Surface Hardening of Steel

Introduction:
The service conditions of many steel components such as cams
and gears, make it necessary for them to possess both hard,
wear-resistant surfaces and, at the same time, tough, shock-resistant
cores.
In plain carbon steels these two different sets of properties exist only in
alloys of different carbon content. A low-carbon steel,
containing approximately 0-1% carbon, will be tough, whilst a high-
carbon steel of 0-9% or more carbon will possess adequate hardness
when suitably heat-treated.
The situation can best be met by employing a low-carbon steel
with suitable core properties and then causing either carbon or
nitrogen to penetrate to a regulated depth into the surface skin; as in the
principal surface-hardening processes of carburising and nitriding.
Alternatively, a steel of medium carbon content and in the
normalised condition can be used, local hardness at the surface then
being introduced by one or other of the name-hardening processes.
In the first case the hardenable material is localised, whilst in
the second case it is the heat-treatment itself which is localised.
There are four main types of the surface hardening which are :-
1. Case hardening
2. Nitriding
3. Flam hardening
4. Induction hardening

The first two types include a change in the chemical structure of the
surface while the other types include a change in the micro constituents as
a result for the positional heat treatment.

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 Engineering Materials

1. Case hardening
Is one of the most used for producing a hard surface on a ductile
steel. It involves the introduction of additional carbon into the surface of
mild steel, effectively producing a composite material consisting of low
carbon steel with a thin case, about 0.5 – 0.7 mm in thickness, of high
carbon steel, this was the principle of case-hardening that have been used
for centuries in the conversion of mild or wrought iron to steel by the
cementation process.
The case-hardening consists in surrounding the component with
suitable carbon material and heating it to above its upper critical
temperature for long enough to produce a carbon enriched layer of
sufficient depth.
Solid, liquid and gaseous carburizing media are used. The nature and
scope of the work involved will govern which media is best to employ.
The case-hardening process has two distinct steps, as shown in figure 1:
 Carburising ( the additional of carbon)
 Heat treatment ( hardening and core refinement)

Figure 1. Case hardening : (a) carburising, (b) after carburising, (c) after quenching
component.

1. Carburising
Carburizing makes use of the fact that low carbon steel absorb
carbon when heated to the austenitic condition various carbonaceous
materials are used in the carburizing process as follows:-

1.1 Carburising in solid media

"Pack-carburising", as it is usually called, involves packing the
components into cast-iron or steel boxes along with the carburizing
material so that a space of approximately 50 mm exists between the
components. The lids are then luted on to the boxes, which are slowly

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 Engineering Materials
heated to the carburizing temperature for between 900 and 950ú C.
they are maintained at this temperature for up to five hours, according
to depth of case required.
Carburising media vary in composition, but consist essentially of
some carbonaceous material, such as wood or bone charcoal or
charred leathers together with an energiser which may account for
up to 40% of the total composition. This energiser is usually a
mixture of sodium carbonate ("soda ash") and barium carbonate,
and its purpose is to accelerate the solution of carbon by the
surface layer of the steel.
It is thought that carburisation proceeds by dissociation of
carbon monoxide which will be present in the hot box. When
the gas comes into contact with the hot steel it dissociates thus:
2CO CO2 + C
The atomic carbon deposited at the surface of the steel
dissolves easily in the metal. In this method of carburizing the
thickness of the surface is between 0.05 – 1.55 mm.
If it is necessary to prevent any areas of the component from being
carburised, this can be achieved by electro-plating these areas with
copper to a thickness of 0-075-0-10 mm; carbon being insoluble
in solid copper at the carburising temperature.
When carburising is complete the components are quenched
or cooled slowly in the box, according to the nature of the subsequent
heat-treatment to be applied.

1.2 Carburising in a liquid bath ( Cyaniding)

Liquid-carburising is carried out in baths containing from 20 to

50% sodium cyanide, together with up to 40% sodium carbonate and
varying amounts of sodium chloride.
This cyanide-rich mixture is heated in pots to a temperature of
870-950° C, and the work, which is contained in wire baskets, is
immersed for periods varying from about five minutes up to
one hour, depending upon the depth of case required.
One of the main advantages of cyanide-hardening is that
pyrometric control is so much more satisfactory with a liquid bath.
Moreover, af t e r treatment the basket of work can be quenched. This
not only produces the necessary hardness but also gives a clean surface
to the components.

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 Engineering Materials
The process is particularly useful in obtaining shallow cases
of 0.1 – 0.25 mm.
Dissociation of the carbon monoxide at the steel surface then takes
place with the same result as in pack-carburising. The nitrogen, in
atomic form, also dissolves in the surface and produces an increase in
hardness by the formation of nitrides as it does in the nitriding
process.
Cyanides are, of course, extremely poisonous, and every
precaution should be taken to avoid inhaling the fumes from a pot.
Every pot should be fitted with an efficient fume-extracting hood.
Likewise the salts s h o u l d 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.

1.3 Carburising by gaseous media

Gas-carburising is carried out in both batch-type and continuous

furnaces. The components are heated at about 900° C for three or four
hours in an atmosphere containing gases which will deposit carbon
atoms at the surface of the components. The most important of these
gases are the hydrocarbons methane CH4 , and propane, C3H8.
They should be of high purity in order to avoid the deposit of oily
soot which impedes carburising. To facilitate better gas circulation
and hence, greater uniformity of treatment the hydrocarbon is mixed
with a "carrier" gas. This is generally an "endothermic" type of
atmosphere made in a generator and consisting of a mixture
containing mainly nitrogen, hydrogen and carbon monoxide.
The relative proportions of hydrocarbon and carrier are adjusted to
give the desired carburizing rate. Thus, the concentration gradient of
carbon in the surface can be "flattened" by prolonged treatment in a
less rich carburising atmosphere. Control of this type is possible only
with gaseous media.
Gas-carburising is becoming increasingly popular, particularly for
the mass production of thin cases. Not only is it a neater and cleaner
process but the necessary plant is more compact for a given output.
Moreover, the carbon of the surface layers can be controlled more
accurately and easily with thickness of about 0.25 – 1.0 mm.
2. Heat treatment after Carburising
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 Engineering Materials
If carburising has been correctly carried out, the core will still be of
low carbon content (0.1 – 0.2% carbon), whilst the case should have a
maximum carbon content of 0.83% carbon (the eutectoid composition),
as shown in figure 2.

Figure 2. Heat-treatment After Carburising.

A Indicates the temperature of treatment for the core, and B the temperature of
treatment for the case.

Considerable gain growth occurs in the material during a carburizing

treatment, therefor a three stage of heat treatment must be given to the
carburized parts to produce the desired final properties. This heat
treatment involves:
2.1 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 870° C for
the core) when the coarse ferrite-pearlite structure will be replaced by
refined austenite crystals. The component is then water-quenched, so
that a fine ferrite-martensite structure is obtained.
The core-refining temperature of 870° C is, however, still high above
the upper critical temperature for the case, so that, at the quenching
temperature, the case may consist of large austenite grains. On

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 Engineering Materials
quenching these will result in the formation of coarse brittle martensite.
Further treatment of the case is therefore necessary.

2.2 Refining the case

The component is now heated to about 760° C, 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
Figure.2). 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.

2.3 Tempering
The component is tempered at about 200 – 220 ° C to relieve any
quenching strains present in the case.

2. Nitriding
The process is used to put a hard, wear-resistant (‫ﻟﺒﻠﻰ‬Ç ‫ﻣﺔ‬æ‫ )ﻣﻘﺎ‬coating on
components made from special alloy steels, for example, drill bushes. The
alloy steels used for this process contain either 1.0% aluminium, or traces
of molybdenum, chromium and vanadium. Nitrogen gas is absorbed
into the surface of the metal to form very hard nitrides. The process
consists of heating the components in ammonia gas at between 500 and
600 °C for upwards of 40 hours.
At this temperature the ammonia gas breaks down and the atomic
nitrogen is readily absorbed into the surface of the steel. The case is
applied to the finished component. No subsequent grinding is possible
since the case is only a few micrometres thick. However, this is no
disadvantage since the process does not affect the surface finish of the
component and the propcess temperature is too low to cause distortion.
Some of the advantages of nitriding are:
 Carking and distortion are eliminated since the processing

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 Engineering Materials
temperature is relatively low and there is no subsequent
quenching.
 Surface hardnesses as high as 1150 HD are obtainable with
'Nitralloy' steels.
 Corrosion resistance of the steel is improved.
 The treated components retain their hardness up to 500 °C
compared with the 220 °Cfor case-hardened plain carbon and low-
alloy steels.
 Resistance to fatigue is good.
 The process is cheap when large number of components are to be
treated.

As disadvantage of nitriding is more expensive than that with case-

hardening, so that the process is economical only when large number of
components are to be treated.
Carbonitriding. This is a surface-hardening process which makes
use of a mixture of carburising gases and ammonia. It is sometimes
known as "dry cyaniding" — a reference to the fact that a mixed
carbide-nitride case is produced as in the ordinary liquid cyanide pro-
cess. The relative proportions of carbon and nitrogen in the case can
be varied by controlling the ratio of ammonia to hydrocarbons in the
treatment atmosphere.

3. Flame hardening
Localised surface hardening can also be achieved in medium- and
high-carbon steels and some cast irons by rapid local heating and
quenching. Figure 3 shows the principle of flame hardening. A carriage
moves over the work piece so that the surface is rapidly heated by an
oxy-acetylene or an oxy-propane flame. The same carriage carries the
water-quenching spray. Thus the surface of the work piece is heated and
quenched before its core can rise to the hardening temperature. This
process is often used for hardening the slideways of machine tools, such
as gears, spindles and pins, that are confidently treated by this process,
since they can be spun between centers.

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 Engineering Materials

Figure 3. Surface hardening with flame hardening (Shorter process).

4. Induction hardening
These processes are similar in principle to flame-hardening, except
that the component is held stationary- whilst the whole of its surface
is heated simultaneously by electro-magnetic induction, as shown in
figure 4. The component is surrounded by an inductor block through
which a high-frequency current in the region of 2ooo Hz, passes. This
raises the temperature of the surface layer to above its upper critical
in a few seconds. The surface is then quenched by pressure jets of
water which pass through holes in the inductor block.
Thus, as in flame-hardening, the induction processes make use of
the existing carbon content (which must be above 0-4%), whilst in
both case-hardening and nitriding an alteration in the composition of
the surges layer of the steel takes place.

Figure 4. Surface hardening with induction hardening.

Table 1.

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 Engineering Materials
Surface hardening methods

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 Engineering Materials

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