Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Fig.

14.10 Magnetic hysteresis loop.

values (B) are plotted vertically, whilst the magnetising force values (//),

calculated from the current passing through the solenoid and the number

of turns of wire on it, are plotted horizontally.

Beginning with a small value of H and gradually increasing it, corre-

sponding values of B and H are plotted, and the curve OP produced. This

is the magnetisation curve, which is continuous until it runs almost parallel

to the horizontal axis at P. This state represents complete magnetic satu-

ration of the specimen.

If we now gradually reduce the magnetising field to zero, the correspond-

ing fall in induced magnetic flux is represented by the line PR. The inter-

cept OR represents the value of the magnetic flux remaining in the steel

when all external magnetising influence is removed. This is termed the

remanent magnetism or remanence

rGm

To demagnetise the steel, and at the same time to measure the amount

of force required to achieve this, a current is once more passed through

the solenoid but in the reverse direction, and gradually increased. This

produces the curve RU, which is called the demagnetisation curve. The

intercept OU corresponds to the amount of force required to completely

demagnetise the steel, and is called, as already stated, the coercive force.

To complete the loop the operations outlined are carried on to negative

saturation and then repeated in the reverse sense.

The hysteresis loop, therefore, represents the lagging of magnetic flux

produced behind the magnetising force producing it. (The word 'hysteresis'

was in fact derived by Ewing from a Greek word meaning 'coming after'.)

The area of the loop is proportional to the amount of energy lost when the

specimen it represents is magnetised or demagnetised, and for permanent

magnets the area of the loop must be as large as possible.

The ultimate criterion of a permanent magnet is the maximum product

of B and H obtained from a point on the demagnetisation curve. In Fig.

14.10 it is shown that by completing the rectangle RXUO and drawing the

diagonal OX, the point Z at which the diagonal cuts the curve RU gives

MAX

POINT

ENERGY PRODUCT

CURVE

an approximation to the point at which the product of YZ and WZ is a

maximum for the curve. This is called

mdX

and corresponds to the

maximum energy the magnet can provide in a circuit external to

itself.

14.36 Fully hardened carbon steels containing about 1.0 % carbon

were originally used for permanent magnets. These were later replaced by

alloy steels containing chromium and tungsten, and later cobalt. The cobalt

steels are the only ones of any practical value today and these have been

superseded by alloys of the Alni-Alnico-Alcomax series. These were

developed from magnetic aluminium-nickel alloys discovered by Japan-

ese workers in 1930.

The Alni-Alnico series have a lower remanence than cobalt steels but a

much higher coercive force. Moreover, they retain magnetism much better

under influences of shock and of heat (Fig. 14.11). The most modern alloys

of the Alcomax-Hycomax series are superior also in remanence. They are

anisotropic materials, that is they have better magnetic properties along a

preferred axis. This effect is obtained by heating the alloy to a high tem-

perature and allowing it to cool in a magnetic field. Groups of atoms

become orientated as the magnet cools and this orientation is frozen in.

It is estimated that the current usage of permanent magnets in domestic

applications averages fifty per household in the Western World. A large

number of these are permanent magnets of the rare-earth type developed

in recent years. Most of them are produced by powder-metallurgy

methods. Amongst the earliest were magnets of the samarium-cobalt

(SmCo

) type followed by alloys containing copper as well as the rare

earths and cobalt. This eventually led to the development of high-energy

magnets of the type Sm(Co-Cu-Fe.x) where x is Zr, Ti or Hf.

In more recent years commercial production of permanent magnets

based on the intermetallic compound Nd

Fe^B has begun. Such new

materials combine very high polarisation coercivity with a very high value

of Z?//max, these values being the most important indications of permanent

magnet performance.

ORIGINAL

FLUX

RETAINED

(°/o)

TEMPERATURE (

Fig.

14.11 The retention of magnetic flux by representative magnet alloys at high tem-

peratures.

6°/oTUNGSTEN

35<Vo COBALT

ALNICO

ALCOMAX H

Table

14.3 Permanent

Magnet

Materials

Characteristics and uses

Obsolete

Largely obsolete—a few

'semi-hard' applications eg

hysteresis

motors

General purposes

Widely used in electrical,

communications and engineering

industries. All are anisotropic

alloys which can only be produced

with

a linear axis of magnetisation.

High performance/low inertia

motors

for NC machine tools; high

performance and/or very small

stepper motors for watches,

camera apertures, rocket

guidance systems, etc.

Magnetic bearings, switches,

servomotors, actuators,

audio-visual and similar consumer

products, disc file systems, etc.

Magnetic

Properties

(kJ/m

)

1.5

7.5

13.5

127

215

(kA/m)

4.4

128

130

600

800

(mT)

900

725

1320

1350

900

1040

800

1100

Special

Details

Cast or sintered

Chill

cast; part

grain

orientation

Full grain

orientation

Cast or sintered

Full grain

orientation

Sintered in argon

Sintered ceramic

Composition

(%)

Other elements

C 0.9; Cr 6; W 5.

Nb 0.6; Si 0.3

Sm 34 (as the

compound

SmCo5)

Nd 16; B 8 (as

the compound

Nd2FeuB)

Type

of Material

Carbon steel

35% Co steel

'Alnico'

'Alcomax 3 SC*

'Columax'*

'Hycomax 3'*

'Hycol 3'*

Rare-earth/cobalt

'Neodure*

Cheap

general

purpose magnets.

Cycle dynamos.

Magnetic

repulsion bearings on Kwh meters.

Small magnetic glandless

couplings for water and gas

meters.

Loudspeakers, door catches,

computer track

shifters

on disc

store devices.

Magnetic

separators; DC motors

for

car heaters and wipers. Larger

motors for washing machines,

lawn mowers, outboard motors,

electrical cars and industrial

drives.

Mechanical properties similar to

steel—tough

and strong.

Used

for

rotors,

speedometers,

general

purpose round bar magnets, etc.

'elongated single domain'

material—iron/cobalt alloy needles

lead

matrix.

Magnetic display boards, eg BBC's

former

Weather Forecast Chart;

refrigerator

door seals and

field

magnets for cheap low

performance motors (toys and

windscreen washers).

135

160

240

160

220

380

370

1200

650

160

230

Sintered ceramic

Cold-worked

Fe-Co

alloy

particles in a

lead

matrix.

Ceramic in a

rubber matrix

BaO.(Fe2O3)6

SrO.(Fe2O3)6

Cr 20 to 30

Pb 69

BaO.(Fe2O3)6

10to

BaI.

'Feroba

2'*

'Feroba

3'*

'Crofeco'*

'Lodex'*

Magnetic

rubber

*Anisotropic alloys and

materials,

properties measured along the preferred axis.

14.37 Special mention must be made of those magnetic materials which

give a 'square' hysteresis loop of the type shown in Fig.l4.12(ii). Such

materials are used in computer memory units. Magnetic saturation to a

moderate value of B is reached quickly and a large proportion of the flux

is retained when the magnetising field is removed; thus it can be used as

a 'memory' to activate systems when called upon to do so. Even with a

small reverse field the strong flux is still available for 'read-out' and when

it is desired to 'scrub' the information this can be done cleanly, leaving the

device ready for remagnetisation. Such a material has a high remanence,

/?rem, and a small but definite coercive force, H

. Ceramics based on Mn-

Mg-Fe oxides and Co-Fe oxides are used.

14.38 Components like transformer cores and dynamo pole pieces need

to be made from a material which is magnetically 'soft' but of high per-

meability. Permeability (\i) changes with the magnetic field, rising to a

maximum and then falling again. Since a transformer core works on alter-

nating current, the core is first magnetised, then demagnetised and then

remagnetised with reverse polarity. This type of cycle produces the closed

curve PRUQST (Fig. 14.10) and the area which it encloses is a measure

of the amount of energy wasted in overcoming remanent magnetism each

time the current is reversed (which occurs many times per second). This

energy is converted into heat, and it is therefore necessary to use materials,

for transformer cores, which give very narrow hysteresis loops, and hence,

a small 'hysteresis loss' in order to reduce energy loss and avoid over-

heating of the core. The differences in properties between a material suit-

able for a permanent magnet and one suitable for a transformer core is

indicated by the different shapes of the hysteresis loops as shown in dia-

grams (i) and (iii) in Fig. 14.12.

Soft iron and iron-silicon alloys are most useful in respect of low hyster-

esis.

The iron-silicon alloys contain up to 4.5 % silicon and practically no

carbon, and are usually supplied in the form of dead-soft sheet from which

transformer-core laminations can be stamped. The use of laminations

Fig.

14.12 Hysteresis loops for representative magnetic materials,

(i) A magnetically 'soft' material (transformer cores), (ii) A 'square loop' material (computer

memory units), (iii) A modern magnetically 'hard' material (permanent magnets). (Note the

compressed H-scale for the latter).

reduces

losses

from eddy currents, which would be more prevalent in a

solid core.

Nickel-iron

alloys are also of importance as low-hysteresis, high-

permeability alloys. 'Permalloy' contains №78 and Fe22 with carbon and

silicon kept as low as possible.

'MumetaF

contains №74; Cu5; Mn up to

1; Fe-bal. Both alloys are used in communications engineering, where the

high permeability

will

utilise to the full the small currents involved. In

addition

to being used for transformer cores, these alloys are used as

shields for submarine cables. Like the iron-silicon alloys, they are used in

the

dead-soft condition, the best treatment consisting of heating to 900

followed by

slow

cooling, and

then

heating to 600

C, followed by cooling

air. This treatment avoids the ordered structure which would otherwise

form in these alloys. Since the magnetic properties are

very

sensitive to

work-hardening these alloys must

always

be used in the annealed state.

all magnetically soft materials properties depend upon the ease of

domain

movement and so work-hardening and the presence of impurities

must be avoided. In iron-silicon alloys the crystal orientation of the sheet

is also controlled. Since magnetisation is easiest in the <100> directions,

the

sheet is prepared with the (100) planes parallel to the plane of the

sheet. This leads to a lower power

loss

in transformer cores.

A number of ceramic-type magnetically-soft materials are now used in

electronics and radio-communications equipment. Most of these materials

are based on the formula

('magnetite' which also occurs naturally

as a magnetic mineral—the iodestone' of the ancient mariners). In this

formula the

ions are replaced by various mixtures of

and Cu

ions and are generally known as 'ferrites'. This term has

direct connection with 'ferrite' which

signifies

a-iron but is a reference

the general formula M-Fe

. They are powder metallurgy products.

Exercises

Discuss

reasons

for and

against

the use of

molybdenum

as a

replacement

for

tungsten

high-speed

steels.

(14.10;

Table

14.1)

reference

to Fig. 14.1

estimate

the %

carbon

solution

austenite

at (i)

900

(ii)

1300

for a

typical

high-speed

steel.

Show

how

this

affects

the

quenching

temperature

for

such

steel.

(14.13;

14.14)

Discuss

the

significance

of Fig. 14.2 on the

heat-treatment

programme

of a

high-speed

steel.

(14.15)

(a)

What

do you

understand

by the

term

'tool

steel'?

(b)

Give

typical

approximate

chemical

compositions

for

tool

steels

suitable

for

thread

rolls

other

components

that

need

great

resistance

wear

and

abrasion.

(c)

Describe

how you

would

heat-treat

such

tool,

mentioning

the

precautions

that

would

necessary

produce

satisfactory

article.

(Table

14.1;

14.14;

14.15)

What

are the

basic

requirements

of a

tool

steel?

Describe

the

heat-treatment

high-speed

tool

steel,

giving

reasons

for

each

step.

Explain

how the

microstructure is related to the service requirements of the tool.

(14.11;

14.14;

14.15)

6. Distinguish between

ferromagnetism,

paramagnetism and

diamagnetism.

Outline

the basic theory which seeks to explain ferromagnetic properties. (14.30 to

14.33)

Explain why the value

mdX

is important in assessing the suitability of a perma-

nent magnet material for service.

Outline developments which have taken place in the formulation of permanent

magnet materials in recent years. (14.35 to 14.37)

8. Define

magnetic permeability

(\i) and sketch representative hysteresis curves for

both magnetically 'hard' and 'soft' materials.

For what purposes are magnetically 'soft' materials used? (14.38)

9. Some magnetic materials are said to give a 'square' hysteresis loop.

Explain what this means and say for what purposes such materials are used.

(14.39)

Bibliography

Berkowski, A. E. and Kellner, E., Magnetism and Metallurgy, Academic Press,

1970.

Hoyle, G., High Speed Steels, Butterworths, 1988.

Rosenberg, H. M., The Solid State, Oxford University Press, 1978.

Thelning, K-E., Steel and its Heat-treatment: Bofors Handbook, Butterworths,

1984.

Wilson, R., Metallurgy and

Heat-treatment

of Tool Steels, McGraw-Hill, 1975.

Cast Irons and Alloy

Cast Irons

15.10 In Victorian times almost anything was likely to be made of cast

iron—street lamps, domestic fireplaces, railings, mock-gothic church

window frames and the ornamental water fountain in the local park. Sadly

many of these relics of the nineteenth century foundryman's art are no

more since most fell victim to the urgent need for steel during the Second

World War. Even as late as the author's childhood toy 'six-shooters' were

of white cast iron, whereas to-day's children play with 'space-age' guns

made of plastics materials. One important reason for the widespread use

of cast iron in Victorian times was the fact that in those days not all

pig iron produced was suitable for conversion into merchantable steel. In

particular those pig irons high in phosphorus and sulphur were suitable

only for ornamental castings which required little strength. Today cast iron

is used exclusively for engineering purposes and its technology, like that

of other alloys, continues to be developed at highly sophisticated levels.

'Spheroidal graphite' cast iron and, more recently, 'compacted graphite'

irons are of this type.

15.11 Ordinary cast iron is very similar in composition and structure

to the crude pig iron from the blast furnace. In most foundries the 'pigs'

are melted, often with the addition of low-grade steel scrap, in a cupola,

any necessary adjustments in composition being made during this melting

process. The relative scarcity and consequent high cost of metallurgical

coke has led some foundries to adopt electric melting, usually by high-

frequency induction furnaces. An obvious advantage of electric melting is

its chemical cleanliness and lack of sulphur contamination as compared

with cupola melting where the iron is in intimate contact with the burning

coke.

Consequently many of the high quality grey cast irons, as well as

spheroidal-graphite irons and alloy cast irons are now melted electrically.

Because production costs of pig iron are relatively low as compared with

other alloys, and since no expensive refining process is necessary, cast iron

is a cheap metallurgical material which is particularly useful where a casting

requiring rigidity, resistance to wear or high compressive strength is neces-

sary. Other useful properties of cast iron include:

(i) good machinability when a suitable composition is selected;

(ii) high fluidity and the ability to make good casting impressions;

(iii) fairly low melting range (1130-1250

C) as compared with steel;

(iv) the availability of high strengths when additional treatment is given

to suitable irons, eg spheroidal-graphite iron, compacted-graphite

irons or pearlitic malleable irons.

The structure and physical properties of a cast iron depend upon both

chemical composition and the rate at which it solidifies following casting.

The Effects of Composition on the Structure of

Cast Iron

15.20 Ordinary cast iron is a complex alloy containing a total of up to

10%

of the elements carbon, silicon, manganese, sulphur and phosphorus;

the balance being iron. Alloy cast irons, which will be dealt with later in this

chapter, contain also varying amounts of nickel, chromium, molybdenum,

vanadium and copper.

15.21 Carbon can exist in two forms in cast iron, namely as free graph-

ite or combined with some of the iron to form iron carbide (cementite).

These two varieties are usually referred to as graphic carbon and combined

carbon respectively, and the total amount of both types in the specimen

of iron as total carbon. In ordinary engineering cast iron the form in which

carbon is present depends largely upon the cooling rate during solidification

and upon the quantity of silicon present. In alloy cast irons those elements

mentioned above will also affect the resultant structure. In fact the effects

of various elements on the microstructure of cast iron is generally the same

as in steels (13.12).

Cementite is a hard, white, brittle compound, so that irons which contain

much of it will present a white fracture when broken, and will have a low

resistance to shock. At the same time they will possess a high resistance

to wear. Such irons are called white irons. The fractured surface of a cast

iron containing graphite, however, will appear grey, and the iron will be

termed a grey iron.

Although the form of the iron-carbon diagram shown in Fig. 11.1 is of

great value to the practical metallurgist it is not a true equilibrium diagram

because in fact cementite is not an 'equilibrium phase'. Cementite is said

to be a metastable phase, that is, it has a natural tendency to decompose

forming a mixture of iron (ferrite) and carbon (graphite). In ordinary steels

this decomposition almost never occurs because in iron supersaturated with

carbon the nucleation of cementite takes place much more readily than

the nucleation of graphite. Once cementite has formed it is quite stable

and for practical purposes can be regarded as an equilibrium phase.

In a simple iron-carbon alloy containing, say, 3% carbon, solidification

would begin with the formation of primary austenite and at 1131°C a

Fig.

15.1 The formation of 'flake' graphite resulting from the growth of eutectic cells. In (iv)

some free cementite will also be present as the solubility of carbon in austenite decreases

from 2.0% at 1131

C to 0.8% at 723°C.

eutectic consisting of austenite and cementite would form. However, engin-

eering cast irons contain sufficient silicon to increase the instability of

cementite to the point where graphite is precipitated from solution instead.

Therefore primary austenite will separate out first until the eutectic tem-

perature (1131°C) is reached, but at that temperature the eutectic which

forms consists of austenite and graphite. This eutectic develops from nuclei

and is in the form of approximately spherical particles known as eutectic

cells. The layers of austenite and graphite develop in roughly radial form,

both being deposited directly from the liquid phase (Fig. 15.1). In Fig.

15.1 and Plate

15.

IA graphite appears to be in the form of separate flakes,

but in fact the eutectic cells are three-dimensional and roughly spherical

in shape so that the graphite layers can be regarded as something like those

shown in Fig. 15.2. That is, rather like a cluster of potato crisps growing

out from a central nucleus.

As in the case of the eutectoid point in steels (13.14) the eutectic point

in cast irons may be displaced to the left by the presence of other elements,

notably silicon. Thus, a cast iron containing less than 4.3% carbon may be

Plate 15.1 15.1A Grey cast

iron.

Graphite flakes in a matrix of pearlite. x 60. Etched in 4% picral.

15.1 B White cast

iron.

Primary cementite (white) and pearlite. x 100. Etched in 4% picral. (Courtesy of BCIRA).

liquid

austenite

pearlite

(i)Eutectic cell nuclei

form.

(it)Eutectic cells grow.

ill)Solidification complete

-graphite in an

austenite matrix.

(iv) Cooled to room temp.

-the austenite matrix

transforms to pearlite.