Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy

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of the main advantages of cyanide-hardening is that pyrometric control is

so much more satisfactory with a liquid bath. Moreover, after treatment

the basket of work can be quenched. This not only produces the necessary

hardness but also gives a clean surface to the components. The process is

particularly useful in obtaining shallow cases of 0.10-0.25 mm, though

case depths up to 0.5 mm are used. Salt-bath carburising is used mainly

for small parts requiring a shallow case depth. It is an economical process

since the rate of heating is rapid due to the high heat-capacity of the liquid

bath and the quick transfer of heat to the work. This reduces the total time

of treatment.

It is believed that sodium cyanide, NaCN, is oxidised at the surface of

the molten bath:

The sodium cyanate, NaCNO, thus formed diffuses into the bath and

decomposes at the surface of the steel:

dissolves in steel

The carbon atoms dissolve interstitially in steel as they do in pack carburis-

ing, but in this process they are accompanied by nitrogen (which is released

in the suitable atomic form). Nitrogen also increases the hardness of the

case by forming nitrides as it does in the nitriding process (19.40).

Cyanides are, of course, extremely poisonous, and every precaution

must be taken to avoid inhaling the fumes from a pot. Every pot should

be fitted with an efficient fume-extracting hood. Likewise the salts should

in no circumstances be allowed to come into contact with an open wound.

Needless to say, the consumption of food by operators whilst working in

the shop containing the cyanide pots should be absolutely forbidden. Dis-

posal of cyanide wastes is also a problem. Effluents containing even small

quantities of cyanides will destroy bacteria used in purifying sewage, conse-

quently the disposal of these wastes must be rigorously controlled.

19.23 Gas Carburising is carried out in both batch-type and continu-

ous furnaces (13.20—Pt. 2) and in recent years has become by far the most

popular method for mass-production carburising, particularly when thin

cases are required. Not only is it a cleaner process but the necessary plant

is also more compact for a given output. Moreover the carbon content of

the case can be controlled more accurately and easily than in either 'solid'

or 'liquid' carburising.

The work is heated at about 900

C for three to four hours in an atmos-

phere containing gases which will deposit carbon atoms by decomposition

at the work piece surfaces. This atmosphere is generally based on the

hydrocarbons methane ('natural gas') CH

; and propane, C

, which are

either partially burnt in the furnace or are diluted with a 'carrier' gas in

Plate 19.1

19.1

A Cross-section through a carburised bar with a case depth of approx

x 3. Etched in 5%

nital.

19.1

B Structure in the region of the surface of a carburised bar, in the as-carburised condition

The original steel contains 0.15% carbon, but the carburised surface is now of eutectoid

composition (0.8% carbon), x 35. Etched in 2%

nital.

order to produce an atmosphere giving the required carbon potential at

the work surface. This carbon potential is in practice the carbon content

maintained in equilibrium in the surface film of the component whilst it is

in contact with the gas atmosphere. A carbon potential of 0.8% is usually

desirable (19.24). Carrier gases are generally of the 'endothermic' type

made in a generator and consisting of a mixture of nitrogen, hydrogen and

19.1A.

19.1B.

carbon monoxide. The active agent in the atmosphere is carbon monoxide

as it is in so-called 'solid' carburising:

2CO ^ CO

+ C (at the work surface) (1)

The atomic carbon thus released dissolves interstitially in the surface of

the austenitic work pieces. Methane present in the atmosphere probably

also releases carbon atoms:

^ 2H

+ C (at work surface) (2)

The 'water gas reaction' may also be responsible for carburising:

CO + H

^± H

O + C (at work surface) (3)

Carbon dioxide formed in (1) probably reacts with methane:

+ CH

^ 2H

+ 2CO (away from the work surface) (4)

The concentration of carbon monoxide is thus maintained so that carburis-

ing continues.

The relative proportions of hydrocarbon and carrier are adjusted to give

the desired carburising rate. Thus, the concentration gradient of carbon in

the surface can be flattened by prolonged treatment in a less rich carburis-

ing atmosphere. Control of this type is possible only with gaseous media.

There are a number of different methods by which the composition of the

carburising atmosphere can be continually monitored, so that the carbon

potential can be accurately controlled.

At present investigational work on 'plasma carburising' is taking place.

This is similar in principle to 'ionitriding' or 'ion implantation' (19.43)

but low-pressure nitrogen is replaced by low-pressure hydrocarbons. This

process, if successfully developed, would lead to a big saving of natural

gas.

It is claimed that a case of one millimetre depth can be produced in

thirty minutes.

19.24 Heat-treatment after Carburising If carburising has been cor-

rectly carried out, the core will still be of low carbon content (0.1-0.2%

carbon), whilst the case should preferably have a carbon content of no

more than 0.8% C (the eutectoid composition). If the carbon content of

the case is higher than this then a network of primary cementite will

coincide with the grain-boundary sites of the original austenite giving rise

to intercrystalline brittleness and consequent exfoliation (or peeling) of the

case during service. Even if this does not occur any cracks arising from the

presence of primary cementite may initiate fatigue failure. Moreover a

case containing 1.0% or more carbon may be soft at the surface after

quenching due to retention of austenite.

After the component has been carburised heat-treatment will be neces-

sary both to strengthen and toughen the core and to harden the case. At

the same time prolonged heating in the austenitic range during carburising

will have introduced coarse grain to the whole structure, so that a heat-

treatment programme which will also refine the grain is desirable if the

optimum properties are to be attained. For components which have been

pack- or liquid-carburised to produce deep cases, a double heat-treatment

is preferable to refine both core and case separately, as well as to harden

the case and strengthen the core.

19.25 Refining the Core The component is first heat-treated with the

object of refining the grain of the core and consequently toughening it.

This is effected by heating it to just above its upper critical temperature

(about 880

C for the core) when the coarse ferrite/pearlite structure will

be replaced by fine austenite crystals. The component is then water-

quenched so that a fine ferrite/bainite/martensite structure is obtained in

the core.

The core-refining temperature of 880

C is, however, still high above the

upper critical temperature for the case, so that, at the quenching tempera-

ture,

the case may consist of large austenite grains. On quenching these

will result in the formation of coarse brittle martensite. Further treatment

of the case is therefore necessary.

19.26 Refining the Case The component is now heated to about

760

so that the coarse martensite of the case changes to fine-grained

austenite. Quenching then gives a fine-grained martensite in the case.

At the same time the martensite produced in the core by the initial

quench will be tempered somewhat, and much will be reconverted into

fine-grained austenite embedded in the ferrite matrix (point C in Fig.

19.3).

The second quench will produce a structure in the core consisting of

martensite particles embedded in a matrix of ferrite grains surrounded by

bainite. The amount of martensite in the core is reduced if the component

is heated quickly through the range 650-760

C and then quenched without

soaking. This produces a core structure consisting largely of ferrite and

bainite, and having increased toughness and shock-resistance.

°c

austenite

• ferrite

ferrite

pearlite

CARBON

(°/o)

Fig.

19.3

Heat-treatment after carburising.

'A' indicates

the

temperature

treatment

for the

core

and 'B' the

temperature

treatment

for

the

case.

Finally, the component is tempered at between 160

C and 220

C to re-

lieve any quenching strains present in the case.

19.27 The above comprehensive heat-treatment may be regarded as

the counsel of perfection and would be that applied to important

components in which cases of considerable depth had been produced and

in which the necessarily prolonged carburising cycle had given rise to coarse

grain. The core-refining process would also cause some reduction in the

high carbon content at the skin to a more acceptable value nearer 0.8%,

thus reducing the chance of primary cementite networks at the surface. For

thinner cases and lower quality work generally, modified heat-treatment is

prevalent.

When thin cases (below 0.5 mm) are involved the component may be

quenched direct from the carburising process with obvious advantages

economically. The shorter carburising time will not have produced grain

growth to the same extent as more prolonged treatment. With cases of

intermediate thickness (0.5-1.25 mm) a component may be cooled slowly

from the carburising temperature and given a single treatment by re-

heating to 820

C as a compromise between the upper and lower critical

temperatures (for the core), water-quenched and finally tempered at

between 160

C and 220

C. The case will then be hard though slightly

coarse-grained whilst the core will consist of a ferrite/bainite/martensite

structure. The ferrite will be rather coarse-grained, remaining from the

original carburising and will not be completely redissolved by heating to

820

Hence the core will not attain maximum toughness.

Gas carburising gives greater flexibility in heat-treatment. Thus the

actual carburising process may be carried out at 900-940

C to give a carbon

content at the surface of 0.8%. When carburising is complete the furnace

temperature can be lowered to 830

C and the work then quenched direct

(Fig. 13.7—Pt. 2).

Case-hardening Steels

19.30 Plain Carbon Steels used for case-hardening commonly contain

up to 0.2% carbon when maximum toughness and ductility of the core are

required. When higher core strength is necessary the carbon content may

be raised to 0.3%, though it is generally more satisfactory to use an alloy

steel in such circumstances.

Up to 1.4% manganese may be included since it aids carburisation by

stabilising cementite. It also increases the depth of hardening but is liable

to induce cracking during quenching. Silicon tends to graphitise cementite

and, since it retards carburisation, is kept below 0.35%. Thus plain carbon

steels used for case-hardening contain up to 0.3% carbon and up to 1.4%

manganese.

19.31 Alloy Steels for case-hardening contain, additionally, up to

4.5Ni;

1.5Cr;

and 0.3Mo. Higher core-strength without serious loss of

either toughness or ductility are obtained by alloying. Moreover, since the

Table

19.1

Case-hardening

Steels

Typical

mechanical properties

core.

Composition

(%)

Characteristics

and

uses

Good core toughness

thin sections—with economy.

General purposes

for

medium-duty work—many types

gears

and

shafts.

carbon-manganese

steel High surface hardness

where

severe shock

free-cutting unlikely,

carbon-manganese

steel

High surface hardness combined

with

core toughness.

High-duty gears, worm gears, crown wheels

and

clutch

gears.

High hardness

and

severe shock—automobile gears,

steering

worm

and

quadrant, overhead valve mechanisms.

Good combination

core strength, shock resistance

and

surface

hardness along with economy—crown wheels,

etc.

For

severe shock

and

high stress—automobile gears,

steering worm, overhead valve mechanisms.

Best

combination

surface hardness, core strength

and

shock

resistance—crown wheels, bevel pins,

aero-reduction gears. Intricate sections which require

air

hardening.

Impact

(J)

Elonga-

tion

(%)

Tensile

strength

(N/mm

)

440

500

750

1005

775

1315

1085

1315

0.14

0.25

0.2

0.85

1.5

0.8

1.2

3.25

1.75

2.0

3.5

4.0

0.45

0.8

1.3

1.4

0.5

0.6

0.5

0.4

0.10

0.15

0.13

0.17

0.13

0.15

970:

Part

045M10

('10

carbon

)

080M15

('15

carbon')

130M15

('15'

carbon-manganese)

214M15('15'

carbon-manganese-

free-cutting)

655M13(3

/4%

nickel-chromium)

655M17(1

/4%

nickel-molybdenum)

822M17(2%

nickel-chrome-molybdenum)

832M13(3V

nickel-chrome-molybdenum)

835M15(4%

nickel-chrome-molybdenum)

critical hardening rates are reduced oil quenching can be used to minimise

the risk of distortion or cracking.

An important function of nickel is to retard grain growth during carburis-

ing and so give a comparatively fine-grained product. For this reason the

core-refining stage in heat-treatment can be dispensed with in some appli-

cations of case-hardened nickel steels with obvious economic advantage,

though this is generally restricted to those in which only a light case has

been produced and in which the presence of a cementite network is

unlikely.

Although nickel retards carburisation and tends to give a softer case

than some plain carbon steels, the case is very wear resistant. Cracking

and exfoliation are also less likely. The addition of chromium increases

hardness and wear-resistance of the case and also stability of the cementite.

It also improves strength of the core with little loss in ductility. Neverthe-

less chromium additions have to be limited because of the tendency of the

metal to promote grain growth with its consequent loss of toughness.

Nitriding

19.40 Nitriding resembles carburising in so far as interstitial penetration

of the solid surface of steel takes place during heating of the work in

contact with the nitriding agent. Whilst in carburising the function of the

carburising agent is to release atoms of carbon at the steel surface, in

nitriding the nitriding agent releases single atoms of nitrogen at the steel

surface so that, in this instance, hardness depends on the formation of hard

nitrides instead of carbides. Whilst it is possible to nitride many types of

steel, high surface hardness is only obtained when using special alloy steels

containing aluminium, chromium, molybdenum or vanadium—elements

which form hard and stable nitrides as soon as they come into contact with

nitrogen atoms at the surface of the work piece.

Fig. 11.7 indicates that iron dissolves up to 0.1% nitrogen at 590

C and

that above this amount it begins to form the hard nitride Fe

N (y'). This

underlines a very important difference between carburising and nitriding.

Whilst carburising must take place when mild steel is in its austenitic state

at about 900

C, nitriding can be achieved with the work in its ferritic state

at about 500

19.41 Since it is conducted at a relatively low temperature, nitriding

is made the final operation in the manufacture of the component, all

machining and core-treatment processes having been carried out pre-

viously. The parts are maintained at 500

C for between 40 and 100 hours,

according to the depth of case required, in a gas-tight chamber through

which ammonia is allowed to circulate. Some of the ammonia dissociates

according to the following equation:

Table

19.2.

Nitriding Steels

Typical mechanical properties

Composition

(%)

Uses

Where maximum surface

hardness

essential coupled

with

very high core strength.

For

maximum surface hardness

and high core strength.

For

maximum surface hardness

combined with reasonably high

core strength.

For

maximum surface hardness

combined with ease

machining before hardening.

Moulds

for

plastics; other

components requiring high

hardness

and

good

finish.

For

ease

machinability

and a

high-class surface.

Ball

races,

etc.,

where high

core

strength

necessary.

Aero

crankshafts, airscrew

shafts,

aero cylinders,

crank-pins

and

journals.

Aero-engine cylinders.

Heat-treatment

(for

core)

Oil-quench from 900

and temper

550-700

Oil quench from 900

and temper

600-700°C.

Oil-quench from 900

and temper

600-700

Oil-quench from 900°C

and temper

600-700°C.

Oil-quench from 900

and

temper

600-700°C.

Oil-quench from 900

and temper

600-700

Oil-quench from 900

and

temper

550-650°C.

Oil-quench from 900

and temper

600-700

Oil-quench from 900

and temper

600-700°C

(case)

1050-1100

750-800

850-900

800-850

Izod

(J)

Elonga-

tion

Tensile

strength

N/mm

1240

927

741

618

927

618

1390

1000

772

Yield

point

N/mm

1000

741

556

463

741

463

772

618

1.1

0.15

0.25

0.2

0.25

1.0

0.5

1.6

2.0

3.0

0.65

0.5

0.45

0.5

0.4

0.3

0.2

0.35

0.18

0.4

0.3

0.2

970

desig-

nation

905M39

905M31

897M39

722M24

DEPTH OF CASE (mm)

Fig.

19.4 The depth of nitriding for a 1% Al steel (905M39) in relation to time of nitriding

at 500

Part of the nitrogen, which is released as single atoms, is absorbed by the

surface of the steel forming nitrides both with iron and with the elements

mentioned above.

Nitriding of plain carbon steels would produce a case of only moderate

hardness (about 400 //

). This is largely because nitrogen diffuses fairly

quickly beneath the surface forming Fe

N (and so netimes Fe2N) dispersed

at greater depths so that surface hardness is reduced. Since aluminium—

and to a lesser extent chromium, vanadium and molybdenum—has a higher

affinity for nitrogen it prevents diffusion of the latter to a greater depth

but instead forms very hard stable aluminium nitride near the surface,

giving an extremely hard but shallow case up to 1.0 mm in depth. Chro-

mium also helps to increase case hardness by the formation of chromium

nitride. Since it forms at a greater depth than does aluminium nitride it

prevents a sudden change in composition from hardened skin to soft core

which would be likely to lead to spalling, or flaking, of the case. In addition

to hardening the case, molybdenum also improves core toughness.

HARDNESS

)

NITRIDING

TIME (h)

ALLOYING ELEMENT (°/o)

Fig.

19.5 The influence of alloying elements on the hardness of 'Nitralloy' type steels after

nitriding.

In each case the steel contains 0.3% C and 0.65% Mn. The effect of nickel is slight because

it does not form nitrides and such increase in hardness as is obtained is due only to solid

solution effects.

When core strength is important aluminium is often omitted from the

steel and a somewhat lower case hardness obtained, dependent upon the

formation of chromium and vanadium nitrides (Fig. 19.5).

19.42 Before being nitrided the work is heat-treated to produce the

required core properties. The normal sequence of operations is:

(a) oil-quenching from 850-930

C, followed by tempering at 550-

700

C depending upon the composition of the steel and the core proper-

ties required;

(b) rough-machining, followed by a stabilising anneal at 550

C for five

hours to remove stresses initiated by the cold-work;

Any parts of the component which are required soft are protected by

coating with a mixture of whiting and sodium silicate.

19.43 lonitriding The concept and principles of ionitriding (ion-

nitriding)—also known as 'plasma nitriding' and 'ion implantation'—were

first established as long ago as the early nineteen-thirties but only in recent

years has the process become commercially established. The principles of

ionitriding are similar to those of ion-plating (21.86) in that ions of the

coating substance are attracted to the surface being coated. The work is

made the cathode in a chamber containing nitrogen under near-vacuum

conditions (1-10 m bar). Under a potential difference of 500-1000 volts

(dc) the low-pressure nitrogen becomes ionised and the

+++

ions so

produced are accelerated towards the negatively-charged work load where,

on impact, they penetrate the surface of the steel. The kinetic energy of

the ions is converted to heat so that the surface is quickly raised to the

nitriding temperature (400-600

C). The work load is completely sur-

rounded by a glow of ionised nitrogen so that uniform treatment is assured.

Depending upon the type of steel and the depth of case required treatment

times range from ten minutes to thirty hours. After treatment the load can

be cooled under low pressure to prevent oxidation and also avoid the risk

of distortion.

The degree of control is superior to that in other nitriding processes and

the properties of the case can be effectively adjusted by alterations in

the conditions of working. Maximum hardness in 'nitralloy-type' steels is

achieved at a treatment temperature of 450

C. The higher the alloy content

the thinner and harder the case since nitrides are formed near to the surface

when aluminium is present.

Ionitriding is already widely used abroad and the throughput of an instal-

lation is about three times that of a comparable orthodox gas-nitriding

plant. The process is currently being used to nitride components as large

as 14 tonnes in mass down to the tiny balls of ball-point pens. Truck engine

crankshafts and other automobile parts, as well as hot- and cold-working

tools and dies are being ionitrided.

19.44 The Advantages of Nitriding are, briefly, as follows:

(i) Since no quenching is required after nitriding, cracking or distor-