MAGNET STEELS -

Magnetic materials used in engineering can be divided into two groups :-

A)  alloys which retain magnetism after the magnetising influence has been removed, i.e. magnetically 'hard' materials used in permanent magnets. The essential characteristics of permanent magnet .materials are : a) coercive force, b) residual magnetism or remanence, c) energy product value.

High Cobalt steels, when correctly heat treated are frequently used in the making of permanent magnets for magnetos, loud speakers, and other electrical machines and instruments. Steels having compositions 15 to 40% Cobalt, 0.4 to 1% Carbon, 1.5 to 9% chromium and upto 10% tungsten possess improved magnetic properties.

B)  alloys which lose most of the induced magnetism when the magnetising field is removed i.e. magnetically soft materials, such as are used in transformer cores and dynamo pole -pieces.

Soft iron and iron-silicon alloys are most useful in respect of low hysteresis and high permeability. The iron-silicon alloys contain upto 4.5% silicon and practically, no carbon, and are usually supplied in the form of dead soft sheets from which transformer-core lamination can be stamped. The use of laminations reduces losses from eddycurrents, which would be more prominent in a solid core.

Nickel - iron alloys are also of importance as low - hysteresis, high permeability alloys. "Permallay" contains 78% nickel and 22% iron with carbon and silicon kept as low as possible. Mumetal contains about 74% nickel, 5% Copper, upto 1% manganese and the balance iron. Both alloys are used in communication engineering where the high permeability will utilise to the full the small currents involved.

CEMENTED CARBIDE -

They are made out of a powdery mixture of tungsten and titanium carbides and metallic cobalt which is first compacted and then sintered, thus employing well known powder metallurgy process.

In their finished form cemented carbides consists of extremely fine grains of tungsten and titanium carbides with a cobalt binder. These alloys are extremely hard with Rockwell hardness number up to 85 and even higher and they retain the hardness at temperatures upto 1000°c.

Most widely used cemented carbide contains 2 to 15% cobalt, and the remainder is tungsten carbide ; and the other grades contain a few percent cobalt and 5, 14, 30 and 60% titanium carbide respectively, the remainder again, being tungsten carbide.

The cutting properties of cemented carbides considerably exceed those of high -speed steel. They are extremely resistant to wear and for this purpose they are valuable for wire - drawing dies and other similar-applications.

STELLITES -

The stellites are intermediates between high -speed steels and cemented carbides, in that they are cast alloys containing tungsten and chromium carbides in a matrix of cobalt and chromium. They are used for roughing cuts on very hard material, but also find applications for use as valve seatings in aero-engine, diesel engines and as dies.

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