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Nickel Chrome Resistance Alloys

Background

The nickel-chromium 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 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 heating elements, so addition levels do not need to be as high.

he 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

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 80:20
NiCr 70:30
NiCr 60:15
NiCr 40:20
NiCr 30:20

108
118
112
105
104

8.3
8.1
8.2
7.9
7.9

17.0
17.0
17.0
19.0
19.0

1200
1250
1100
1050
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
- Available in Wires / Strip / Ribbon Form

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