WELDING AND COATING METALLURGY

1 INTRODUCTION 4
1.1 ROLE OF CARBON IN STEEL 4
1.2 WELDABILITY OF STEELS 5
2 CRYSTAL STRUCTURE 7
2.1 SOLUBILITY OF CARBON 8
3 IRON - IRON CARBIDE PHASE DIAGRAM 9
3.1 AUSTENITE (γ) 10
3.2 FERRITE (α) 11
3.3 PERITECTIC 11
3.4 PEARLITE 12
3.4.1 PEARLITE GROWTH 13
3.5 PRO-EUTECTOID FERRITE 14
3.6 PHASE TRANSFORMATIONS IN LOW ALLOY STEELS 15
3.7 GRAIN GROWTH 16
3.8 NON-EQUILIBRIUM COOLING 17
3.9 MARTENSITE - EFFECT OF RAPID COOLING 18
3.10 BAINITE 19
4 TRANSFORMATION DIAGRAMS 20
4.1 TIME TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS 20
4.2 CONTINUOUS COOLING TRANSFORMATION (CCT) DIAGRAMS 23
4.2.1 CRITICAL COOLING RATES 24
4.2.2 DETERMINING CCT DIAGRAMS 24
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4.3 EFFECT OF ALLOYING ELEMENTS 24
4.4 M
s

and M
f
TEMPERATURES 25
5 HARDENABILITY / WELDABILITY OF STEELS 26
5.1 CARBON EQUIVALENT (CE) & WELDABILITY 28
5.2 TEMPERING – EFFECTS OF REHEATING 29
5.3 SECONDARY HARDENING 30
6 HYDROGEN CRACKING RELATED TO WELDABILITY 32
6.1 LAMELLAR TEARING 34
7 REHEAT CRACKING IN THE HAZ 36
8 WELDING STEELS CONSIDERED DIFFICULT 38
8.1 PROCEDURAL CONSIDERATIONS 38
8.2 POST WELD HEAT TREATMENT (PWHT) 38
8.3 THE HEAT AFFECTED ZONE (HAZ) 38
8.3.1 LOSS OF TOUGHNESS IN THE HAZ 41
8.4 PREHEAT & CARBON EQUIVALENT 41
8.4.1 SEFERIAN GRAPH 42
9 SUMMARY 44
10 GENERAL ASPECTS CONCERNED WITH WEAR 45
PROTECTIVE COATINGS 45
11 SELECTING THE OPTIMUM WEAR RESISTANT SOLUTION 46
12 METHODS OF DEPOSITION 47
13 WELDING PROCEDURAL GUIDELINES 49
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13.1 BASE METAL CONSIDERATIONS 49
13.1.1 WELDABILITY FACTORS 49
14 APPLICATION OF WEAR PROTECTIVE COATINGS 50
14.1 BASE METAL PREPARATION 50

14.2 PREHEAT 50
14.3 BUILD-UP 51
14.4 APPLICATION TECHNIQUE 51
14.5 COOLING PROCEDURE 51
14.6 FINISHING 51
15 WEAR PATTERNS AND PRODUCT SELECTION 52
15.1 WEAR PATTERNS FOR ABRASIVE AND IMPACT WEAR 52
15.2 WEAR PLATES AND GROUSER BARS 60
15.2.1 WEAR PLATES 60
15.2.2 GROUSER BARS (EG, BARS FOR REBUILDING WORN 60
16 SURFACING ALLOYS 62
16.1 CHROMIUM CARBIDE WEARFACING ALLOYS 62
16.2 WORK HARDENING ALLOYS (AUSTENITIC MANGANESE STEEL) 63
16.3 IRON BASED BUILD UP AND WEARFACING ALLOYS 64
16.4 TUNGSTEN CARBIDE WEARFACING ALLOYS 65
16.5 Ni BASED WEARFACING ALLOYS 66
17 GRADING OF WEAR RESISTANCE OF HARDFACING ALLOYS 67
18 METHODS OF WEAR PROTECTION - SUMMARY 68
19 REFERENCES 69
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WELDING AND COATING METALLURGY

1 INTRODUCTION

Steels form the largest group of commercially important alloys for several reasons:
♦ The great abundance of iron in the earth’s crust
♦ The relative ease of extraction and low cost
♦ The wide range of properties that can be achieved as a result of solid state transformation such as

alloying and heat treatment
1.1 ROLE OF CARBON IN STEEL

Steels are alloys of iron with generally less than 1% carbon plus a wide range of other elements. Some of
these elements are added deliberately to impart special properties and others are impurities not completely
removed (sometimes deliberately) during the steel making process. Elements may be present in solid
solution or combined as intermetallic compounds with iron, carbon or other elements. Some elements,
namely carbon, nitrogen, boron and hydrogen, form interstitial solutions with iron whereas others such as
manganese and silicon form substitutional solutions. Beyond the limit of solubility these elements may
also form intermetallic compounds with iron or other elements. Carbon has a major role in a steels
mechanical properties and its intended use as illustrated in Figure 1.
As the carbon concentration is increases carbon steel, in general, becomes stronger, harder but less ductile.

This is an important factor when a steel is required to be welded by joining or surfacing.
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Figure 1 Role of Carbon in Steel

1.2 WELDABILITY OF STEELS

When considering a weld, the engineer is concerned with many factors such as design, physical
properties, restraint, welding process, fitness-for-purpose etc., which can conveniently be summarized as
the base materials “weldability”. Weldability can be defined as “the capacity of a metal to be welded
under the fabrication conditions imposed into a specific, suitably designed structure, and to perform
satisfactorily in the intended service.”

Welding is one of the most important and versatile means of fabrication and joining available to industry.
Plain carbon steels, high strength low alloy (HSLA) steels, quench and tempered (Q&T) steels, stainless

steels, cast irons, as well as a great many non-ferrous alloys such as aluminium, nickel and copper are
welded extensively. Welding is of great economic importance, because it is one of the most important
tools available to engineers in his efforts to reduce production, fabrication and maintenance costs.
A sound knowledge of what is meant by the word “weld” is essential to an understanding of both welding
and weldability. A weld can be defined as a union between pieces of metal at faces rendered plastic or
liquid by heat, or pressure, or both, with or without the use of filler metal. Welds in which melting
occurs are the most common. The great majority of steels welded today consist of low to medium carbon
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steel (less than 0.4%C).Practical experience over many years has proved that not all steels are welded with
ease. For example, low carbon steels of less than 0.15%C can be easily welded by nearly all welding
processes with generally high quality results. The welding of higher carbon steels or relatively thick
sections may or may not require extra precaution. The degree of precaution necessary to obtain good
quality welds in carbon and alloy steels varies considerably. The welding procedure has to take into
consideration various factors so that the welding operation has minimal affect on the mechanical
properties and microstructure of the base metal.

The application of heat, generally considered essential in a welding operation, produces a variety of
structural, thermal and mechanical effects on the base metal being welded and on the filler metal being
added in making the weld. Effects include:
♦ Expansion and contraction (thermal stresses etc.)
♦ Metallurgical changes (grain growth etc.)
♦ Compositional changes (diffusion effects etc.)

In the completed weld these effects may change the intended base metal characteristics such as strength,
ductility, notch toughness and corrosion resistance. Additionally, the completed weld may include defects
such as cracks, porosity, and inclusions in the base metal, heat affected zone (HAZ) and weld metal itself.
These effects of welding on any given steel are minimized or eliminated through changes in the detailed
welding techniques involved in producing the weld.



It is important to realize that the suitability of a repair weld on a component or structure for a specific
service condition depends upon several factors:
♦ Original design of the structure, including welded joints
♦ The properties and characteristics of the base metal near to and away from the intended welds
♦ The properties and characteristics of the weld material
♦ Post Weld Heat Treatment (PWHT) may not be possible

As discussed, a steels weldability will be dependent upon many factors but the amount of carbon will be a
principal factor. A steels weldability can be categorized by its carbon content as shown in Table 1.
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Table 1 Common Names and their Typical Uses for Carbon Steel

COMMON NAME %C TYPICAL
HARDNESS
TYPICAL USE WELDABILITY
Low C steel
<0.15 60 Rb Sheet, strip, plate Excellent
Mild Steel
0.15 – 0.30 90 Rb Structural shapes,
plate, bar
Good
Medium C Steel
0.30 – 0.50 25 Rc Machine parts, tools Fair
(preheat & postheat
normally required;
low H

2

recommended)
High C Steel
0.50 – 1.00 40 Rc Springs, dies, rails Poor – Fair
(preheat and post
heat; low H
2

recommended)

In order to understand the physical and chemical changes that occurs in steels when they are welded, a
basic understanding of the metallurgy of steels is necessary.
2 CRYSTAL STRUCTURE

Iron has the special property of existing in different crystallographic forms in the solid state. Below
910°C the structure is body-centred cubic (bcc). Between 910°C and 1390°C iron changes to a face-
centred cubic (fcc) structure.

Figure 2 Transformation of crystal structure for iron showing contraction occurring at 910°C.

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Figure 3 BCC Crystal Structure





Figure 4 FCC Crystal Structure

Above 1390°C and up to the melting point at 1534°C the structure reverts back to body-centred
cubic form. These are known as allotropic forms of iron. The face-centred cubic form is a close-
packed structure being more dense than the body-centred cubic form. Consequently iron will
actually contract as it is heated above 910°C when the structure transformation takes place.
2.1 SOLUBILITY OF CARBON

The solubility of carbon in the bcc form of iron is very small, the maximum solubility being only
about 0.02 wt.% at 723°C. Figure 5 shows there is negligible solubility of carbon in iron at
ambient temperature (less than 0.0001 wt.%). Since steels nearly always have more carbon than
this, the excess carbon is not in solution but present as the intermetallic compound iron-carbide
Fe
3
C known as cementite.
`
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Figure 5 Solubility of carbon in
α
(bcc) iron as a function of temperature


In contrast the fcc form of iron dissolves up to 2% carbon, well in excess of the usual carbon
content of steels. A steel can therefore be heated to a temperature at which the structure changes
from bcc to fcc and all the carbon goes into solution. The way in which carbon is obliged to
redistribute itself upon cooling back below the transformation temperature is the origin of the wide
range of properties achievable in steels.
3 IRON - IRON CARBIDE PHASE DIAGRAM


Fundamental to a study of steel metallurgy is an understanding of the iron – iron carbide phase
diagram. The diagram commonly studied is actually the metastable iron – iron carbide system.
The true stable form of carbon is graphite, but except for cast irons this only occurs after
prolonged heating. Since the carbon in steels is normally present as iron carbide, it is this system
that is considered. Figure 7 shows the iron – iron carbide system up to 6 wt.% carbon. We will
now consider several important features of this diagram.

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Figure 6 The iron-iron carbide equilibrium phase diagram

A eutectic is formed at 4.3% carbon. At 1147°C liquid of this composition will transform to two
solid phases (austenite + cementite) on cooling. This region is important when discussing cast
irons but is not relevant to steels.

TALLURGY2
3.1 AUSTENITE (
γ
)

This region in which iron is fcc, identified in Figures 7 and 8, dissolves up to 2% carbon. This
phase is termed austenite or gamma phase. With no carbon present it begins at 910°C on heating
but with 0.8% carbon it starts at 723°C. When a steel is heated into the austenite region all carbon
and most other compounds dissolve to form a single phase (i.e. normalizing).
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Figure 7 The austenite region of the iron-iron carbide diagram showing
maximum solubility of up to 2%C

3.2 FERRITE (
α
)

The region shown in Figure 9 where carbon is dissolved in bcc iron is very narrow, extending to
only 0.02% carbon at 723°C. This phase is termed ferrite or alpha phase. Although the carbon
content of ferrite is very low other elements may dissolve appreciably in it so ferrite cannot be
considered as “pure iron”.

Figure 8 The ferrite region of the iron-iron carbide diagram
3.3 PERITECTIC

The region at the top left portion of the phase diagram enlarged in Figure 10 is where the iron
reverts back to the bcc structure known as delta ferrite. Here again the solubility for carbon is
low, only 0.1 wt.% at 1493°C. The part of the diagram at 0.16% carbon having the appearance of
an inverted eutectoid is called a peritectic. At this point a two phase mixture of liquid and solid
(austenite) transforms on cooling to a single phase solid of austenite. This portion of the phase
diagram will not be discussed in detail, but it should be recognized since it has been invoked to
explain various hot cracking phenomena in welding.

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Figure 9 Peretectic region of the iron-iron carbide diagram
3.4 PEARLITE

At 0.8% carbon and 723°C a eutectoid is formed as illustrated in Figure 11. This is similar to the
eutectic transformation but involves a solid phase transforming into two different phases on
cooling (ferrite and cementite). This eutectoid mixture is called pearlite. Figure 12 shows how
the two phase constituents that make up pearlite are formed. Note that pearlite is only one of

many phases that can be produced from ferrite and cementite (depending on cooling rate).
Cementite (iron carbide) itself is very hard - about 1150 Hv – but when mixed with the soft ferrite
layers to form pearlite, the average hardness of pearlite is considerably less.




Figure 10 The eutectoid point on the iron-iron carbide diagram
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Figure 11 Schematic View of how pearlite is formed in an approx. 0.4%C steel


This region of the phase diagram (where carbon concentration is less than 0.8%) is of the most
interest to a study of steels and their weldability which will be discussed in more detail later.

3.4.1 PEARLITE GROWTH

A steel with 0.8 wt.% carbon, it will be recalled, transforms on cooling through 723°C to the two
phase eutectoid constituent pearlite. In pearlite the two phases ferrite and cementite are mixed
closely together in fine layers. As the ferrite contains very little carbon while the cementite has
6.7%, carbon atoms must diffuse to the growing cementite plates as shown in Figure 13.


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Figure 12 Schematic View of different pearlite growth rates

The distance they can diffuse, and hence the spacing of the plates, depends on how fast the pearlite
is growing. A fast growth rate means less time for diffusion and a finer pearlite results. Figure 14
shows a typical pearlite microstructure.


Figure 13 Typical Lamellar Appearance of Pearlite. Mag:X1500

3.5 PRO-EUTECTOID FERRITE

If the steel has less than 0.8 wt.% carbon (termed hypo-eutectoid steel) ferrite will be formed first
from the austenite. The example in Figure 15 shows a steel of 0.4 wt.% carbon. This ferrite is
called pro-eutectoid ferrite because it transforms first on cooling as illustrated in Figure 15. As
transformation continues and the temperature drops, the remaining austenite becomes richer in
carbon. At 723°C the steel comprises ferrite and the remaining austenite (which contains 0.8wt.%
carbon). With further cooling, the austenite then transforms to pearlite producing a final structure
in the steel of pro-eutectoid ferrite and pearlite.


Figure 14 Phase Transformation on Cooling a 0.4%C Steel
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The amounts of pro-eutectoid ferrite and pearlite can be estimated by application of the lever rule
(see references for more detailed information). For a 0.4 wt.% carbon steel about 50% will be
ferrite and 50% pearlite. Similarly a steel of more than 0.8 wt.% carbon (from 0.8 wt.% up to 1.8
wt.% carbon is termed hyper-eutectoid steel) first transforms to cementite (i.e. pro-eutectoid
carbide) with the remaining austenite forming pearlite as shown in Figure 16.



Figure 15 Phase Transformation on Cooling a 1.2%C Steel
3.6 PHASE TRANSFORMATIONS IN LOW ALLOY STEELS

Figure 17 shows the appearance of a polished and etched section of an approximately 0.6wt.%
carbon steel. You can see that the pro-eutectoid ferrite has formed initially at the austenite grain
boundaries, nucleation taking place at several points around each austenite grain. Since each
region of ferrite becomes an individual grain, its grain size will be very much smaller than that of
the parent austenite. Ferrite continues to form and grow until the final transformation of
remaining austenite to pearlite. The ferrite does not always appear as neat, equiaxed grains as
shown in Figure 17, but can occur as long spikes from the grain boundaries or even nucleate
within the austenite grain. This can occur quite markedly from the welding process due to the
cooling rates imposed by the heat input (i.e. travel speed).



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Figure 16 Prior Austenite Boundaries Showing Pro-Eutectoid Ferrite

On reheating the steel the process reverses and the pearlite and ferrite grains transform back into
single phase austenite to form completely new grains. The temperature required to get complete
transformation depends on the carbon level as seen from the phase diagram (see Figures 7 and
15)and ranges from 910°C for zero carbon to 723°C for 0.8 wt.% carbon.

3.7 GRAIN GROWTH

Heating to higher temperatures than those necessary to get complete transformation causes the
austenite grains to grow. The final size of the austenite grains depends not only on the

temperature reached but also on the type of steel. Some steels containing small precipitates such
as aluminium and vanadium nitride retain small grain size up to high temperatures. These are
known as fine grained steels. Steels can be deliberately made as coarse grain or fine grain. Fine
grained steels are tougher and are more commonly specified for most structural applications.

The effect of austenizing temperature on grain size is shown in Figure 18. It shows that although
grain growth is restricted in a fine grain steel, at a sufficiently high temperature the precipitates
dissolve and the steel behaves as a coarse grain steel. Thus at sufficiently high temperature, grain
growth can occur with subsequent loss of toughness. This is an important consideration in the
HAZ associated with welding.


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Figure 17 Schematic Effect of Temperature on Grain Growth for Coarse and Fine Grained Steels
3.8 NON-EQUILIBRIUM COOLING

The phases and microstructures predicted by the iron – iron carbide diagram occur in steels cooled
very slowly. In addition the diagram assumes that carbon is the only alloying element present in
the steel. With the addition of other common alloying elements such as manganese, silicon,
nickel, titanium, molybdenum, chromium etc., the phase diagram can still be used except that it
will be distorted and the lines may move to slightly different locations.
Figure 18 Effect of Various Element Additions on the Recrystallization Temperature


For example the presence of alloy elements changes the recrystallization (eutectoid) temperature
as shown in Figure 19. In structural steels the concentration of alloys is generally quite small
(austenitic manganese steels are an exception containing over 12 wt.% manganese) and the basic
iron – iron carbide phase diagram is not distorted very much from equilibrium conditions.


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3.9 MARTENSITE - EFFECT OF RAPID COOLING

The rate of cooling has a major effect on the types of microstructures formed and unless the steel
cools slowly the iron – iron carbide phase diagram cannot be used. The reason is that the
transformation of austenite to pearlite requires the diffusion of carbon to the sites of growing
carbon, a process which takes time. We saw how a faster cooling rate produced finer pearlite.
With even faster cooling rates less time is available for diffusion and pearlite cannot form.
Alternative microstructures form with their exact morphology depending on just how quickly the
steel cools. In a water quench, for example, the cooling rate is so rapid there is no time for any
diffusion, and the carbon remains trapped in the same place as it was in the austenite. A rapid
quench cannot suppress the crystal structure change from fcc to bcc but the presence of trapped
carbon in the bcc phase distorts it to a tetragonal shape, as indicated in Figure 20, rather than a true
cubic structure. This is called martensite.

Figure 19 Schematic Transformation of Austenite (BCC) To Martensite (Tetragonal) With Increasing %C



The amount of carbon influences the amount of distortion in the crystal structure as shown in
Figure 20. This in turn affects the hardness of the martensite as shown in Figure 22.

Under the microscope as shown in Figure 21 martensite has the appearance of a mass of needles.
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Figure 20 Martensite Microstructure



Martensite can be very hard and brittle when it contains appreciable amounts of carbon. The
hardness
depends almost exclusively on the carbon

content with other elements having little effect as illustrated in
Figure 22.
Figure 21 Effect of Carbon and Alloying on the Hardness of Martensite

The formation of martensite can occur in the HAZ adjacent to a weld deposit due to the fast
cooling rates imposed by the welding process. This is discussed in more detail in Section !!

3.10 BAINITE

Intermediate between a rapid quench that produces martensite, and a slow cool producing pearlite,
other constituents may form particularly in alloy steels. The most important of these is bainite.

Figure 22 Microstructure of lower bainite (X1000)

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Bainite is still a two phase mixture of ferrite and iron carbide but unlike the cementite plates in
pearlite the carbide in bainite is spherical. Bainite formed above 300°C contains relatively coarse
particles of the Fe
3
C form of iron carbide (cementite) and is termed upper bainite. When formed
below 300°C bainite has a much finer structure with the carbides tending to form striations across
the ferrite laths. This is termed lower bainite. The carbides in lower bainite are Fe
2.4
C known as
epsilon (ε) carbide. Some steels in the bainitic condition may possess ductility and toughness

superior to that shown by the same steel in the Q&T condition.

4 TRANSFORMATION DIAGRAMS

Since the iron – iron carbide phase diagram is only valid for very slow cooling rates, alternative
diagrams for determining the constituents present in a more rapidly cooled steel have been
developed. There are two types:
♦ Time Temperature Transformation (TTT) curves where the steel sample is held at a constant
temperature until transformation is complete.
♦ Continuous Cooling Transformation (CCT) curves where the steel sample is cooled from the
austenitic region at different cooling rates.

Although these diagrams are principally designed for the foundry metallurgist and heat treater etc.,
they are an excellent tool for use by welding engineers where fast cooling rates need to be
evaluated near to the welded area.

4.1 TIME TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS

Consider heating a sample of steel until it is fully austenitic then quenched to some temperature
below the equilibrium transformation temperature as shown in Figure 24.


Figure 23 Schematic Representation of TTT

If we hold the steel at this temperature we find there is a delay before transformation begins and a
further elapse of time while transformation takes place. The delay depends on the temperature at
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which the steel is held and we can plot this information on a diagram of temperature against time
for a given steel composition.


Figure 24 Schematic TTT Curve for Carbon Steel

An example of such a time-temperature-transformation (TTT) diagram for a carbon steel is shown
in Figure 25. Note that at high temperatures (Figure 26) the steel transforms to pro-eutectoid
ferrite followed by pearlite.

Figure 25 TTT Curve Illustrating High Temperature Transformation of Pro-Eutectoid Ferrite

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At lower temperatures less pronounced pro-eutectoid ferrite is formed and the pearlite is finer. At
about 550°C the pearlite forms in the shortest time and there is no pro-eutectoid ferrite (Figure
27).

Figure 26 TTT Curve Illustrating Pearlite Transformation

Cooling down to below this range (approximately 450°C) transformation to bainite occurs, taking
a longer time for lower temperatures (Figure 28).

Figure 27 TTT Curve Illustrating Transformation to Bainite

At a sufficiently fast cooling down to low temperature martensite can begin to form (Figure 29).
Note that it forms almost instantaneously and does not grow as a function of time. For each steel
specification there is a fixed temperature M
s
at which martensite starts to form and a fixed
temperature M
f
at which transformation is complete. The percentage of martensite formed

therefore depends only on the temperature to which the steel is rapidly cooled c and not on how
long it is held there. If the composition of the steel is known, the M
s
temperature can be calculated
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(see Section !!!). Note that for some compositions the M
f
temperature can be below ambient
temperature.
Figure 28 TTT Curve Illustrating Martensite Formation

4.2 CONTINUOUS COOLING TRANSFORMATION (CCT) DIAGRAMS

Now consider the case of continuous cooling. We may superimpose a cooling curve on the TTT
diagram as illustrated in Figure 30 in order to get an idea of what microstructures form, but it is
more accurate to use a diagram established under continuous cooling conditions. The CCT
diagram is slightly different from the TTT curve.
Figure 29 Cooling Curves Superimposed onto TTT Curve for Typical Carbon Steel


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4.2.1 CRITICAL COOLING RATES

You should note that in plain carbon steels bainite generally will not form during continuous
cooling because of the shape of the TTT diagram. The bainite region is tucked under the pearlite
area so a cooling curve either hits the pearlite curve or misses it completely as shown in Figure 30.
At cooling rates fast enough to miss the nose of the curve martensite is formed.


This is an important concept since the cooling rate at which martensite can form in a HAZ
strongly influences the risk of cracking during welding and gives an indication of a steels
“weldability”. This will be discussed in more detail in Section 8.


4.2.2 DETERMINING CCT DIAGRAMS

The exact shape of a CCT curve depends on the chemistry of the steel and on the heating and
cooling cycles. CCT diagrams are available for numerous carbon and alloy steels and if desired
can even be established for specific weld metal.

4.3 EFFECT OF ALLOYING ELEMENTS

Alloy elements have significant effects on the shape of the CCT and TTT diagrams which allow
different microstructures to be produced in alloy steels. Chromium and molybdenum, for
example, shift the top (pearlite) part of the curve to the right i.e. to longer times, thus exposing the
bainite region. Steels containing these elements such as 4135 can produce bainite on continuous
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cooling.
Figure 30 CCT Curve for 4135 Steel

The entire TTT curve may also shift to the right with additions of certain elements (e.g.
chromium, vanadium, molybdenum and others) to greater times allowing martensite to form at
much slower cooling rates. This increases the “hardenability” of the steel, but also increases the
risk of cracking from welding if proper precautions are not taken.

4.4 M
S
AND M

F
TEMPERATURES

The other notable effect of alloy element addition is to change the martensite start (M
s
) and
martensite finish (M
f
) temperatures. Increasing the carbon content, for example, depresses the M
s

to lower temperatures as shown in Figure 32.

Figure 31 Schematic Diagram Showing the Influence of %C on Martensitic Start Temperature

Other elements affect martensite formation and the combined affect can be approximated by the
following equation:

M
s
(°C) = 550 – 350 ×%C - 40×%Mn - 35×%V - 20×%Cr - 17×%Ni -10×%Cu - 10×%Mo - 5×%W + 15×%Co +
30×%Al

As mentioned earlier, if the M
f
is below ambient temperature, martensite transformation is not
completed and the steel contains retained austenite.
Note that since the product of various alloy element additions affects the Ms, we can affect a steels
hardenability by using small quantities of several alloy elements rather than a large quantity of one
element, for example, carbon. This is important when designing a steel for not only hardenability

but also its weldability.
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