Magnetic Material1

The habits of iron

Iron and steel can be induced to show a range of magnetic effects. Hard steel once magnetized tends to remain so. Soft iron is easily magnetized but its magnetized state disappears when the field inducing it is removed.The electric currents produced magnetic fields and that changing magnetic fields, when linked with a conductor, produced electric effects. The extension of this experience is that two electric current systems each producing a magnetic field will react on each other and produce interactive forces just like permanent magnet do.

Studies by Bethe showed that only when atomic parameters fall into a certain Studies by Bethe showed that only when atomic parameters fall into a certain range will spontaneous co-operative alignment of electron spin become range will spontaneous co-operative alignment of electron spin become energetically favorable. Thermodynamic experience indicates that whenever a energetically favorable. Thermodynamic experience indicates that whenever a lower energy state is available Nature will preferentially occupy it. lower energy state is available Nature will preferentially occupy it. Iron, cobalt and nickel are the prime candidates for so-called Iron, cobalt and nickel are the prime candidates for so-called ferromagnetism. Non-ferromagnetic materials exhibit a weak response to ferromagnetism. Non-ferromagnetic materials exhibit a weak response to magnetizing forces, either reinforcing or opposing them. Such paramagnetic magnetizing forces, either reinforcing or opposing them. Such paramagnetic and diamagnetic effects are important to physical theories but do not yield and diamagnetic effects are important to physical theories but do not yield forces of engineering importance.forces of engineering importance.

Manganese

Iron

Cobalt

Nickel

Gadolinium

Ratio of atomic separation to unfilled shell diameter

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HISTORY OF THE DEVELOPMENT OF ELECTRICAL STEELS

In the earliest times, cast iron and wrought iron were used since they were convenient and available, but the permeability of cast iron is not very good, as seen in Figure 2.

0 200 400 600 800 1000 1200 1400

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Cast Iron

Cast Steel

Mild Steel

Silicon free electrical steel

Figure 2

At room temperature, iron has a body centred cubic lattice and within a At room temperature, iron has a body centred cubic lattice and within a crystal of iron in which electron spins have spontaneously aligned to give crystal of iron in which electron spins have spontaneously aligned to give self saturation, some directions are much more easily magnetisable than self saturation, some directions are much more easily magnetisable than others. Figure 3 shows a representation of the iron lattice and plots the others. Figure 3 shows a representation of the iron lattice and plots the relative magnetisability of various arranged so that the magnetic relative magnetisability of various arranged so that the magnetic properties of the overall material represents an average of the various properties of the overall material represents an average of the various crystal direction.crystal direction.

(a)

[100] [111]

[110]

[111]

[100]

[110]

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Bsat

Bsat/ √3

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Applied field (kA/m)

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Figure 3 (a) Body centred cubic lattice: [111] is a body diagonal;[110] is a face Figure 3 (a) Body centred cubic lattice: [111] is a body diagonal;[110] is a face diagonal; [100] is a cube edgediagonal; [100] is a cube edge

(b) The [100] direction is the easy cube edge direction; [100] is the (b) The [100] direction is the easy cube edge direction; [100] is the hard cube face diagonal direction; [111] is he hardest cube body hard cube face diagonal direction; [111] is he hardest cube body diagonal directiondiagonal direction

Tools to improve magnetic efficiency

Lamination

Raised resistivity

Purification

Grow large grains

Orientation of grains

Removal of stress

Tensile coatings

Lamination Lamination usage for restraint of eddy current and raising of resistivity promotes rapid flux penetration usage for restraint of eddy current and raising of resistivity promotes rapid flux penetration and full usage of the iron. and full usage of the iron.

Relationship between Relationship between electrical resistivity and electrical resistivity and silicon content silicon content

Adding silicon has Adding silicon has disadvantages. These disadvantages. These are:are:

1.1. Cost of ferrosilicon to Cost of ferrosilicon to bebe

added to iron is moreadded to iron is more expensive : Silicon isexpensive : Silicon is available commercially inavailable commercially in the form of ferrosilicon.the form of ferrosilicon.2. Greatly increased2. Greatly increased brittleness induced bybrittleness induced by added silicon.added silicon.3.3. Decrease in the totalDecrease in the total flux available from ironflux available from iron due to dilution by silicon.due to dilution by silicon.

3210

Silicon (%)

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The addition of siliconThe addition of silicon raised resistivityraised resistivity

Isat

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Isat Wall Wall

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PurificationPurificationAny non metallic inclusions in steel impair its magnetic properties. This Any non metallic inclusions in steel impair its magnetic properties. This arises because when magnetisation is being changed domain walls must arises because when magnetisation is being changed domain walls must move, and they are pinned and obstructed by non metallic inclusions. move, and they are pinned and obstructed by non metallic inclusions. Domain walls carrier energy implicit in its existence and if a hole is produced Domain walls carrier energy implicit in its existence and if a hole is produced in a wall due to its straddling inclusions then this will constitute energy well in a wall due to its straddling inclusions then this will constitute energy well as energy is needed to create wall area as the wall is pulled away.as energy is needed to create wall area as the wall is pulled away.

Effect of alloying elements on resistivity and saturation Effect of alloying elements on resistivity and saturation magnetisation [after Foley et al. BISRA]magnetisation [after Foley et al. BISRA]

Grow large grainsGrow large grainsWhen steel is held at a high temperature, e.g. 800-1000oC, grains grow at the expense of each other When steel is held at a high temperature, e.g. 800-1000oC, grains grow at the expense of each other so that overall grain size increases. With large grains the presence of the grain boundary is less pr unit so that overall grain size increases. With large grains the presence of the grain boundary is less pr unit volume. Grain boundaries are prime domain wall pinning sites so that large grains reduce the overall volume. Grain boundaries are prime domain wall pinning sites so that large grains reduce the overall mount of obstruction of wall movement per unit volumemount of obstruction of wall movement per unit volume . .

Orientation of grainsOrientation of grains were called Goss orientation is used. Norman P. Goss developed grain were called Goss orientation is used. Norman P. Goss developed grain orientation processing in the 1930s in the USA after finding that suitably applied cold rolling and heat orientation processing in the 1930s in the USA after finding that suitably applied cold rolling and heat treatment regimes led to the selective growth of grains having the easy directions of magnetisation in treatment regimes led to the selective growth of grains having the easy directions of magnetisation in the strip rolling directionthe strip rolling direction. .

Removal of stressRemoval of stressStressed regions in steel constitute domain wall pinning sites and are to be avoided, so that stress Stressed regions in steel constitute domain wall pinning sites and are to be avoided, so that stress relieving heat treatments are often used to optimise magnetic properties. In general stress, and relieving heat treatments are often used to optimise magnetic properties. In general stress, and particularly compressive stress, impairs magnetic properties. The strained displacement of atoms in particularly compressive stress, impairs magnetic properties. The strained displacement of atoms in stressed regions affects the crystallite response to magnetisation and efforts to remove stress are well stressed regions affects the crystallite response to magnetisation and efforts to remove stress are well rewarded. rewarded.

Tensile coatingsTensile coatingsNot all stress is unfavourable and for some steel (e.g. transformer steels) coating which induce tension Not all stress is unfavourable and for some steel (e.g. transformer steels) coating which induce tension in the underlying strip can be helpful. in the underlying strip can be helpful.

RANGE OF MATERIALS

PropertiesPropertiesComposition. Added silicon can range from near zero up to 3+%. In general higher Composition. Added silicon can range from near zero up to 3+%. In general higher silicon which restrains eddy currents reduces loss, is harder and cost more to make. silicon which restrains eddy currents reduces loss, is harder and cost more to make. Resistivity and as annealed hardness were varies with the percentage of silicon. Resistivity and as annealed hardness were varies with the percentage of silicon. Added silicon reduces Added silicon reduces BsatBsat and impairs high field permeability. The absence of and impairs high field permeability. The absence of carbon, sulphur and titanium is desirable; desulphurization is possible but expensive.carbon, sulphur and titanium is desirable; desulphurization is possible but expensive.Thickness. The thinner the steel the more effectively eddy currents are restrained and Thickness. The thinner the steel the more effectively eddy currents are restrained and the lower the core losses. However laminations pile of a given height contains less the lower the core losses. However laminations pile of a given height contains less metal I made of 0.35 mm steel rather than 0.65 mm steel due to the effect of the extra metal I made of 0.35 mm steel rather than 0.65 mm steel due to the effect of the extra surfaces on the stacking factor. The effect of this is to reduce the effectivesurfaces on the stacking factor. The effect of this is to reduce the effective Bsat Bsat. . Thinner steel costs more to produce and requires more press blows to make Thinner steel costs more to produce and requires more press blows to make lamination stacks of a given size.lamination stacks of a given size.Coated or not. Coatings are expensive to apply. Needless coating reduces the space Coated or not. Coatings are expensive to apply. Needless coating reduces the space factor and may cause weld ability problems.factor and may cause weld ability problems.State of heat treatment. Material can be finally annealed or semi processed and State of heat treatment. Material can be finally annealed or semi processed and awaiting a final anneal after stamping.awaiting a final anneal after stamping.Hardness. Material can be soft due to being finally annealed and having very little alloy Hardness. Material can be soft due to being finally annealed and having very little alloy content, content,

ApplicationApplicationSmall machine generally demand a low price steel. Steel at 0.65 mm unalloyed, Small machine generally demand a low price steel. Steel at 0.65 mm unalloyed, awaiting final anneal with no formal coating could be typical. Larger machines awaiting final anneal with no formal coating could be typical. Larger machines benefit from a higher percentage of silicon and definite coating. Very large benefit from a higher percentage of silicon and definite coating. Very large machines use 3+ percent silicon and a good coating, perhaps followed by a further machines use 3+ percent silicon and a good coating, perhaps followed by a further apply last varnish when interlaminar emfs will be large.apply last varnish when interlaminar emfs will be large.

Working conditionsWorking conditionsThe force between rotor and stator depends on The force between rotor and stator depends on BB22 , so that the best power output , so that the best power output per volume, higher per volume, higher BB values tend to be used. If operating cyclic values tend to be used. If operating cyclic BmaxBmax values are values are too high then losses rise rapidly and permeability falls off, so that compromise too high then losses rise rapidly and permeability falls off, so that compromise BmaxBmax value will be used. Careful attention should be given to the value will be used. Careful attention should be given to the BmaxBmax at which at which purchase and sale agreed. A steel having lower loss at purchase and sale agreed. A steel having lower loss at BmaxBmax = 1.5 T (a typical = 1.5 T (a typical specification induction) may perform worse at specification induction) may perform worse at BmaxBmax =1.8 T if it has a higher =1.8 T if it has a higher percentage of silicon than an unalloyed grade would do. High silicon grades look percentage of silicon than an unalloyed grade would do. High silicon grades look best under test at medium inductions, but perform less well at higher inductions. best under test at medium inductions, but perform less well at higher inductions. Particularly in Germany, specifications at Particularly in Germany, specifications at BmaxBmax = 1.0 T were common as a carry = 1.0 T were common as a carry over from long past hot rolled high silicon steels and were confusingly over from long past hot rolled high silicon steels and were confusingly inappropriate when applied to steel aimed at 1.5 – 1.8 Tinappropriate when applied to steel aimed at 1.5 – 1.8 T Bmax Bmax operation. operation.