Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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aluminium, silicon and chromium, ionic mobility is low and since these

oxides tend also to be dense they offer good protection against further

oxidation. The first three of these elements tend to have deleterious effects

on the mechanical properties of steels in the quantities necessary to impart

corrosion resistance so that chromium—sometimes assisted by small

amounts of silicon—is used. These elements form barrier films of oxide

which adheres tenaciously to the surface.

13.72 The considerable amount of chromium required to effect oxida-

tion resistance unfortunately also favours rapid grain growth. Hence nickel

is often added to counteract this as in many other steels already dealt with

here.

However, nickel must be absent from those steels which are in con-

tact with atmospheres containing sulphur gases, since brittle grain boun-

dary films of nickel sulphide would be formed (21.20):

Ni-+!Ni

+! +2e~

S + 2e--+!S--j

nickel sulphide

13.73 The chromium present contributes to some extent to the high-

temperature strength by ordinary substitutional solid solution. However,

creep strength is generally increased by 'stiffening' the alloy with small

amounts of tungsten, molybdenum, vanadium, titanium, niobium, alu-

minium or nitrogen. Reduction in creep at high temperatures is then

achieved by dispersion hardening provided by the small particles (8.62) of

the carbides of niobium, tungsten, molybdenum, vanadium or titanium;

the nitrides of aluminium, titanium or niobium; and also NiAk.

Tungsten, molybdenum and cobalt also render steels resistant to temper-

ing effects at high temperatures as they do in high-speed steel (14.10), and

thus raise the limiting creep stress.

In some cases precipitation hardening can be used to strengthen the

alloy provided that at the working temperature, non-coherent precipitation

(9.92) does not occur since this would result in loss of strength and an

increase in brittleness. Generally, steels working at high temperatures are

either austenitic or austenitic/ferritic in structure so that brittle martensite

will not form each time the component cools, otherwise cracking in the

martensitic areas would be likely due to internal stress arising from non-

uniform cooling.

13.74 Short-time creep tests (5.72) are often employed in evaluating

heat-resistant alloys. Design is frequently based on the stress which will

produce a creep rate of 0.0001% per hour (1% extension in the gauge

length in 10 000 hours) at the maximum operating temperature. Safe work-

ing stresses are often based on 50% of this stress. However, such a value

can only be applied under conditions of direct axial static loading and

relatively uniform temperature. When impact loading or rapid temperature

changes are involved higher safety factors must be used, ie

33V3%

or even

25%

of the creep stress as derived above. The compositions and properties

of some typical heat-resisting steels are given in Table 13.6.

Manganese Steels

13.80 Although the CIS is the largest producer of manganese, South

Africa also mines considerable quantities of the metal, followed by Brazil,

Gabon, Australia, India, Mexico, China and Ghana. The bulk of Britain's

imports are from South Africa, Brazil and Ghana. Interest is currently

focused on the large deposits of ferromanganese nodules which occur in the

main basins of the Pacific, Atlantic and Indian Oceans at depths between 4

and 5 km. The origin of these nodules appears to be biological as well as

chemical.

13.81 Manganese is very widely used in steel production both for deoxi-

dation and desulphurisation of the molten steel. It is generally added as

ferromanganese (80Mn; 6C; bal.Fe) or as spiegeleisen (5-20Mn; 5C;

bal.Fe) and eliminates most of the oxygen and sulphur:

FeO + Mn

—»

Fe 4- MnO (volatilises or joins the slag)

FeS + Mn

—>

Fe + MnS (insoluble in iron—so joins slag)

In order that these reactions proceed as shown an excess of manganese

must be added and the excess remains in the steel. Such amounts are

usually less than 1.0%, and it is only when the manganese content exceeds

this amount that it is regarded as a deliberate alloying element. Of recent

years,

however, there has been a tendency to increase the manganese

content of ordinary carbon steels at the expense of a little carbon, thus

giving improvements in respect of ductility and Izod values, in both the

normalised and heat-treated conditions. The susceptibility of low-carbon

steels to brittle fracture is reduced by raising the manganese carbon ratio,

using up to 1.3% manganese (5.53). Similarly free-cutting steels contain

up to 1.7% manganese so that sulphur will be present as manganese sul-

phide (MnS) globules which aid machining (6.63).

CARBON

fpfo)

MANGANESE ftb)

Fig.

13.12 A Guillet-type diagram illustrating the relationship between manganese and the

structure and uses of low-manganese steels.

AUSTENlTE + CEMENTITE

MARTENSITE +

BAINITE + CEMENTITE

PEARLITE

plain

carbon

tools

safe limit

for

water quench

rails

optimum strength and toughness

after normalising

structural

steels

forgings

shafting

plates

sheets

boiler

plates

carburising

steels

13.82 Like nickel, manganese stabilises austenite by raising the A

tem-

perature and depressing the A

temperature; but, unlike nickel, manganese

has a very powerful stabilising effect on carbides without itself being a very

strong carbide former. It does however form stable Mn

C. Manganese also

has a considerable strengthening effect on the ferrite and also increases

the depth of hardening to a useful degree. Despite these advantageous

properties, the low-carbon, low-manganese steels are not widely used,

though manganese is used to some extent to replace nickel in low-alloy

steels.

13.83 As recorded at the beginning of this chapter, the development

of high-manganese steel in 1882 by Sir Robert Hadfield was one of the

landmarks in the history of alloy steels. Hadfield's steel contained approxi-

mately 12.5% manganese and 1.2% carbon, and, both as far as compo-

sition and heat-treatment are concerned, this is basically the same steel as

that used to-day. It is austenitic in type, though if the steel is allowed to

cool slowly to room temperature some precipitation of carbides will take

place. The steel is therefore water-quenched from 1050

C in order to keep

the carbon in solution.

Being austenitic, high-manganese steel is extremely tough and shock-

resistant, and although relatively soft, it nevertheless wears extremely well.

The reason for this apparently paradoxical situation is the mechanical dis-

turbance causes the surface layers to become extremely hard so that the

resultant component has a soft, though shock-resistant core and a very

hard, wear-resistant case. Abrasion of the surface will cause the Brinell

figure to rise from 200 to 550. The reason for this phenomenon is still

uncertain. Some claim that mechanical disturbance of the austenite causes

martensite to form at the points of high stress concentration, whilst others

have suggested that the cause is simply work-hardening of the austenite.

There is little positive evidence of martensite formation and it seems most

probable that strain-hardening occurs as a result of mechanical dis-

turbance.

High-manganese steel is available as castings, forgings or hot-rolled sec-

tions,

but is very difficult to machine because of its tendency to harden as

°c

CARBON (<Yo)

Fig.

13.13 The effect of manganese in displacing the eutectoid point in carbon steel. Like

nickel,

manganese depresses both /\i and A

temperatures.

Table

13.7

Manganese

Steels

Uses

Automobile

axles, crankshafts,

connecting rods, etc., where a

cheaper steel is required.

Automobile

and general

engineering where a carbon steel

not completely satisfactory, but

where the expense of a

nickel-chromium steel is not

justified.

Plug gauges; thread and precision

gauges; blanking dies; press tools;

reamers; taps; plastics moulds;

cams; general purpose low-cost,

oil-hardening steel.

Jaw-crushing

machinery;

liner

plates for ball- and rod-mills, parts

for

gyratory crushers.

Dredging

equipment;

tumblers,

buckets,

bucket lips, heel plates.

Track

work;

points and crossings.

Digger teeth, crane track wheels,

elevator chains, axle-bar liners,

bottom

plates for magnets and

where non-magnetic properties are

required.

Japanese Nonmagne

30—an

austenitic steel of low magnetic

permeability. Used in development

nuclear fusion reactors &

magnetic levitation railways.

Heat-treatment

Oil-quench

from

860

(water-quench for

sections

over 38 mm dia.).

Temper as required.

Oil-quench from

840-860

temper at

550-650

and cool in oil

air.

Oil-quench

from

770

temper at

200-300

Finish by quenching from

1050

to keep the

carbides in solution.

Heat at

700

for 2 h and

then water-quench

Typical mechanical properties

Brinell

620

Case-550

Core-200

190

Izod

(J)

145

Reduc-

tion

area

(%)

Elonga-

tion

(%)

Tensile

strength

(N/mm

)

587

711

849

835

Yield

point

(N/mm

)

355

510

345

Compo-

sition

(%)

Condition

Normalised

28.5 mm bar,

oil-quenched and

tempered at

600

Hardened and

tempered

Water-quenched

from

1050

Water-quenched

1.5

2.0

12.5

14.0

Cr-2.0

Ni-2.0

0.25

0.35

1.0

1.2

0.6

RS. 970

desig-

nation

150M28

150M36

soon as this is attempted. Despite this drawback, no really suitable substi-

tute has been found for Hadfield steel, for applications where toughness

combined with resistance to conditions of extreme abrasion are required,

eg rock-crushing machinery, dredging equipment and track-work points

and crossings. The austenitic core can be further toughened by the addition

of carbide-forming elements, such as chromium and vanadium.

Some manganese steels are shown in Table 13.7.

Steels Containing Tungsten

13.90 Up to the middle of the eighteenth century the name 'tungsten' (or

'heavy stone') implied the mineral scheelite, but in 1781 the renowned

chemist Scheele showed that scheelite contained a peculiar acid which he

called tungstic acid. Two years later metallic tungsten was isolated by J. J.

y Don Fausto d'Elhuyar. To-day tungsten is mined in many countries, the

leading producers being China, the CIS, Bolivia, South Korea, the USA,

North Korea, Thailand, Australia, Portugal and Canada. The bulk of

British imports are from Portugal. The principal ore of tungsten has always

been known as 'wolfram' but IUPAC (International Union of Pure and

Applied Chemistry) suggests that in future this name should be used to

describe the metal

itself.

However, in this edition we have decided to retain

the original name of 'tungsten'.

13.91 Tungsten dissolves in both y- and a-iron but, having a BCC

structure, tends to stabilise a (ferrite). It therefore raises the A

tempera-

ture and, like chromium, forms a closed y-loop in the phase diagram.

Unlike chromium, however, it inhibits grain growth and therefore has a

grain-refining effect. It also reduces decarburisation during working and

heat-treatment. Both of these features are useful since high temperatures

are involved during the heat-treatment of all tungsten steels.

Tungsten has a high affinity for carbon forming the extremely hard

and very stable carbides W2C and WC and, in steel, a double carbide

Fe4W2 C. These carbides dissolve very slowly in steel when the latter is

heated, and then only at very high temperatures. Once in solution the

dissolved tungsten renders transformations extremely sluggish. Therefore,

when successfully hardened, steels containing tungsten have a great resist-

ance to tempering conditions and can be heated in the range 600-700

before carbides begin to precipitate and softening sets in. For this reason

tungsten is an important constituent of most high-speed steels (14.10) and

hot-working die steels in which it develops high-temperature ('red') hard-

ness following suitable heat-treatment.

13.92 Since the carbides of tungsten are so hard, tungsten is commonly

used in small amounts in other tool steels (Table 13.8). It is also used in

heat-resisting steels and alloys in which it assists in raising the creep

strength at high temperatures.

Plate 13.2 13.2A High-manganese steel (12.5% manganese; 1.2% carbon).

Water quenched from 1050

C. Austenite. x 100. Etched in 2%

nital.

(Courtesy of Messrs

Hadfields Ltd, Sheffield)

13.2B High-speed steel (18% tungsten; 4% chromium; 1% vanadium).

Air quenched from 1250

C. Carbide globules in an austenite/martensite matrix, x 500.

(Courtesy of Messrs Edgar Allen & Co Ltd, Sheffield)

13.2C High-speed steel (18% tungsten; 4% chromium; 1% vanadium).

Air quenched from 1250

C and then 'secondary hardened' at about 600

C. Austenite is

transformed and the matrix etches darker, x 500. (Courtesy of Messrs Edqar Allen & Co

Ltd, Sheffield).

13.2c

13.2B.

13.2A.

Table

13.8

Steels Containing Tungsten (Other than

High-speed

Steels)

Uses

Dies; stay-taps; delicate broaches;

milling

cutters; plugs;

gauges;

circular

cutters;

fine press tools

and

master tools

Extrusion

dies, mandrels

and

noses for

aluminium

and

copper alloys.

Hot-forming, piercing, gripping

and

heading tools. Brass-forging

and

hot-pressing dies. Other hot-working

operations.

Hand

and

pneumatic chisels, caulking

tools

and

general boilermaker's tools;

punches; dies; blanking tools

and

shear

blades. Coal-cutter picks.

Cold-heading

and nail-making dies. Engraving

punches; hot-blanking tools.

Hot-forging

dies.

Heat treatment

Oil-quench

from

790

and

temper

200-250°C

Pre-heat

800

soak

and

then heat

quickly

1020

then air-cool.

Temper

for

1V2

hours

at 540-620

C-

Oil-quench

from

880-920

temper

200-300

for cold-working tools;

temper

at 400-600

C for

hot-working

tools.

Composition

(%)

Other

elements

0.85

1.20

0.4

0.5

1.35

2.25

1.9

0.4

0.2

1.5

0.75

0.5

5.0

1.1

1.8

0.2

1.0

0.6

1.0

0.9

0.35

0.5

0.53

4659

desig-

nation

BO1

BH12

Type

steel

Tool steel

Hot-working

die

steel

Shock-resisting

hot-

and cold-working tool

steel

Hot-extrusion

mandrels for steels and

non-ferrous

metals. Hot-forging tools. Die

holders.

Zinc die-casting dies.

Extrusion

dies. Hot-piercing punches.

Brass-stamping dies. Die-casting dies.

Hot-forging

dies and punches for making

bolts,

nuts,

rivets

and similar small

components where tools reach high

temperatures. Hot-forging dies for

copper alloys. Extrusion dies, mandrels,

pads and liners for the extrusion of

copper alloys. Die-casting dies for

copper alloys and for pressure

die-casting of aluminium alloys.

Pre-heat to

750

and then to

1000

Quench in oil.

Temper

650

for 30

minutes.

Pre-heat to

850

and then to

1150

Quench in oil. Temper at

550-680

Pre-heat to

850

and then heat

rapidly

1150-1200

oil-quench (or

air-cool

small sections); temper for 2-3

hours

600-700

Ni 2.25

Co 4.25

0.4

2.25

4.25

10.0

0.3

2.25

0.35

0.4

0.5

0.4

0.5

0.85

4.25

2.85

3.75

0.3

0.2

0.3

0.28

0.4

0.3

BH19

BH21

Hot-working

tool steel

Hot-working die steel

Steels Containing Cobalt

13.100 Much of the World's supply of cobalt is mined in Zaire whilst

significant amounts are produced in Morocco, Zambia and Canada.

Britain's main sources of supply are Zaire and Zambia.

Cobalt is chemically very similar to nickel and the ores of these metals

are generally associated with each other. Until recently cobalt was used

principally in permanent-magnet alloys (14.36) and in 'super' high-speed

steels (14.11). In the latter it gives a useful increase in red-hardness by

promoting extreme sluggishness in transformation.

13.101 Cobalt is an essential constituent of most 'maraging' steels

developed since the late nineteen-fifties. These are high-strength steels an

important group of which contains 18Ni; 8-12Co; 3-5Mo and small

amounts of titanium and aluminium. Carbon is present in very small

amounts.

If such a steel is solution treated at 820

C to absorb precipitated com-

pounds, uniform austenite is produced. On cooling in air, an iron-nickel

martensite is formed, but unlike ordinary tetragonal martensite (12.22)

this is of a lath-like BCC form and is much softer and tougher than ordinary

martensite. If the structure is now 'age-hardened' at 480

C for three or

more hours coherent precipitates of intermetallic compounds such as (Ti,

Al or Mo)Ni

are formed and a high tensile strength up to 2400N/mm

developed. Although the crystal structure of TiNi

is basically hexagonal

there is a high degree of 'matching' between its (0001) planes and the (110)

crystallographic planes of the martensite matrix (Fig. 13.14). Consequently

Fig.

13.14 Illustrating the 'near-matching', and hence coherency, of the (0001) planes of

TiNi

with the (110) planes of iron-nickel martensite.

(HO)planes of martensite

(OOOD-or basal-plane of

TiNi

Table

13.9 Maraging

Steels

Containing Cobalt

Typical Mechanical Properties

Charpy Impact

(J)

Elongation

(%)

7.5

Tensile

strength

(N/mm

)

1465

1775

1965

2460

1730

0.2%

Proof

Stress

(N/mm

)

1430

1740

1930

2390

1650

Heat

treatment

Solution treated at 820

C for one

hour,

air cooled and then aged at

480

C for three hours.

Solution treated at 820

C for one

hour,

air-cooled

and aged at

480

C for twelve hours.

Solution treated at 1150

C for four

hours,

air-cooled and aged at 480

for

three hours.

Composition (%)

0.01

0.1

0.15

0.05

0.2

0.4

0.6

1.8

0.3

3.75

4.6

8.5

12.5

17.5

Type

18NM400

18Ni1700

18NM900

18Ni2400

17Ni1600

(cast)