Heating Element Alloys

Nickel Chrome Resistance Alloys


Background of Nickel Chrome Resistance Alloys

The nickel-chromium (NiChrom) system shows that chromium is quite soluble in nickel. This is a maximum at 47% at the eutectic temperature and drops off to about 30% at room temperature. A range of commercial alloys is based on this solid solution. Such alloys have excellent resistance to high temperature oxidation and corrosion and good wear resistance.

Heating Elements

A marked increase in electrical resistivity is observed with increasing chromium additions. An addition level of 20% chromium is considered the optimum for electrical resistance wires suitable for electrical heating elements. This composition combines good electrical properties with good strength and ductility, making it suitable for wire drawing. Commercial grades include Nickel-chromium and NiCr. Small modifications of to this composition may be made to optimise it for particular applications.

The addition of the appropriate reactive alloying elements will affect the properties of the scale. The operating conditions of the alloy will largely influence the composition that should be used. Table 1 outlines the compositional differences between alloys used for intermittent and continuous usages.

Table 1. Suitable compositions for heating elements used intermittently and continuously.

ElementIntermittentContinuous
Cr2020
Si1.50.5
Ca0.10.05
Ce0.05-
NiBalanceBalance

While the compositional changes have a negligible effect on mechanical properties, higher additions of reactive elements tend to prevent flaking of the scale during cyclic heating and cooling. This effect is less of an issue with continuously operating electrical heating elements, so addition levels do not need to be as high.

The binary 90/10 Ni/Cr alloy is also used for heating elements, and has a maximum operating temperature of 1100°C.

Nickel-Chromium: Heating Element and Resistance Alloys

Alloy Description

  • NiCr 80:20 – Industrial furnaces (up to 1200 °C), electric cooking equipment, precision resistors.
  • NiCr 70:30 – Industrial furnaces (up to  1230 °C) with alternating oxidizing/reducing atm., precision resistors.
  • NiCr 60:15 – Industrial furnaces (up to 1100 °C), electrically heated equipment, high-resistance and potentiometer resistors.
  • NiCr 40:20 – Domestic heating appliances (up to 1050 °C), furnaces in carburising or semi-reducing atmosphere.
  • NiCr 30:20 – Industrial furnaces (up to 1050 °C), heating elements of cooking equipment.

Condition of Supply

NiCr alloys are available in following conditions:

  1. Bright Annealed
  2. Oxidized Annealed
  3. Bright Drawn

Properties

 Specific Resistance
(Electrical Resistivity)
m W-cm
Density
g/cm3
Linear Expansion
Coeff.  bet RT &
1000 °C  (10-6/K)
Maximum Operating
Temperature (°C)

(in air)
NiCr 80:201088.317.01200
NiCr 70:301188.117.01250
NiCr 60:151128.217.01100
NiCr 40:201057.919.01050
NiCr 30:201047.919.01000

For Resistance (in Ohm/m) of any size, devide the above resistivity with (cross-sectional area in mm2 of that size x 100).

R (ohm/m) =     Resistivity (michrohm-cm)
                                 A (in mm2) x 100

Size Range

 WireStripRibbon
Dia (mm)12 – 0.15--
Width (mm)-5 - 1000.5 – 5
Thickness (mm)-0.5 - 30.15  &  Above

Mechanical Properties at Room Temperature

 NiCr 80:20 NiCr 70:30 NiCr 60:15 NiCr 40:20 NiCr 30:20
Tensile strength
Min-Max (N/mm2)
700-900800-950700-850700-850 650-800
Elongation %
(L0=100 mm)
25-3525-3025-3525-3525-35

Chemical & Physical properties of Electrical Resistance Alloys

Physical CharacteristicUnit NiCr
80/20
NiCr
60/15
NiCr
40/20
NiCr
30/20
FeCrAl
135

Nominal Composition
Ni %79603730-
Cr %2015202020
Al----5
Fe %-Balance Balance Balance Balance
Other Si +Si +2 Si +2 Si ++
Specific Resistance µ Ώ. Cm.108 / 110112105104137
Density Gm/cm38.4128.2477.957.907.20
Maximum operating
temperature of element
°C12001100105010001250
Melting Point Approx °C14001350138013901510
Temperature Coefficient
of Resistance x 10-6/°c
Km6017024025070
Range °C20-100020-50020-100020-100020-1000
Specific Heat (20°C)J/Kg °C435450460500460
Linear Expansion
Coefficient (T=20-1000°C)
10-6 /°C1717191914
Tensile Strength
MIN/MAX
N/mm2690690690690640
N- mm217001700170017001500
Elongation %3030303016

Electrical Resistance Nickel Alloy - Available in Wires / Strip / Ribbon Form

Article coutsey of JLC Electromet Pvt. Ltd., India

High-Temperature Alloys The NiChrome alloy has better resistance to hot corrosion and oxidation than inexpensive iron-nickel-chromium alloys. Hence, it is mostly used for cast and wrought parts for high-temperature applications. This alloy is ideal for applications that are prone to oxidation.

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