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 and 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.
Nickel Copper Alloys

These alloys are sometimes referred to as MONEL or NICORROS and contain nickel with copper and small amounts of iron and manganese. A typical alloy is the 400 grade (UNS N04400). This 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 alloys are used where a higher strength is required compared to pure nickel. They have a wider range of environments where they resist corrosion but in some specialised applications, such as strong alkali contaminant, nickel would be superior.

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

These 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.

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, and NIMONIC,

The more recent 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.

Table 1. Compositions of some common nickel alloys.

UNS No	Al	Cr	Co	Fe	Mo	Ni	Nb+Ta	Ti	W
N10276		15.5		5.5	16	57			3.8
N06600		15.5		8		76
N06625		21.5			9	61	3.6
N08800		21		46		32.5
N07718	0.5	19		18.5	3	52.5	5.1
N07090	1.5	19.5	16.5			60		2.5

Table 2. Typical properties of some common nickel alloys.

UNS No	Condition	UTS (MPa)	0.2 Proof Stress (MPa)	Elong. On 5cm (%)	Hardness (HB)	1000hr Ruptire Str. 750°C (MPa)
N10276	Annealed	790	415	50	184	N/a
N06600	Annealed	550-690	210-430	55-35	120-170	38
N06625	Annealed	930	520	45	180	160
N08800	Annealed	520-690	210-410	60-30	120-184	70 (700°C)
N07718	Age Hardened	1350	1180	17	382	170

This list is far from exhaustive and enquiries 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 alloys are used extensively where changes in mechanical properties with temperature could be a problem, such as in precision springs. The alloys are also used where a metal/glass seal is required.

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 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.
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.