Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Scale Indenter Total force (kgf)

A Diamond cone 60

V steel ball 100

C Diamond cone 150

D Diamond cone 100

Vs"

steel ball 100

F Vi

" steel ball 60

G V

" steel ball 150

Vs"

steel ball 60

K W steel ball 150

Of these, scale C is probably the most popular for use with steels.

Fig.

2.10 The Rockwell Diamond Cone lndentor.

2.45 The Shore Scleroscope (Greek: 'skleros'—hard) tests the

material very near to its surface. The instrument embodies a small dia-

mond-tipped 'tup' which is allowed to fall from a standard height inside a

graduated glass tube. The height of rebound is taken as the hardness index.

Since the Shore Scleroscope is a small, portable instrument, it is very useful

for the determination of hardness of large rolls, castings and gears, and

other large components which could not easily be placed on the testing

tables of any of the more orthodox testing machines.

The development of digital display units has enabled very small portable

hardness testers of the indentation type to be manufactured. One of these

consists of a small motorised probe which, when pressed against the surface

of the test piece, makes a minute diamond impression using a force of

only 8.4 N. Consequently such a test is virtually non-destructive and the

instrument can be used in the most remote corners of the factory, hangar

or repair yard. At the same time a high accuracy of ± 15 VPN is claimed

over the hardness range of 50 to 995 VPN. Such instruments have largely

replaced the Shore Scleroscope in terms both of accuracy and adaptability.

Table 2.3 gives representative hardness numbers, together with other

mechanical properties, for some of the better-known metals and alloys.

O»2 mm

radius

Table 2.3 Typical mechanical properties of some metals and alloys

Metal or Alloy

Lead

Aluminium

Duralumin

Magnesium-6AI/

1Zn

Copper

70/30 Brass

Phosphor bronze

(5%

tin)

Mild steel

0.45% carbon

steel

4Ni/Cr/Mo steel

18/8 stainless

steel

Grey cast iron

Titanium

(commercially

pure)

Titanium alloy

(4Sn/4AI/4Mo

0.5Si)

Condition

Soft sheet

Wrought and

annealed

Extruded and

fully

heat-treated

Extruded bar

Wrought and

annealed

Annealed

Deep-drawn

j Rolled and

annealed

Hard-rolled

Hot-rolled

sheet

Normalised

0.7%

Proof

Strength

(N/mm

)

275

170

370

120

650

270

420

Water-quenched

and tempered

at 600

Air-hardened

and

tempered at

300

Softened

As cast

Annealed

sheet

Precipitation

hardened

540

1200

185

370

1200

Tensile

Strength

(N/mm

)

430

300

216

320

465

340

710

400

665

780

1550

525

300

450

1390

Specific

Strength

(N/mm

)

1.54

21.8

154

167

24.1

37.6

54.6

38.1

79.6

50.8

84.7

99.4

198

66.3

40.5

100

309

Young's

Modulus

(kN/mm

)

130

100

101

210

oc\c\

225

220

150

120

150

Specific

Modulus

(kN/mm

)

1.37

25.9

25.4

26.7

14.5

11.7

11.3

26.7

QC A

28.7

27.8

20.3

26.7

33.3

Elongation

(%)

Hardness

(Brinell)

115

132

188

100

152

200

444

170

250

Impact

Value

(IZOd)(J)

Bold type denotes maximum value in that property (where relevant).

Fig.

2.11 The Avery-Denison Universal impact-testing machine.

This machine can be used for either Charpy or Izod impact tests. For Izod tests, the

pendulum is released from the lower position, to give a striking energy of 170 J; and for the

Charpy test it is released from the upper position, to give a striking energy of 300 J. (The

scale carries a set of graduations for each test.) The machine can also be used for

impact-tension tests.

Impact Tests

2.50 Impact tests indicate the behaviour of a material under conditions

of mechanical shock and to some extent measure its toughness. Brittleness

—and consequent lack of reliability—resulting from incorrect heat-

treatment (13.42) or other causes may not be revealed during a tensile test

but will usually be evident in an impact test.

2.51 The Izod Impact Test In this test a standard notched specimen

is held in a vice and a heavy pendulum, mounted on ball bearings, is

allowed to strike the specimen after swinging from a fixed height. The

striking energy of 167 J (120 ft lbf) is partially absorbed in breaking the

specimen and, as the pendulum swings past, it carries a pointer to its

highest point of swing, thus indicating the amount of energy used in fractur-

ing the test piece.

TEST-PIECE

CLAMPING

LEVER

CHUTE FOR BROKEN

TEST-PIECES

PENDULUM

RELEASE

LEVER

IZOO TEST-

PIECE

PENOULUM REST STOP

SECTION THROUGH

PENDULUM BO8

AT X-Y

IZOD RELEASE

POSITION

PENDULUM

STRIKER

CHARPY RELEASE POSITION

SCALE

2.52 The Charpy Test, developed originally

on the

Continent

but now

gaining favour

Britain, employs

test piece mounted

as a

simply-

supported beam instead

of in the

cantilever form used

in the

Izod test

(Fig.

2.12).

The

striking energy

is 300 J (220 ft lbf).

2.53

To set up

stress concentrations which ensure that fracture does

occur, test pieces

are

notched.

It is

essential that notches always

stan-

dard,

for

which reason

standard gauge

used

test

the

dimensional

accuracy

of the

notch.

Fig. 2.12

shows standard notched test pieces

for

both

the

Izod

and

Charpy impact tests.

Fig.

2.12

Dimensions

standard test pieces

for

both Izod

and

Charpy tests.

the

Izod test piece, notches 28mm apart

may be cut in

three different faces

that

representative value

obtained.

2.54

The

results obtained from impact tests

are not

always easy

interpret,

and

some metals which

are

ductile under steady loads behave

brittle materials

in an

impact test.

mentioned above, however,

the

impact test gives

good indication

how reliable

the

material

likely

be under conditions

mechanical shock. These tests

are

most likely

to be

specified

for

constructional steels

medium-carbon content.

Other Destructive Tests

2.60 These

are

often designed specifically

for the

measurement

some

property peculiar

to a

single class

material

or to

assess

the

suitability

a material

for a

special purpose.

2.61

The

Erichsen Cupping Test (11.54-Pt.

II) is

closely connected

with

the

ductility

of a

material

but is in

fact designed

assess

its

deep-

drawing properties.

2.62 Compression Tests

are

used

measure

the

capability

of a

cast

iron

carry compressive loads.

standard test cylinder

tested

in com-

pression, usually employing

tensile testing machine running

'reverse'.

2.63 Torsion Tests

various types

are

sometimes applied

materials

wire

and rod

form.

IOmm SQUARE

HAMMER

ROOT

RADIUS

HAMMER

VICE

SQUARE

IZOD

DETAILS

NOTCH

FOR

BOTH TESTS

CHARPY

Non-destructive Tests

2.70 The mechanical tests already mentioned are of a destructive nature

and are subject to the availability of separate test pieces which are reason-

ably representative of the production material. Thus, wrought products

such as rolled strip, extruded rod and drawn wire are generally uniform in

mechanical properties throughout a large batch and can be sampled with

confidence. Parts which are produced individually, however, such as cast-

ings and welded joints, may vary widely in quality purely because they are

made individually and under the influence of many variable factors. If the

quality of such components is important and the expense justified—as in

the case of aircraft castings—it may be necessary to test each component

individually by some form of non-destructive test. Such tests seek to detect

faults and flaws either at the surface or below it, and a number of suitable

methods is available in each case.

The Detection of Surface Faults

2.80 It is often possible to detect and evaluate surface faults by simple

visual examination with or without the use of a hand magnifier. The pres-

ence of fine hair-line cracks is less easy to detect by visual means and some

aid is generally necessary. Such surface cracks may be associated with

the heat-treatment of steel or, in a welded joint, with contraction during

cooling.

2.81 Penetrant Methods In these methods the surface to be examined

is cleaned and then dried. A penetrant fluid is then sprayed or swabbed

on the surface which should be warmed to about 90

C. After sufficient

time has elapsed for the penetrant to fill any fissures which may be present

the excess is flushed from the surface with warm water (the surface tension

of the water is too high to allow it to enter the narrow fissure). The test

surface is then carefully dried, coated with fine powdered chalk and set

aside for some time. As the coated surface cools, it contracts and penetrant

tends to be squeezed out of any cracks, so that the chalk layer becomes

stained, thus revealing the presence of the cracks. Most penetrants of this

type contain a scarlet dye which renders the stain immediately noticeable.

Aluminium alloy castings are often examined in this way.

Penetrants containing a compound which fluoresces under the action of

ultra-violet light may also be used. This renders the use of messy chalk

unnecessary. When the prepared surface is illuminated by ultra-violet light,

the cracks containing the penetrant are revealed as bright lines on a dark

background. Penetrant methods are particularly useful for the examination

of non-ferrous metals and austenitic (non-magnetic) steels.

2.82 Magnetic Dust Methods consist in laying the steel component

across the arms of a magnetising machine and then sprinkling it with a

special magnetic powder. The excess powder is blown away, and any cracks

or defects are then revealed by a bunch of powder sticking to the area on

Fig.

2.13 The penetrant method of crack detection.

(i) The cleaned surface is coated with penetrant which seeps into any cracks present, (ii)

Excess penetrant is removed from the surface, (iii) The surface is coated with chalk. As the

metallic surface cools and contracts, penetrant is expelled from the crack to stain the chalk.

each side of the crack. Since the crack lies across the magnetic field, lines of

force will become widely separated at the air gap (Fig. 2.14) and magnetic

particles will align themselves along the lines of force.

Fig.

2.14 The principles of magnetic particle crack detection.

The Detection of Internal Defects

2.90 Internal cavities in the form of blow holes or shrinkage porosity may

be present in castings of all types, whilst wrought materials may contain

slag inclusion and other subcutaneous flaws. Welded joints, by the nature

of their production methods, may contain any of these defects. Since metals

are opaque to light, other forms of electromagnetic radiation of shorter

wavelength (X-rays and y-rays) must be used to penetrate metals and so

reveal such internal discontinuities. Although the railway wheeltapper was,

for some obscure reason, always 'good for a laugh' at the mercy of the

professional comedian, he was in fact using a 'sonic' method of testing the

continuity of structure of the wheel and modern sophisticated methods of

ultra-sonic testing use similar principles.

2.91 X-ray Methods X-rays are used widely in metallurgical research

in order to investigate the nature of crystal structures in metals and alloys.

Their use is not confined to the research laboratory, however, and many

firms use X-rays in much the same way as they are used in medical radiogra-

phy, that is, for the detection of cavities, flaws and other discontinuities in

castings, welded joints and the like.

penetrant

fissure

(i)

(H)

(Hi)

chalk

component

magnetic field

X-rays

are

produced when

stream

high-velocity electrons impinges

metal target.

Fig. 2.15

illustrates

the

principle

of an

X-ray tube

which

filament supplies free electrons. Being negatively charged these

electrons

are

accelerated away from

the

cathode towards

the

anode (some-

times called

the

'anti-cathode')

by the

high potential difference between

the electrodes. Collision with

the

anode produces X-rays.

The

containing

tube

under vacuum,

as the

presence

of gas

molecules would obstruct

the

passage

relatively small electrons. Nevertheless only about

1% of the

energy expended produces X-rays

the

remainder being converted

heat.

Consequently

the

anode must

water-cooled.

For

greater output

X-rays (above

1 MeV)

other types

generator such

as the

'linear gener-

ator'

'Linac' have

to be

used.

Fig.

2.15

Radiography

of a

casting using X-rays.

The penetrating power

electromagnetic radiation generally, depends

upon

its

frequency. Thus radiation

at the

ultra-violet

end of the

visible

spectrum will penetrate

our

skin

to a

depth

less than

1 mm but

will

nevertheless produce painful radiation burns

(and

possibly skin cancer)

we sunbathe carelessly. X-rays, having

much higher frequency than

light, will penetrate more deeply

and the

'harder'

the

X-rays

(ie the

higher

the frequency)

the

greater

the

depth

penetration. X-rays used

metal-

lurgical radiography

are

harder than those used

medicine,

and are

better

able

penetrate metals.

At the

same time their properties make them

much more dangerous

human body tissue,

and

plant producing radiation

of this type needs

to be

carefully shielded

order

prevent

the

escape

of stray radiations which would seriously impair

the

health

operators.

cooling

water

VACUUM

cathode

tungsten

target

X-RAY

TUBE

shield

copper

anode

filament

glass tube

x-rays

casting

cavity

photographic film

resulting

negative

image

cavity

Like light, X-rays travel in straight lines, but whilst metals are opaque

to light they are moderately transparent to X-rays, particularly those of

high frequency. Fig. 2.15 illustrates the principle of radiography. A casting

is interposed between a shielded source of X-rays and a photographic film.

Some of the radiation will be absorbed by the metal so that the density of

the photographic image will vary with the thickness of the metal through

which the rays have passed.

2.92 X-rays are absorbed logarithmically—

/ =

e-*

Where I

and / are the intensities of incident and emergent radiation

respectively, d the thickness and pi the linear coefficient of absorption of

radiation, (i is lower for radiation of higher frequency.

A cavity in the casting will result in those X-rays which pass through the

cavity being less effectively absorbed than those rays which travel through

the full thickness of metal. Consequently the cavity will show as a dark

patch on the resultant photographic negative in the same way that a greater

intensity of light affects an ordinary photographic negative.

A fluorescent screen may be substituted for the photographic film so

that the resultant radiograph may be viewed instantaneously. This type of

fluoroscopy is obviously much cheaper and quicker but is less sensitive

than photography and its use is generally limited to the less-dense metals

and alloys.

2.93 y-ray Methods can also be used in the radiography of metals. Since

they are of shorter wavelength than are X-rays, they are able to penetrate

more effectively a greater thickness of metal. Hence they are particularly

useful in the radiography of steel, which absorbs radiation more readily

than do light alloys.

2.94 y-rays constitute a major proportion of the dangerous radiation

emanating from 'nuclear waste' and from the fall-out of nuclear explosions.

Initially naturally-occurring radium and radon (18.70) were used as a

source of y-rays but artificially activated isotopes of other elements are

now generally used. These activated isotopes are prepared by bombarding

the element with neutrons in an atomic pile. A nucleus struck by a neutron

absorbs it and then contains an excess of energy which is subsequently

released as y-rays. Commonly used isotopes are shown in Table 2.4. Of

these iridium-192 and cobalt-60 are the most widely used in industry.

2.95 Manipulation of the isotope as a source of y-radiation in metallur-

gical radiography is in many respects more simple than is the case with

X-rays, though security arrangements are extremely important in view of

the facts that y-radiation is 'harder' than X-radiation and that it takes place

continuously from the source without any outside stimulation. All y-ray

sources are controlled remotely, generally using a manual wind-out system

(Fig. 2.16). When not in use the isotope is stored in a shielded container

of some y-ray absorbent material such as lead. Because they are 'harder'

than X-rays, y-rays can be used to radiograph considerable thicknesses of

steel. Since the radiation source is small and compact and needs no external

Fig.

2.16 y-radiography manual remote wind-out system,

(i) y-ray source exposed; (ii) y-ray source stored.

power supply, y-radiation equipment

very portable

and can be

used

examine materials

situ,

welded joints

motor-way bridges.

All forms

ionising radiation such

X-rays and y-rays

are

very harmful

all

living tissue

and

their

use in the UK is

governed

by the

Factories

Act—The Ionisating Radiations (sealed sources) Regulations 1969'.

2.96 Ultra-sonic Methods

marine navigation

the old

method

'Swinging

the

lead'

was

used

determine

the

depth

of the

water under

the boat. This

was

replaced

in the

technological

age by a

'sonic' method

in which

signal

was

transmitted from

the

boat down through

the

water.

The time interval which elapsed between transmission

and

reception

of the

'echo'

was a

measure

of the

depth

of the

ocean bed.

The

ultrasonic testing

of metals

somewhat similar

principle. Ordinary sound waves

(of fre-

quencies between

30 and 16 000 Hz)

tend

bypass

the

small defects

are

dealing with

metallic components

and

ultra-sonic frequencies

(between

0.5 and 15

MHz)

are

used

for

metals inspection.

When

ultrasonic vibration

transmitted from one medium

another

some reflection occurs

at the

interface.

Any

discontinuity

in a

structure

will therefore provide

reflecting surface

for

ultrasonic impulses

(Fig.

2.17).

probe containing

electrically-excited quartz

barium titanate

(i)

(ii)

manual

wind-out

flexible

cable-drive

isotope

'safe'

probe

V-ray

source

radiation

shielding

Table

2.4

y-ray sources used

industry

Isotope

Cobalt-60

Caesium-137

Thulium-170

Ytterbium-169

lridium-192

Symbol

137

170

T-S

169

192

Half-life

5.3 years

33 years

127 days

31 days

74 days

Relative energy

output

terms

y-radiation

1.3

0.35

0.0045

0.021

0.48

Typical uses

50-200mm

steel

25-100mm

steel

2-13mm

aluminium

2-13mm

steel

10-90mm

steel

Fig.

2.17 The detection of a fault in plate material by ultrasonics.

In (i) the impulse is reflected from the lower surface of the plate; whilst in (ii) it is reflected

from a defect. Measurement of the time interval between transmission of the impulse and

reflection of the echo determines the depth of the fault.

crystal which can both transmit and receive high-frequency vibrations is

used to traverse the surface of the material to be examined (Fig. 2.18.).

The probe is coupled to a pulse generator and to a signal amplifier which

transfers the resultant 'image' to a CRT (cathode-ray tube).

2.97 In satisfactory material the pulse will pass from the probe unim-

peded through the metal and be reflected from the lower inside surface at

A back to the probe, then acting as receiver. Both transmitted pulse and

echo are recorded on the CRT and the distance, ti, between peaks is pro-

portional to the thickness, t, of the test piece. If any discontinuity is

encountered such as a blow-hole, B, then the pulse is interrupted and

reflected as indicated. Since the echo returns to the receiver in a shorter

time an intermediate peak appears on the CRT trace. Its position relative

to the other peaks gives an indication of the depth of the fault beneath the

surface.

Fig. 2.18 shows separate crystals being used for transmitter and receiver

but, as mentioned above, in many modern testing devices a single crystal

fulfils both functions. Different types of probe are available for materials

of different thickness and this method is particularly useful for examining

material—such as rolled plate—of uniform thickness.

Fig.

2.18 Basic principles of ultrasonic testing.

The values of 'd' and T in the test piece are proportional to the values of 'di

and V shown

on the CRT.

metal

plate

defect

(ii)

(i)

PULSE

GENERATOR

SIGNAL

AMPLIFIER

TIME BASE

CRT

PROBE

TEST

PIECE

transmitted

pulse

echo

from B

echo

from A