Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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High-speed steel is therefore heated further, so that more carbide is

absorbed by the austenite in accordance with the increase in solubility of

the carbide, as indicated by the sloping phase boundary ES in Fig. 14.1.

Quenching from temperatures above 840

C will produce increasing red-

hardness with rise in temperature, due to more carbon and associated

alloying elements being taken into solution and retained in solution, thus

rendering the resultant martensite more sluggish to tempering influences.

Even when the solidus temperature is reached at S (approximately 1330

C),

no more than 0.6% carbon has been absorbed and the remainder exists as

isolated globules of complex carbides (Pl. 13.2). During the heating process

most of the chromium carbide has gone into solution by the time 1100

has been reached and at the quenching temperature of, say, 1250

C, car-

bides amounting to 10% of the total remain undissolved. These are princi-

pally the double carbides Fe

C and Fe

C and, because vanadium

has a very strong carbide-forming tendency, practically all of the VC.

14.14 Therefore, in order to obtain the highest cutting efficiency in a

high-speed steel, it must be hardened from a temperature little short of

the solidus SD, at which fusion commences. This temperature will vary

with steels of different composition but is usually between 1170° and

1320

C. Heat-treatment at such temperatures has its attendant difficulties,

°c

UNSTABLE

AUSTENITE

COMPLEX

CARBIDES

7O%

TRANSFORMATION TO MARTENSITE

18-4-1

HS.

STEEL

TIME (SEC) (LOGARITHMIC SCALE)

Fig.

14.2 TTT curves for an 18-4-1 high-speed steel.

The initial structure consists of saturated austenite together with some undissolved carbides.

The line C

indicates the commencement of precipitation of primary carbides (since the steel

is not of eutectoid composition). E

and E

indicate the start and finish of eutectoid ('pearlite')

formation.

shows the start of bainite formation whilst the associated broken line indicates

the time required for 60% transformation to bainite. Note that at room temperature the

transformation of austenite to martensite is incomplete (70% at 50

C so that air- or oil quench-

ing will produce a mixture of martensite and retained austenite.

and requires some skill and experience in management, as well as adequate

plant. Grain growth and oxidation can be excessive at high temperatures

unless the conditions under which the steel is heated are suitably con-

trolled.

Oxidation is effectively prevented by heating the tool in a slightly car-

burising atmosphere; whilst grain growth is minimised by using a two-

chamber furnace. The tool is first pre-heated in the lower-temperature

chamber to about 850

C, and then transferred to the higher-temperature

chamber (or to a high-temperature salt bath) in which it is heated rapidly

to the hardening temperature. The time for which the tool is maintained

at the higher temperature must be strictly controlled if grain-growth and

decarburisation are to be avoided. Sections below 10 mm in thickness

should not be in the high-temperature compartment for more than four

minutes whilst sections of 50 mm should be heated for no more than ten

minutes. As soon as it has been in the high-temperature chamber for the

allotted time it is oil-quenched or cooled in an air blast according to the

composition of the steel (Table 14.1). Pre-heating reduces the time during

which the surface of the tool would otherwise be in contact with the high-

temperature conditions.

Modern trends are towards the use of vacuum furnaces for the large-scale

heat-treatment of some high-speed steels, particularly 'molybdenum 562'.

The tools are heated to 1250

C in a chamber maintained at high vacuum,

the resistance-heating elements being of graphite which will not oxidise

under such conditions. When the quenching temperature is reached the

vacuum chamber is purged with high-velocity inert gas so that hardening

of the charge is achieved. Such a process has obvious advantages over

traditional methods of heat-treatment:

1) a bright, oxide-free finish is retained;

2) the tool surface suffers neither loss nor gain of carbon;

3) repeatability and uniformity of control are possible;

4) hazardous fumes and toxic wastes are absent.

14.15 Due to the sluggishness produced in the austenite

—»

martensite

transformation by the presence of alloying elements, the quenched high-

speed steel will contain considerable amounts of retained austenite (Fig.

14.2).

It is necessary to transform this to martensite, and this can be accom-

plished by sub-zero treatment or, more generally, by tempering. Low-

temperature tempering between 300 and 400

C reduces the hardness

slightly but increases the toughness. On tempering between 400 and 600

the hardness increases again, often to a figure higher than the original (Fig.

14.3),

though this will be accompanied by some reduction in toughness.

This phenomenon, known as 'secondary hardening', is associated with the

presence of vanadium. Secondary hardening is usually carried out by heat-

ing the tool for one-half to three hours at 550-600

C according to the

composition of the steel. Some austenite may still be present following

tempering, and for this and other reasons double tempering is now gener-

ally used. The retained austenite is 'conditioned' during the first tempering

process and some of it changes to martensite on cooling. During the second

tempering process the transformation goes still further towards completion

Table 14.1 High-speed

Steels

Characteristics

and

uses

General shop practice—all-round

work,

moderate duties. Also for

some

cold-working

tools

(blanking

tools and shear blades).

Lathe, planer and

shaping

tools;

millers and gear cutters. Router

bits; hobs; reamers; broaches;

taps; dies; drills;

slitting

discs.

Hacksaws; bandsaws; etc.

Roller-bearings at high

temperatures, e.g. gas turbines.

Large

hobs and

milling

cutters,

automatic lathe tools, accurate

form-tools,

large broaches, etc.

Holds its edge longer than the 18%

tungsten

type of

steel.

Lathe, planer and shaper tools on

very

hard materials and for extra

heavy

work, e.g. high-tensile

steels, hard-cast irons, new and

brake-hardened railway tyres, etc.

Austenitic steels which

work-harden

quickly.

Hardness

(VPN)

800-860

800-890

830-950

800-900

Heat-treatment

Composition

(%)

Secondary-hardening

treatment

Double temper at 565°C

for

1 hour

Quench

in oil or

air

blast

from:

1250-1300

1290-1310°C.

1280-1320

1290-1320

5.5

0.5

0.25

1.2

1.4

1.2

14.0

18.2

21.5

18.5

4.0

4.2

4.5

0.65

0.75

0.78

0.8

BS4659

designation

BT1

BT20

BT4

Type

of steel

14%

Tungsten

18%

Tungsten

(18-4-1)

22%

Tungsten

6% Cobalt

Lathe, planer

and

shaper

tools,

milling cutters, hobs,

twist

drills, etc.,

for

exceptionally

hard materials.

Has

maximum

red-hardness

and

toughness.

Suitable

for

severest machining

duties—manganese

steel, steels

over

1200

N/mm

T.S.,

close-grained cast irons.

Extremely

hard

due

high

vanadium

but

also

tough.

Excellent red-hardness

and

edge

retention.

High accuracy

and

finish

—form tools, turning, milling,

shaping. Also cutting gas-turbine

alloys.

For very hard materials—

gas-turbine alloys,

etc.

Tool

bits,

form

tools, milling cutters,

etc.

Roughly

equivalent

standard

18-4-1

high-speed steel,

but

tougher.

Used

for

drills, reamers,

taps, milling cutters, punches,

threading dies, cold-forging dies.

general-purpose

Molybdenum-type high-speed

steel—drills, taps, reamers,

cutters.

susceptible

decarburisation during

heat-treatment than ordinary

18-4-1.

Similar uses

to 12%

Cobalt-21

Tungsten

high-speed

steel.

900-950

900-1000

900-990

850-900

830-900

830-935

Double temper

560

for

hour

Double temper

560

for

hours

Double temper

565°C

for

hour

Triple

temper

530

for

hour

1300-1320°C.

1230-1300

1250

1200-1230

1180-1210

12.0

10.0

5.0

8.75

8.25

0.5

0.7

0.5

5.0

9.0

9.5

1.5

2.0

5.0

1.9

1.25

1.1

21.5

18.8

12.5

17.5

6.5

1.6

1.65

4.75

4.5

5.0

4.25

3.75

0.8

0.78

1.5

0.83

0.8

1.0

BT6

BT15

BM2

BM1

BM42

12% Cobalt

12%Tungsten-

5% vanadium

18%Tungsten-

5% vanadium

Molybdenum

"562"

9% molybdenum

molybdenum-

8% cobalt

as more of the retained austenite changes to martensite. This second tem-

pering process also relieves internal stresses which have been set up as

austenite transformed to martensite during the first tempering process. In

the case of high-speed steel the initial quench from a high temperature

tends not to produce internal stresses, since an adequate cushion of soft

austenite is retained. The first tempering process causes much of the

retained austenite to transform to hard martensite and this may lead to the

introduction of internal micro-stresses.

The final structure of a correctly heat-treated high-speed steel will consist

of isolated globules of complex carbide, in a matrix of martensite which

contains sufficient dissolved alloying elements to render it sluggish to tem-

pering influences. The traditional type of 18-4-1 high-speed steel, when

correctly hardened, will retain a high hardness to about 550

C whilst the

more expensive cobalt steels will retain a similar hardness for a further

100

C (Fig. 14.3). Although high-speed steels are generally noted for their

use in machining operations under severe conditions, their high strength

at elevated temperatures makes them useful for hot-working tools of other

types,

eg hot shears and punches for the hot-forming of nuts. The wear

of such steels can be further improved by nitriding (19.40) after normal

heat-treatment has been completed.

TEMPERING TEMPERATURE (

Fig.

14.3 The relationship between hardness and tempering temperature for high-speed

steels.

(A) Represents the 12% cobalt type, and (B) the ordinary 18% tungsten type.

Cemented Carbide and Other 'Cermet' Cutting

Materials

14.20 Mention has already been made (6.50) of the production of cutting

tools by powder metallurgy, and though, strictly speaking, not ferrous

alloys, these materials will be discussed here because of their close associ-

ation with high-speed steels.

Cemented carbide tools are prepared by sintering at high temperature

and pressure, in an atmosphere of hydrogen, a mixture of tungsten carbide

HARDNESS

(VPN)

powder with powdered cobalt. Sometimes tungsten carbide is replaced by

powdered carbides of other refractory metals, tantalum, molybdenum,

titanium and also by silicon carbide. In each case cobalt (or some other

tough metal) forms a ductile bonding matrix between the carbide particles.

The resultant structure resembles that of a high-speed steel, in that

spherical particles of carbide are held in a tough matrix, but whereas

the properties of a high-speed steel are developed by heat-treament, the

structures of cemented carbide materials are produced by mechanical

methods.

14.21 In other 'cermets', which are sintered products similar in prin-

ciple to the cemented carbides mentioned above, the object is to combine

the high-temperature strength and hardness of suitable metallic oxides,

nitrides or borides with sufficient ductility provided by a metallic bonding

element, thus giving a material with considerable resistance to thermal and

mechanical shock and suitable for jet-engines or other high-temperature

applications as well as for cutting tools. Typical hard-phase/ductile-bond

cermets are shown in Table 14.2.

Table 14.2 Some Typical Cermets

Group

Carbides

Oxides

Borides

Hard Phase

Tungsten carbide, WC

Titanium carbide, TiC

Molybdenum carbide, M02C

Silicon carbide, SiC

Chromium carbide, Cr3C2

Aluminium oxide, AI2O3

Magnesium oxide, MgO

Chromium oxide, Cr2O3

Titanium boride, TiB2

Chromium boride, Cr3B2

Molybdenum boride, M02B

Bonding Metal or Alloy

Cobalt

Molybdenum, cobalt or nickel/

chromium alloy

Cobalt or nickel

Cobalt or chromium

Nickel

Cobalt, iron or chromium

Magnesium, aluminium,

iron,

cobalt or nickel

Chromium

Cobalt or nickel

Nickel

Nickel or nickel/chromium

Uses

Cutting tool bits.

Gauge blocks, slip gauges,

wire-drawing dies, jet-engine

parts

Throw-away' tool bits,

sparking-plug bodies, rocket

motor and jet-engine parts.

Cutting-tool tips.

Magnetic Properties and Materials

14.30 In the vicinity of either a magnetised material or an electrical con-

ductor carrying a current, a compass needle is acted upon by a couple

which tends to make it rotate. This couple is attributed to the presence

of a magnetic field of force surrounding the magnetised material (or the

current-carrying conductor). The strength of the

magnetic

field at any point

is denoted by H (Fig. 14.4). This magnetic field will give rise to magnetic

induction in any material upon which it acts and may be conveniently

produced either by a 'permanent magnet' or by a current-carrying solenoid

which, as Ampere illustrated, is equivalent to a series of magnets. The

quantity, magnetic induction, is denoted by B and is related to the applied

field strength by the equation:

B = \iH

where ^i is the magnetic permeability and is generally not constant for a

material. If the value of \i is high then a relatively smaller applied field,

//, is required to induce a given field, B, within the material.

Whilst iron is the metal generally associated with magnetism three other

transition metals—cobalt, nickel and the 'rare earth' gadolinium—are also

very strongly magnetic,

ferromagnetic as it is termed. Nevertheless other

elements possess weak magnetic properties. Many metals are paramag-

netic, that is they are very weakly attracted by a strong magnetic field.

Consequently a paramagnetic metal like chromium when placed in a mag-

netising field carries a slightly greater magnetic flux than would a corre-

sponding vacuum. Some metals, and all non-metals, are diamagnetic, that

is they are weakly repelled by a magnetic field. Thus diamagnetic copper

would carry a slightly smaller magnetic flux than the corresponding vacuum

Fig.

14.4 The magnetic flux in a material is B = \iH, where H is the magnetic field in the

solenoid and ^i is the permeability of the material.

Fig.

14.5 The three types of magnetism.

In diamagnetic substances like copper the atoms have no resultant moment whilst in para-

magnetic substances (like chromium) there is no resultant field although the individual atoms

have magnetic moments. In ferromagnetic materials domains are re-aligned during magnet-

isation.

Cu atom Cr atom

domain

boundary

Fc atoms

unmagnetised

(lii)ferromagnetic

movement of

domain boundary

(Bloch wall)

(ii) paramagnetic(i) diamagnetic

magnetised

under the influence of a magnetising field. Only those metals, alloys and

mixtures which exhibit ferromagnetism are useful as magnetic materials.

14.31 Ferromagnetism The relationship between an electric current

and a magnetic field was demonstrated by Ampere and it is now considered

that this magnetic field originates from the 'spin' of electrons. In a stable

atom or ion the electrons can be thought of as particles moving in orbitals

around the nucleus. At the same time each electron behaves as though it

were spinning on its own axis and it is this spin which generates the mag-

netic field.

Pauli's Exclusion Principle indicates that in a stable atom not more than

two electrons are able to occupy the same energy level. These two electrons

will possess opposite 'spins' so that in effect their fields cancel out (Fig.

14.6(i)). In a large number of a elements electrons are 'paired' in their

orbitals in this way so that such elements will be diamagnetic. Atoms of

some of the transition elements, however, have only partly-filled d sub-

shells (Table 1.4) so that unpaired electrons will be present. Consequently

in an atom more electrons will spin in one direction than in the other

(Fig.l4.6(ii)) and such an atom will have a magnetic moment. For this

reason the ferromagnetic elements and the strongly paramagnetic elements

are found in the transition groups in the Periodic Table. In the elements

iron, cobalt, nickel and gadolinium the magnetic moments are large enough

and the ions suitably spaced in the crystal structure so that when they

become magnetically aligned (during 'magnetisation') a powerful magnetic

field is produced.

Fig.

14.6 Unfilled 'd' sub-shells as the basis for ferromagnetism.

14.32 Magnetic properties are very dependent upon atomic spacing. If

the atoms are too far apart the forces between them are too small to resist

the effects of thermal agitation in throwing spins out of alignment so that

there is no resultant field. If the atoms are very close together then inter-

atomic forces are too great to permit alignment of spins. Favourable con-

ditions appear to be obtained when the atomic radius is between about 1.5

and 2.0 times the radius of the sub-shell containing the un-paired electrons.

Thus iron, cobalt, nickel and gadolinium fulfil these requirements whilst

other transition metals such as titanium, chromium and manganese are

only just outside the range. In fact when manganese contains sufficient

interstitially-dissolved nitrogen to increase its atomic spacing by an

adequate amount it becomes ferromagnetic. Similarly the interatomic spac-

ings in some ceramic materials like BaO.6Fe2C>3 make them suitable as

ferromagnetic materials. Modern magnetic alloys contain varying pro-

unfillcd

electron

position

portions of iron, cobalt, aluminium, nickel, copper, niobium, chromium

and titanium; whilst sintered ceramics contain boron, barium, samarium

and other 'rare earths' such as neodymium. Currently magnets based on

neodymium are replacing those containing samarium because of the

greater abundance of the former and its consequent lower cost.

14.33 Within the crystal of a ferromagnetic material are small regions

known as magnetic domains. These are about 0.01 mm in width and micro-

scopic examination using a colloidal suspension of magnetite (which be-

haves like minute iron filings) shows them to be of a shape as indicated in

Fig. 14.7. Inside each domain the electron spins are aligned, but since the

alignment varies from one domain to the next the resultant fields cancel

and the material appears un-magnetised. If an external magnetic field is

applied then the energy of these domains which are aligned against the

applied field is raised so that they begin to reverse direction. In this way

the favourably oriented domains grow at the expense of the others by a

movement of boundaries which separate them. The boundary zones (called

'Bloch walls') move so that domains become similarly oriented and a

resultant field is set up (Fig. 14.8)

If this alignment is retained when the magnetising field is removed the

material is said to be magnetically hard but in many cases the magnetism

is not retained and the material is said to be

magnetically

soft. Whether or

not a material is magnetically hard or soft depends to a large extent on its

physical condition. Magnetic hardness generally coincides with physical

hardness, possibly because residual strains oppose randomisation of

domains once the material has been magnetised. The presence of inclusions

or holes gives rise to regions of low energy and these tend to anchor Bloch

walls.

Since they are difficult to move the material is more difficult to

magnetise but, once magnetised, the magnetism is retained.

High magnetic 'coercivity' can also be promoted by using very small

needles which have a thickness less than the natural domain wall thickness

of the material. These needles are spontaneously magnetised and the direc-

tion of magnetisation cannot be changed because domain walls are absent.

Some of the modern permanent-magnet materials consist of suitably

aligned very small needles of iron which are then permanently embedded

in a plastics matrix.

Fig.

14.7 Magnetic domains in an

'unmagnetised' ferromagnetic

material.

Fig.

14.8 Re-alignment within a Bloch

wall.

Bloch

wall

Fig.

14.9 The effects of a magnetic field on magnetically 'soft' and

'hard'

materials. The

arrows represent domains.

When

ferromagnetic material

has

been magnetised

saturation

increase

temperature causes

corresponding increase

randomisation

so that

the

magnetism progressively decreases reaching zero

at the

Curie

point (769°C, 1120

and

358°C

for

iron, cobalt

and

nickel respectively)

when

the

material ceases

to be

ferromagnetic

and

becomes paramagnetic.

This change

reversible

cooling,

769°C

a-iron

^±

p-iron

14.34

permanent magnet

designed

retain

high intensity

magnetisation

for an

indefinite period provided

it is not

subjected

unusually high demagnetising influences.

The

actual magnetic strength,

flux, in

permanent magnet

dependent upon

its

design

and the

steel

other alloy from which

it is

made.

The

essential characteristics

perma-

nent-magnet materials

are:

(a) coercive force;

(b) residual magnetism

remanence;

Coercive force refers

to the

ability

of the

magnet

withstand demag-

netising influences. Remanence

is the

amount

magnetism remaining

the magnet after

the

magnetising field

has

been switched

off. The

energy

product value

is the

quantity

magnetic energy possessed

by the

magnet,

and

calculated from

the

demagnetising curve

(Fig. 14.10).

The mag-

netising force

expressed

ampere-turns

per

metre', whilst

the SI

unit

of induced flux density

is the

tesla (T), symbolised

by H

and

respectively.

14.35

will

now

consider what

known

as the

hysteresis loop.

Fig.

14.10 shows

typical hysteresis loop (PRUQST)

and

magnetisation curve

(OP)

for a

permanent magnet. When

piece

steel

placed

in a

solenoid

with

current passing through

it,

magnetic flux

induced

in the

steel.

The

amount

this flux

can be

measured

webers

(Wb) and the

flux density

in teslas

(T),

where

Wb = Vs and T =

Wb/m

Fig. 14.10 magnetic flux

unmagnctiscd

in magnetising

field

magnetising field

removed

(ii) magnetically 'hard' material.

(i) magnetically 'soft' material.