Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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Table

13.3b

High

nickel-chromium

stainless

steels

Uses

Low-cost

ferritic

stainless iron—

beet sugar industries, coal mining,

etc

Particularly

suitable for domestic

and decorative purposes.

'weld-decay'

proof

steel

(fabrications may be used in the

as-welded condition). Used

extensively

in plant for the

manufacture of

nitric

acid. An

austenitic steel.

deep-drawing austenitic alloy—

suitable for table-ware and kitchen

equipment.

low-carbon alloy suitable for

deep-drawing.

Used

mainly for

domestic purposes, etc.

Typical mechanical properties

Heat-

treat-

ment

Brinell

160

170

180

225

160

Izod

(J)

143

104

Reduc-

tion area

(%)

Elonga-

tion (%)

Tensile

strength

(N/mm

)

550

618

896

649

803

579

618

Yield

point

(N/mm

)

335

278

803

278

402

232

Condition

Soft

Softened

Cold-rolled sheet

Softened

Cold-rolled

Softened

Composition (%)

Other

elements

Ti 0.25

Si 0.5

Ti 0.6

11.5

18.0

12.5

18.5

0.75

8.5

12.5

10.0

0.75

0.8

<0.03

0.05

0.03

BS 970 (Pt 4)

designation

302S25

321S20

304S15

Immune from weld-decay,

due to

the stabilising effect

niobium.

Used

welded

plant

where

corrosive

conditions

are

very

severe,

eg in

nitric acid plant

Molybdenum improves

the

corrosion-resistance

such

reagents

sulphuric

and

phosphoric

acids, chloride

solutions

and

organic

acids, such

acetic

acid.

free-cutting stainless steel

which

the

machinability

increased

by the

presence

manganese

and

sulphur.

Fittings

marine

environments.

Chemical

process plant-pumps,

valves;

etc.

(Nitronic

50)

•o

Very hostile

conditions,

eg, power-

station

condenser tubes

using

sea

water

ferritic

steel

austenitic

steel

175

180

175

104

618

587

1690

620

263

278

232

515

275

Softened

Cold-worked

Soft

1.0

2.75

0.6

0.3

0.6

0.25

2.2

0.2

N 0.3

3.5

18.0

17.5

18.0

21.2

27.5

10.0

13.0

8.5

12.5

1.2

0.8

1.5

5.0

0.04

0.05

0.04

<0.025

347S17

320S17

325S21

precipitation of chromium carbide particles leads to impoverishment of the

surrounding austenite in chromium, so that electrolytic action (21.30) is

induced between the chromium-depleted region and the adjoining chro-

mium-rich regions. It is therefore necessary to heat the alloy to 1050

and then quench it, in order to retain the carbon in solid solution. Carbide

precipitation leads to a defect usually known as 'weld-decay' in fabricated

articles (20.93). This defect may be reduced by adding small amounts of

titanium, molybdenum or niobium; elements which have a strong carbide-

forming tendency and thus 'tie up' most of the carbon as carbides at a high

temperature, so that this carbon is no longer available to form chromium

carbide during slow-cooling following a welding process. In this way any

variation in chromium content across the microstructure is avoided and

post-welding treatment is unnecessary.

13.47 High-strength Stainless Steels The development of the aero-

space industries created a demand for high-strength stainless steels and

these alloys are now finding use in many other fields. In some austenitic

stainless steels part of the nickel content has been replaced by manganese

to reduce cost, manganese like nickel stabilising austenite. These steels are

sometimes strengthened by the addition of nitrogen which, unlike carbon,

is less likely to introduce corrosion problems. Steels of this type are further

strengthened by cold-work. In the USA the 'Nitronic' austenitic steels are

in this class. 'Nitronic 33' contains 18Cr; 12Mn; 3.2Ni; 0.32N and 0.05C

and has roughly double the yield strength of a conventional 18-8 stainless

steel in the annealed form. Examination of this composition in relation to

the Schaeffler diagram (Fig. 13.7) indicates that it will remain completely

austenitic after welding and that the formation of martensite will be

unlikely. 'Nitronic 50' contains 21.2Cr; 12.5Ni; 5Mn; 2.2Mo; 0.2Nb; 0.2V;

0.3N and 0.04C. It has a high resistance to corrosion and after cold-work

attains a tensile strength of 1690 N/mm

Some stainless steels are precipitation hardened and may be either mar-

tensitic or austenitic in structure. Martensitic steels of this type usually

contain 17Cr; 4Ni; 4Cu and small amounts of molybdenum, aluminium or

niobium. They are solution treated at about 1000

C and then air-cooled to

give a martensitic structure. Subsequent precipitation treatment at 500-

600

C results in the formation of coherent precipitates based on compounds

of nickel with molybdenum, aluminium or niobium which further

strengthen the martensitic structure. Austenitic steels of this type contain

17Cr; 10Ni and up to 0.25P. These are precipitation hardened at 700

C to

produce coherent precipitates based on carbides and phosphorus com-

pounds which will introduce lattice strain and inhibit the movement of

dislocations.

The compositions, properties and uses of some of the more important

nickel-chromium steels are given in Tables 13.3a and b.

Steels Containing Molybdenum

13.50 The USA is the dominant producer of molybdenum, much of her

output being a by-product of several copper mines; Canada, Chile, CIS

and China are other significant producers. At the end of last century practi-

cally the whole of the world production of molybdenum was used in making

the chemical reagent ammonium molybdate for the analytical determi-

nation of phosphorus in iron, steel, and fertilisers. Today the principal use

of molybdenum is in the manufacture of alloy steels.

As mentioned earlier in this chapter (13.42), one of the main uses of

molybdenum is to reduce the tendency to 'temper-brittleness' in low-

nickel, low-chromium steels. Additions of about 0.3% molybdenum are

usually sufficient in this respect, and the resultant steels retain a high

impact value, irrespective of the rate of cooling after tempering.

Among alloy steels the 'nickel-chrome-moly' steels possess the best all-

round combination of properties, particularly where high tensile strength

combined with good ductility is required in large components. These steels

are much less influenced by the mass effects of heat-treatment, the transfor-

mation rates of the nickel-chromium steels being still further reduced by

the presence of molybdenum, which therefore contributes considerably to

depth of hardening.

13.51 Molybdenum is also added to chromium steels to produce a gen-

eral improvement in machinability and mechanical properties, whilst nick-

el-molybdenum steels are very suitable for case-hardening. Molybdenum

dissolves in ferrite which it strengthens considerably and also forms a hard,

stable carbide M02C as well as double carbides such as Fe

Mo2C and

Fe2iMo2C6. Small amounts of molybdenum are very effective in reducing

the transformation rates, particularly austenite

—»

pearlite, ie the nose of

the TTT curve is displaced considerably to the right. Molybdenum raises

the high-temperature strength and creep resistance of high-temperature

alloys and also enhances the corrosion resistance of stainless alloys particu-

larly to chloride solutions. In modern high-speed steels (14.10) much of

the tungsten has been replaced by molybdenum. Molybdenum-bearing

steels are listed in Table 13.4.

Steels Containing Vanadium

13.60 In 1801 Del Rio found a new metal in a Mexican ore. He called it

'erythronium'—from the Greek erythros meaning 'red '—because it

formed red compounds. Later, Sefstrom discovered what he thought was

a new metal in some Swedish iron ore and named it 'vanadium' (after

the Scandinavian goddess, Vanadis) but almost immediately the eminent

chemist Wohler identified Sefstrom's vanadium as being Del Rio's

erythronium. To-day South Africa is the leading producer of the metal,

followed by the USA, CIS, Finland and Norway.

13.61 Plain vanadium steels are used only to a limited extent, but

Table 13.4

Steels

Containing Molybdenum

Uses

substitute for more highly

alloyed

nickel-chromium

steels.

Railway

engineering:

connecting-rods, spring hanger

and slide-bar bolts, draw

gears, etc.

General

engineering:

parts of

harvesters, dredging

machines, crushers, rods for

well-boring. Has very good

fatigue-

and shock-resisting

properties.

Suitable for crank-shafts and

connecting-rods,

stub axles,

etc.

As a substitute for 3%

nickel steel (it machines more

readily and is

free

from

temper

brittleness). Zinc die-casting

dies.

Heat-treatment

Oil-quench

from

830-850

temper

550-650°C

and

cool

in oil or air.

Oil-quench

from

860-900

temper

600-650

cool

air.

Oil-quench from

840-860

temper

500-700

and

cool

in oil or air.

Typical

mechanical properties

Reducn.

Izoc

area

Brinell

293

20?

179

311

201

(J)

(%)

Elong-

ation

(%)

Tensile

strength

(N/mm

)

1010

700

617

1080

695

Yield

point

(N/mm

)

800

525

463

895

525

Ruling

Section

(mm)

150

28.5

150

Condition

Oil-quenched

and tempered

600

Oil-quenched

and

tempered

Oil-quenched

and tempered

600

Composition (%)

0.27

0.25

0.3

1.1

0.55

1.6

0.65

0.35

0.2

0.4

970

desig-

nation

605M56

785M19

709M40

Type

Steel

Mn-Mo

steel

Ni-Mn-

steel

1%Cr-

steel

Automobile

and

general-engineering

components requiring

tensile

strength

700-1000

N/mm

Differential shafts, crankshafts

and other highly-stressed parts

where fatigue

and

shock-resistance

are

important.

In the

lightly

tempered condition

suitable

for

automobile

and

machine-tool

gears.

(It can be

surface-hardened effectively

cyanide bath.)

Parts

thin section where

maximum ductility

and

shock-resistance

are

required,

eg connecting-rods, inlet

valves, cylinder studs,

valverockers.

air-hardening steel

for

aero-engine

connecting-rods,

valve mechanisms, gears

differential

shafts

and

other

highly-stressed parts.

Can be

further

surface-hardened

cyanide

carburising.

High creep stress

up to

400

pressure vessels; heat

exchanger; reactor vessels;

gas-turbine

and

power-plant

components.

Oil-quench

from

830-850

temper

550-660

cool

air.

Oil-quench

from

830-850

'light

temper

180-200

'full'

temper

550-650

cool

oil or air.

Oil-quench

from

820-840

temper

550-650

cool

oil or air.

Air-harden

from

820-840

temper

150-200

C, and

cool

in air.

Normalise

930-980

stress-relieve

590-610

for

hours.

291

"200

440

223

310

250

440

1005

690

1544

770

1080

850

1700

571

895

525

1470

590

895

680

1470

432

28.5

150

28.5

150

100

150

63.5

Oil-quenched

from

850

and

tempered

600

Oil-quenched

and tempered

200

Oil-quenched

and tempered

600°C

Oil-quenched

and tempered

600

Air-hardened

and tempered

200

Normalised

950

then

stress-relieved

for

hours

600

0.2

0.3

0.25

0.5

0.45

1.1

0.8

1.25

0.03

0.75

1.5

3.25

4.25

1.35

0.55

0.6

0.45

0.6

0.4

0.3

0.10

945M38

817M40

830M31

835M30

/4%

Mn-Ni-

Cr-Mo

steel

/2%

Ni-Cr-Mo

steel

Ni-Cr-Mo

steel

/4%

Ni-Cr-Mo

steel

/2%

Mo-boron

steel

chromium-vanadium steels containing up to 0.2% vanadium are widely

used for small and medium sections. The mechanical properties resemble

those of low nickel-chromium steels but usually show a higher yield stress

and percentage reduction in area. Chromium-vanadium steels are also

easier to forge, stamp and machine, but are more susceptible to mass

effects of heat-treatment than corresponding nickel-chromium steels of

similar section.

13.62 Like nickel, vanadium is a very important grain refiner in steels

and as little as 0.1% is effective in restricting grain growth during normal

hardening processes. Vanadium is present in the microstructure as finely

dispersed carbides and nitrides and since these do not dissolve at normal

heat-treatment temperatures, their presence acts as a barrier to grain

growth. If the steel is over-heated so that the particles of carbide and

nitride go into solution, then grain growth immediately takes place rapidly.

13.63 Vanadium has a strong carbide-stabilising tendency and forms

the carbide VC. It also stabilises martensite and bainite on heat-treatment

and increases the depth of hardening. Since it combines readily with oxygen

and nitrogen it is often used as a 'scavenger' or 'cleanser' in the final stage

of deoxidation to produce a gas-free ingot. One of the most important

effects of vanadium is that it induces resistance to softening at high tem-

peratures provided that the steel is first heat-treated to absorb some of the

vanadium carbide into solid solution. Consequently vanadium steels are

used for hot-forging dies, extrusion dies, die-casting dies and other tools

operating at elevated temperatures. Most high-speed steels (14.10) contain

vanadium, sometimes in amounts up to 5.0%. Some steels containing vana-

dium are shown in Table 13.5.

Heat-resisting Steels

13.70 Steels required for service at high temperatures are used for such

components as exhaust valves for aero-engines; racks for enamelling

stoves; conveyor chains; furnace arch and floor plates; recuperator tubes;

rotors for gas turbines; annealing boxes; pyrometer sheaths; retorts; super-

heater element supports; burner nozzles; etc. The main requirements of

such steels are:

(i) resistance to oxidation by gases in the working atmosphere;

(ii) a sufficiently high strength (creep resistance) at the working tem-

perature

(iii) freedom from structural changes at the working temperature such

as the formation of grain-boundary precipitates which would cause

embrittlement.

13.71 The surface of iron scales readily at temperatures above 700

and the layer of oxide produced adheres only loosely to the surface. More-

over the mobility of metallic and non-metallic ions through the oxide film

is high (21.21) and increases with temperature, resulting in continual

destructive oxidation. In a number of oxides, notably those of beryllium,

Table

13.5

Steels

Containing

Vanadium

Uses

Cold-drawing dies,

etc.

Coining

and

embossing dies

(12.70—Part

Die-casting dies

for

zinc-base alloys.

Hot-forging dies

for

steel

and

copper

alloys

where excessive temperatures

are

not

reached. Extrusion dies

and

mandrels

for

aluminium alloys.

Pressure

and

gravity dies

for

aluminium

die-casting.

Fine press

tools.

Deep-drawing

dies

and

forming dies

for

sheet metal. Hobbing

dies. Thread-rolling dies. Wire-drawing

dies

and

wortle plates.

Blanking

dies,

punches

and

shear blades

for

hard

metals.

Cold-heading dies.

Refractory

moulds and other uses where

resistance

abrasion

necessary.

Heat-treatment

Water-quenched from 850

Temper

required.

Pre-heat

700

and

then

oil-quench

from

850

Temper

required.

Pre-heat

850

and

then

1000

C; soak

for 10-30

minutes

and

air-cool; temper

550-650°C

for 2

hours.

Pre-heat

850

and

then

1000

C; soak

for 14-45

minutes

and

quench

in oil or

air; temper

200-400

for

30-60

minutes.

Pre-heat

800

and

then

1030

C air-cool; temper

180-450

for 2-3

hours.

Water-quench from 770

Temper

150-300

Oil-

air-quench from 940

Temper

150-220

Hard-

ness

(VPN)

575

600

375

800

700

825

Composition

(%)

Condition

Hardened

and

tempered

Hardened and—

(a)

Tempered

550

(b)

Tempered

650

Hardened and—

(a)

Tempered

200

{b)

Tempered

400

Hardened

and

tempered

150

0.2

0.45

1.0

0.5

0.25

3.6

4.5

1.35

1.5

1.0

0.8

0.4

1.1

0.75

5.0

13.0

12.5

0.4

5.25

0.25

0.8

0.3

0.2

0.4

0.25

1.0

0.2

0.3

0.7

1.0

0.57

0.4

0.35

1.6

1.9

1.4

2.3

Type

steel

/4%Vsteel

/4%

Cr-V

steel

Cr-Mo-V

steel

High

C-high Cr

steel

Vanadium

tool steel

Cr-5% V

steel

Table

13.6 Heat-resisting Steels

Typical

uses

Steam-turbine and other

castings.

creep-resisting steel for

turbine castings working at high

pressures.

Conveyor chairs and skids,

heat-treatment boxes,

recuperator tubes, exhaust

valves

and other furnace parts.

Maximum

working

tempera-

ture

CC)

450

510

820

Typical mechanical properties at various

temperatures

Elonga-

tion

(%)

Tensile

strength

(N/mm

)

463

510

Yield

stress

(N/mm

)

278

309

Testing

tempera-

ture

CC)

Condition

cast

Forged or rolled

Composition (%)

Other

elements

Mo 0.5

0.25

25.0

0.4

20.0

0.8

0.6

0.3

0.2

0.1

0.15

Relevant

specifi-

cations

BS 3100:

1398

Grade

BS 3100:

1398

Grade

AISI

Series

311

low-cost, heat-resisting steel

used mainly for valves.

valve steel which can be

precipitation-hardened.

850

1001

909

816

339

1001

847

755

616

385

0.2% Proof

stress

801

708

585

185

585

447

385

323

216

200

400

600

200

400

600

800

Oil-quenched

1050

tempered

800

Water-quenched

1200

then

air-cooled

from

800

(Precipitation

treatment)

Nb 2.5

N 0.45

4.0

8.0

21.0

0.5

9.0

3.5

0.2

0.45

0.5

BS 970

(Pt.

4):

401S45

970

(Pt.

4):

352S52

fairly

cheap

grade of

heat-resisting steel with a good

combination of properties.

Oil- and gas-fired installations

—heat-treatment furnaces,

nozzle burners, salt pots,

super-heater supports.

Used

the

cast, forged or rolled

form.

casting alloy. Not suitable for

use when highly sulphurous

gases are present. Useful for

conditions

involving cyclic

thermal shock.

casting alloy suitable for

carburising and quenching

equipment.

wrought steel. Hearth plates,

trays

and muffles.

Quenching baskets, etc. Good

wear-resistance at high

temperatures, hence used in

cement-production equipment.

1000 (air)

950 (flue

gases)

1050

1100 (air)

1100

1200

698

543

527

465

248

155

48.3

15.4

11.0

69.0

20.7

15.9

45.5

15.4

11.7

543

0.1%

Proof

stress

347

262

248

217

139

1.0%

Proof

stress

270

200

400

600

800

900

800

1000

1050

800

1000

1050

800

1000

1050

Air-cooled

from

1100

cast

Quenched

from

1100°Cto

retain

carbides

solution

cast

Co 50.0

11.0

12.5

35.0

60.0

20.0

19.0

23.0

15.0

13.0

25.0

28.0

1.0

0.8

1.0

0.8

1.5

1.2

2.0

0.8

0.1

0.3

0.45

0.35

0.07

0.05

AISI

Series

302B

AISI

Series

309

BS 1501

(Pt.

3):

310S24; AISI

Series 310