Nickel Chrome (NiCr) 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.
| Element | Intermittent | Continuous |
|---|---|---|
| Cr | 20 | 20 |
| Si | 1.5 | 0.5 |
| Ca | 0.1 | 0.05 |
| Ce | 0.05 | - |
| Ni | Balance | Balance |
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:
- Bright Annealed
- Oxidized Annealed
- 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 80:20 | 108 | 8.3 | 17.0 | 1200 |
| NiCr 70:30 | 118 | 8.1 | 17.0 | 1250 |
| NiCr 60:15 | 112 | 8.2 | 17.0 | 1100 |
| NiCr 40:20 | 105 | 7.9 | 19.0 | 1050 |
| NiCr 30:20 | 104 | 7.9 | 19.0 | 1000 |
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
| Wire | Strip | Ribbon | |
|---|---|---|---|
| Dia (mm) | 12 – 0.15 | - | - |
| Width (mm) | - | 5 - 100 | 0.5 – 5 |
| Thickness (mm) | - | 0.5 - 3 | 0.15 & 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) |
700-900 | 800-950 | 700-850 | 700-850 | 650-800 |
| Elongation % (L0=100 mm) |
25-35 | 25-30 | 25-35 | 25-35 | 25-35 |
Chemical & Physical properties of Electrical Resistance Alloys
| Physical Characteristic | Unit | NiCr 80/20 |
NiCr 60/15 |
NiCr 40/20 |
NiCr 30/20 |
FeCrAl 135 |
|---|---|---|---|---|---|---|
Nominal Composition |
Ni % | 79 | 60 | 37 | 30 | - |
| Cr % | 20 | 15 | 20 | 20 | 20 | |
| Al | - | - | - | - | 5 | |
| Fe % | - | Balance | Balance | Balance | Balance | |
| Other | Si + | Si + | 2 Si + | 2 Si + | + | |
| Specific Resistance | µ Ώ. Cm. | 108 / 110 | 112 | 105 | 104 | 137 |
| Density | Gm/cm3 | 8.412 | 8.247 | 7.95 | 7.90 | 7.20 |
| Maximum operating temperature of element |
°C | 1200 | 1100 | 1050 | 1000 | 1250 |
| Melting Point Approx | °C | 1400 | 1350 | 1380 | 1390 | 1510 |
| Temperature Coefficient of Resistance x 10-6/°c |
Km | 60 | 170 | 240 | 250 | 70 |
| Range °C | 20-1000 | 20-500 | 20-1000 | 20-1000 | 20-1000 | |
| Specific Heat (20°C) | J/Kg °C | 435 | 450 | 460 | 500 | 460 |
| Linear Expansion Coefficient (T=20-1000°C) |
10-6 /°C | 17 | 17 | 19 | 19 | 14 |
| Tensile Strength MIN/MAX |
N/mm2 | 690 | 690 | 690 | 690 | 640 |
| N- mm2 | 1700 | 1700 | 1700 | 1700 | 1500 | |
| Elongation | % | 30 | 30 | 30 | 30 | 16 |
Electrical Resistance Nickel Alloy - Available in Wires / Strip / Ribbon Form
Article coutsey of JLC Electromet Pvt. Ltd., India