Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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The effect of this prolonged annealing is to cause the cementite to break

down, but instead of forming coarse graphite flakes, the carbon is precipi-

tated in the form of small 'rosettes' of 'temper carbon'. A fractured section

will thus be black, hence the term 'blackheart'. The final structure, which

consists entirely of ferrite and finely divided temper carbon, is soft, readily

machinable and almost as ductile as cast steel. Blackheart malleable cast-

ings find particular application in the automobile industries because of the

combination of castability, shock-resistance and ease of machining they

afford. Typical uses include rear-axle housings, wheel hubs, brake shoes,

pedals, levers and door hinges.

15.72.2.

Whiteheart Malleable

Iron castings are manufactured from white

iron of which the following composition is typical:

Carbon 3.3%

Silicon 0.6%

Manganese 0.5%

Sulphur 0.25%

Phosphorus 0.1%

In this process the castings are heated in contact with some oxidising

material, such as haematite ore, for between 70 and 100 hours at a tempera-

ture of about 1000

During annealing, the carbon at the surface of the casting is oxidised by

contact with the hematite ore, and lost as carbon dioxide. This causes more

carbon to diffuse outwards from the core, and, in turn, this is lost by

oxidation. Thus, after treatment a thin section may be completely ferritic,

and on fracture present a steely white appearance; hence the name

'whiteheart'. Heavier sections will not be completely decarburised, and so,

whilst the outer layer is ferritic, this will merge into an inner zone contain-

ing some pearlite, and at the extreme core some nodules of temper carbon.

The surface layer may exhibit some oxide penetration.

Thin-sectioned components requiring high ductility are often made in

the form of whiteheart malleable castings. Examples include fittings for

gas,

water, air and steam pipes; bicycle- and motorcycle-frame fittings;

parts for agricultural machinery; switchgear equipment; and parts for tex-

tile machinery.

15.72.3.

Pearlitic Malleable Iron is produced from raw material similar

in composition to that of blackheart malleable iron, though in some cases

alloying elements may be used to stabilise the required pearlitic matrix in

the final structure. When an unalloyed iron is used it is first malleabilised

either fully or partially at about 950

C to cause adequate breakdown of

the primary cementite. It is then reheated to 900

C so that carbon will

dissolve in the austenite present at that temperature. Subsequent treatment

consists either of air-cooling (to produce a pearlitic matrix) or some other

form of heat treatment designed to give a bainitic or tempered-martensite

type of matrix.

If an alloyed iron is used it is often given a normal malleabilising treat-

*Whiteheart malleable castings of greater cross-section are likely to be stronger since the core will

contain more pearlite.

ment, the presence of carbide-stabilising elements causing retention of the

pearlitic matrix during this process.

The final structure will consist of rosettes of temper carbon in a matrix

of pearlite, bainite or tempered martensite according to the final heat-

treatment given. Tensile strengths in the region of 775 N/mm

are possible

and in many respects the product can compete with cast steel, despite the

extra cost of the heat-treatment processes.

15.73 Compacted-graphite (CG) Irons These materials have their

origins in work carried out by Morrogh at BCIRA* many years ago and

are characterised by graphite structures (Pl.

15.4B)—and

consequently also

physical properties—which are intermediate between those of ordinary

grey flake-graphite irons and those of SG iron. With appropriate chemical

treatment the graphite flakes produced are short and stubby and have

rounded edges. This has led to the term 'vermicular iron' being used in the

USA, though the flakes are not quite 'wormlike' as the title implies.

CG iron is produced when molten iron of near-eutectic composition is

first de-sulphurised in the ladle and then treated at 1400

C with a single

alloy containing appropriate amounts of magnesium, titanium and cerium

so that the resultant iron contains Mg (0.015-0.03%), Ti (0.06-0.13%)

and Ce (a trace). The presence of magnesium tends to produce spheroidal

graphite (as it does in SG iron) and this is controlled by the restraining

tendency of titanium which makes the amount of the magnesium less

critical.

The mechanical properties are roughly intermediate between those of

grey iron and those of SG iron, whilst the resistance to scaling and 'growth'

at high temperatures are very good. Since in cast irons generally graphite

* British Cast Iron Research Association, Alvechurch, Birmingham.

Table 15.4 Mechanical Properties (Minimum Values) of Malleable Cast Irons.

Type

White-

heart

malleable

BS 309

Black-heart

malleable

BS 310

Pearlitic

malleable

3333

Grade

W-410/4

W-340/3

B-340/12

B-310/10

B-290/6

P-690/2

P-570/3

P-540/5

P-510/4

P-440/7

Diam. of

test piece

(mm)*

0.5% proof

stress

(N/mm

)

190

230

250

200

190

170

540

420

340

310

270

Tensile

Strength

(N/mm

)

350

390

410

270

310

340

310

290

690

570

540

510

440

Elongation

(%)

Hardness (H$ \

(not mandatory)

229 max.

149 max.

241-285

197-241

179-229

170-229

149-197

Table

15.5 Compositions, Properties and

Uses

of Some

Typical

Alloy Cast

Irons

Properties

and uses

Cylinder blocks, brake drums, clutch

casings,

etc.

'inoculated' cast iron.

Piston rings for aero, automobile and

diesel

engines. Has wear-resistance and long life.

Automobile crankshafts. Hard, strong and

tough.

Good resistance to wear and heat-cracks.

Used for brake drums and clutch plates.

Crankshafts

for diesel and petrol

engines.

Good strength, shock-resistance and

vibration-damping capacity.

Mechanical

properties

Brinell

230

300

220

300

Tensile

strength

278

355

371

278

448

Compositon

(%)

Other

elements

Vanadium

0.17

Molybdenum

0.9

Copper

1.25

Molybdenum

0.8

Copper

0.15

0.32

0.9

0.65

0.10

1.5

0.5

0.35

1.75

0.17

0.50

0.09

0.10

0.05

0.08

0.05

0.8

0.6

0.8

0.6

0.7

2.1

1.4

2.8

2.1

2.0

2.1

3.2

2.8

3.6

3.1

3.4

2.9

Type

of iron

Chromidium

Ni-Tensyl

Wear-resistant

Ni-Cr-Mo

iron

Heat-resistant

Wear-

and shock-

resistant

martensitic iron used to resist severe

abrasion, e.g. chute plates in coke plant

Chemical

plant handling sulphur or chloride

solutions.

Austenitic, non-magnetic and

corrosion-resistant.

Pump castings, handling concentrated brine;

an austenitic, corrosion-resistant alloy.

austenitic, corrosion-resistant alloy, pump

components, flue gas dampers.

austenitic, corrosion-resistant alloy.

Resistant to high temperatures.

Abrasion-resistant

White

irons

600

140

130

330

210

450

500

170

216

263

232

170

Copper

6.0

Molybdenum

1.3

Molybdenum

1.5

Molybdenum

0.75

1.5

2.0

5.0

15.0

20.0

25.0

4.5

21.0

15.0

18.0

11.0

0.100.05

0.5

1.0

6.0

1.0

1.1

1.3

2.1

5.0

1.5

5.0

0.5

3.3

2.8

2.9

2.0

2.8

2.5

2.7

2.6

3.0

Ni-hard

Ni-resist

Nicrosilal

No-mag

SiIaI

High

Chromium

4844:

Grade

Table 15.6

Some

Flake

and SG Austenitic

Alloy

Cast

Irons

covered by BS3648

Typical uses

Non-magnetic pressure covers for

turbine generator sets; housings for

switch

gear;

insulator

flanges,

terminals and ducts.

Pumps

handling alkalis; vessels for

alkalis—in soap, food and plastics

manufacture

Pumps; valves; compressors; turbo

supercharger housings; gas exhaust

manifolds.

Good corrosion resis. even to

dilute

sulphuric acid—pump parts; flue

dampers

Pump parts and flue gas dampers

subjected

to high mechanical stress.

Low

thermal expansion—parts

requiring high dimensional accuracy.

Machine tools, scientific instruments,

glass moulds.

Typical

mechanical properties

Typical

Composition (%)

Hardness

(H*)

135

150

170

180

210

130

155

Compressive

strength

(N/mm

)

735

770

560

630

Elong.

(%)

2.5

2.0

30.0

Tensile

strength

(N/mm

)

180

425

190

420

205

440

150

390

0.2%

proof

strength

(N/mm

)

235

230

275

225

2.0

5.0

13.0

20.0

30.0

35.0

7.0

1.0

3.25

1.9

5.0

1.5

3.0

2.5

2.4

Grade*

Flake

(L)/'

//SG. (S)

L-NiMn 13-7

S-NiMn

13-7

L-NiCr

20-2

S-NiCr

20-2

L-NiSiCr

30-5-5

S-NiSiCr30-5-5

L-Ni 35

S-Ni

"These particular compositions, selected from B.S.3648, can be used as 'flake-graphite' irons or can be treated to give spheroidal-graphite structures. The

considerable improvement in mechanical properties obtained by the use of S.G. iron is apparent both in terms of strength and ductility.

is a better conductor of heat than is the iron matrix, it follows that CG

irons have a thermal conductivity only slightly less than grey iron but better

than nodular graphite irons. All of these factors make CG iron attractive

as a heat-resisting material and it was developed originally for the manufac-

ture of ingot moulds and vehicle brake components. In modern engineering

there are many other applications, eg gear pumps, eccentric gears, fluid

and air cylinders, discharge manifolds, etc.

15.74 The choice between ordinary grey iron, SG iron and malleable

iron for a specific application is, as with most materials problems, dictated

by both economics and technical requirements. Grey iron is, of course,

lowest in cost and also the easiest in which to produce sound castings. With

regard to the relative merits of the two high-strength competitors, SG iron

and malleable iron, the choice may be less easy. Malleable iron is the more

difficult to cast since at this stage it will have a white structure. This also

involves limitations as to cross-section as compared with SG iron. More-

over, the cost of the malleabilising process makes the product a little more

expensive than SG iron. However, it is sometimes necessary to anneal SG

iron in order to improve its machinability, and in this case malleable iron

may prove more attractive cost-wise.

Alloy Cast Irons

15.80 The microstructural effects which alloying elements have on a cast

iron are, in most cases, similar to the effects these elements have on the

structure of a steel. Alloying elements can therefore be used to improve

mechanical properties, to refine grain, to increase the hardness by stabilis-

ing cementite and forming other harder carbides; and to stabilise, when

desirable, martensitic and austenitic structures at ambient temperatures.

Improvements in resistance to atmospheric corrosion and also deterio-

ration at high temperatures are also attainable.

Plate 15.6 15.6A A martensitic alloy cast

iron.

Graphite in a matrix of martensite. x 600. Etched in 4% picral. (Courtesy of BCIRA)

15.6B An austenite alloy cast iron (Ni-resist).

Graphite and carbides in an austenite matrix, x 100. Etched in 4% picral. (Courtesy of

BCIRA).

15.81 Nickel is a common alloying element and, as in steel, it tends

to promote graphitisation. At the same time it has a grain-refining effect,

so that whilst nickel will help to prevent chilling in thin sections, it will

also prevent coarse grain in thick sections. Nickel also reduces the tendency

of thin sections to crack.

15.82 Chromium is a strong carbide stabiliser, and so inhibits the

formation of graphite. Moreover, the carbides formed by chromium are

more stable and less likely to graphitise under the application of heat than

is iron carbide. Irons containing chromium are therefore less susceptible

to growth.

As in the low-alloy steels, the disadvantages attendant on the use of

either nickel or chromium separately are overcome by using them in con-

junction in the ratio of two to three parts of nickel to one part of chromium.

15.83 Molybdenum, when added in small amounts, dissolves in the

ferrite, but in larger amounts forms double carbides. It increases the hard-

ness of thick sections and promotes uniformity of microstructure. Impact

values are also improved by amounts in the region of 0.5% molybdenum.

15.84 Vanadium promotes heat-resistance in cast irons, in so far as

the stable carbides which it forms do not break down on heating. Strength

and hardness are increased, particularly when vanadium is used in conjunc-

tion with other alloying elements.

15.85 Copper is only sparingly soluble in iron, and has a very slight

graphitising effect. It has little influence on the mechanical properties, and

its main value is in improving the resistance to atmospheric corrosion.

15.86 Martensitic irons, which are very useful for resisting abrasion,

usually contain 4-6% nickel and about 1% chromium. Such an alloy is

Nihard, included in Table 15.5. It is martensitic in the cast state, whereas

alloys containing rather less nickel and chromium would need to be oil-

quenched in order to obtain a martensitic structure. Austenitic irons usu-

ally contain between 10 and 30% nickel and up to 5% chromium. These

are corrosion-resistant, heat-resistant, non-magnetic alloys. Some of them

are treated to produce structures containing spheroidal instead of flake

graphite. Table 15.6 indicates the degree of improvement in both strength

and ductility which arise from this additional treatment.

Exercises

What factors control the structure of cast iron? Explain how these principles

are used in the foundry to control the strength of grey-iron castings. (15.20

and 15.30)

Discuss the influence of the following elements on the structure and properties

of cast iron: (i) silicon; (ii) manganese; (in) sulphur; (iv) phosphorus. (15.22

to 15.25)

Describe briefly the methods available to control the size and shape of graphite

in cast iron, and explain how the mode of occurrence of the graphite affects

the properties. (15.22; 15.30;

15.71;

15.73)

Explain fully what is meant by the term growth as applied to cast irons. Discuss

methods which are used to overcome this phenomenon. (15.50)

Cast iron is a cheap, easily produced engineering material, but ordinary grey

cast iron has relatively poor mechanical properties. Discuss, in general terms,

the methods employed to overcome these limitations.

(15.71;

15.72; 15.73)

6. Discuss the effect of chemical composition and cooling rate on the structure

and properties of cast irons. Briefly describe one method for producing (a)

malleable iron and (b) nodular iron. (15.20; 15.30; 15.72; 15.71)

Most cast irons consist of a dispersion of graphite in a steel-like matrix. Illus-

trate this statement by sketches of microstructures and discuss the variations

in properties that can arise, by reference to: (a) pearlitic grey cast iron; (b)

blackheart malleable iron; (c) SG iron; (d) compacted-graphite iron. Indicate

briefly the treatment necessary to produce blackheart malleable iron, SG cast

iron and compacted-graphite cast iron. (15.40; 15.72;

15.71;

15.73)

8. How are malleable castings produced? What are the most important engineer-

ing properties of these materials? (15.72)

9. Discuss the role of microstructure in determining the general properties of cast

iron. (15.20; 15.30; 15.70)

10.

Discuss the uses of nickel, chromium and molybdenum in cast irons. (15.50;

15.81 to 15.83)

Bibliography

Angus, H. T., Cast Iron: Physical and

Engineering

Properties,

Butterworths, 1960.

Copper Development Association, Copper in Cast Iron, 1964.

Hume-Rothery, W., The Structures of Alloys of Iron, Pergamon, 1966.

BS 1452: 1977

Specification

for Grey Iron Castings.

BS 6681: 1986

Specifications

for Malleable Cast Iron.

BS 2789: 1985 Iron Castings with Spheroidal or Nodular Graphite.

BS 1591: 1983

Corrosion-resistant

High Silicon Iron Castings.

BS 3468: 1986 Austenitic Cast Iron.

Copper and the

Copper-Base Alloys

16.10 Copper was undoubtedly the first useful metal to be employed by

Man. In many countries it is found in small quantities in the metallic

state and, being soft, it was readily shaped into ornaments and utensils.

Moreover, many of the ores of copper can easily be reduced to the metal,

and since these ores often contain other minerals, it is very probable that

copper alloys were produced as the direct result of smelting. It is thought

that bronze was produced in Cornwall by acc

;

dentally smelting ores

containing both tin- and copper-bearing minerals in the camp fires of

ancient Britons.

As a schoolboy the author was taught that the very protracted period of

Man's history when progress was extremely slow, namely the 'Stone Age',

was followed by a 'Bronze Age' when primitive technology developed

relatively quickly. It is now fairly certain that the Bronze Age was preceded

by a comparatively short Copper Age but, because copper corrodes more

quickly than bronze, few artefacts of that period have been recovered by

archaeologists. It is fairly certain that the Egyptians used copper com-

pounds for colouring glazes some 15 000 years ago and they may have been

the first to extract the metal. In Europe copper production seems to have

begun in the Balkans some 6000 years ago, and very soon metal workers

were hardening copper by the addition of small amounts of arsenic for the

manufacture of knives and spear heads. This may well have been the first

instance, in Europe at least, of the deliberate manufacture of an alloy.

16.11 At the end of the eighteenth century practically all the world's

requirement of copper was smelted in Swansea, the bulk of the ore coming

from Cornwall, Wales or Spain. Deposits of copper ore were then dis-

covered in the Americas and Australia, and subsequently imported for

smelting in Swansea. Later it was found more economical to set up smelting

plants near to the mines, and Britain ceased to be the centre of the copper

industry.

Climbers and ramblers who ascend Snowdon by the popular Pyg track

will pass the ruins of an old copper-mine, perched on the mountain side

below the Crib Goch ridge. This small mine was one of many located in

the mountainous areas of Britain, but all have long since ceased pro-

duction. Their output was insignificant compared with that of a modern

mine like that at Chuquicamata, in Chile. Nevertheless supplies of the

higher grades of copper ore are diminishing and the demand for yet higher

outputs has meant that low-grade, less economical sources have to be

worked. Until a couple of decades ago the world tonnage of copper pro-

duced annually was second only to that of iron but now copper ranks third,

having been overtaken by aluminium.

Although the USA is still the largest producer of copper, as of many

other metals, her output is being closely approached by those of the CIS

and Chile. Canada, Zambia, Zaire and Peru are also leading producers,

whilst in Europe the outputs of Poland and Yugoslavia are significant. The

bulk of Britain's supply of copper comes from Zambia and Canada.

Properties and Uses of Copper

16.20 A very large part of the world's production of metallic copper is

used in the unalloyed form, mainly in the electrical industries. Copper has

a very high specific conductivity, and is, in this respect, second only to

silver, to which it is but little inferior. When relative costs are considered,

copper is naturally the metal used for industrial purposes demanding high

electrical conductivity.

16.21 The Electrical Conductivity of Copper The International

Annealed Copper Standard (IACS) was adopted by the International Elec-

trochemical Commission as long ago as 1913, and specified that an

annealed copper wire 1 m long and of cross-section 1 mm

should have a

resistance of no more than 0.017241 ohms at 20

C. Such a wire would be

said to have a conductivity of 100%. Since the standard was adopted in

1913 higher purity copper is now commonly produced and this explains

the anomalous situation where electrical conductivities of up to 101.5%

are frequently quoted.

As indicated in Fig. 16.1, the presence of impurities reduces electrical

conductivity. To a lesser degree, cold-work has the same effect. Reduction

in conductivity caused by the presence of some elements in small amounts

is not great, so that up to 1% cadmium, for example, is added to telephone

wires in order to strengthen them. Such an alloy, when hard-drawn, has a

tensile strength of some 460 N/mm

as compared with 340 N/mm

for

hard-drawn, pure copper, whilst the electrical conductivity is still over 90%

of that for soft pure copper. Other elements have pronounced effects

on conductivity; as little as 0.04% phosphorus will reduce the electrical

conductivity to about 75% of that for pure copper.

16.22 The Commercial Grades of Copper include both furnace-

refined and electrolytically refined metal. High-conductivity copper (usu-

ally referred to as OFHC or oxygen-free high conductivity) is of the highest

purity, and contains at least 99.9% copper. It is used where the highest