Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Tin is added in amounts up to 1.0% in order to improve corrosion-

resistance, particularly in naval brass and Admiralty brass for condenser

tubes.

Such small quantities of tin are retained in solid solution. Alterna-

tively, small amounts of arsenic (0.01-0.05%) may be added to 70-30

brass destined for the manufacture of condenser tubes, as it is said to

improve corrosion-resistance and inhibit dezincification.

Lead,

in amounts

of the order of 2.0%, is added to improve machinability. It is insoluble

in brass, and exists as small globules, which cause local fractures during

machining (6.66). Aluminium is sometimes added, in amounts up to 2.0%,

to brass for the manufacture of naval condenser tubes, since it imparts

excellent corrosion-resistance, particularly to impingement' attack. Nickel

is retained in solid solution, and small amounts may be added to brass to

improve corrosion-resistance. The now obsolete twelve-sided threepenny

pieces were made from a brass containing 20% zinc, 1% nickel and the

balance copper.

16.35 The Hot-working a + |3' brasses Whilst the a-brasses are

specifically cold-working alloys they are generally hot-worked in the

'breaking-down' stages; but a + (3' alloys, containing not more than 60%

copper are shaped almost entirely by hot work.

The only important 'straight' brass in this group is 60-40 brass, formerly

known as 'Muntz metal'. As already mentioned, the a-phase is entirely

absorbed into the (3-phase when the alloy is heated to about 750

C, so that

the best hot-working temperature range is while cooling between 750 and

650

during which range the a-phase is being deposited. The mechanical-

working process breaks down the a-phase into small particles as it is

deposited, and prevents the reintroduction of the coarse Widmanstatten

structure.

16.36 Free-cutting Brasses of the 60-40 type contain about 2.0%

lead, whilst a similar alloy of higher purity is used extensively for hot-

forging where a machining operation is to follow. Up to 0.15% arsenic

may be added to those brasses destined for the manufacture of water

fittings as it is known to improve corrosion-resistance and inhibit dezincifi-

cation.

16.37 High-tensile Brasses are misleadingly called Manganese

Bronze, possibly because the manganese they often contain produces an

oxidised-bronze effect on the surface of extruded rod. These brasses con-

tain 54-62% copper, up to 7.0% other elements and the balance zinc. In

addition to being hot-working alloys, they are also used in the cast form

for such applications as marine propellers, water-turbine runners, rudders,

gun mountings and sights, and locomotive axle boxes. The wrought sec-

tions are used for pump rods, and for stampings and pressings for auto-

mobile fittings and switch gear. The tensile strength is increased, by the

addition of these other elements, to as much as 700 N/mm

in the chill-cast

or forged condition. The additions usually in amounts up to 2.0% each,

include manganese, iron and aluminium; whilst up to 1.0% tin may also

be included to improve corrosion-resistance.

Details of some of the more important wrought brasses are given in

Table 16.1.

The Tin Bronzes

16.40 Bronzes containing approximately 10% tin were probably the first

alloys to be used by Man. In Britain bronze articles almost four thousand

years old have been found, and during the Roman occupation of Britain

the copper-mines of Cumberland and Wales were in a state of rapidly

increasing production. Centuries before the Roman invasion, however,

Phoenician traders from Tyre and Sidon brought their ships to Cornwall

in search of tin.

One of the significant factors in the early Roman conquests was

undoubtedly the bronze sword, and it is thought that in even earlier times

metal-workers realised that a high tin content, in the region of 10%, pro-

duced a hard bronze whilst less tin gave a softer alloy. The relatively high

cost of copper—and particularly tin—coupled with competition from other

new and improved materials has led to a decline in the wide use of bronzes

in the modern world.

16.41 The relationship between the equilibrium diagram and the actual

microstructure produced for a given alloy is rather more complex in the

case of tin bronzes than with the brasses. The rate of diffusion of copper

and tin into each other is much lower than it is with copper and zinc. This

is indicated by the wide range of composition at any temperature, between

the liquidus AL and the solidus AS (Fig. 16.4), and leading to a high

°c

LIQUID

(K. + LIQUID

LIQUID

TIN PER CENT

Fig.

16.4 The copper-tin equilibrium diagram.

degree of coring during the actual solidification process. Moreover, struc-

tural changes below approximately 400

C take place in copper-tin alloys

with extreme sluggishness. Both of these factors mean that a cast bronze,

cooled to ambient temperature under normal industrial conditions, will

rarely exhibit the structure indicated by the equilibrium diagram.

In short, whilst the peritectic reaction (a —»(3) at 789°C and the eutectoid

transformations (|3 -» a + y) at 586°C and (y -> a + 6) at 520

C will take

place as indicated by the diagram during ordinary rates of cooling, the

eutectoid transformation (6 —> a + e) at 350

C would occur only under

conditions of extremely slow cooling, such as would never be encountered

industrially. Hence the phase 8 (Cu

Sn) is never seen in the structure of a

cast bronze containing more than 11.0% tin. Further, due to the slow rate

of diffusion of copper and tin atoms below 350

C, the precipitation of 8

from a in alloys containing less than 11.0% tin, in accordance with the

phase boundary XY, will not occur. For practical purposes the reader can

therefore ignore that part of the equilibrium diagram below 400

C and

assume that whatever structure has been attained at 400

C will persist to

room temperature under normal industrial rates of cooling.

16.42 As with brasses, the a-phase, being a solid solution, is tough and

ductile, so that a-phase alloys can be cold-worked successfully. The

6-phase, however, is an intermetallic compound of composition equivalent

to Cu

iSn

, and is a hard, brittle, blue substance, whose presence renders

the a + 5 bronzes rather brittle. The 6-phase must, therefore, be absent

from alloys destined for any degree of cold-work.

Due to heavy coring arising from slow rates of diffusion, cast alloys with

as little as 6.0% tin will show particles of 6 at the boundaries of the cored

a crystals. The skeletons of the a-phase crystals will be much richer in

copper than the nominal 94%, thus making the outer fringes correspond-

ingly richer in tin to such an extent that the 6-phase is formed. In order to

make such an alloy amenable for cold-work, the 6-phase can be absorbed

by prolonged annealing (say six hours at 700

C), which will promote dif-

fusion so that equilibrium is attained in accordance with the equilibrium

diagram and a uniform a-phase structure produced. Subsequent air-cooling

—or even furnace-cooling at the usual industrial rates—will be too rapid

to permit precipitation of any of the s-phase when the phase boundary

XY is reached; and so the uniform a structure will be retained at room

temperature. By using such initial heat-treatment to produce a uniform a

structure it is possible to cold-work bronzes containing as much as 14%

tin, though in general industrial practice only alloys with up to 7% tin are

produced in wrought form.

The tin bronzes can be classified as follows:

16.43 Plain Tin Bronzes These comprise both wrought and cast

alloys, the former usually containing up to 7% tin and the latter as much

as 18% tin. The wrought alloys contain none of the 6-phase, the absence

of which makes them amenable to shaping by cold-working operations.

These alloys are usually supplied as rolled sheet, drawn rod or drawn

turbine blading.

The cast alloys are used mainly for bearings, since the structure fulfils

the requirements for that duty, namely, hard particles of the S-phase,

which will resist wear, embedded in a matrix of the a-phase, which will

resist shock.

16.44 Phosphor-bronzes Most of the tin bronzes mentioned above

contain small amounts of phosphorus (up to 0.05%) residual from deoxi-

dation which is carried out before casting. They are often incorrectly called

phosphor-bronzes. True phosphor-bronze contains phosphorus as a delib-

erate alloying addition, generally present in amounts between 0.1 and

1.0%.

Wrought phosphor-bronzes contain up to 8.0% tin and up to 0.3% phos-

phorus, and, like the plain tin bronzes, are supplied in the form of rod,

wire and turbine blading. The phosphorus not only increases the tensile

strength but also, it is claimed, improves the corrosion-resistance.

Cast phosphor-bronzes contain up to 13.0% tin and up to 1.0% phos-

phorus, and are used mainly for bearings and other components where a

low friction coefficient is desirable, coupled with high strength and tough-

ness.

The phosphorus is usually present in these alloys as copper phos-

phide, C113P, a hard compound which forms a ternary eutectoid with the

a- and 6-phases.

16.45 Bronzes Containing Zinc These, again, comprise both wrought

and cast alloys. The wrought alloys are used chiefly for the manufacture

of coinage and contain up to 3.0% tin and up to 2.5% zinc. In recent years

the tin content of British 'copper' coinage has been reduced from the

pre-war figure of 3.0% to as low as 0.5%. This began as a war-time measure

when the Japanese occupation of Malaya cut off our main supplies of tin;

and has continued since because of the high prices which have prevailed

for the metal. The replacement of tin by some zinc cheapens the alloy,

zinc being only about a tenth the price of tin. A subsidiary function of zinc

is that, like phosphorus, it acts as a deoxidiser and forms zinc oxide, ZnO,

which floats to the top of the melt. The wrought bronzes containing zinc

are all a-phase alloys with similar structures to those of straight tin bronzes

of like compositions.

The best known of the cast alloys is 'Admiralty gunmetal', or 88-10-2

gunmetal, containing 10% tin and 2% zinc. It is no longer used in naval

ordnance, but is still widely employed where a strong, corrosion-resistant

casting is required. Its structure is similar to that of a straight tin bronze

containing 11% tin, so that, due to considerable coring, a lot of a + 5

eutectoid will be present. The zinc not only cheapens the alloy and acts as

a deoxidiser but also improves casting fluidity.

16.46 Bronzes Containing Lead Up to 2.0% lead is sometimes added

to bronzes as well as to brasses in order to improve machinability. Larger

amounts are added for some special bearings; and such bronzes permit 20%

higher loading than do lead-base or tin-base white metals. The thermal

conductivity of these bronzes is also higher, so that they can be used at

higher speeds, since heat is dissipated more quickly. With normal lubrica-

tion they have excellent wear resistance but, equally important, the seizure

resistance is high since lead will function temporarily as a lubricant should

normal lubrication fail. Leaded bearing bronzes contain up to 20% lead

Plate 16.3 16.3A 5% Tin bronze, as-cast.

Large crystals of cored a solid solution, x 100. Etched in ammonia/hydrogen peroxide.

16.3B 5% Tin bronze, cold-worked, annealed and cold-worked again.

Uniform a crystals showing large numbers of strain bands, x 250. Etched in ammonia/

hydrogen peroxide.

16.3C 5% Tin bronze, cold-worked and annealed.

Twinned a crystals with the effects of strain removed by annealing, x 250. Etched in

ammonia/hydrogen peroxide.

16.3c.

16.3B.

16.3A.

Table

16.2 Tin

Bronzes,

Phosphor-bronzes

and Gunmetal

Characteristics and uses

Coinage

Bronze. Used for

British

'copper' coinage.

Low-tin

Bronze. Used where good elastic properties

combined with resistance to corrosion and corrosion

fatigue are necessary. Widely used as springs and

instrument

parts.

Drawn

Phosphor-bronze.

Generally

used

in the

work-hardened condition. Useful for engineering

components subjected to friction. Also for steam-turbine

blading and other corrosion-resisting applications.

Cast

Phosphor-bronze. A suitable alloy for general

sand-castings. With phosphorus raised to 0.5% (BSS

1400/PB1/C)

the alloy is a standard phosphor-bronze

for

bearings. It is often supplied as cast sticks for turning

small bearings, etc.

High-tin

Bronze.

Used

for bearings subjected to heavy

compression

loads—bridge and turntable bearings, etc.

Admiralty

Gunmetal. Widely used for pumps, valves

and miscellaneous castings, particularly for marine

purposes because of its corrosion-resistance.

Also

used

for

statuary because of good casting properties.

85-5-5-5

Leaded

Gunmetal or Red

Brass.

Often

used

as a substitute for Admiralty gunmetal and also where

pressure

tightness is required.

Useful where lubrication is likely to

fail.

Also

in aero and

automobile

construction. For carrying heavy loads a steel

backing is necessary.

Typical mechanical properties

Hard-

ness

(VPN)

200

210

180

Elonga-

tion (%)

Tensile

strength

N/mm

324

726

340

741

355

695

278

170

293

216

175

200

0.2% Proof

stress

N/mm

120

600

150

620

150

620

140

154

140

110

Condition

Annealed

Hard

Annealed

Hard

Annealed

Hard

Sand-cast

Chill-cast

Composition (%)

Other

elements

Pb 5

1.5

0.5

0.1

Up to

1.0

3.75

5.5

95.5

Rem.

Relevant

specifica-

tions

2870:

PB101

2870/4:

PB102

BS 1400:

CT1/B

BS 1400:

Gl/C

BS 1400:

LG2/A

BS 1400:

LB5/B

Plate 16.4 16.4A Phosphor bronze containing 10% tin and 0.5% phosphorus, as-cast.

Cored a (dark) with an infilling of a, 6 and Cu

P eutectoid. The irregular dark regions are

shrinkage cavities, x 150. Etched in ammonia/hydrogen peroxide.

16.4B The same bronze at higher magnification showing the nature of the eutectoid.

At this higher magnification the primary a appears light, x 1100. Etched in ammonia/hydro-

gen peroxide.

(LB5) and are used for aero and automobile crankshaft bearings. 'Red

brass'

(LG2) is also used occasionally as a bearing metal but more often

for pressure-tight castings (Table 16.2.)

Aluminium Bronze

16.50 Part of the copper-aluminium constitutional diagram is shown in

Fig. 16.5. As in the case of the copper-zinc alloys, further structural

changes occur in some of these alloys if they are cooled very slowly indeed

16.4A.

16.4B.

ALUMINIUM

(°h)

Fig. 16.5 The

copper-aluminium

constitutional diagram.

The

microstructural sketches

indicate

the types of structure normally found in alloys produced

commercially. (See also Plate 16.5).

liquid

cooled

slowly

quenched

from

tempered

at 5OO°C

OC+

eutectoid

ineOC

+• Y^

under laboratory conditions below 400

C. However, such very slow cooling

rates are never encountered in industrial production methods and we can

assume that the a + 72 structure persists in alloys containing between 9.4

and 16.2% aluminium which are cooled reasonably slowly ('furnace

cooled') to ambient temperatures. Consequently that part of the consti-

tutional diagram below 500

C can be ignored for our purposes and is not

shown here.

Like the brasses the aluminium bronzes can be divided into two main

groups; the cold-working and the hot-working or casting alloys respect-

ively. The constitutional diagram indicates that a solid solution (a) contain-

ing up to 9.4% aluminium at room temperature, is formed. Like the other

a solid solutions based on copper, it is quite ductile. With more than 9.4%

aluminium the phase y

is formed. This is an intermetallic compound of

the formula Cu

and, in common with compounds of this type, is very

hard and brittle, resulting in an overall brittleness of alloys containing the

-phase.

16.51 Further inspection of the constitutional diagram reveals simi-

larities between it and the iron-carbon diagram. The two a-phases are

analogous; the (3-phase solid solution of the copper-aluminium diagram

corresponds to the y (austenite) phase of the iron-carbon diagram; and

the a +

eutectoid is similar to the ferrite + cementite eutectoid (pearlite)

of the steels. As a result of these similarities in the positions of the phase

fields in the respective diagrams, a 10% aluminium bronze can be heat

treated in a manner parallel to that of steel so that a martensite-type

transformation occurs. Nevertheless it should be realised that the crystal-

lography of the aluminium bronzes is different from that of the correspond-

ing steels.

Consider a 10% aluminium bronze; this will consist entirely of the phases

a and y

if it is allowed to cool slowly in a furnace to ambient temperature.

If it is reheated the a + y

eutectoid is transformed to the solid solution (3

when the eutectoid temperature (565°C) is reached, and as the temperature

rises further, the a-phase is absorbed until at about 900

C the structure

consists entirely of the solid solution (3. Water-quenching from this tem-

perature produces a structure consisting of the phase p'. This is not shown

in the equilibrium diagram, since, like martensite in steels, it is not an

equilibrium phase. The P'-phase is hard and brittle like martensite, and is

in fact very similar in microstructural appearance. Tempering this P'-phase

at 500

C causes the precipitation of a fine agglomerate of the phases a and

, closely resembling the tempered martensite of a steel treated in a paral-

lel manner.

In fact the thermal equilibrium characteristics of 10% aluminium bronze

resemble more closely those of an air-hardening alloy steel than those of

a plain carbon steel. For example, if the 10% aluminium bronze is air

cooled from the P-phase field ('normalised') then the resultant structure is

likely to be either P' ('martensitic') or a bainitic type of structure containing

finely precipitated y

. The a +

('pearlitic') structure will only be obtained

by annealing, followed by furnace cooling to ambient temperature. Ex-

tremely slow cooling under controlled laboratory conditions may cause a

Table

16.3

Aluminium

Bronzes

Characteristics and uses

Cold-worked for decorative purposes,

imitation jewellery, etc. Also

useful

in various

engineering applications, especially in tube

form.

Excellent resistance to corrosion and to

oxidation

on heating. Tubes for heat exchrs.

Suitable

for

chemical-engineering

applications, especially at moderately elevated

temperatures.

These

alloys are cold-worked to a limited

extent only and are

shaped

by hot-working

processes, including forging. The properties

quoted are for the hot-worked condition. The

materials can be heat-treated by quenching

and tempering (16.51).

Low

magnetic permeability imparted by Si—

developed for components for Navy mine

counter-measure vessels. Also for

commercial uses.

Popular die-casting alloy.

Most

common alloy for aluminium-bronze

castings.

The casting version of CA107.

Typical mechanical properties

Hard-

ness

(VPN)

220

100

180

215

200

115

160

Elonga-

tion

(%)

Tensile

strength

N/mm

386

772

432

556

726

600

575

690

480

0.2% Proof

stress

N/mm

123

587

154

247

463

400

200

300

185

Condition

Annealed

Hard

Hot-worked

Forged

Cast

Composition (%)

Other

elements

Mn and/or Ni

up to 4.0

Fe, Mn and

Ni up to 2.0

Mn up to 0.5

Si-2.2

& Mn up

1.0 each.

Ni-5.5 Mn

up to 1.5

Si-2.2

Fe + Ni = 4.0

0.6

2.5

4.5

0.6

9.5

6.2

9.5

6.2

Rem.

Relevant

specifications

2870/5:

CA101

2870/5:

CA102

2872:

CA103

2872:

CA104

2872:

CA107

BS 1400:

AB1

BS 1400:

AB2

BS 1400:

AB3