WIS5 Weldability jdhgzfshjsjdhe.ppt 1.ppt

Copyright © 2003 TWI Ltd
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World Centre for Materials Joining Technology
Welding Inspection
Welding Inspection
Weldability of Steels
Weldability of Steels
By
Mohd Faisal Yusof

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Plain Carbon Steels
Steels are classified into groups as follows
1. Low Carbon Steel 0.01 – 0.3% Carbon
2. Medium Carbon Steel 0.3 – 0.6% Carbon
3. High Carbon Steel 0.6 – 1.4% Carbon
Plain carbon steels contain only iron & carbon as main alloying
elements, traces of Mn, Si, Al, S & P may also be present
Classification of Steel

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Iron is an element that can exist in 2 types of cubic structures,
depending on the temperature. This is an important feature
A most important function in the metallurgy of steels, is the
ability of iron to dissolve carbon in solution
The carbon atom is very much smaller than the iron atom and
does not replace it in the atomic structure, but fits between it
Iron atoms Carbon atoms
The following basic foundation information on metallurgy will
not form any part of your CSWIP examination.

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α Alpha iron
This structure occurs below 723 °C and is
body centred, or BCC in structure
It can only dissolve up to 0.02% Carbon
Also known as Ferrite or BCC iron
At temperatures below Ac/r 1, (LCT) iron exists like this
Compressed representation could appear like this

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γ Gamma iron
This structure occurs above the UCT in
Plain Carbon Steels and is FCC in structure.
It can dissolve up 2.06% Carbon
Also called Austenite or FCC iron
At temperatures above the Ac/r 3, (UCT) iron exists like this

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If steel is heated and then cooled slowly in equilibrium, then
exact reverse atomic changes take place
If a steel that contains more than 0.3% Carbon is cooled quickly,
then the carbon does not have time to diffuse out of solution,
hence trapping the carbon in the BCC form of iron.
This now distorts the cube to an irregular cube, or tetragon
This supersaturated solution is called Martensite and is the
hardest structure that can be produced in steels

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Martensite can be defined as:
A supersaturated solution of carbon in
BCT iron (Body Centred Tetragonal)
It is the hardest structure that can be
thermally produced in steels
If some steels are cooled quickly their structure looks like this

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Ferrite:  Low carbon solubility. Maximum 0.02%
Austenite:  High carbon solubility. Maximum 2.06%
Solubility of Carbon in BCC & FCC phases of steels
Martensite: The hardest phase in steels, which is produced
by rapid cooling from the Austenite phase
It mainly occurs below 300 °C

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IRON CARBON DIAGRAM

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TTT DIAGRAM

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Diagram showing the Relationship between Carbon Content,
Mechanical Properties, Microstructure and Uses of Plain Carbon
Steels in the Normalised Condition

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An Alloy steel is one that contains more than
Iron & Carbon as a main alloying elements
Alloy steels are divided into 2 groups
1. Low Alloy Steels < 7% extra alloying elements
2. High Alloy Steels > 7% extra alloying elements

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(a) substitutional (b) interstitial
Solid solution

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Carbon: Major element in steels, influences
strength,toughness and ductility
Manganese: Secondary only to carbon for strength
toughness and ductility, secondary deoxidiser and also acts
as a desulphuriser.
Silicon: Primary deoxidiser
Molybdenum: Effects hardenability, and has high creep
strength at high temperatures. Steels containing
molybdenum are less susceptible to temper brittleness than
other alloy steels.
Chromium: Widely used in stainless steels for corrosion
resistance, increases hardness and strength but reduces
ductility.
Nickel: Used in stainless steels, high resistance to corrosion
from acids, increases strength and toughness
Steel Weld Metallurgy

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 Aluminium:Deoxidiser,grain refinement
 Sulphur: Machineability
 Tungsten: High temperature strength
 Titanium: Elimination of carbide precipitation
 Vanadium: Fine grain – Toughness
 Copper: Corrosion resistance and strength

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Increased strength: C, Si, Cu, Mn, Mo (also Nb and V;
their exact effect depends on other factors also
such as the rolling temperature and time, amount
of carbon and nitrogen present, etc.)
Hardening capacity: C, Mn, Mo, Cr, Ni, Cu
Toughness: Ni, grain refinement (achieved via the presence of Nb, V, Al, Ti)
Elevated Temperature Properties: Cr, Mo, V
Atmospheric corrosion Resistance: Cu, Ni

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The type and number of elements present in the
material
The temperature reached during welding and or
PWHT.
The cooling rate after welding and or PWHT
The grain structure of steel will influence its weldability,
mechanical properties and in-service performance. The grain
structure present in a material is influenced by:

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Cooling Rate
Cooling Rate
The cooling rate of the weld zone depends on the following factors:
•Weld heat : Also call arc energy, is the amount of electrical
energy that is supplied to the welding arc
over a given weld length ( an inch or mm)
•Thickness of material
•Preheating

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Heat Affected Zone
Heat Affected Zone
The parent material undergoes microstructure changes
due to the influence of the welding process. This area,
which lies between the fusion boundary and the unaffected
parent material, is called the heat affected zone (h.a.z.).

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Heat Affected Zone
Heat Affected Zone

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Heat Affected Zone
Heat Affected Zone
 Material composition
 Cooling rate, fast cooling higher hardness
 Arc energy, high arc energy wider HAZ
 The HAZ can not be eliminated in a fusion weld
The extent of changes will be dependent upon the
following :-

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Arc energy
Arc energy
Arc energy = 1.6 kJ/mm
Amps = 200 Volts = 32
Travel speed = 240 mm/min
Arc energy= Amps x volts
Travel speed mm/sec X 1000
Arc energy= 200 X 32 X 60
240 X 1000

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High arc energy - slow cooling
 Low toughness
 Reduction in strength
Arc Energy
Arc Energy
Low arc energy - fast cooling
 Increased hardness
 Hydrogen entrapment
 Lack of fusion

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Carbon Equivalent
Carbon Equivalent
 The CE of steel primarily relates to its hardenability.
 Higher the CE, lower the weldability
 Higher the CE, higher the susceptibility to brittleness
 The CE of a given material depends on its alloying
elements
 The CE is calculated using the following formula
CE = C + Mn + Cr + Mo + V + Cu + Ni
6 5 15
Hardenability:The relative ability of a ferrous alloy to form martensite
when quenched from high temperatures.

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 Weldability can be defined as the ability of a material to
be welded by most of the common welding processes,
and retain the properties for which it has been designed.
 A steel which can be welded without any real dangerous
consequences is said to possess Good Weldability.
 Hardenability influence the weldability.The higher the
hardenability the poorer the weldability.
Weldability
Weldability

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Weldability
Weldability
Weldability is a function of many inter-related
factors but these may be summarised as:
Composition of parent material
 Joint design and size
 Process and technique
 Access

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Weldability
Weldability
It is very difficult to asses weldability in absolute terms
therefore it is normally assessed in relative terms

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Weldability
Weldability
There are many factors which affect weldabilty e.g. material type, welding
parameters amps, volts travel speed, heat input.

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Weldability
Weldability
Other factors affecting weldabilty are welding position and
welding techniques.

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Weldability
Weldability
Basically speaking weldabilty is the ease with which a material or
materials can be welded to give an acceptable joint

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Cracks
Cracks

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Process Cracks
Process Cracks
Hydrogen induced cold cracking
(HICC)
Solidification cracking (Hot Tearing)
Lamellar tearing
Weld Decay

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Cracks
Cracks
When considering any type of crack mechanism,
three elements must be present for it’s
occurrence:
 Stress: stress is always present in weldments,
through local expansion and contraction.
 Restraint: may be a local restriction, or through the
plates being welded.
 Susceptible microstructure: the structure is often
made susceptible to cracking through welding, e.g high
hardness

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Hydrogen
Hydrogen
Cracks
Cracks

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Hydrogen Cracking
Hydrogen Cracking
Hydrogen causes general embrittlment and in welds may
lead directly to cracking,

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A combination of four factors is necessary to cause HAZ hydrogen cracking

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Hydrogen Cracking
Characteristics
 Also known as hydrogen induced cold cracking ,
delay cracking , underbead cracking and chevron.
 Hydrogen is the major influence to this type of
cracking.
 Source of hydrogen may be from moisture or
hydrocarbon such as grease , paint on the parent
material, damp welding fluxes or from condensation
of parent material
 Hydrogen is absorbed by the weld pool from the arc
atmosphere.

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• During cooling, much of this hydrogen escapes
from the solidified bead by the diffusion but some
also diffuses into the HAZ of the parent metal.
• Type of cracking is intergranular along grain
boundaries or transganular
• Requires susceptible grain structure, stress and
hydrogen and low temperature is reached.
• Most likely in HAZ for Carbon Manganese steel
and in weld metal for HSLA steel.

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Hydrogen induced
weld metal
cracking
Hydrogen induced
HAZ cracking
Hydrogen Cracking
Hydrogen Cracking
Micro Alloyed Steel Carbon Manganese Steel

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Hydrogen Cracking
Factors responsible:
Hydrogen cracking occurs when the conditions outlined in 1 – 4
occur simultaneously :
1.Susceptible grain structure – hardness value > 350 V.P.N
That part of HAZ which experiences a high enough temperature for
the parent steel to transform rapidly from ferrite to austenite and
back again,produces microstructures which are usually harder and
more susceptible to hydrogen embrittlement.
2.Hydrogen level - > 15 ml/100g
This is inevitably present, derived from moisture in the fluxes used
in welding and from other sources.

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Hydrogen Cracking
3.Temperature < 200o
C for any steel and < 150o
C for
structural steel.
The greatest risk of cracking occurs when temperatures
near ambient are reached and cracking may thus take
place several hours after welding has been completed
( normally after 72 hours )
4.Stress > 50% yield strength of parent metal
These arise inevitably from thermal contractions during
cooling and may be supplemented by other stresses
developed as a result of rigidity in the parts to be joined.

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 Pre heat, removes moisture from the joint preparations, and slows down the
cooling rate
 Ensure joint preparations are clean and free from contamination
 The use of a low hydrogen welding process such as TIG or MIG/MAG
 The use of Nickel and Austenitic filler metal
 Ensure all welding is carried out under controlled environmental conditions
 Ensure good fit-up as to reduced stress
 The use of a PWHT with maintaining the pre- heat temperature
 Avoid poor weld profiles
 Use low hydrogen electrodes and baked as per manufacturer instructions
Hydrogen Cracking
Hydrogen Cracking
Precautions for controlling hydrogen cracking

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Solidification
Solidification
Cracks
Cracks

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Solidification Cracking
Characteristics
 Also known as hot cracking or center line cracking or crater
cracking and liquation cracking
Solidification cracking is intergranular type of cracking that
is along the grain boundaries of the weld metal.
It occurs during the terminal stages of solidification,when the
stresses developed across the adjacent grains exceed the
strength of the almost completely solidified weld metal.
Impurities such as sulphur and phosphorous and carbon
pick - up from parent metal increase the risk of cracking
High joint restraint which produce high residual stress will
increase the susceptibility to this type of cracking.

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• Occurs during weld solidification process from
liquidus to solidus and at the last area to solidified.
• Steels with high sulphur content (low ductility at
elevated temperature ) whereby produce hot
shortness to the weld metal
• FeS form films at the grain boundaries whereby
reduce the strength of the weld metal.
• Addition of manganese will form MnS and forms
globules instead of films( FeS)
• Occur longitudinally down center of weld
• Welding process that most susceptible to this type
of cracking are SAW and MIG/MAG with spray
transfer due to high dilution rate.

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Intergranular liquid
film along the grain
boundary

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Weld Centerline

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Factors responsible :
Metallurgical factors
a) Freezing temperature range –higher freezing range more
susceptible to solidification cracking due to presence of FeS
b) Primary solidification Phase – Less than 5% delta ferrite
c) Surface tension – concave more susceptible than convex
weld shape
d) Grain structure of fusion zone – Coarse columnar grain more
susceptible especially with high energy welding process.
Mechanical factors
a) Contraction stresses – Thicker material more susceptible.
b) Degree of restraint – poor fit - up

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 Use low dilution welding process
The use of high manganese and low carbon content fillers
Control sulphur ,keep below 0.06%
Maintain a low carbon content
Minimise the amount of stress / restraint acting on the joint during
welding
The use of high quality parent materials, low levels of impurities
Use proper joint design, use Single J instead of single V
Clean joint preparations, free from oil, paints and any other sulphur
containing product.
Joint design selection depth to width ratios
Precautions for controlling solidification cracking

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Lamellar
Lamellar
Tearing
Tearing

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Y

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Lamellar Tearing
Characteristics
 Lamellar tearing has a step like appearance due
to the solid inclusions linking up under the
influences of welding stresses
Occurs at beneath of HAZ or near HAZ
 It forms when the welding stresses act in the
short transverse direction of the material (through
thickness direction)
 Low ductile materials containing high levels of
impurities are very susceptible

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• Occur only in rolled direction of the parent material
• Associated with restrained joints subjected to through
thickness stresses on corners and tees
• Presence of elongated stringers such of nonmetallic
inclusion such as silicates and sulfides parallel to steels
rolling plane will produce poor through thickness ductility
of the plate.
• Tearing will triggered by this such non metallic inclusion
near the weld or it just outside HAZ during weld
contraction.
Lamellar Tearing

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Step like appearance
Cross section
Lamellar Tearing
Lamellar Tearing

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Susceptible joint types
Tee fillet weld Tee butt weld
(double-bevel)
Corner butt weld
(single-bevel)
Lamellar Tearing
Lamellar Tearing

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Critical area
Critical
area
Critical area
Lamellar Tearing
Lamellar Tearing

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Lamellar Tearing
Precautions for controlling lamellar tearing
 The use of high quality parent materials, low levels of impurities
( Z type material )

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Friction Welded Caps
Short Tensile Specimen
Through
Thickness
Ductility
Sample of Parent Material
A test for a materials susceptibility to lamellar tearing
Short Tensile Tests
Short Tensile Tests
The results are given as a STRA value
Short Transverse Reduction in Area

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Lamellar Tearing
( Z type material )
 Change joint design

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Lamellar Tearing
Lamellar Tearing
Modifying a Tee joint to avoid lamellar tearing
Susceptible
Susceptible Improved
Non-susceptible
Non-susceptible
Gouge base metal
and fill with weld
metal before welding
the joint
Susceptible Less susceptible
Prior buttering of the joint
with a ductile layer of weld
metal may avoid lamellar
tearing

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Lamellar Tearing
Lamellar Tearing
Modifying a corner joint to avoid lamellar tearing
Susceptible Non-Susceptible
Prior welding both plates
may be grooved to avoid
lamellar tearing
An open corner joint may
be selected to avoid
lamellar tearing

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Lamellar Tearing
( Z type material )
 Change joint design
 Minimise the amount of stress / restraint acting on the joint
during welding
 The use of buttering runs with low strength weld metal
 Hydrogen precautions e.g use low hydrogen electrodes
 Shift welding process such as Electro slag welding
 Use forging or casting joint.
 Place soft filler wire between the joint e.g T joint to reduce
stresses during expansion and contraction of weld metal.
 Pre heating helps on removal of Hydrogen on the plate.

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Weld Decay
Weld Decay

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Weld Decay
Characteristics
 Weld decay may occurs in unstabilized austenitic stainless
steels with carbon content above 0.1%
 Also known as knife line attack or crack
 Chromium carbide precipitation takes place at the critical
range of 450o
C-850o
C (sensitising temperature )
 At this temperature range carbon is absorbed by the
chromium, which causes a local reduction in chromium
content by promoting chromium carbides.
 Loss of chromium content results in lowering the materials
resistance to corrosion attack allowing rusting to occur

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Precautions for Weld Decay
Precautions for Weld Decay
 The use of a low carbon grade stainless steel e.g.
304L, 316L, 316ELC with carbon content < 0.03%
 The use of a stabilized grade stainless steel e.g.
321, 347, 348 recommended for severe corrosive
conditions and high temperature operating conditions
 Standard grades may require PWHT, this
involves heating the material to a temperature
over 1100o
C and quench the material, this
restores the chromium content at the grain boundary,
a major disadvantage of this heat treatment is the
high amount of distortion

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Fatigue
Fatigue
Cracks
Cracks

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Fatigue Testing
Fatigue Testing

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Fatigue Cracks
Fatigue Cracks
 Fatigue cracks occur under cyclic stress
conditions
 Fracture normally occurs at a change in section, notch
and weld defects i.e stress concentration area
 All welded materials are susceptible to fatigue cracking
 Fatigue cracking starts at a specific point
referred to as a initiation point
 The fracture surface is smooth in appearance
sometimes displaying beach markings
 The final mode of failure may be brittle or
ductile or a combination of both

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Initiation points / weld defects
Fatigue fracture surface
smooth in appearance
Secondary mode of failure
ductile fracture rough fibrous
appearance
Fatigue Cracks
Fatigue Cracks

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Fatigue Cracks
Fatigue Cracks
A fatigue failure
on a small bore
pipe work

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Precautions against Fatigue Cracks
Precautions against Fatigue Cracks
Toe grinding, profile grinding.
The elimination of poor profiles
The elimination of partial penetration welds and
weld defects
Operating conditions under the materials
endurance limits
The elimination of notch effects e.g. mechanical
damage cap/root undercut
The selection of the correct material for the service
conditions of the component

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1) Characteristic Fatigue fracture.
2) 45° Ductile fracture
7 x Initiation points at fillet weld toes
2
2 1
1

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1) Characteristic Fatigue radius & fracture.
Initiation point at weld toe.
2) 45° Ductile fracture. 3) Small area of plain
strain.
1
2
3
2

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= Initiation at points in weld root
1) Characteristic Fatigue radius (Beach marks)
a b c
2) Plain strain effect
3
3
3
3
3) Ductile fracture
2
2

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2) Ductile shear & shear lips.
Initiation point.
2
2
2
Initiation point is on the edge of the shaft.
1

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Initiation point at the toe of the
weld.
1) Brittle fracture.
Characteristic chevrons.
2) Areas of shear where
planes of fracture join.
1
2 2
2

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1) Characteristic chevrons.
Direction of fracture initiation, but
the point is off the specimen.

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1) Characteristic Fatigue radius and fracture.
1 2
Initiation point at the machined notch.

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Initiation point at the machined notch.
3) Area of plain strain effect.
2) Ductile fracture with shear lips.
2
2
2
1
2
2
3

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Any Questions?
Any Questions?

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QU 1. Briefly discuss the four essential factors for hydrogen
cracking to occur
Questions
Questions
QU 2. State four precautions to reduce the chance of hydrogen
cracking
QU 3. In which type of steel is weld decay is experienced and
state how it can be prevented
QU 4. State the precautions to reduce the chances of
solidification cracking
QU 5. State four the essential factors for lamellar tearing to
occur

WIS5 Weldability jdhgzfshjsjdhe.ppt 1.ppt

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WIS5 Weldability jdhgzfshjsjdhe.ppt 1.ppt