in electric ignition systems, such as electric matches and model rocket igniters.

Industrial and hobby hot wire foam cutters use nichrome wire.

Nichrome wire is commonly used in ceramic as an internal support structure to help some elements of clay sculptures hold their shape while they are still soft. Nichrome wire is used for its ability to withstand the high temperatures that occur when clay work is fired in a kiln.

Nichrome wire can be used as an alternative to platinum wire for flame testing by colouring the non-luminous part of a flame to detect cations such as sodium, potassium, copper, calcium etc.

The alloy tends to be expensive due to its high nickel content. Distributor pricing is typically indexed to commodity market prices for nickel.

Other areas of usage include motorcycle silencers, in certain areas in the microbiological lab apparatus, and as the heating element of plastic extruders by the RepRap 3D printing community.

For heating, resistance wire must be stable in air when hot. Nichrome wire forms a protective layer of chromium oxide. [1]

Properties

The properties of nichrome vary depending on its alloy. Figures given are representative of typical material and are accurate to expressed significant figures. Any variations are due to different percentages of nickel or chromium.

Material property Value Units
Modulus of elasticity2.2 × 1011Pa
Specific gravity8.4g/cm3
Density8400kg/m3
Melting point1400°C
Electrical resistivity at room temperature1.0 × 10−6 to 1.5 × 10−6Ωm
Specific heat450Jkg−1°C−1
Thermal conductivity11.3Wm−1°C−1
Thermal expansion14 × 10−6°C−1
Standard ambient temperature and pressure
used unless otherwise noted.

Table 1: Resistance per inch (Ω), closed helix, 80/20 alloy.

[2]
Wire Gauge
(B&S No. / AWG)		 Outside Diameter of Helix (inches)
3/4 5/8 1/2 3/8 1/4 7/32 3/16 5/32 1/8 3/32 1/16 1/32
14 .446 .365 .283 .202 .121 .101
15 .638 .523 .408 .293 .178 .148 .120
16 .895 .735 .575 .415 .255 .215 .175 .135
17 1.32 1.08 .851 .617 .383 .325 .266 .208 .150
18 1.89 1.56 1.22 .891 .559 .475 .392 .309 .226
19 2.60 2.14 1.69 1.23 .779 .665 .551 .438 .324
20 3.72 3.07 2.42 1.78 1.13 .967 .805 .644 .482
21 4.53 3.58 2.63 1.68 1.45 1.21 .971 .733 .496
22 4.98 3.67 2.36 2.03 1.70 1.37 1.05 .719
23 7.02 5.18 3.34 2.88 2.42 1.96 1.51 1.05
23 7.02 5.18 3.34 2.88 2.42 1.96 1.51 1.05
24 4.69 4.05 3.41 2.78 2.14 1.60 .865
25 6.87 5.94 5.02 4.10 3.17 2.25 1.32

[2]

Table 2: Current (A) vs. temperature characteristics, straight wire.

Showing approximate amperes necessary to produce a given temperature. Applying only to straight wires stretched horizontally in free air.

Wire Gauge
(B&S No. / AWG)		 Diam. Inches 400°F
204°C		 600°F
316°C		 800°F
427°C		 1000°F
538°C		 1200°F
649°C		 1400°F
760°C		 1600°F
871°C		 1800°F
982°C		 2000°F
1093°C
12 .081 11.34 15.91 20.27 25.53 31.77 39.03 46.73 54.80 63.01
13 .072 9.73 13.53 17.21 21.61 26.89 33.06 39.60 46.41 53.31
14 .064 8.34 10.50 14.59 18.30 22.76 28.01 33.56 39.31 45.11
15 .057 7.15 9.78 12.38 15.50 19.26 23.73 28.44 33.30 38.17
16 .051 6.13 8.31 10.50 13.11 16.30 20.10 24.10 28.20 32.30
17 .045 5.31 7.18 9.13 11.30 13.90 16.90 20.30 23.60 27.00
18 .040 4.66 6.26 7.90 9.75 11.96 14.51 17.37 20.48 23.08
19 .036 4.09 5.46 6.84 8.41 10.30 12.45 14.87 17.78 19.73
20 .032 3.58 4.77 5.92 7.25 8.86 10.69 12.72 15.43 16.87
21 .0285 3.14 4.16 5.13 6.26 7.63 9.17 10.88 13.40 14.40
22 .0253 2.76 3.63 4.44 5.40 6.56 7.87 9.31 11.63 12.33
23 .0226 2.42 3.16 3.84 4.67 5.65 6.76 7.97 10.09 10.54
24 .020 2.12 2.76 3.32 4.01 4.86 5.80 6.82 8.76 9.01
25 .0179 1.84 2.42 2.90 3.44 4.15 4.97 5.86 6.96 7.72
26 .0159 1.58 2.09 2.52 3.00 3.61 4.31 5.06 5.97 6.63
27 .0142 1.34 1.80 2.19 2.62 3.14 3.73 4.37 5.12 5.69
28 .0126 1.18 1.55 1.90 2.28 2.73 3.23 3.77 4.39 4.88
29 .0113 1.02 1.34 1.65 1.99 2.37 2.80 3.25 3.76 4.39
30 .010 .875 1.16 1.43 1.74 2.06 2.43 2.81 3.22 3.59

.040 through .010 based on coiling on an arbor .12 diameter and stretched to twice the close wound lengths. [2]

Table 3: Cold resistance (Ω at 75°F) and wire gauge vs. power output (W) at operating voltage (V).

In the following table, the alloy named Nichrome V is specified as: 19-21% Ni, 2.5% Mn (max), 1.0% Fe (max), 0.75-1.6% Si, 0.15% C (max), balance Cr.[2]

Power Output (W)
(Upon reaching operating temperature)
Resistance (Ω), Nichrome V, 75°F. Resistance (Ω), Nichrome, 75°F. Recommended Wire Gauge (B&S No./AWG)
110-120 V 220-240 V 110-120 V 220-240 V 110-120 V 220-240 V
100 123.52 494.09 118.10 472.40 Max. 26-30 Min. Max. 29-33 Min.
150 82.347 329.38 78.732 314.93 26-30 29-33
200 61.761 247.04 59.050 236.20 25-29 28-32
250 49.409 197.64 47.240 188.96 24-28 27-31
300 41.174 164.69 39.366 157.46 24-28 27-31
350 35.291 141.16 33.742 134.97 23-27 26-30
400 30.881 123.52 29.525 118.10 22-26 25-29
450 27.449 109.80 26.244 104.98 20-24 23-27
500 24.704 98.817 23.620 94.479 20-24 23-27
550 22.458 89.832 21.472 85.889 19-23 22-26
600 20.586 83.345 19.683 79.730 19-23 22-26
650 19.004 76.016 18.170 72.679 18-22 21-25
700 17.646 70.584 16.871 67.486 18-22 21-25
750 16.468 65.874 15.745 62.982 18-22 21-25
800 15.440 61.766 14.762 59.055 18-22 21-25
850 14.532 58.128 13.894 55.577 17-21 20-24
900 13.724 54.897 13.122 52.487 17-21 20-24
950 13.002 52.009 12.431 49.726 17-21 20-24
1000 12.352 49.409 11.810 47.240 16-20 19-23
1050 11.764 47.055 11.247 44.989 16-20 19-23
1100 ..229 44.918 10.737 42.946 16-20 19-23
1150 10.741 42.964 10.270 41.078 15-19 18-22
1200 10.2936 41.174 9.8418 39.367 15-19 18-22
1250 9.8817 39.527 9.4479 37.792 14-18 17-21
1300 9.5016 38.006 9.0845 36.338 14-18 17-21
1350 9.1497 37.599 8.7480 35.992 13-17 16-20
1400 8.8229 36.292 8.4356 34.743 13-17 16-20
1450 8.5188 34.075 8.1449 32.579 12-16 15-19
1500 8.2347 32.938 7.8732 31.493 12-16 15-19
2000 6.1761 24.704 5.9050 23.619 10-14 13-17
2500 4.9409 19.764 4.7240 18.896 9-13 12-16
3000 4.1174 16.469 3.9366 15.746 8-12 11-15

[2]

Additional properties

Approximate current (A) to heat a straight oxidized wire to a given temperature[3]
AWG DIA-IN 400°F 1000°F 2000°F
8 .128 22.4 52 128
10 .102 16.2 37.5 92
12 .081 11.6 26.5 65
22 .0253 2.9 5.6 12.5
32 .0080 0.68 1.36 2.76
40 .0031 0.24 0.43 0.79
Ohms per foot @ 20°C[3]
AWG DIA-IN NiCrA NiCrC
10 .102 0.06248 0.06488
12 .081 0.09907 0.1029
22 .0253 1.015 1.055
32 .0080 10.16 10.55
40 .0031 67.64 70.24
Increase in resistance with temperature[3]
°F °C NiCrA NiCrC
68 20 0 0
600 315 3.3% 5.2%
1000 538 6.3% 8.6%
2000 1093 6.0% 10.5%
NiCrA
Chemical Composition: 80% Ni, 20% Cr
Approx. Melting Point: 1400°C
NiCrC
Chemical Composition: 61% Ni, 15% Cr, bal. Fe
Approx. Melting Point: 1350°C

See also

References

  1. ↑ "Advanced Topic: Oxidation Resistant Materials" (PDF). Sciences Education Foundation. General Atomics. 2002.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 "Wire cable specifications". Pelican Wire. Archived from the original on 2012-09-20. Retrieved 2013-11-24. 
  3. ↑ 3.0 3.1 3.2 "Nichrome 80 & Other Resistance Alloys - Technical Data & Properties". wiretron.com, Wiretronic Inc.

External links

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