Metallurgy

Mohammud Hanif Dewan,
Maritime Lecturer & Trainer, Bangladesh
METALLURGY

DUCTILITY
• A metal is ductile when it may be drawn out in tension without
rupture.
• Wire drawing depends upon ductility for its successful
operation.
• A ductile metal must be both strong and plastic
• With many materials ductility increase rapidly with heat.
• Is the property of a material which enables it to be drawn
easily into wire form
• The percentage elongation and contraction of area, as
determined from a tensile test are a good practical measures
of ductility
• Ability to undergo permanent change in shape without rupture
or loss of strength if any force applied.
5/28/2015
Mohd. Hanif Dewan, Chief Engineer and
Maritime Lecturer & Trainer, Bangladesh.
2

MALLEABILITY
• The ability to be hammered or rolled out without
cracking.
• Very few metals have good cold malleability, but
most are malleable when heated to a suitable
temperature
• The material that can be shaped by beating or
rolling is said to be malleable.
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ELASTICITY
• The elasticity of a metal is its power of returning
to its original shape after deformation by force.
• The ability to return to the original shape or size
after having been deformed or loaded.
• All strain in the stressed material disappears
upon removal of the stress.
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PLASTICITY
• The property of flowing to a new shape under
pressure/stress and retaining on the new shape
after removal of pressure/stress.
• This is a rather similar property to malleability, and
involves permanent deformation without rupture.
• It is opposite to elasticity
• The ability to deform permanently when load is
applied.
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Modulus of Elasticity E defined as the ratio of
tensile stress to strain and determined in a tensile
test.
Modulus of Rigidity G defined as the ration of
shear stress and strain and determined in a torsion
test.
Bulk Modulus K defined as the ration of pressure
and volumetric strain and found with specialised
equipment for liquids.
Poisson’s ratio ν defined as the ratio of two
mutually perpendicular strains and governs how
the dimensions of a material change such as
reduction in diameter when a bar is stretched.
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TOUGHNESS
• Resistance to fracture by blows.
• The materials usually have high tenacity combined with
good or fair ductility.
• Toughness decreases with heating.
• A combination of strength and the ability to absorb energy
or deform plastically.
• A condition between brittleness and softness.
• A materials ability to sustain variable load conditions
without failure..
• Materials could be strong and yet brittle but a material is
tough has strength
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HARDNESS
• The hardness of a metal is a measure of its ability to
withstand scratching, wear and abrasion,
indentation by harder bodies, etc.
• The machine ability and inability to cut are also
hardness property which is important for workshop
process.
• Hardness also decreased by heating
• A material’s resistance to erosion or wear will
indicate the hardness of the material
• A material’s ability to resist plastic deformation
usually by indentation
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HARDNESS MATERIALS LIST:
Hard materials are diamonds and glass. Soft materials are copper
and lead. Hardness is measured by comparing it to the hardness
of natural minerals and the list is called the Moh scale. The list
runs from 1 to 10 with 1 being the softest ands 10 the hardest.
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10 Diamond
9 Corundum
8 Topaz
7 Quartz
6 Feldspar
5 Apatite
4 Fluorite
3 Calcite
2 Gypsum
1 Talc

BRITTLENESS
• Opposite of toughness.
• A brittle material breaks easily under a sharp
blow, although it may resist a steady load quite
well.
• Brittle materials are neither ductile or malleable,
but they often have considerable hardness.
• As a lack of ductility
• Strong materials may also be brittle
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STIFFNESS/RIGIDTY
- This is the property of resisting deformation within
the elastic range and for ductile materials is
measured by the Modulus of Elasticity. A high E
value means that there is a small deformation for a
given stress.
- The property of a solid body to resist deformation,
which is sometimes referred to as rigidity.
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Strength
• The greater the load which can be carried the
stronger the material and strength of the
material will be higher.
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Tensile strength
• This is the main single criterion with reference to
metals.
• This is the ability of a material to withstand
tensile loads without rupture when the material
is in tension
• It is a measure of the material’s ability to
withstand the loads upon it in service.
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If the material is ductile, we look for the point at which it
starts to stretch like a piece of plasticine. This point is
called the yield point and when it stretches in this manner,
we call it PLASTIC DEFORMATION.
If the material is not ductile, it will snap without becoming
plastic. In this case, we look for the stress at which it snaps
and this is called the ULTIMATE TENSILE STRENGTH.
Most materials behave like a spring up to the yield point
and this is called ELASTIC DEFORMATION and it will
spring back to the same length when the load is removed.
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The tensile test is carried out with a standard sized specimen and the force required to stretch it,
is plottedagainsttheextension. Typical graphs are shown below.

Ultimate tensile strength (UTS)
(Tensile strength or Ultimate Strength)
- It is the maximum stress that a material can withstand
while being stretched or pulled before failing or breaking.
Tensile strength is not the same as compressive strength
and the values can be quite different.
- UTS is usually found by performing a tensile test and
recording the engineering stress versus strain. The highest
point of the stress-strain curve (see point 1 on the
engineering stress/strain diagrams below) is the UTS.
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Stress vs. Strain
curve typical of
aluminum.
1 Ultimate Strength
2 Yield Strength
3 Proportional Limit
Stress
4 Rupture
5 Offset Strain
(usually 0.002)
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Compressive Strength
• This is the ability of a material to withstand Compressive (squeezing) loads
without being crushed when the material is in compression.
Shear Strength
• This is the ability of a material to withstand offset or traverse loads without
rupture occurring.
Fatigue Strength
• This is the property of a material to withstand continuously varying and
alternating loads.
Yeild Strength
The stress a material can withstand without permanent deformation.
Torsional Strength
This governs the stress at which a material fails when it is twisted and a test
similar to the tensile test is carried out, only twisting the specimen instead of
stretching it. This is a form of shearing.
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• Heat treatment is a general term referring to a cycle of
heating and cooling which alters the internal structure of a
metal and thereby changes its properties
• Metal and alloys are heat treated for a number of
purposes however the primarily to:-
1. Increase their hardness and strength
2. To improved ductility
3. To soften them for subsequent operations (cutting etc)
4. Stress relieving
5. Eliminate the effects of cold work
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HEAT TREATMENT OF STEEL
The mechanical properties of materials can be changed by
heat treatment. Let’s first examine how this applies to
carbon steels.
CARBON STEELS
In order to understand how carbon steels are heat treated
we need to re-examine the structure. Steels with carbon fall
between the extremes of pure iron and cast iron and are
classified as follows.
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All metals form crystals when they cool down and change from liquid
into a solid. In carbon steels, the material that forms the crystals is
complex. Iron will chemically combine with carbon to form IRON
CARBIDE (Fe3C). This is also called CEMENTITE. It is white, very
hard and brittle. The more cementite the steel contains, the harder and
more brittle it becomes.
When it forms in steel, it forms a structure of 13% cementite and 87%
iron (ferrite) as shown. This structure is called PEARLITE. Mild steel
contains crystals of iron (ferrite) and pearlite as shown. As the %
carbon is increased, more pearlite is formed and at 0.9% carbon, the
entire structure is pearlite.5/28/2015
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NAME
Dead mild
CARBON %
0.1 – 0.15
TYPICAL APPLICATION
pressed steel body panels
Mild steel
Medium carbon steel
High carbon steels
Cast iron
0.15 – 0.3
0.5 – 0.7
0.7 – 1.4
2.3 – 2.4
steel rods and bars
forgings
springs, drills, chisels
engine blocks

1538
1130
2.0
oC
695
910
0.4 0.8 1.2
AUSTENITE
AUSTENITE + FERRITE
FERRITE + PEARLITE
HYPO-EUTECTOID STEELS
PEARLITE
Mixture of Ferrite &
Cementite
EUTECTOID STEELS
AUSTENITE
AUSTENITE + CEMENTITE
AUSTENITE + CEMENTITE
HYPER-EUTECTOID STEELS
IRON – CARBON EQUILIBRIUM DIAGRAM
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IRON IRON-CARBON DIAGRAM
Ferrite
Austenite
Steel Cast iron
Pearlite
Pearlite and
Cementine
Pearlite and
Carbide
Eutectic
eutectoid

1538
1130
2.0
oC
695
910
0.4 0.8 1.2
AUSTENITE
FERRITE + CEMENTITE
AUSTENITE + CEMENTITEAUSTENITE + FERRITE
FERRITE
+
PEARLITE
CEMENTITE
+
PEARLITE
AUSTENITE + LIQUID
IRON – CARBON EQUILIBRIUM DIAGRAM
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AUSTENITE
• A solid solution of Carbon in face-centred
cubic iron (Allotropic), containing a maximum
0f 1.7 % carbon at 1130oC
• It is soft, ductile and non-magnetic and also
exist in the plain carbon steel above the
upper critical range.
• It may however occur at room requirement,
however, occur at room temperatures in
certain alloy steels
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FERRITE
• Ferrite is nearly pure iron.A solid solution of Carbon
in body-centred cubic  iron, containing a maximum
of 0.04 % Carbon at 695oC.
• At room temperature, small amounts of manganese,
silicon and other elements may be dissolved in iron
as well as up to 0.007 % Carbon.
• Found only in Hypoeutectoid steel
• It is softest constitute of steel and very ductile and
readily cold-worked
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CEMENTITE
• A hard brittle compound of iron and Carbon with
the formula Fe3C
• The hardest constituent of steel
• This may exist in the free state usually as a grain
boundary film, or as a constituent of the
eutectoid pearlite
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PEARLITE
• This is the eutectoid structure consisting of
alternate lamination of ferrite and
cementite.
• It contains 0.83% Carbon and is formed by
the breakdown of the austenite solid
solution at 695oC
• The properties of pearlite are harder and
stronger than ferrite, but softer and more
ductile than cementite
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If the carbon is increased further, more cementite is
formed and the structure becomes pearlite and
cementite as shown.
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HEAT TREATMENT of CARBON STEELS
Steels containing carbon can have their properties (hardness,
strength, toughness etc) changed by heat treatment. Basically if
it is heated up to red hot and then cooled very rapidly the steel
becomes harder. Dead mild steel is not much affected by this but
a medium or high carbon steel is.
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Principle of heat treatment of steel
• Metals are never heated to the melting point in heat
treatment.
• Therefore, all the reactions within the metal during the
heating and cooling cycle, take place while the metal is
in the solid state
• During ordinary heat treating operations, steel is seldom
heated above 983oC.
• In using the iron-iron carbide diagram, we need only to
concern ourselves with that part which is always solid
steel.
• The area where the Carbon content is 2% or less and
the temperature is below 1130oC
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COOLING RATE
• Cooling rate is the most important part of heat treatment.
• Different cooling rates are now considered as they have a
significant effect on the properties of the metal.
SLOW COOLING
• Austenite is transformed to course pearlite.
• Slightly more rapid cooling may produce fine pearlite in which
the layers of ferrite and cementite are thinner.
INTERMEDIATE COOLING
• Austenite transforms to a material called Bainite instead of
the usual pearlite.
• When etched, Bainite gives a dark appearance and shows a
circular or needle like form.
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FAST COOLING
• By quenching in water, the transformation of
austenite is suppressed until about 318oC at which
point a new constituent called Martensite(quite brittle)
begins to form instead of the Bainite or pearlite of
slower cooling rate.
• As the temperature drops lower, the transformation
become complete.
• This temperature vary with the alloy content of the
steel
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TIME TEMPERATURE TRANSFORMATION
• In order to obtain steels with the desired
properties, we must have some control over
the transformation process, and this is
indicated in the TTT diagram
• TTT diagram are used to predict the
metallurgical structure of a steel sample
which is quenched in the austenite region
and held to constant elevated temperature
below 729oC.
• This is known as Isothermal transformation
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Time (sec)
oC
0
760
725
650
590
540
430
316
260
190
90
TIME TEMPERATURE TRANSFORMATION DIAGRAM
Ferrite
form
Pearlite
starts
Pearlite
forms
Pearlite is
complete
Coarse
Pearlite
Fine
Pearlite
Bainite
forming
Upper
Bainite
Lower
Bainite
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TIME TEMPERATURE TRANSFORMATION
• However since heat treatment usually
involves continuous cooling, TTT diagrams
are not directly applicable but can be
modified to be useful in at least a qualitative
way for continuous cooling condition
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THE AFFECT OF PROCESSING and
MANIPULATION ON METALS
When a metal solidifies grains or crystals are
formed. The grains may be small, large or long
depending on how quickly the material cooled and
what happened to it subsequently. Heat treatment
and other processes carried out on the material
will affect the grain size and orientation and so
dramatically affect the mechanical properties. In
general slow cooling allows large crystals to form
but rapid cooling promotes small crystals. The
grain size affects many mechanical properties
such as hardness, strength and ductility.
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MANIPULATIVE PROCESSES
These are processes which shape the solid
material by plastic deformation. If the process is
carried out at temperatures above the
crystallisation temperatures, then re-crystallisation
occurs and the process is called HOT WORKING.
Otherwise the process is called COLD WORKING.
The mechanical properties and surface finish
resulting are very different for the two methods.
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HOT ROLLING
This is used to produce sheets, bars and sections. If the
rollers are cylindrical, sheet metal is produced. The hot slab
is forced between rollers and gradually reduced in
thickness until a sheet of metal is obtained. The rollers may
be made to produce rectangular bars, and various shaped
beams such as I sections, U sections, angle sections and T
sections. Steel wire is also produced this way. The steel
starts as a round billet and passes along a line of rollers. At
each stage the reduction speeds up the wire into the next
roller. The wire comes of the last roller at very high speeds
and is deflected into a circular drum so that it coils up. This
product is then used for further drawing into rods or thin
wire to be used for things like springs, screws, fencing and
so on.
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COLD ROLLING
The process is similar to hot rolling but the metal is
cold. The result is that the crystals are elongated in
the direction of rolling and the surface is clean and
smooth. The surface is harder and the product is
stronger but less ductile. Cold working is more
difficult that hot working.
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FORGING
In this process the metal is forced into shape by
squeezing it between two halves of a die. The dies may
be shaped so that the metal is simply stamped into the
shape required (for example producing coins). The dies
may be a hammer and anvil and the operator must
manipulate the position of the billet to produce the
rough shape for finishing (for example large gun
barrels).
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COLD WORKING
Cold working a metal by rolling, coining, cold forging or drawing leaves
the surface clean and bright and accurate dimensions can be
produced. If the metal is cold worked, the material within the crystal
becomes stressed (internal stresses) and the crystals are deformed.
For example cold drawing produces long crystals. In order to get rid of
these stresses and produce “normal” size crystals, the metal can be
heated up to a temperature where it will re-crystallise. That is, new
crystals will form and large ones will reduce in size.
If the metal is maintained at a substantially higher temperature for a
long period of time, the crystals will consume each other and fewer but
larger crystals are obtained. This is called “grain growth”.
Cold working of metals change the properties quite dramatically. For
example, cold rolling or drawing of carbon steels makes the stronger
and harder. This is a process called “work hardening”.
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HOT WORKING
Most metals (but not all) can be shaped more easily
when hot. Hot rolling, forging, extrusion and drawing is
easier when done hot than doing it cold. The process
produces oxide skin and scale on the material and
producing an accurate dimension is not possible.
Hot working, especially rolling, allows the metal to re-
crystallise as it is it is produced. This means that
expensive heat treatment after may not be needed.
The material produced is tougher and more ductile.
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LIQUID CASTING AND MOULDING
When the metal cools it contracts and the final product is
smaller than the mould. This must be taken into account in
the design.
The mould produces rapid cooling at the surface and
slower cooling in the core. This produces different grain
structure and the casting may be very hard on the outside.
Rapid cooling produces fine crystal grains. There are many
different ways of casting.
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SAND CASTING
A heavy component such as an engine block would be cast
in a split mould with sand in it. The shape of the component
is made in the sand with a wooden blank. Risers allow the
gasses produced to escape and provide a head of metal to
take up the shrinkage. Without this, the casting would
contain holes and defects.
Sand casting is an expensive method and not ideally suited
for large quantity production. Typical metals
used are cast iron. Cast steel and aluminium alloy.
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DIE CASTING
Die castings uses a metal mould. The molten metal
may be fed in by gravity as with sand casting or forced
in under pressure. If the shape is complex, the
pressure injection is the best to ensure all the cavities
are filled. Often several moulds are connected to one
feed point. The moulds are expensive to produce but
this is offset by the higher rate of production achieved.
The rapid cooling produces a good surface finish with
a pleasing appearance. Good size tolerance is
obtained. The best metals are ones with a high degree
of fluidity such as zinc. Copper, aluminium and
magnesium with their alloys are also common.
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CENTRIFUGAL CASTING
This is similar to die casting. Several moulds are
connected to one feed point and the whole
assembly is rotated so that the liquid metal is
forced into the moulds. This method is especially
useful for shapes such as rims or tubes. Gear
blanks are often produced this way.
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MACHINING
Machining processes involve the removal of
material from a bar, casting, plate or billet to form
the finished shape. This involves turning, milling,
drilling, grinding and so on. Machining processes
are not covered in depth here. The advantage of
machining is that is produces high dimensional
tolerance and surface finish which cannot be
obtained by other methods. It involves material
wastage and high cost of tooling and setting.
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Heat treatment Methods
• Annealing
• Normalizing
• Hardening
• Tempering
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Annealing
• heat treatment that alters the
microstructure of a material causing
changes in properties such as strength,
hardness, and ductility
• It the process of heating solid metal to
high temperatures and cooling it slowly so
that its particles arrange into a defined
lattice

Stages in annealing
Heating to the desired temperature ,
Holding or soaking at that temperature,
Cooling or quenching ,usually to room
temperature .
• In practice annealing concept is most widely
used in heat treatment of iron and steals

Purpose of annealing
• It is used to achieve one or more of the
following purpose .
1. To relive or remove stresses
2. To include softness
3. To alter ductility , toughness, electrical,
magnetic.
4. To Refine grain size
5. To remove gases
6. To produce a definite microstructure .

Application
Annealing process is employed in following
application
• Casting
• Forging
• Rolled stock
• Press work ….

3 Types of Annealing:
I. Process Annealing
II. Full annealing
III. Spheroidising
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i. Process Annealing
 Carried out on cold-worked low carbon steel
sheet or wire in order to relieve internal stress
and to soften the metals.
• The steel is heated to 550 to 650oC below the
critical point.
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Increase in ductility reduce in TS & hardness

ii. Full Annealing
 It carried out on hot-worked and cast steels in
order to obtain grain refinement with high
ductility.
 It also produces a softer steel with better
machinability
• For steels
– heating above critical point (30 - 50oC) then
- holding at this temperature for a time (thickness)
- followed by slow cooling usually in furnace.
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iii. Spheroidizing Annealing
 To remove coarse pearlite and making machining process
easy .
 It forms spherodite structure of maximum soft and ductility
easy to machining and deforming.
• The process is limited to steels in excess of 0.5% carbon.
This steel can be softened by annealing at 650 – 750oC just
below the lower critical point, when the cementite of the
pearlite balls up or spheroidizes.
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Defects uncontrolled temperature
i. Overheating
 Heated above the actual temperature or to long
maintained at annealing temperature: austenite grain
growth will occur and make the metal weak and brittle
ii. Burning
 If heated above the upper critical point to temperature,
Brittles films of oxide are formed which make the steel
unsuitable. For further use and must be remelted.
iii. Under annealing
 The original pearlite will have change to several small
austenite grains.
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NORMALIZING
 For hypoeutectoid steels
- heating above critical point (30 - 50oC)
- holding at this temperature for a time (thickness) &
- followed by cooling in still air.
• Produces maximum grain refinement and
consequently the steel slightly harder and stronger
than a fully annealed steel.
• However the properties will vary with section
thickness
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HARDENING
 Hardening is process in which Medium and High
carbon steels (0.4 – 1.2%) is heated to a
temperature above the critical point (until red
hot), held at this temperature and quenched
(rapidly cooled) in water, oil or molten salt baths.
• Hardening producing a very hard and brittle
metal. At 723 Deg C, the ferrite changes into
Austenite structure.
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TEMPERING
Tempering is a process of heat treating, which is used to
increase the toughness of iron-based alloys.
To remove some of the brittleness from hardened steels,
tempering is used. The metal is heated to the range of 220-
300 deg C and cool in the air.
• Tempering is usually performed after hardening, to reduce
some of the excess hardness, and is done by heating the
metal to some temperature below the critical temperature
for a certain period of time, then allowed to cool in still air.
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QUENCHING
• To harden by quenching, a metal (usually steel or cast
iron) must be heated into the austenitic crystal phase
and then quickly cooled.
• Quenching Media:
 Brine (water and salt solution)
 Water
 Oil
 Air
 Turn off furnace
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CASE HARDENING
• Low carbon steels cannot be hardened by
heating due to the small amounts of carbon
present. So, Case hardening seeks to give a
hard outer skin over a softer core on the metal.
• The addition of carbon to the outer skin is known
as carburising.
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• When temperature region 200 – 450oC the
martensite decomposes into ferrite and the
precipitation of the fine particles of carbide occurs
known. as troostite
• At higher temperatures 450 – 650oC the carbide
particles coalesce thus producing fewer and larges
particles which provide fewer obstacles to
dislocations resulting further increasing toughness
while decrease in strength and hardness and known
as sorbite.
• Sorbite is ideal for components subject to dynamic
stresses such as crankshaft and connecting rod
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SORBITEMARTENSITE TROOSTITE
200 400 600
oC
Hardness
200
800
600
400
1000
EFFECT OF TEMPERING
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ALLOYS
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Nickel
- One of the most widely used alloying elements in
steel. In amounts 0.50% to 5.00% its use in alloy
steels increases the toughness and tensile
strength without detrimental effect on the ductility.
Chromium
- Gives resistance to wear and abrasion.
Chromium has an important effect on corrosion
resistance and is present in stainless steels in
amounts of 12% to 20%.

ALLOYS
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•Molybdenum
- Increases hardenability, toughness to
quenched/tempered steels. It also improves the
strength of steels at high temperatures (red-
hardness).
•Vanadium
- Steels containing vanadium have a much finer
grain structure than steels of similar composition
without vanadium.

CREEP
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• Creep is strain increase with time
under constant load.
• Creep is temperature dependent – the
higher the temperature the greater the
effect

FRETTING
A type of wear that occurs between tight-fitting
surfaces subjected to cyclic relative motion of
extremely small amplitude. Usually, fretting is
accompanied by corrosion, especially of the very
fine wear debris.
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FRETTING CORROSION
The accelerated deterioration at the interface
between contacting surfaces as the result of
corrosion and slight oscillatory movement between
the two surfaces.

IMPURITIES
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Sulphur
– The presence of free sulphur in a steel
product is detrimental to its properties,
especially toughness.
Phosphorous
– Its presence in steel is usually regarded as
an undesirable impurity due to its embrittling
effect, for this reason its content in most
steels is limited to a maximum of 0.050%.

Welding Metallurgy
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Heat Affected Zone Welding
Concerns
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Heat Affected Zone Welding
Concerns
 Changes in Structure Resulting in Changes in Properties
 Cold Cracking Due to Hydrogen
Two major concerns occur in the heat affected zone which
effect weldability these are,
a.) changes in structure as a result of the thermal cycle
experienced by the passage of the weld and the resulting
changes in mechanical properties coincident with these
structural changes, and
b.) the occurrence of cold or delayed cracking due to the
absorption of hydrogen during welding.
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First let’s review the thermal cycles experienced in the heat
affected zone as a result of the passage of the weld. The
figure illustrated here shows the temperature vs time curve at
various distances from the weld metal. Note that almost
every thermal cycle imaginable occurs over this short
distance of the heat affected zone. Thus a variety of
structural and property variations are expected.
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Look At Two Types of Alloy Systems
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There are two types of alloy systems which we will
consider, those which do not have an allotropic
phase change during heating like copper, and
those which have an allotropic phase change on
heating like steel. We will first consider those
materials which do not have an allotropic phase
change. The top schematic illustrates this type of
material. We will however consider that this
material has been cold worked (not the elongated
cold worked grains present in the base material
(region A). The weld metal is represented by
region C, and the heat affected zone is region B.
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Note that the heat of welding has effected the structure of this material
even though there are no allotropic transformations. Recall that cold
worked structures undergo recover, recrystalization and grain growth when
heated to ever increasing temperatures. So it is in this material. As we
traverse from the cold worked elongated grains in the unaffected base
metal, we come to a region where the cold worked grains undergo
recovery and then shortly there after they recrystalize into fine equaled
new grains. Traversing still closer to the weld region we note grain growth
where the more favorably oriented grains consume neighboring grains and
grain growth occurs. The grains within the weld epitaxially nucleate from
the grains in the heat affected zone at the fusion boundary, and grain
growth continues into the solidifying weld metal making very large grains.5/28/2015
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Introductory Welding Metallurgy,
AWS, 1979
Cold Worked Alloy Without Allotropic Transformation
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One of the factors that occur when cold worked grains
recrystalize and grain grow occurs we have already
discussed, and that is the material softens. Thus the heat
affected zone and weld metal will not hold the same
strength level as the cold worked base metal. Another
consequence of increased grain size is perhaps equally
important and that is that the larger grains are more brittle.
A “Charpy” impact test is used to determine how much
impact energy a structure will absorb over various
temperature ranges. Note that the larger grain size
material will become brittle and not absorb much of an
impact load even at temperatures around room
temperature and above.
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Welding Precipitation
Hardened Alloys Without
Allotropic Phase Changes
Welded In:
• Full Hard Condition
• Solution Annealed
Condition
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A second way of strengthening materials without allotropic
phase changes is by precipitation strengthening. (The first
we just discussed was cold working). Recall that in
precipitation strengthening, the base metal is solutionized,
rapidly cooled and then aged at some moderately elevated
temperature to promote precipitate formation. There are
two ways that precipitation hardened material can be
welded. One is to weld on the full hard, that is the already
aged base metal. The second is to weld on material which
has been solution annealed and rapidly cooled, but not yet
given the ageing heat treatment. In either case, when
welding, the heat affected zone will see some additional
time at temperature (varied temperature over the distance
of the HAZ) as illustrated above, and this will effect the
aged or overaged condition of the precipitates.
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Annealed upon
Cooling
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When welding on the already aged (full hard) material,
the unaffected base metal will have aged precipitates
that are just the right size for strengthening. The heat
affected zone, on the other hand, will experience some
additional heating. In the region farthest from the weld
the heat will be sufficient to overage the precipitates
with the resulting loss in strength. In regions closer to
the weld, the heat will be so excessive that the
temperature will exceed the two phase region and the
single phase solutionizing region on the phase
diagram will be entered. Again, a loss in strength will
occur, but this region at least might be able to be re-
aged to recover some strength.
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Let us now turn our attention to the materials which do
have an allotropic phase change during heating. A typical
material like steel is ferrite at low temperatures and
transforms to austenite when heated. Each time the
material goes through one of these phase changes, new
finer equaled grains grow starting from the grain
boundaries of the previous grains present. So in the case
of cold worked steels in the base metal, the elongated cold
worked grains will undergo recovery, recrystalization and
grain growth just as discussed above. But now the
recrystallized grains at higher temperature will undergo the
allotropic phase change, reducing the grain size again
which then is followed by grain growth at still higher
temperature (nearer the weld). This variation in grain
structure is schematically shown in the lower figure above.
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AWS, 1979
Steel Alloys With Allotropic Transformation
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This illustration shows the various regions in the heat
effected zone and what microstructure would be predicted
as related to the iron-carbon phase diagram. Note that at
the far extent of the element in the base metal, ferrite and
commentate arte expected. Closer to the weld some dual
phase ferrite austenite will occur at temperature of welding.
Closer yet we would expect single phase austenite, and
then maybe some austenite of delta ferrite and liquid
mixtures until at the maximum temperature the liquid phase
would be present as the welding arc traverses. These are
the structures at temperature, but we now must consider
what happens during cooling.
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We have already seen that the cooling rate from
welding can vary depending upon a number of
weld variables. The two most important are
preheat and heat input. The cooling rate is fastest
when no preheat and low heat input are used to
make the weld. On the other hand, the cooling
rate is slowest when high preheat and high heat
input are employed.
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AWS, 1979
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As we have learned before, the cooing rate from austenite can
effect the room temperature structure as defined by the
continuous cooling transformation diagram. Rapid cooling results
in non-equilibrium hard brittle martensite. Slow cooling results in
some higher temperature transformation products such as
bainite, ferrite and pearlite which tend to be softer. Examining
two welding procedures here, one with no preheat (number 1)
and the other with preheat (number 2) we find some differences
in structure. The no preheat weld has a narrower HAZ and rapid
cooling and the austenite transforms to martensite on cooling
giving a hard martensite peak near the fusion line. The weld with
preheat has a wider HAZ, a slower cooling rate producing ferrite
pearlite and bainite and the fusion line peak is softer. There is
also more outer HAZ region grain growth and overaging so that
the softening in the HAZ is greater. Thus, once again, welding
procedures have to be carefully tailored for the material being
welded.
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How does the hydrogen get into the heat effected
zone where the cold cracking is often observed?
Liquid metal can absorb more hydrogen than solid
austenite, and austenite more than ferrite. When
welds are made on wet material or with wet
electrodes, the hydrogen is absorbed into the
liquid. As the liquid solidifies, if forces some of the
hydrogen which it is trying to get rid of into the
surrounding hot austenite. If there is still too much
to be absorbed even in a supersaturated solid,
some hydrogen porosity may form in the weld
metal, a sure sign that poor procedures were
followed.
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During cooling, the cooler material tries to push
hydrogen out while at the same time the solidifying
weld metal tries to push hydrogen out. Note that
the large grained region of the HAZ which just may
have the hardest most susceptible martensitic
microstructure thus acquired hydrogen from both
directions and a supersaturated condition exists
there.
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The hydrogen then slowly diffuses to any location
where is can relieve the stress of being stuck in the
lattice in the supersaturated condition. The hydrogen
atoms are often carried by dislocation and the
preferred site for collection is often inclusions. At this
point, they can either weaken the surrounding
structure or the hydrogen atoms can recombine and
form molecular hydrogen gas and exert an internal
pressure. As this pressure grows, the crack slowly
expands until a critical size is reached and
catastrophic failure occurs. This takes time at low
temperature , thus the common name of cold cracking
or delayed cracking applies.
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The time after welding has an effect. As time
proceeds, the hydrogen diffuses away from the
high concentration in the most critical portion of the
heat affected zone. If hydrogen diffuses away
before the critical crack length is reach, the weld
has occurrence of some micro cracks but
catastrophic failure does not occur. On the other
hand, if hydrogen diffusion is slower than that
failure may occur. Elevated temperature post weld
treatment will allow fast hydrogen diffusion and
may help in the reduction of cold cracking.
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Dickinson5/28/2015
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The above diagram summarizes the discussions
about delayed cracking. The red regions are crack
sensitive regions while the blue represents the
safe region. Materials with high hardenabilty will
promote the formation of martensite, and materials
with high carbon content will produce a harder
martensite. Increases in heat input and preheat
and stress reliving practices increases the safety
against hydrogen delayed cracking. And the
decrease in hydrogen in the welding process
likewise increases the safety region.
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Why Preheat?
• Preheat reduces the temperature
differential between the weld region and
the base metal
– Reduces the cooling rate, which reduces the
chance of forming martensite in steels
– Reduces distortion and shrinkage stress
– Reduces the danger of weld cracking
– Allows hydrogen to escape
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1030.1.1.5.1.T9.95.12

Using Preheat to Avoid Hydrogen
Cracking
• If the base material is preheated, heat flows more
slowly out of the weld region
– Slower cooling rates avoid martensite formation
• Preheat allows hydrogen to diffuse from the metal
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Cooling rate T - Tbase)2
Steel
Cooling rate T - Tbase)3
T base
T base

Why Post-Weld Heat Treat?
• The fast cooling rates associated with welding often
produce martensite
• During postweld heat treatment, martensite is
tempered (transforms to ferrite and carbides)
– Reduces hardness
– Reduces strength
– Increases ductility
– Increases toughness
• Residual stress is also reduced by the postweld heat
treatment
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Carbon and Low-Alloy Steels
0.1.1.5.1.T10.95.12

Postweld Heat Treatment and
Hydrogen Cracking
• Postweld heat treatment (~ 1200°F) tempers any
martensite that may have formed
– Increase in ductility and toughness
– Reduction in strength and hardness
• Residual stress is decreased by postweld heat
treatment
• Rule of thumb: hold at temperature for 1 hour
per inch of plate thickness; minimum hold of 30
minutes
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Steel

Base Metal Welding Concerns
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Lamellar Tearing
• Occurs in thick plate subjected to high transverse
welding stress
• Related to elongated non-metallic inclusions, sulfides
and silicates, lying parallel to plate surface and
producing regions of reduced ductility
• Prevention by
– Low sulfur steel
– Specify minimum ductility levels in transverse
direction
– Avoid designs with heavy through-thickness direction
stress
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Cracking in Welds
0.1.1.5.2.T14.95.12

Improve Cleanliness
Improve through thickness properties
Buttering
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This illustrates how the rolled out inclusions (mainly MnS)
can de-bond from the base metal matrix and under the
action of short transverse (through thickness) stresses they
can actually link to form a stepped like fracture. Improving
cleanliness of the steel during steel processing, and
improving through thickness properties by steel making
processed line calcium or rare earth treatment which
produces inclusions which to not roll out a long stringer
during plate processing can help. Also laying a weld bead
on top of the plate which has lower strength and improved
ductility before welding the attachment can help by letting
the weld bead take the shrinkage stresses rather than
transmitting them into the base plate.
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Multipass Welds
• Heat from subsequent passes affects the structure and
properties of previous passes
– Tempering
– Reheating to form austenite
– Transformation from austenite upon cooling
• Complex Microstructure.
• In a multi-pass weld, the heating and cooling cycles of one
pass are superimposed upon those of previous passes.
Portions of previous passes are heated high enough to form
austenite again, and upon cooling this austenite once again
can transform to ferrite and pearlite or to martensite. Some
portions of previous weld passes will not transform to
austenite but will be tempered by the heat from subsequent
passes. All in all, this leads to a rather complicated structure
in multi-pass welds.
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Multipass Welds
• Exhibit a range of
microstructures
• Variation of
mechanical properties
across joint
• Postweld heat
treatment tempers
the structure
– Reduces property
variations across the
joint
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Steel

Reheat Cracking
• Mo-V and Mo-B steels susceptible
• Due to high temperature embrittlement of the
heat-affected zone and the presence of residual
stress
• Coarse-grained region near fusion line most
susceptible
• Prevention by
– Low heat input welding
– Intermediate stress relief of partially completed welds
– Design to avoid high restraint
– Restrict vanadium additions to 0.1% in steels
– Dress the weld toe region to remove possible areas of
stress concentration5/28/2015
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Cracking in Welds
0.1.1.5.2.T15.95.12

Steels containing molybdenum or vanadium resist creep at
elevated-temperature. These steels, along with thick
sections of high-strength, low-alloy steels, are subject to
reheat cracking in combination with residual stress and low
creep-ductility in the HAZ.
During postweld heat treatment, cracks form along the
grain boundaries in the HAZ, particularly in the coarse-
grained region near the fusion line.
Defects at the weld toe can promote reheat cracking;
therefore, grinding or peening the weld toe can help
prevent this cracking.
The cracked area must be heat treated to restore ductility
prior to repair. Then it can be cut out beyond the ends of
the cracks and rewelded.
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Knife-Line Attack in the HAZ
• Cr23C6 precipitate in HAZ
– Band where peak
temperature is 800-
1600°F
• Can occur even in
stabilized grades
– Peak temperature
dissolves titanium
carbides
– Cooling rate doesn’t
allow them to form again
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Weld
HAZ
Knife-line attack
Stainless Steel

A discrete band in the heat affected zone of the austenitic
stainless steel welds experiences peak temperatures in the
800°-1600°F temperature range associated with
sensitization.
Chromium carbide precipitation in this region can lower the
chromium content near the grain boundaries to less than
12%, thereby causing sensitization.
Stabilized grades can also suffer from knife-line attack.
Elevated temperatures in the heat-affected zone can
dissolve titanium and niobium carbides. The fast cooling
rates in the welded joint do not allow these carbides to
reform. This leaves excess free carbon, which can then
form chromium carbides.
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WELDING FAULTS
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Root Faults
For deep vee multi run welds the first run or root weld is critical to the
quality of the welds laying on top. Typical faults may be caused by too
high or low a current of too large a rod.

Fusions Faults
The three main causes of this is too low current for rod, too high a
travel rate or when too small a rod is used on a cold surface.
Bead Edge Defects
normally in the form of under cutting or edge craters. The main
cause for this is incorrect current setting. Too high will lead to
undercutting, too low to edge craters. Similar efects may occur at
the correct current due to incorrect arc length. Edge faults are
particularly common in vertical welding or 'weave' welding. The
general cause for the latter being a failure to pause at the
extremes of the weave. Edge defects are stress raisers and lead
to premature weld failure.
Porosity
May have many causes the most common being moisture in the
rod coating or in the weld joint. Poor rod material selection is also
a factor
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Heat Cracks
this is a destructive fault caused generally due to incompatiablity of
the Weld material and weld Rod. Indeed in some cases the
material may be deemed unweldable. Heat cracks occur during or
just after the cooling off period and are caused by impurites in the
base metal segrateing to form layers in the middle of the weld. The
layers prevent fusion of the crystals. The two main substances
causing this are Carbon and Sulphur. A switch to 'basic'
electrodes may help.
Anouther cause is temsion acroos the weld which , even without
segregation in the weld, cause a crack. This occurs during a
narrow critical temerpature range as the bead coagulates. During
this period the deformation property is small, if the shrinkage of the
base material is greater than the allowed stretch of the weld then a
crack will result. One method of preventing this is to clamp the
piece inducing a compressive force on the weld during the cooling
period
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Shrinkage Cracks
Thes form due to similar effect of allowed weld deformation being less
than base metal shrnkage although it is not associated with the critical
temerpature rang above and therefore cannot be elleviated by
compression. The use of 'basic' electrodes can help
Hydrogen cracks
This is generally associated what either hardened material or material
hardened during the welding process. The hydrogen source can be
moisture, oil, grease etc. Ensuring that the rod is dry is essential and
preheating the weld joint to 50'C will help. The cracking occurs adjacent
to the weld pool and allied to the tension created during the welding
porcess will generate a through weld crack.
Slag Inclusion
This common fault is caused by insufficient cleaning of the weld between
runs. If necessary as well as using a chipping hammer and brush grind
back each weld run with an angle grinder. Once the slag is in the weld it
is near impossible to removed it by welding only5/28/2015
120

Metallurgical Testing
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Non-Destructive Testing
- This is carried out on components rather than on test
pieces, they are designed to indicate flaws occurring
due or after manufacture. They give no indication of
the mechanical properties of the material.
- Surface flaws may be detected by visual means
aided by dye penetrant or magnetic crack detection.
- Internal flaws may be detected by X-ray or ultrasonic
testing.
- In addition to this there are special equipment able to
exam machine finish.
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Liquid Penetrant Methods
- The surface is first cleaned using an volatile cleaner and
degreaser.
- A fluorescent dye is then applied and a certain time
allowed for it to enter any flaws under capillary action.
Using the cleaning spray, the surface is then wiped clean. -
- An ultra violet light is shone on the surface, any flaws
showing up as the dye fluoresce.
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Dye penetrant method
- The surface is cleaned and the low viscosity
penetrant sprayed on.
- After a set time the surface is again cleaned.
- A developer is then used which coats the surface
in a fine white chalky dust, then the dye seeps out
and stains the developer typically a red colour.
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Magnetic crack detection
- A component is place between two poles of a magnet.
- The lines of magnetism concentrate around flaws.
- Magnetic particles are then applied, in a light oil or dry sprayed, onto
the surface where they indicate the lines of magnetism and any
anomalies/ abnormalities like the below figures.
Limitation of Magnetic test:
This method of testing has a few limitations.
- Firstly it cannot be used on materials which cannot be magnetised
such as austenitic steel and non-ferrous metals.
- Secondly it would not detect a crack which ran parallel to the lines of
magnetism.
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- When a material is tested under a tensile load, it
changes shape by elongating.
- Initially the extension is in proportion to the increasing
tensile load.
- If a graph is plotted showing extension for various
loads, then a straight line is obtained at first.
- If the loading is continued the graph, deviates as
shown.
- Within the limit of the straight line, if the load is
removed the material will return to its original length
which is elastic limit of the specimen.
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Tensile Testing Method

- When the test piece reaches the Yield point (Yu), there is a
failure of the crystalline structure of the metal, not along the
grain boundaries as it has been the case, but through the grains
themselves. This is known as “slip”.
- A partial recovery is made at the lower yield point (YL), then
the extension starts to increase.
- If the load is removed at any stage along the “Load-Extension”
curve after Yield Point (YL), the material will have a
corresponding permanent deformation. This termed
“permanent set”.
- Maximum loading occurs at the “ultimate Load” (S).
and after Yield Point (YL) to Ultimate Load (S) is the plastic limit.
- Ultimate Load (S) to Breaking point (B) this stage local wasting
or extension will start which termed “necking”. Normally this starts
at about the centre of the specimen and will rapidly be followed by
failure up to breaking point (B). 129
Tensile Testing Method

Proof Stress:
For a material which does not have a marked yield point such
as Aluminium, there is a substitute stress specified. This is
termed “the proof stress”.
- Proof stress is determined from a load/extension or
stress/strain graph.
131

Proof Testing Method
• Hard steels and non-ferrous metals (Aluminium)
do not have defined yield limit, therefore a stress,
corresponding to a definite deformation, (0.1% or
0.2%) is commonly used instead of yield limit.
This stress is called proof stress or offset yield
limit (offset yield strength):
• σ0.2%= F0.2% / S0
• The method of obtaining the proof stress is
shown in the picture.
• As the load increase, the specimen continues to
undergo plastic deformation and at a certain
stress value its cross-section decreases due to
necking .
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• At point S in the Stress-Strain Diagram the stress
reaches the maximum value, which is called ultimate
tensile strength (tensile strength):
σt= FS / S0
• Continuation of the deformation results in breaking the
specimen - the point B in the diagram (from Ultimate
load S to breaking point B)
• The actual Stress-Strain curve is obtained by taking into
account the true specimen cross-section instead of the
original value.
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135

Creep Testing
- Creep tests are carried out
under controlled temperature
over an extended period of
time in the order of
10,000hrs.
- The test piece is similar to
the type used for tensile
tests and creep is usually
thought of as being
responsible for extensions
of metal only. In fact creep
can cause compression or
other forms of deformation
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- Temperature of the test is at recrystallization point around
400oC. For other metals the recrystallization temperature is
different (200oC for copper and room temperature for tin and
lead).
- At the start of the test the initial load must be applied
without shock.
- This load, normally well below the strength limit of the
material, will extend the test piece slowly.
- The load is kept steady through the test and the
temperature is maintained accurately.
- Extension is plotted and is seen to proceed in three distinct
stages.
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HARDNESS TESTING
The basis of the Brinell hardness
testing is the resistance to
deformation of a surface by a
loaded steel ball.
Oil is pumped into the chamber
between the pistons until there is
sufficient pressure to raise the
Weight so that it is floating. The ball
is now forced into the specimen
material at the same force. The
loading for steel and metals of
similar hardness is 3,000Kg. The
load is allowed to act for 15 sec to
ensure that plastic flow occurs.
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- The surface diameter of the indentation is measured
with the aid of a microscope which is traversed over
the test piece on a graduated slide with a vernier.
- Cross wires in the microscope, enable the operator
to accurately align the instrument.
- Both the loading and ball diameter (10mm) are
known, by measuring the indentation diameter the
hardness can be calculated.
For softer materials the loading is reduced, Copper
being 1000Kg and Aluminium 500Kg. The diameter of
the indentation must be less than half the ball
diameter.
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- The thickness of the specimen must be not less than
10x the depth of the impression. The edge of the
impression will tend to sink with the ball if the surface
has been work hardened; otherwise the local
deformation will tend to cause piling up of the metal
around the indent
If the hardness test is used on very hard materials, the
steel ball will flatten. This method is not reliable for
reading over 600. It is used in preference to other
methods where the material has large crystals, e.g.
Cast iron.
Mild Steel 130, Cast Iron 200, white cast iron 400,
nitrided surface 750.
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Under low temperature conditions , impact or shock loading on a
material can cause cracking in a material which is normally
ductile at room temperature.
To find out the Critical stressing in a material, Griffith
equation,
sc = Kic / ж Pc
where, sc = the critical stress in a material
Kic = the fracture toughness of a material
Pc = the micro-crack length within the materials
141
BRITTLE FRACTURE TESTING

BRITTLE FRACTURE TESTING
- The presence of these micro cracks (porous
materials or defects) can act to cause
transcrystalline type failures with a bright crystalline
appearance.
- Testing is carried out via the Charpy notched piece
test at various temperatures between -200o to
+200oC
- To reduce the effects of brittle fracture the carbon
content in carbon steels is kept as low as practical.
- Grains within the materials are kept as small as
possible by heat treatment and normalizing.
- Alloying elements may also be added.
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144
Transition Temperatures
• As temperature decreases a ductile material can become
brittle - ductile-to-brittle transition
– The transition temperature is the temp at which a
material changes from ductile-to-brittle behavior
• Alloying usually increases the ductile-to-brittle
transition temperature. FCC metals remain ductile down
to very low temperatures. For ceramics, this type of
transition occurs at much higher temperatures than for
metals.

Factors which affect the transition temperature are
1. Elements:
- Carbon, silicon, phosphorus and sulphur raise the
temperature.
- Nickel and manganese lower the temperature.
2. Grain size:
- the smaller the grain size the lower the transition
temperature, hence grain refinement is beneficial.
3. Work hardening:
- this appears to increase transition temperature.
4. Notches:
- possibly occurring during assembly e.g. weld defects or
machine marks.
- Notches can increase tendency to brittle fracture.
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18/80 stainless steel
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146
It is this property of stainless steel that makes it so suitable
for use in LPG carriers. Hardness Testing

ANY QUESTION?
THANK YOU!
5/28/2015
147

Metallurgy

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