Industrial uses of Nichrome & Resistance Alloys

While almost any conductive wire can be used for heating, most metals conduct electricity with great efficiency. This requires the metals to be formed into thin, delicate wires to create enough resistance to generate heat. When most metals are heated, they oxidize quickly, making them brittle and prone to breaking when heated in air. Nichrome wire, however, develops an outer layer of chromium oxide, which makes the wire thermodynamically stable in air, mostly impervious to oxygen, and protects the heating element from further oxidation.

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With its high-temperature strength and good workability, Nichrome is an ideal material for demanding applications in the electric appliance industry, such as hair dryers and heat guns. It is also commonly used in electronic cigarettes (e-cigarettes) and other vaping (vape) applications.

Other common applications for Nichrome alloys include ironing machines, water heaters, soldering irons, metal-sheathed tubular elements, cartridge elements, quartz tube heaters, infrared emitters, and other precision heating element applications.

60/16 Nichrome offers good corrosion resistance, oxidation properties, and form stability. Typical applications for 60/16 Nichrome are in tubular heating elements.

Nickel Chrome (NiChrome) Alloys

Chemical Formula

Ni/Cr Alloys

Topics Covered

Background

Oxidation Resistance

Heating Elements

Thermocouples

High Temperature Corrosion Resistant Alloys

Wear Resistant Alloys

Background

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

Oxidation Resistance

The introduction of small amounts (<7%) of chromium to nickel increases the sensitivity of the nickel-chromium alloy to oxidation. This is because the diffusion rate of oxygen in the scale is increased. This trend reverses after addition levels increase above 7% chromium and increases up to an addition level of approximately 30%. Above this level, there is little change.

Oxidation resistance can be attributed to the formation of a highly adherent protective scale. The adherence and coherence of the scale can be improved by adding small amounts of other reactive elements such as zirconium, silicon, cerium, calcium, or similar. The scale thus formed is a mixture of nickel and chrome oxides (NiO and Cr2O3). These combine to form nickel chromite (NiCr2O4), which has a spinel-type structure.

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 Chrome NiChrome and BrightRay. Small modifications of this composition may be made to optimize it for particular applications.

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Figure 1. Electrical resistivity as a function of chromium content for nickel-chromium alloys

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 lesser with continuously operating 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. Other uses for this alloy are thermocouples.

Thermocouples

Thermocouples like the 90/10 nickel-chromium alloy are commonly used in conjunction with a 95/5 Ni/Al alloy. This combination, called chromel-alumel, has a maximum operating temperature of 1100°C. However, this combination can become susceptible to drift around 1000°C due to preferential oxidation after prolonged usage. The addition of silicon has been found to overcome this effect. Commercial grades include Nicrosil (containing 14% Cr and 1.5% Si) and Nisil (containing 4.5% Si and 0.1% Mg).

High Temperature Corrosion Resistant Alloys

The 80/20 nickel-chromium alloy is often used for wrought and cast parts for high-temperature applications, as it has better oxidation and hot corrosion resistance compared to cheaper iron-nickel-chromium alloys. This alloy is highly suited to applications that are subject to oxidation.

In applications encountering fuel ashes and deposits such as alkali metal salts like sulfates, higher chromium content alloys are more suitable. This is because fuel ashes react with the oxide scale. Ashes containing vanadium are particularly aggressive and have a fluxing effect on the scale, increasing the susceptibility of the alloy to oxidation attack.

In sulfur-containing environments, chromium sulfide (Cr2S3, melting point 1550°C) is formed preferentially to nickel sulfide. Formation of nickel sulfide is preferred, as it hinders the formation of the nickel/nickel-sulfide eutectic which has a low melting point. Eventually, local chromium supplies can be exhausted, leaving sulfur to react with nickel to form the low-melting point eutectic compound, leading to liquid phase attack. This form of attack leaves wart-like growths on the alloy's surface. Due to the preferential formation of chromium sulfides, higher chromium-containing nickel-chromium alloys are more resistant to this type of attack.

Nickel/chromium alloys containing more than 30% chromium have a two-phase structure which consists of α-chromium and γ-nickel. The α-chromium phase is brittle and hence the alloy decreases in ductility with increasing chromium content. Properties for some binary alloys are given in Table 2. The addition of about 1.5% niobium induces improved strength and ductility, while reducing embrittlement after high-temperature exposure, provided impurities such as carbon, nitrogen, and silicon are controlled.

Table 2. Tensile and ductility properties for some nickel-chromium alloys at room temperature.

Cr Content (%)

Tensile Strength (MPa)

Elongation (%)

35

480

62

50

540-680

7-24

60

800-1000

1-2

Alloys with chromium contents up to approximately 35% are suitable for hot working. Above this level, they are generally only suited to casting. Some ductility gains can be achieved by the addition of zirconium or titanium. Inconel 671, which contains 48% Cr and 0.35% Ti, is such an alloy used in applications including duplex tubing for coal-fired superheating tubing.

Wear Resistant Alloys

Wear mechanisms are complex, but high hardness and good corrosion resistance contribute to good wear resistance. Nickel-chromium alloys provide an economical alternative to materials such as weld-deposited cobalt-chrome alloys with additions of carbon and tungsten, which are commonly used in wear-resistant applications. An example of a nickel-chromium alloy for this type of application contains 8-12% Cr, 0.3-1.0% C, 3-4% Si, 1.5-2.5% B, 1-4% Fe, and the balance Ni. A coating of this material, deposited by inert gas-shielded arc techniques, would be in the range of 40-50 Rockwell C.

Property Table of Nickel Chromium Alloys

Nickel Chrome Alloys Composition: Ni/14-46Cr + some combination of Fe, Mo, Cu, Co, Si, Ti, W, Al, and others

Property

Minimum Value (S.I.)

Maximum Value (S.I.)

Units (S.I.)

Minimum Value (Imp.)

Maximum Value (Imp.)

Units (Imp.)

Atomic Volume (average)

0.0065

0.0072

m3/kmol

396.654

439.371

in3/kmol

Density

7.75

8.65

Mg/m3

483.817

540.002

lb/ft3

Energy Content

40

200

MJ/kg

4333.55

21667.7

kcal/lb

Bulk Modulus

110

205

GPa

15.9541

29.7327

106 psi

Compressive Strength

170

2100

MPa

24.6564

304.579

ksi

Ductility

0.005

0.7

0.005

0.7

Elastic Limit

170

2100

MPa

24.6564

304.579

ksi

Endurance Limit

130

1150

MPa

18.8549

166.793

ksi

Fracture Toughness

65

150

MPa.m1/2

59.153

136.507

ksi.in1/2

Hardness

1000

6000

MPa

145.038

870.227

ksi

Loss Coefficient

9e-005

0.0013

9e-005

0.0013

Modulus of Rupture

170

2100

MPa

24.6564

304.579

ksi

Poisson's Ratio

0.26

0.325

0.26

0.325

Shear Modulus

55

100

GPa

7.97707

14.5038

106 psi

Tensile Strength

330

2300

MPa

47.8625

333.587

ksi

Young's Modulus

150

245

GPa

21.7557

35.5342

106 psi

Glass Temperature

K

°F

Latent Heat of Fusion

275

320

kJ/kg

118.228

137.575

BTU/lb

Maximum Service Temperature

1070

1473

K

1466.33

2191.73

°F

Melting Point

1475

1710

K

2195.33

2618.33

°F

Minimum Service Temperature

0

0

K

-459.67

-459.67

°F

Specific Heat

380

500

J/kg.K

0.294066

0.386929

BTU/lb.F

Thermal Conductivity

8

17

W/m.K

14.9763

31.8246

BTU.ft/h.ft2.F

Thermal Expansion

9

16

10-6/K

16.2

28.8

10-6/°F

Breakdown Potential

MV/m

V/mil

Dielectric Constant

Resistivity

84

240

10-8 ohm.m

84