Nickel Alloys Stainless Steels, Nickel Copper Alloys, Nickel Chromium Alloys, Low Expansion Alloys and Magnetic Alloys

Topics Covered

• Background
• Stainless steels
• Nickel Copper Alloys
• Nickel Chromium Base Alloys
• Low Expansion Alloys
• Magnetic Alloys

Background

Nickel alloys are used extensively because of their corrosion resistance, high temperature strength and their special magnetic and thermal expansion properties.

The major alloy types that are used are:

• Iron-Nickel-Chromium alloys
• Stainless Steels
• Copper-Nickel alloys and Nickel-Copper alloys
• Nickel-Chromium Alloys
• Nickel-Chromium-Iron alloys
• Low Expansion Alloys
• Magnetic Alloys

Stainless steels

The majority of the stainless steels contain 8-10% nickel. In all cases it is the combination of chromium with the nickel that does the job. Stainless steels are also useful as fire retardant materials since they retain their strength to higher temperatures than structural steel.

The most common stainless steel is the 304 grade with 8% nickel and 18% chromium and the balance iron. This is used for such common items as spoons and forks, saucepans and kitchen sinks. Where extra corrosion resistance is required, such as for roofing in marine applications, type 316 is used. This has about the same amount of nickel and chromium as 304 but with 3% of molybdenum added. The balance is again iron.

There are many other stainless steels to cover the wide spectrum of demands of engineers and architects such as nickel plated steel (NPS).

Nickel Copper Alloys

These nickel copper alloys are sometimes referred to as NiCu or MONEL or NICORROS and contain nickel with copper and small amounts of iron and manganese. A typical nickel copper alloy is the 400 grade (UNS N04400). This nickel copper alloys contains 63% nickel minimum, 28-34% copper, and a maximum of 2% manganese and 2.5% iron. There are also a small number of impurities kept at limited values to ensure the metal's properties are not harmed.

These nickel copper alloys are used where a higher strength is required compared to pure nickel. Nickel copper alloys have a wider range of environments where they resist corrosion but in some specialised applications, such as strong alkali contaminant, nickel or commercially pure nickel would be superior.

Nickel copper alloys find wide application in oil refining and marine applications where long corrosion-free life is required. Because of good thermal conductivity of nickel copper alloys, they are frequently are used for heat exchangers where sea water is one of the fluids concerned.

Nickel Chromium Base Alloys

These nickel chromium base alloys are used extensively in applications where heat resistance and/or corrosion resistance is required. In some members of the group, where conditions are less demanding, some nickel is replaced by iron to decrease the overall cost.

Metals fail at high temperatures by both oxidation (scaling) and through a loss in strength. Alloys in this class are designed to resist failure from both of these mechanisms. Nickel alloys are not suitable for high temperature sulphur rich environments.

Where corrosion resistance is significant, molybdenum is used as an alloying addition in nickel chromium based alloys.

This group of alloys are frequently sold under trade name specifications but most are listed in the Unified Numbering System. Common trade names are Hastelloy, Incoloy, Inconel, Nicrofer, NiChromM and NiMonic.

The more recentnickel chromium base alloys in these groups also have a wide range of ancillary elements added to give special properties - some of these can be quite complicated and require very close control over composition and heat treatment.

This list is far from exhaustive and enquiries to nickel alloy manufacturars should be made for specialised high temperature or corrosive situations.

All metals 'creep' under stress at high temperature and in their manufactured form, components may deform. This deformation could cause failure. Nickel alloys have higher strength and longer life at elevated temperature than most alloys. This makes them ideal for such parts as blades and disks in gas turbine engines. The designer however, must determine the expected life of each component and use the appropriate creep and rupture strength in the design.

Low Expansion Alloys

There are a group of nickel-iron controlled expansion alloys where the expansion coefficient is low and constant over a range of temperatures.

These nickel iron low expansion alloys are used extensively where changes in mechanical properties with temperature could be a problem, such as in precision springs. These nickel iron alloys are also used where a metal/glass seal is required called as Glass Sealing Alloys.

One example is the alloy containing 48% nickel and the balance iron (UNS K94800). This alloy has the following expansion coefficients:

20-100°C: 8.5 x 10^-6m/m.°C
20-400°C: 8.3-9.3 x 10^-6m/m.°C

This nickel iron alloy has an expansion co-efficient designed to match that of soda-lime and soft lead glasses and thus provides a sound glass/metal seal that will not crack because of differential expansion between the two materials. Another such seal is provided by dumet wires for lamps

Magnetic Alloys

There is a requirement for materials with high magnetic permeability to minimise the power requirements to generate a strong magnetic field, such as that required in tape recorder heads and for magnetic shielding around precision cathode ray display devices.

These high permeability materials are complex alloys based on nickel with a range of composition possibilities. A typical composition could be: 70-80% nickel with small amounts of molybdenum and/or copper and the balance iron. This alloy would be expected to have a maximum relative permeability of between 50,000 and 100,000. Common trade names of this group are MU-METAL and PERMALLOY.

There is also a requirement for materials with a constant permeability over a range of magnetic flux densities. This is required in telephone equipment and electrical fitters where a variation in permeability would result in distortion. These alloys are generally known as the PERMINVAR alloys and have compositions ranging around 45% nickel, 30% iron and 25% cobalt.

Trade Names and Owners of Nickel Alloys

For more information on this source please visit The Institute of Materials Engineering Australasia.