Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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lead, cadmium, tin and iron need to be strictly controlled in order to

preserve dimensional stability. Whilst ZA8, like the BS1004 alloys, can be

pressure die cast both ZA12 and ZA27 are more suitable for gravity die

casting.

18.25 These alloys, like the BS1004 alloys, undergo changes both in

mechanical properties and dimensions as a result of an ageing process

subsequent to being cast. As is indicated in the zinc-aluminium equilib-

rium diagram (Fig. 18.2) a eutectoid change occurs in these alloys at 275°

when the solid solution |3' transforms to a pearlite-like eutectoid of a and

|3.

However the rapid cooling which prevails during die casting suppresses

the eutectoid change. This is further aggravated by the presence of copper

which also retards the eutectoid transformation.

Subsequently ageing occurs as the |3' phase—unstable at ambient tem-

peratures—gradually reverts to the a + |3 eutectoid with passage of time.

This leads to a general deterioration in mechanical properties and at the

same time a change in volume. The ZA27 alloy is sometimes heat treated

by cooling down slowly from above the eutectoid temperature in order to

produce a fine lamellar eutectoid (cf. normalising in steel—11.60) and as a

result much improved toughness and ductility. The ductile/brittle transition

temperature is also depressed from 0

C to

—

In order to stabilise it

against volume changes the ZA27 alloy can be aged for a short time just

below the eutectoid temperature.

18.26 The corrosion resistance of zinc-base alloys is adequate for most

atmospheric environments. However, the availability of zinc anodising

(21.90) and other protective finishes make them useful in aggressive marine

environments such as oil platforms in the North Sea. One such treatment,

chrornate

passivation, involves immersion of the castings in a solution con-

taining sulphuric acid and sodium dichromate.

°c

liquid

Fig.

18.2 The zinc-aluminium thermal equilibrium diagram.

4O°hA\

Nickel-Chromium High-temperature Alloys

18.30

The

main feature

of the

nickel-chromium alloys

their ability

resist oxidation

elevated temperatures.

further suitable alloy additions

are made

the

strength

increased under conditions

stress

high

tem-

peratures. Alloys rich

nickel

and

chromium have

high specific resist-

ance

electricity, which makes them admirable materials

for the

manufacture

resistance wires,

and

(because

their low rate

oxidation

at high temperatures) heater elements

many kinds capable

working

at temperatures

up to

bright

red

heat. Such alloys

(the

Brightray series)

are included

Table

18.4.

18.31

further group

high-temperature, nickel-chromium-base

alloys

are

those containing iron

and,

sometimes, molybdenum

and

tung-

sten.

these

the

Inconel series

has

been long established. Inconel

600

contains 76Ni;

16Cr; and 7Fe and is

resistant

many acids

and

alkalis

as well

oxidising atmospheres

high temperatures. Since

retains

reasonable strength

high temperatures

it is

used

for

furnace equipment

including retorts, muffles, heat-treatment trays, supports

and

nitriding

Table 18.4 High-temperature Resistance Alloys

Composition(%)

Tensile strength of

annealed rod

N/mm

911

726

664

402

Resistivity

(7(T

Om)

103

106

Maximum

working temp.

CC)

1150

950

700

300

Uses

Heaters

for

electric

furnaces, cookers,

kettles,

Immersion

heaters, hair dryers,

toasters,

etc.

Similar uses

above,

but

for

goods

lower

quality—also

for

soldering-irons, tubular

heaters, towel rails,

laundry irons,

etc.

Cheaper-quality

heaters working

at low

temperatures,

but

mainly

as a

resistance

wire

for

motor-starter

resistances,

etc.

Limited

use for low-

temperature heaters

such

bed-warmers,

etc.

Mainly

as a

resistance wire

for

instrument shunts,

field regulators

and

cinema

arc

resistances.

Table

18.5 Some High-temperature 'Superalloys'

Typical

mechanical

properties

at 650

CComposition (%) (BaI. -Ni)

Uses in jet aircraft construction

Casings and rings; sheet-metal

fabrications

(combustion

chambers, hot-gas ducts, exhaust

systems,

silencers, thrust

reversers,

etc.)

Discs; turbine blades and vanes;

shafts;

casings and rings.

Discs;

blades and vanes for both

compressors

and turbines; shafts;

casings and rings.

Discs;

turbine blades and vanes.

Turbine blades and vanes.

Casings and rings; sheet metal

fabrications

(combustion

chambers, hot-gas ducts, exhaust

systems,

silencers, thrust

reversers,

etc.)

Elongation

(%)

Tensile

strength

(N/mm

)

600

870

900

1100

1110

800

0.2% Proof

stress

(N/mm

)

325

700

740

750

770

470

Other elementsAl

1.3

1.4

4.7

0.5

0.4

2.3

2.5

1.2

2.1

3.5

5.9

Relevant

specifications

(BS)

HR5;

HR203;

HR403;

2HR504.

2HR1;

2HR201;

2HR401;

2HR601.

2HR2;

2HR202;

HR402;

2HR501;

2HR502;

2HR503.

HR3

HR4

HR10;

HR206.

Alloy

Nimonic 75

Nimonic 8OA

Nimonic 90

Nimonic 105

Nimonic 115

Nimonic 263

Compressor blades and vanes;

discs; shafts; casings and rings.

Casings and rings; sheet-metal

fabrications

(combustion

chambers, hot-gas ducting,

exhaust

systems, silencers,

thrust

reversers,

etc.)

Discs.

Compressor blades and vanes;

discs;

shafts; casings and rings.

Turbine blades and vanes.

Shafts; casings and rings.

Sheet metal fabrications

(combustion chambers; hot-gas

ducting; exhaust systems;

silencers; thrust reversers, etc.)

1000

620

770

1060

1350

1050

810

310

560

720

1000

950

Fe-35

Fe-18W-0.6

Fe-35

Fe18.5 Nb-5

Y2O3-O.6

Fe-42

Nb-3

Y2O3-0.5

0.3

1.2

2.1

0.4

0.3

0.7

4.5

2.9

1.2

2.2

3.5

0.9

0.5

1.4

0.5

5.7

3.3

3.1

1.5

12.5

18.5

HR53;

HR404.

HR6;

HR204.

HR55;

HR207.

Nimonic 901

Nimonic PE13

Nimonic PE16

Nimonic

PK33

Nimonic AP.1

lnconel 718

lnconel

MA754

lncoloy

903

lncoloy

MA956

boxes,

as well as for exhaust manifolds and electric heating elements for

cookers. Hastelloy C (59Ni; 17Mo; 15Cr; 5Fe; and 4W) is resistant to

corrosion by oxidising acids such as nitric and sulphuric, as well as to

oxidising and reducing atmospheres up to 100O

C; whilst Hastelloy X

(51Ni; 22Cr; 18Fe; 9Mo) is a high-temperature alloy maintaining good

oxidation resistance and reasonable strength up to 1200

C making it useful

in furnace equipment and combustion systems of jet aircraft.

18.32 By far the most spectacular of the nickel-chromium-base alloys,

however, are those of the Nimonic series. Nimonic is a trade name coined

by Messrs. Henry Wiggin, the manufacturers of these alloys, which have

played a leading part in the construction of jet engines. They are basically

nickel-chromium alloys which will resist oxidation at elevated tempera-

tures,

but which have been further stiffened for use at high temperatures

by the addition of small amounts of titanium, aluminium, cobalt, molyb-

denum and carbon in suitable combinations. 'Nimonics' and 'super-alloys'

generally must have a high creep resistance at high temperatures. Since

creep proceeds by the thermally-activated movement of dislocations

through the lattice, the basic requirements for high creep resistance are a

high elastic modulus and low diffusion rates within the lattice at high

temperatures. The presence of one or more of the elements mentioned

above will provide solute atoms in which the atom size produces sufficient

lattice distortion to increase the elastic modulus, and at the same time have

low diffusion rates.

The most important strengthening mechanism depends upon the forma-

tion of coherent precipitates—generally designated y'—within the lattice

of the nickel-base matrix. These coherent precipitates are based on the

intermetallic compounds TiNi

, AlNb, or NbNi

which, like the nickel-base

matrix have a FCC structure. However, there is a very small difference

(less than 1%) in lattice dimensions between matrix and compound and

since the mismatch is slight the compounds are able to form coherent

precipitates within the nickel-base matrix which remain stable at high tem-

peratures. Any carbides present form as finely-dispersed particles which

strengthen the grain boundary regions and inhibit intergranular fracture.

Some of these alloys are dispersion strengthened (8.62) by the inclusion

of particles of yttrium oxide Y

. Uniform distribution of such insoluble

particles cannot be achieved by normal casting processes. Hence the pow-

dered base alloy is ground with Y

powder in a ball mill to promote

uniform mixing and 'mechanical alloying' of the components. The blend is

then hot extruded to consolidate the alloy which may be shaped by further

hot-working processes.

The result is a series of alloys which will withstand oxidation and mech-

anical stresses at working temperatures in excess of 1000

C. In addition to

their use in jet engines, they find application as thermocouple sheaths,

tubular furnaces for the continuous annealing of wire, jigs for supporting

work during brazing or vitreous enamelling, gas retorts, extrusion dies and

moulds for precision glass manufacture. Some of the current Nimonic

alloys together with other Wiggin 'super alloys' are shown in Table 18.5

whilst the relationship between strength and temperature for some of these

Fig.

18.3 The relationship between tensile strength and temperature for some important

'Nimonic' high-temperature alloys.

18.33 Nickel-base Corrosion-resistant Alloys containing molyb-

denum, iron, usually chromium and sometimes small amounts of tungsten

and copper are used at ordinary temperatures in the chemical industries

where they are in contact with concentrated acids and other extremely

corrosive liquids. At ordinary temperatures up to 20

C molybdenum dis-

solves in nickel in the solid state and this increases both corrosion resistance

and strength. The Hastelloy series of alloys is probably the most important

in this group. Hastelloy B (65Ni; 28Mo; 5Fe) is used for transporting and

storing hydrochloric acid, phosphoric acid and other non-oxidising acids;

whilst Hastelloy C (59Ni; 17Mo; 15Cr; 5Fe; 4W) has a high resistance

to a wide range of corrosive media including oxidising liquids and those

containing chlorides. It is also reasonably resistant to concentrated sul-

phuric acid at ambient temperatures and to phosphoric acid both cold and

boiling.

Bearing Metals

18.40 Bearings are devices used to transmit loads between relatively-

moving surfaces. They may be either plain bearings involving sliding con-

tact between surfaces or more complex bearings using costly ball- or

roller-components. In this instance we shall consider alloys used for plain

bearings.

TENSILE

STRENGTH

(N/mm*)

alioys is shown in Fig. 18.3. It should be appreciated, however, that the

true criteria to usefulness will be high creep strengths at elevated tempera-

tures,

available with these alloys.

TEMPERATURE (

If lubrication were always perfect and bearing surfaces accurately fitted

so that 'running in' was unnecessary, then almost any metal would suffice

as a bearing provided that it was strong enough to carry the load. In fact

it is often difficult to align a long shaft accurately enough to run in a series

of bearings. Consequently a relatively soft, malleable material is preferable

so that it will align itself to the journal when under pressure. This prevents

the build-up of stress at any 'high spots' on the surface so that the journal

is less likely to be scored by grit in the lubricant. Nevertheless the bearing

must be strong enough and hard enough, preferably with a low-friction,

wear-resistant surface. This set of properties is generally not obtained in

a single-phase alloy for, whilst solid solutions are tough, ductile and strong

they are also rather soft; and whilst intermetallic compounds are hard,

they are also brittle and weak. Hence, traditionally, bearing metals have

been two-phase alloys, eg white metals and bronzes. In these materials

harder, low-friction particles are held in a malleable, soft solid-solution (or

eutectic) matrix. This matrix tends to wear, providing channels which assist

the flow of lubricant.

Another desirable characteristic of a bearing metal is its ability to form

'soaps'

with the lubricant, whilst low overall hardness will minimise damage

to the shaft starting and stopping, when oil pressure may be low and the

tendency towards pressure-welding of the journal to the bearing surface at

its greatest.

Bearing materials are of three main types:

(i) Two-phase alloys of the traditional types, eg white metals and

bronzes;

(ii) Two-phase alloys in which one phase can become a lubricant in

extreme circumstances, eg leaded bronzes and aluminium-tin alloys;

(iii) Single-phase materials, eg nylon and PTFE (polytetrafluoro-

ethene).

18.41 Copper-base Bearing Alloys include the phosphor-bronzes

(containing from 10 to 13% tin and 0.3 to 1.0% phosphorus) and the plain

tin bronzes (containing from 10 to 15% tin). Both types of bronze satisfy

the structural requirements of a bearing metal, since they contain particles

of the hard intermetallic compound 6 (O131 Sn

) embedded in a tough

matrix of the solid solution a. These alloys are very widely used for bear-

ings of many types where the loading is heavy.

For small bearings and bushes of standardised sizes, sintered bronzes

are often used. These are generally of the self-lubricating type, and are

made by mixing copper powder and tin powder together, in the proportions

of a 90-10 bronze, with the addition of graphite. The mixture is then

compacted and sintered (6.53—Part I and 6.50—Part II) to produce a

semi-porous, self-lubricating alloy suitable for low-duty bearings in sizes

up to about 75 mm diameter shaft.

Leaded bronzes (Table 16.2) are used for main bearings in aero-engines,

and for automobile and diesel crankshaft bearings. They have a high resist-

ance to wear, and their good thermal conductivity enables them to keep

cool when running. Should lubrication fail the lead is extruded under pres-

sure,

as overheating sets in, and forms a lubricating film which prevents

seizure—a necessary feature with aircraft bearings. These alloys contain

5-30% lead and since lead is insoluble in molten bronze these bearings

are often centrifugally cast to prevent excessive segregation of the lead.

Alternatively, segregation is reduced by adding small amounts of nickel.

Some of these bearings consist of sintered porous copper infiltrated with

lead. Corrosion of the lead by sulphur compounds in the lubricant is pre-

vented by overlaying the surface with a coating of lead-indium or tin-

indium which is encouraged to diffuse into the bearing surface by low-

temperature heat-treatment.

18.42 White Bearing Metals may be either tin-base or lead-base. The

former, which represent the better-quality white bearing alloys, are often

called Babbitt metals, after Isaac Babbitt, their original patentee. All of

these alloys contain between 3.5 and 15.0% antimony, much of which

combines with some of the tin present to form an intermetallic compound

SbSn. This compound forms cubic crystals, usually called cuboids (see

Plate 18.1) which are hard and of low-friction properties.

Let us assume that we have a plain tin-antimony alloy containing 10%

antimony, as being typical of a basic bearing-metal analysis. Fig. 18.4

shows the tin-rich end of the tin-antimony equilibrium diagram. Our 10%

alloy will begin to solidify at X by depositing cuboids of the hard intermet-

allic compound SbSn. These SbSn crystals are of lower specific gravity than

the liquid which remains, and they therefore tend to float to the surface of

the melt. At 246°C a peritectic reaction takes place, and the solid solution a

is formed as an in-filling between the cuboids. As the temperature falls, the

solubility of antimony in tin decreases from about 10.3% at the peritectic

temperature to 3.5% at room temperature, so that if cooling is slow

enough, more cuboids of SbSn are precipitated uniformly within the solid

solution a. Nevertheless, the segregation of the primary cuboids of SbSn

at the surface of the cast bearing is a serious fault. It can, however, be

minimised in one of the following ways:

°c

liquid

Ma +SbSn

OC + SbSn

ANTIMONY (°fo)

Fig.

18.4 Part of the tin-antimony thermal equilibrium diagram.

(a) By rapidly cooling the cast alloy when possible. The metal then

solidifies completely before the cuboids have had an opportunity to rise

to the surface.

(b) By adding up to 3.5% copper to the alloy. This combines with

some of the tin, forming a network of needle-shaped crystals of Cu

which precipitates from the melt before the cuboids of SbSn begin to

form. This network traps individual cuboids as they develop and prevents

their movement towards the surface, thus ensuring a uniform cast struc-

ture.

Moreover, the compound Cu

, being hard, improves the general

bearing properties.

Lead is added in the interests of cheapness, and this forms solid solutions

of limited solubility with both tin and antimony. These solid solutions form

a eutectic structure. In some of the cheap bearing metals, eg. Magnolia

metal, tin is omitted completely. The cuboids then consist of almost pure

antimony, embedded in a eutectic matrix of antimony-rich and lead-rich

solid solutions. These lead-rich alloys are intended only for low pressures.

The more important white bearing metals are indicated in Table 18.6.

18.43 Aluminium-tin Bearing Alloys The more recent philosophy of

using a duplex structure consisting of two interlocking networks—one

strong and the other soft and capable of acting as a lubricant under

occasional high contact pressures—led to the development of alu-

minium-tin alloys as bearings. These alloys contain approximately

80Al-20Sn. The two metals form a eutectic series (Fig. 18.5) in which the

eutectic contains only 0.5% aluminium and solidifies at 228°C so that the

alloy in question solidifies over a range of more than 400

C and segregation

is a considerable danger. Nevertheless, by a combination of cold-rolling

Table 18.6 White Bearing Metals

BS 3332

designation

Composition (%)

68.5

1.5

Characteristics and uses

High unit loads and high temperatures.

Main bearings in automobile and aero engines.

British Rail—moderately severe duties.

Railway carriage work. Pumps, compressors and general

machinery.

Railway wagon bearings, mining machinery and electric

motors.

Magnolia Metal—Light loads and speeds at higher

temperatures.

Under-water bearings.

Plate 18.1

18.1

A Vertical section through a small ingot of bearing metal (8% antimony;

32%

lead;

60% tin).

A slow rate of cooling has enabled the cuboids of SbSn to float to the surface, where they

have formed a layer, x 4. Etched in 2%

nital.

18.1 B Vertical section through a similar alloy (with the addition of 3% copper).

Although cooled at the same rate as the ingot in Plate

18.1

A, segregation of the SbSn

cuboids has been prevented by the needles of Cu

which formed first, x 4. Etched in 2%

nital.

18.1 C Bearing metal containing 10% antimony; 60% tin; 27%

lead;

and 3% copper.

Cuboids of SbSn (light) and needles of Cu

(light) in a matrix consisting of a eutectic of

tin-rich and lead-rich solid solutions, x 100. Etched in 2%

nital.

18.1A.

18.1B.

18.1c.