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Tuesday, 6 August 2019

Hard Magnetic Materials Examples

Hard Magnetic Materials with Examples

Magnetically hard materials are used for making permanent magnets. Such materials have

(a) Large area of B-H curve
(b) Large coercive force and magnetic reluctance
(c) High saturation values
(d) Magnetisation is difficult when compared to soft magnetic materials.

When carbon is added in a material the area of hysteresis loop increases and permanent magnets are prepared with the addition of tungsten, cobalt, chromium etc. Following are the examples of main hard magnetic materials used for making permanent magnets:

Tungsten Steel :

It is made up of 5% to 6% tungsten, 0.6% to 0.8% carbon, 0.5% manganese. The balance is iron. Its saturation flux density is 1.3 Wb/m2 and coercivity 8000 At/m. It is tough and resistance to wear and shock. It is so hard that it requires a special care during heat treatment to avoid cracks and distortion. It has high magnetic retentivity. It is suitable for making permanent magnets for dynamos, motors etc.

Chromium Steel :

It contains 2.5% to 6% chromium, 0.6% to 0.8% carbon, 0.5% manganese, and the rest iron. Its magnetic properties are identical with that of tungsten steel. Its saturation flux density is 1.5 Wb/m2 and coercivity 8000 At / m. It is hard and tough. It requires good amount of care during hardening as it is liable to break due to high carbon content.

Alnico :

It contains 10% of aluminium, 15% nickel, 5% copper, 20% cobalt, and the rest iron. It possess high magnetic properties. It is lighter in weight and has a high value of B-H product. It cannot be easily machined but grinding can be done. It is brittle. Its coercive force is 105 At / m. Magnets are manufactured by casting or by sintering. It is used in portable and light instruments. It has more rectangular hystresis loop and high value of remanence. It is available in many grades each possessing varying properties. Its saturation flux density is 1.2 Wb / m2.

Cunife :

It contains 60% copper, 20% nickel and 20% iron. It is malleable and ductile and can be easily machined, punched and cold rolled. It is used for making magnets of small size.

Hypernic :

It is an alloy of 50% nickel and 50% iron. It is annealed in an atmosphere of hydrogen to improve its permeability. Its saturation flux density is 1.6 Wb / m2 and coercivity is 4 At / m. Its permeability ranges from an initial value of 4000 to minimum value of 80,000. Its density is 8.25 g/cm3 and resistivity is 35 x 10-2 ohm-m. Its curie temperature is 500°C and has high magnetic properties, with low hysteresis loss. It is used in current transformers and instruments. It is also used in loading coils placed at intervals in long distance telephone lines to make the electrical impulse distinct.

Invar :

It is also an alloy of nickel and iron with 62% of iron and 38% of nickel. It is not affected much due to rapid magnetic reversals of alternating current supply and has a very low value of hysteresis loss. It possess thermal stability having small thermal expansion. It is used in relays and instruments.

Materials for Permanent Magnet:

Magnetic Material
General Property
Carbon Steel
Tungsten Steel
Chromium Steel
30% Cobalt Steel
Cast grind
Hard Brittle
Wire strip
Ductile & Malleable

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