Thermal Processing/Heat Treatment of
Steel
Lecture 9
Heat Treatment Fundamentals

It is an operation or combination of operations involving



heating a metal or alloy in its solid state to a certain temperature
holding it there for some times, and
cooling it to the room temperature at a predetermined rate to obtain desired
properties.

All basic heat-treating processes for steels involve the transformation of
austenite.

the nature and appearance of these transformation products determine
the physical and mechanical properties of heat treated steels.
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Heating Period
Holding
Heating steel to above critical temperature range
(A3 or Acm) in order to form single-phase austenite.

Rate of heating is usually less important than other
factors, except for
[1] highly stressed materials, or
[2] thick-sectioned materials.
Temperature

Usually slow heating rate is preferable.
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Time
Holding / Soaking Period
Holding
Holding at the austenitizing temperature for
complete homogenization of structure.

Usually 1 hour per inch section is enough for holding.
Temperature

Time
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Cooling Period

Cooling rate determines the nature of transformation products of austenite.

Depending on cooling rate,
common heat treatment of steels are classified as:
[1] annealing
[2] normalizing
[3] hardening
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Annealing of Steels

Annealing is a heat treatment process that consists of heating to and holding at a
suitable temperature followed by cooling slowly through the transformation range
preferably in the furnace, primarily for the softening of metallic materials.

Generally, in plain carbon steels, full annealing (commonly known as annealing) produces
ferrite-pearlite structures.

Purposes of annealing:





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May be to refine grain
Inducing ductility, toughness, softness
Relieve residual stresses
Improving electrical and magnetic properties
In some cases to improving machinability
Humayun Kabir, Dept of MME, BUET
1.Full Annealing



Heating and holding steels to austenitizing temperature, and then cooling very
slowly through the transformation range preferably in the furnace.
Improves ductility
utilized in low- and medium-carbon steels that will be machined or will
experience extensive plastic deformation during a forming operation.
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Humayun Kabir, Dept of MME, BUET
2.Stress Relief Annealing

Heating and holding steels to below lower critical temperature, and then
cooling to room temperature (sub-critical annealing).

Relieve residual stresses due to heavy machining/grinding or other coldworking processes, non-uniform cooling (during weld/casting), and phase
transformations.

Distortion and warpage may result if these residual stresses are not removed.
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Humayun Kabir, Dept of MME, BUET
3.Process Annealing







Similar to stress relief annealing.
Process annealing is a heat treatment that is used to negate the effects of cold
work.
i.e; to soften and increase the ductility of a previously strain-hardened metal.
It is commonly utilized during fabrication procedures that require extensive
plastic deformation, to allow a continuation of deformation without fracture or
excessive energy consumption.
Structure refined by a process of recovery and recrystallization.
Ordinarily a fine-grained microstructure is desired
Designed to restore ductility of steels between processing steps and facilitate
further cold working.
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Humayun Kabir, Dept of MME, BUET
4.Spheroidizing Annealing



Make very soft steels for good machining (for hypereutectoid steels).
Prolonged heating breaks pearlite and cementite network and spheroids
of cementite in ferrite matrix forms.
Both sub-critical and inter-critical annealing practices are used.



10
Prolonged heating at a temp. just below the lower critical temp.
Heating and cooling alternately between temperatures that are just above and just
below the lower critical line.
Heating to a temperature above the lower critical line and then either cooling very
slowly in the furnace or holding at a temperature just below the lower critical line.
Humayun Kabir, Dept of MME, BUET
4.Spheroidizing Annealing

The spheroidized structure is desirable when minimum hardness, maximum
ductility or maximum machinability is important.

Low carbon steels are seldom spheroidized for machining because in the
spheroidized condition they are excessively soft and gummy. The cutting tool
will tend to push the material rather than cut it.
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Temperature Range
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Annealing of Hypoeutectoid Steels

Since cooling is very slow,
annealing comes closest to
follow the iron – iron carbide
equilibrium diagram
AUSTENITE
austenite
910C

(c)
A3
FERRITE +
AUSTENITE
Temperature
(b)
ferrite
pearlite
723 C
A1
0.80
Pearlite Austenite

FERRITE + PEARLITE
(a)
13

0.2
wt.% C
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(d)
Annealing of Hyper-eutectoid Steels


Refinement of the grain size of hypereutectoid steel will occur about 30°C
(50°F) above the lower-critical-temperature (A1) line.
Heating above this temperature will coarsen the austenitic grains, which, on
cooling, will transform to large pearlitic areas.

The microstructure of annealed hypereutectoid steel will consist of coarse
lamellar pearlite areas surrounded by a network of proeutectoid cementite.

Because this excess cementite network is brittle and tends to be a plane of
weakness, annealing should never be a final heat treatment for hypereutectoid
steels.
The presence of a thick, hard, grain boundary will also result in poor
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machinability.
 A careful estimation of the proportions of pearlite and/or ferrite present in an
annealed steel can be used to determine the approximate carbon content of the
steel:
Wt.% C = (0.80) x (%Pearlite area) + (0.008) x (%Ferrite area)
Wt.% C = (0.80) x (%Pearlite area) + (6.67) x (%Cementite area)
 An approximate tensile strength of a hypoeutectoid steel can also be determined
in a similar manner:
Approx. Tensile Strength, psi = (120,000) x (%Pearlite area)
+ (40,000) x (%Ferrite area)
Tensile strength of hypereutectoid steels can not be estimated similarly, since their strengths
are determined by the cementite network only.
Humayun Kabir, Dept of MME, BUET
Problem
Microstructure of an annealed steel sample is found to contain 25% ferrite area and 75%
pearlite area. Identify the steel and determine its approximate tensile strength.
Wt.% C in steel = (0.8) 0.75 + (0.008) 0.25 = 0.602 %
Since the carbon content is less than 0.80, the eutectoid composition, the sample is a
hypoeutectoid steel.
Approx. Tensile Strength = (120,000) 0.75 + (40,000) 0.25 psi
= 100,000 psi
Humayun Kabir, Dept of MME, BUET
Thermal Processing/Heat Treatment of
Steel
Lecture 10
 Normalizing is done by heating the steel approximately 55°C above the upper
critical (A3 and Acm), followed by cooling to room temperature in still air.
The main purpose of normalizing heat treatment is to produce harder and
stronger steel than full annealing.
 Other Purposes of normalizing
 Modifying and refining cast dendritic structure
 Refining grains and homogenising the structure
 Inducing toughness
 Improving machinability
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Temperature Range
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 Normalizing produces a more harder and stronger steel than full annealing (due to a
faster cooling rate).
❑ Thus, normalizing is often used as a final heat treatment (whereas
annealing cannot!!).
 The initial cementite network is required to be dissolved completely into
austenite.
❑ For this reason, both hypo- and hypereutectoid steels are always
heated to complete austenite region.
Humayun Kabir, Dept of MME, BUET
Effect of Faster Cooling Rate
 Cooling rate is no longer under equilibrium conditions
 Iron – iron carbide phase cannot be used to predict the proportions of pearlite and proeutectoid
constituents (ferrite or cementite)
 Approximate carbon content cannot be determined knowing the proportions of pearlite and
proeutectoid constituents (ferrite or cementite)
 There is less time for the formation of proeutectoid constituents




There will be less ferrite/cementite and more pearlite
Reduces the continuity of cementite network
Sometimes, formation of proeutectoid constituents is suppressed altogether
Hypereutectoid steels show increased strength (due to lesser cementite)
Humayun Kabir, Dept of MME, BUET
Effect of Faster Cooling Rate
 Austenite transformation condition is changed
 The austenite transformation temperature is decreased; the faster the cooling rate, the lower the
temperature of austenite transformation
 The eutectoid point is shifted towards lower C content for hypoeutectoid steels and towards higher C
content for hypereutectoid steels
Cementite
Ferrite
 Fineness of pearlite is increased(Fine-grained pearlitic steels are
tougher than coarse-grained ones)
 The soft ferrite is held close by the hard cementite plates
 Stiffness of ferrite is increased
(so it will not yield as easily)
 Thus, the hardness is increased
ANNEALED
coarse lamellar
pearlite
NORMALISED
medium lamellar pearlite
To conclude, normalising produces a finer and more abundant pearlite structure than is obtained by
annealing, which results in a harder and stronger steel
Humayun Kabir, Dept of MME, BUET
Effect of carbon and heat
treatment on properties of plaincarbon steels
 Under slow or moderate cooling rates
❑
❑
C atoms are able to diffuse out of the austenite structure
The iron atoms then moves slightly to become b.c.c (body centered cubic)
 This gamma to alpha transformation is a diffusion controlled process,
which requires slow cooling
Growth
Nucleation
&
(Time Dependent)
 With a still further increase in cooling rate:
❑
❑
❑
Insufficient time is allowed for C atoms to diffuse out of austenite solution
C atoms remained trapped inside austenite
The structure cannot become b.c.c. (Austenite cannot transforms into ferrite).
# The resultant structure is called Martensite.
Humayun Kabir, Dept of MME, BUET
 Hardening is done by heating the steel approximately to
❑
50 C above the upper critical temperatures (A3 line)
(for hypeutectoid steels).
❑
50 C above the lower critical temperatures (A3,1 line)
(for hypereutectoid steels)
followed by drastic cooling to room temperature.
 Purposes of hardening:


to improve hardness
to improve wear resistance
Humayun Kabir, Dept of MME, BUET
What happens during hardening?
 If cooling rate is very fast (as in during water quenching):
❑
austenite cannot transforms into ferrite due to insufficient time is given for C atoms to diffuse out of
austenite.
❑
most of the C atoms remain trapped in austenite to distort the structure; (Although some movement of iron
atoms takes place). The c-axis becomes elongated and the cubic structure is transformed into a
tetragonal structure.
❑
a supersaturated solid solution of C atoms trapped in a body-centred tetragonal (BCT) structure is formed.
❑
this structure is called MARTENSITE.
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What happens during hardening?
Structure of martensite (BCT) where the vertical axis
is slightly expanded because of the trapped carbon atoms
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What happens during hardening?
 The highly distorted lattice structure is the prime reason for the high
hardness of martensite .
 The basic aim of most hardening operations is to obtain 100% full
martensitic structure. The minimum cooling rate that is required to produce a full
martensitic structure(avoid the formation of any of the softer products of
transformation) is called the critical cooling rate (CCR).
 Since atoms of martensite (BCT) are less densely packed than in austenite
(FCC), an expansion occurs during this transformation.
Humayun Kabir, Dept of MME, BUET
What happens during hardening?
 This expansion during the formation of martensite produces high localized
stresses which result in plastic deformation of the matrix.
 After drastic cooling (quenching), Martensite appears microscopically as
needle or acicular structure, sometimes described as a pile of straw.
 In most steels, the martensitic structure appears vague and unresolvable.
 In high-carbon alloys where the background is retained austenite, the acicular
structure of martensite is more clearly defined.
Humayun Kabir, Dept of MME, BUET
What does martensite look like?
Martensite needles (black) in
retained austenite (white background)
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Characteristics of martensitic transformation
 Diffusionless (time independent) transformation.
❑ Small volume of austenite suddenly changes its crystal structure into martensite by a combination of
two shearing actions; No change in chemical composition.
(athermal transformation, not isothermal transformation!!)
❑ Transformation depends upon the decrease in temperature; start at
MS temperature, ends at MF temperature.
Temperature
 Transformation proceeds only during cooling
❑ The amount of martensite formed with decreasing temperature is not
linear.
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MS
MF
Per cent Martensite
Characteristics of martensitic transformation
 The start of martensite
transformation cannot be
suppressed.
❑ MS or MF temperature cannot be changed by
changing cooling rate.
❑ MS temperature is a function of composition only.
❑ Martensite transformation never completes; there is
always some retained austenite in the structure.
Ms(°F)= 1000-(650 x %C) - (70 x %Mn) - (35 x
%Ni) - (70 x %Cr) - (50 x %Mo)
Humayun Kabir, Dept of MME, BUET
Characteristics of martensitic transformation
 Martensite is a metastable structure.
❑ It will change into a stable ferrite (BCC), if given the
opportunity.
 Need sufficient carbon to obtain
extreme hardness.
❑ The max. hardness obtainable from a steel depends on
the C content only.
❑ 60 Rockwell C @ 0.4%C and then start to level off
❑
❑
65 Rockwell C @ 0.8 %C.
The leveling off is due to the greater tendency to retain
asutenite in high carbon steels .
Humayun Kabir, Dept of MME, BUET
Thermal Processing/Heat Treatment of
Steel
Lecture 11
 Formation of a full martensitic structure is desirable in order to have the maximum hardness.
 But we can’t always produce martensite throughout an entire part.
➢ The part geometry,
➢ alloy content/composition and
influence the formation of martensite.
➢ quenching medium
 Hardenability is a term that is used to describe the ability of an alloy to be hardened by the formation of
martensite as a result of a given heat treatment i.e; the ability of steel to form a full martensitic
structure throughout the part is called hardenability.
(Not to be confused with hardness!!)
Humayun Kabir, Dept of MME, BUET
Hardenability of Steels
 Hardenability is not “hardness,” which is the resistance to indentation;
 Rather, hardenability is a qualitative measure of the rate at which hardness drops off
with distance into the interior of a specimen as a result of diminished martensite content.
A steel alloy that has a high hardenability is one that hardens, or forms martensite, not
only at the surface but to a large degree throughout the entire interior.
The most widely used methods of determining hardenability is the end quench
hardenability test or the Jominy test.
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Humayun Kabir, Dept of MME, BUET
Hardenability of Steels
The Jominy End- Quench Test:
❖ With this procedure, except for alloy composition, all factors that may influence the depth to
which a piece hardens (i.e., specimen size and shape, and quenching treatment) are maintained
constant.
❖ A cylindrical specimen 25.4 mm (1.0 in.) in diameter and 100 mm (4 in.) long is austenitized at a
prescribed temperature for a prescribed time.
❖ After removal from the furnace, it is quickly mounted in a fixture as diagrammed in Figure 11.11a.
❖ The lower end is quenched by a jet of water of specified flow rate and temperature. Thus, the
cooling rate is a maximum at the quenched end and diminishes with position from this point along
the length of the specimen.
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Hardenability of Steels
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Humayun Kabir, Dept of MME, BUET
Hardenability of Steels
The Jominy End- Quench Test:
❖ After the piece has cooled to room temperature, shallow flats 0.4 mm (0.015 in.) deep are ground
along the specimen length and Rockwell hardness measurements are made for the first 50 mm (2
in.) along each flat (Figure 11.11b);
❖ for the first 12.8 mm hardness readings are taken at 1.6-mm intervals, and for the remaining 38.4
mm, every 3.2 mm. A hardenability curve is produced when hardness is plotted as a function of
position from the quenched end.
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Hardenability Curve
❖ The quenched end is cooled most rapidly and exhibits the maximum hardness; 100% martensite is
the product at this position for most steels.
❖ Cooling rate decreases with distance from the quenched end, and the hardness also decreases.
❖ Thus, a steel that is highly hardenable will retain large hardness values for relatively long distances; a
steel with low hardenability will not.
❖ With diminishing cooling rate, more time is allowed for carbon diffusion and the formation of a
greater proportion of the softer pearlite, which may be mixed with martensite and bainite.
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Hardenability Curve
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Quenching medium
 Severity of cooling medium influences the cooling rate.
Air  slow cooling rate  low hardness
Oil  moderate cooling rate  moderate hardness
Water  fast cooling rate  high hardness
Part geometry
 Thicker the sample, more variation in the cooling rate between the centre and surface of the sample.
Centre  slow cooling rate  low hardness
Surface  faster cooling rate  high hardness
Alloy Content
 Addition of alloying elements slows down the diffusion process, thereby making it easier for the steel to form
martensite.
Humayun Kabir, Dept of MME, BUET
 In the as-quenched condition, martensite, in addition to being very hard, is too brittle for most
applications. The formation of martensite also leaves high residual stresses in the steel.
any internal stresses that may have been introduced
during quenching have also a weakening effect.
Toughness (ft-lb)
Hardness (RC)
100
80
Hardness
60
40
20
0
0
200
400
600
800
Tempering Temperature (C)
 Tempering is done almost immediately after hardening to relieve residual stresses and to improve
ductility and toughness. The increase in ductility is attained at the sacrifice of some hardness or
strength.
Humayun Kabir, Dept of MME, BUET
 In tempering, the hardened steel is heated and held to a temperature (which is below the lower
critical) for a specified time period, and then cooled to room temperature.
 The selection of heating temperature depends upon desired properties.
 Normally, tempering is carried out at temperatures between 250 and 650°C (480 and 1200°F);
internal stresses, however, may be relieved at temperatures as low as 200°C (390°F). This tempering
heat treatment allows, by diffusional processes, the formation of tempered martensite, according to
the reaction
martensite (BCT, single phase)
tempered martensite (α + Fe3C phases )
where the single-phase BCT martensite, which is supersaturated with carbon, transforms to the
tempered martensite, composed of the stable ferrite and cementite phases.
Humayun Kabir, Dept of MME, BUET
What happens during tempering?
 During tempering, the excess carbon atoms, trapped in martensite, gradually come out as extremely fine
cementite particles and the metastable BCT martensite transforms into stable BCC ferrite.
 The resulting microstructure of tempered martensite consists of extremely small and uniformly
dispersed cementite particles embedded within a continuous ferrite matrix.
Humayun Kabir, Dept of MME, BUET
 In general, over the broad range of tempering temperatures, hardness decreases and toughness
increases as the tempering temperature is increased. This is true if toughness is measured by
reduction of area in a tensile test.
 However, this is not entirely true if the notched bar such as Izod or Charpy impact test is used as a
measure of toughness.
 Most steels actually show a decrease in notched-bar toughness when tempered between 400°F and
800 °F, even though the piece at the same time loses hardness and strength.
 The reason for this decrease in toughness is not fully understood.
Humayun Kabir, Dept of MME, BUET
Humayun Kabir, Dept of MME, BUET
 The tempering range of 400 to 800°F is a dividing line between applications that require high hardness
and those requiring high toughness.
 If the principal desired property is hardness or wear resistance, the part is tempered below 400° F
 If the primary requirement is toughness, the part is tempered above 800°F
Humayun Kabir, Dept of MME, BUET
Humayun Kabir, Dept of MME, BUET
Surface Hardening of Steel
Lecture 12
 Numerous industrial applications require a hard, wear-resisting surface (called the case), and a
relatively soft, tough inside (called the core). Examples: roll, camshaft, gears, etc.
 Such a combination of properties of the same sample is impossible in any ordinary HT method
(such as annealing/normalizing/hardening).
 Consequently, several special heat treatment processes have been developed, called surface heat
treatment or case hardening.
1.
2.
3.
4.
5.
Carburizing
Nitriding
Cyaniding and carbonitriding
Flame hardening
Induction hardening
Transmission
Gears/Shafts
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 In this method, the carbon content of the surface of the steel is increased.
 Since the objective of case hardening is to produce a hard case and a tough core, the first
consideration is to select steel capable of producing a tough core.
 Thus, the steel should have low carbon (less than ~ 0.20%) content (unable to produce martensite).
 When the carbon content of the surface is raised, it becomes hardenable during subsequent heat
treatment.
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 During carburizing, steel is heated to an austenitizing temperature (usually about 925°C/1700°F)
 Presence of suitable carbon medium, where C diffuses into the steel surface
 Therefore, a surface layer of high carbon (about 1.2% C) is built up.
 Since the core is of low carbon content, the carbon atoms trying to reach equilibrium will begin to
diffuse inward.
 After diffusion has taken place for the required time, depending on the desired case depth, the
sample is removed from the furnace and cooled.
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 If the part is furnace cooled and examined microscopically, the carbon gradient will be visible in the
gradual change of the structure.
Core direction from the Surface
Surface
Fig. 1: Structure of 0.2% carbon steel pack carburized at 925 C for 6 h and furnace cooled.
5
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In carburizing, low-carbon steel is placed in an atmosphere that contains substantial amounts of carbon monoxide.
Hence, the following reaction takes placeFe + 2CO = Fe(C) +CO2
The carburization reaction is reversible and proceeds to the left, removing carbon from the surface layer. Thus, in
the presence of CO2 gas in the atmosphere, the removal of carbon, known as decarburization, occurs.
Fig. 2: Decarburized ferrite layer on the surface of a high-carbon annealed steel.
6
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Other decarburization reactions are
Fe (C) + H2O = Fe + CO + H2
Fe (C) + O2 = Fe + CO2
As a result, the surface will be depleted in carbon and will not transform into martensite on subsequent
hardening.
Decarburization can be prevented by using a suitable endothermic gas mixture (about 40% N2, 40% H2, and 20%
CO prepared by reacting hydrocarbon gas with air) to protect the surface of the steel from oxygen, carbon
dioxide, and water vapor.
7
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 Pack carburizing (using solid carburizing materials)
Parts are packed in a closed container (Heat resistant) with a mixture of hard coke/charcoal and
barium/sodium/calcium carbonate (used as an energizer).
Carbon cannot diffuse into steel directly. It reacts with oxygen first to form CO, then reacts with steel.
Mixtures in the form of coarse particles or lumps are used so that sufficient air is trapped to form CO after
sealing the container.
A typical reaction is:
BaCO3 = BaO + CO2 ; CaCO3 = CaO + CO2
Na2CO3 = NaO + CO2 ; CO2 + C = CO
Fe + 2CO  Fe (C) + CO2.
;
The commercial carburizing compound consists of hardwood charcoal, coke, and about 20 percent of
barium carbonate as an energizer.
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 Gas carburizing (using suitable hydrocarbon gases). Parts heated in methane, propane, natural gas, or
vaporized fluid hydrocarbon atmosphere, along with a carrier gas.Typical reaction:
CH4 + Fe  Fe(C) + H2
CH4 = Catomic + H2
Catomic +Fe  Fe(C) + H2
CO2 + C = CO
Fe + 2CO  Fe (C) + CO2
 Liquid carburizing (using fused baths of carburizing salts). Parts are placed in a bath of fused salts
containing sodium cyanide and alkaline earth salts (also known as a bath of molten cyanide).
Salts react with cyanide to form alkaline earth metal cyanide, which reacts with iron.Typical reactions:
2 NaCN + BaCl2 =2 NaCl + Ba(CN)2
Ba(CN)2 + Fe  Fe(C) + BaCN2.
9
Humayun Kabir, Dept of MME, BUET
The heat treatment of carburized steel is complex because of the variation of carbon content that
occurs in one piece of steel.
If steel is cooled slowly after carburizing from the carburized temperature, the surface hardness will be
low since the structure will effectively be that of annealed high-carbon steel, Fig.1.
Since steel is carburized in the austenite region, direct quenching from the carburized temperature will
harden the case and core if the cooling rate exceeds Critical Cooling Rate (CCR).
Direct quenching of coarse-grained steels often leads to brittleness and distortion, so this treatment
should only be applied to fine-grained steels.
10
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The heat treatment given to carburized steel depends on
(1) the carburizing temperature used,
(2) the composition of the core and case, and
(3) the purpose for which the part is intended or the properties that must be obtained to meet
the specifications.
 When a carburized part is hardened, the case will appear as a light martensite zone followed by a
darker transition zone.
 The effective case (or hard case) is measured from the outer edge to the middle of the dark zone.
From the nature of the carbon gradient, the hard case contains a portion above 0.40 percent carbon
and approximately equal to two-thirds of the total case.
 The distance from the surface to the point where hardness level HRC 50 or a carbon content of
about 0.4 weight percent.
11
Humayun Kabir, Dept of MME, BUET
Cases containing carbon and nitrogen are produced in liquid salt baths (cyaniding) or by the use of gas
atmosphere (carbo-nitriding).
N imparts inherent hardness by forming hard nitride compounds, and increased C content makes the
steel surface hardenable during quenching.
Cyaniding is closely related to carburizing. Liquid carburizing may be distinguished from cyaniding by
the character and composition of the case produced. The cyanide case is higher in nitrogen and lower
in carbon; the reverse is true of liquid carburized cases.
The heating temperature is about 760 – 875 °C (1400-1600˚F), generally lower than that used in
carburizing.
Exposure time is shorter, resulting in thinner cases (up to 0.01 inch for cyaniding and up to 0.03 inch
for carbo-nitriding).
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Humayun Kabir, Dept of MME, BUET
The case usually contains about 0.5 – 0.8 % C (carbon content is lower than that produced by
carburizing) and 0.5 % N.
The proportion of nitrogen and carbon in the case produced by a cyanide bath depends on the bath’s
composition and temperature (prominent).
Typical bath composition is: 30% sodium cyanide, 40% sodium carbonate, and 30% sodium chloride.
General reactions to take place:
2NaCN + 2O2 = Na2CO3 + CO + 2N
2CO = CO2 + C
2NaCN + O2 = 2NaCNO
NaCN + CO2 = NaCNO + CO
3NaCNO = NaCN + Na2CO3 + C + 2N
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Humayun Kabir, Dept of MME, BUET
Carbonitriding is a modification of gas carburizing
Adding anhydrous ammonia gas to the furnace atmosphere causes both C and N to be absorbed
simultaneously by the steel surface at the carbonitriding temperature.
Actually, carbonitriding is a modification of carburizing, and the name “nitrocarburizing” is more
descriptive.
Although a wide variety of gas mixtures are used, the typical composition is 15 % anhydrous ammonia
(provides nitrogen to the surface), 5 % natural gas (enriching gas and is the primary source of the
carbon added to the surface), 80 % carrier (mixture of N2, H2, and CO; endothermic) gas.
The heating temperature range is 650 – 885 °C, lower than those used for gas carburizing. Case depth
rarely exceeds 0.02 in (due to lower heating temperatures).
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Humayun Kabir, Dept of MME, BUET
This is a process for case hardening of alloy steel in an atmosphere consisting of a mixture in suitable
proportions of ammonia gas and dissociated ammonia.
The effectiveness of the process depends on the formation of nitrides in the steel by the reaction
of nitrogen with certain alloying elements.
Although at suitable temperatures and with the proper atmospheres, all steels can form iron nitrides;
The best results are obtained in steels containing one or more of the major nitride-forming alloying
elements like Al, Cr, or Mo.
The parts to nitride are placed in an airtight container through which the nitriding atmosphere is
supplied continuously while the temperature is raised and held between 925˚F (496˚C) and 1050˚F
(565˚C).
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Humayun Kabir, Dept of MME, BUET
The nitriding cycle is quite long, depending upon the case depth desired. A 60h-cycle will give a case
depth of approximately 0.024 inches at 975˚F (524˚C).
A nitride case consists of two distinct zones.
In the outer zone, the nitride-forming elements, including iron, have been converted to nitrides. This
layer is known as the white layer.
In the zone beneath this white layer, alloy nitrides only have been precipitated.
The white layer is brittle and tends to chip or spall from the surface if it has a thickness over 0.0005
inches.
Thicker white layers must be removed by grinding after nitriding.
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Humayun Kabir, Dept of MME, BUET
Since nitriding is performed at relatively low temperatures and no quenching (and tempering) is
required, distortion is reduced to a minimum.
However, some growth does occur due to the increase in volume of the case.
This growth is constant and predictable for a given part and cycle, so in most cases, parts may be
machined very close to the final dimensions before nitriding. This is an advantage of nitriding over
carburizing.
Some complex parts that cannot be case hardened satisfactorily by carburizing have been nitrided
without difficulty.
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Humayun Kabir, Dept of MME, BUET
Wear resistance is an outstanding characteristic of the nitride case and is responsible for its selection
in most applications.
The hardness of a nitride case is unaffected by heating to temperatures below the original nitriding
temperature.
Fatigue resistance is also an important advantage.
Although it is sometimes indicated that nitriding improves the corrosion resistance of steel, this is true
only if the white layer is not removed.
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Humayun Kabir, Dept of MME, BUET
Disadvantages of nitriding include
1.
the long cycles usually required,
2.
the brittle case,
3.
the use of special alloy steels if maximum hardness is to be obtained,
4.
the cost of ammonia atmosphere, and
5.
the technical control required.
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Humayun Kabir, Dept of MME, BUET