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Prepared by Dr.A.Vinoth Jebaraj
Solid state welding Fusion Arc Welding
Gas Welding Laser Welding
 Welding metallurgy deals with the interaction of different metals (similar
& dissimilar) and interaction of metals with atmospheric gases within a
short period of time (i.e. fraction of seconds).
Solubility of
atmospheric gases
and the effect of
shielding gases
with molten
weldment
Solid state
transformation
during cooling
after welding
Microstructural
changes in
weldment and
HAZ after
welding
Influence of
welding
parameters on
welding process
Effect of
impurities in
the weld
Changes in
Mechanical
& Corrosion
properties
Molten weld pool  semi solid weld  fully solidified weldment
What will happen, when
the weld metal is in hot
liquid state ?
1. No distinct structure
2. No orderliness in the
arrangement of atoms
3. High degree of mobility between
atoms due to heat energy
involved in welding.
When molten weld cools, atoms loose their energy and their mobility and
formed into a definite patterns.
These patterns are arranged in a three dimensional form and forms a
crystalline solid.
Classification of commercial welding processes
Gas Welding Electric Arc welding High density beam
welding
Oxyacetylene welding SMAW
GTAW, PAW
GMAW, FCAW
SAW, ESW
EBW
LBW
All the welding processes
involves these operations
Liquid/
Solid
interface
Solid/Solid
interface
FSW & FW
& RSW
10 W/cm2
10000 W/cm2
App. 6000°C
App. 3300°C
App. 20000 to 30000°C
50 W/cm2
Factors affecting the heat input needed for welding
Dimensions of the parent metal (Thickness)
Thermal conductivity
Preheating temperature of the base metal
Melting point
Rate of heat input
Electrode angle with respect to welding direction
Types of welding processes
High Arc Energy Welding Processes
Gas Tungsten Arc Welding (GTAW)
TIG welding Setup
Weld Bead using
TIG welding
Position of electrode and filler
metal during welding
 Low deposition rate and slow
speed welding. But clean weld.
Three different Polarities in welding
DCEN  straight polarity  more power (about two-thirds) is
located at the work end of the arc and less (about one-third) at the
electrode End  narrow and deep weld
DCEP  reverse polarity  heating effect of electrons is now at the tungsten
electrode  shallow weld  used for welding thin sheets of strong oxide-forming
materials  The positive ions of the shielding gas bombard the work piece,
knocking off oxide films and producing a clean weld surface
AC  Good penetration and oxide cleaning action both can be obtained
Plasma Arc welding (PAW):
Similar to GTAW Orifice gas as well as
shielding gas  converging action of orifice
gas nozzle  arc expands only slightly with
increasing arc length
Comparison between a plasma
arc and a gas tungsten arc
1. Gas plasma, 2. Nozzle protection,
3. Shield Gas, 4. Electrode, 5.
Nozzle constriction, 6. Electric arc
Gas Metal Arc Welding (GMAW) or Metal Inert Gas Welding (MIG)
Most clean weld process &
High deposition rate &
high productivity
Weld Bead using
MIG welding
MIG Welding
Setup
Shielded Metal Arc Welding (SMAW) or MMAW or Stick welding
The flux coating of the electrode disintegrates, giving off vapors that serve
as a shielding gas and providing a layer of slag, both of which protect the
weld area from atmospheric contamination.
Protection  Deoxidation  Arc stabilization  Metal addition
• Submerged Arc Welding: Arc welding process that uses a continuous,
consumable bare wire electrode, arc shielding is provided by a cover of
granular flux.
Used for welding thick plates due to high current input – Arc is not visible –
Arc efficiency is high
Flux Cored Arc Welding (FCAW)
Similar to GMAW  Both shielding gas and flux coated electrode are used
in FCAW
Laser Beam Welding (LBW)
Laser weld in 13-
mm-thick
A633 steel
Beam Temperature: 20000°C
Power density  1010 W/m2
Beam diameter  0.3 – 0.8 mm
Beam Temperature: 30000°C
Other Welding Processes
Friction Stir Welding
Resistance welding
Spot welding
Applications: welding of ship hulls,
storage tanks, and bridges.
Extremely high deposition rate.
One single pass is enough for even high
thickness.
Weld heat input is very high.
 weld can be done only in vertical
position.
large heat input leads to low toughness in
the weld because of coarser grain size in
the weldment and HAZ.
Gas Welding
Neutral flame oxygen and acetylene are mixed in equal amounts primary
combustion (Chemical reaction between oxygen and acetylene in the inner cone) 
products of primary combustion (CO and H2) react with O2 and forms CO2 and
H2O  secondary combustion area (protection envelop) preventing oxidation.
Reducing flame  excessive acetylene  greenish acetylene feather between inner
and outer envelop  used for welding aluminum alloys and carbon steel
Oxidizing flame  excessive oxygen  presence of unconsumed oxygen  used
for welding brass  because copper oxide covers the weldment and prevents zinc
evaporation from the weldment.
Welding process Filler metal Nature of Shielding Heat Input
GTAW
Non consumable tugnsten
electrode & Filler metal
Argon/ Helium High
GMAW Consumable filler wire Argon/ Helium/ CO2 High
SMAW
Consumable flux coated
electrode
Flux coated electrode High
FCAW
Consumable flux coated
electrode
Flux coated electrode High
SAW Consumable bare electrode Granular Flux High
LBW Autogenous or Consumable Argon/ Helium Low
EBW Autogenous or Consumable No shielding / vacuum Low
FSW Autogenous welding No shielding
Solid state
process
SW Autogenous welding No shielding
Solid state
process
Gas Welding Consumable electrode No shielding Very High
Welding process Activation energy Power density Temperature
GTAW Electric Arc Transfer Medium 6000°C
GMAW Electric Arc Transfer Medium 6000°C
SMAW Electric Arc Transfer Medium 6000°C
FCAW Electric Arc Transfer Medium 6000°C
SAW Electric Arc Transfer High 6000°C
LBW Radiation Transfer High 10000°C
EBW Radiation Transfer High 20000°C
PAW Gas Transfer High 6000°C
SW Electric current Solid state welding --
Gas Welding Gas Transfer Low 3000°C
Efficiency in welding and Heat input:
Where ,
Q = Heat transfer rate from the heat source to the work piece
Qnominal = Nominal power of the heat source
Always efficiency is less than one [η˂1] due to the lose of heat to the
surroundings during welding.
Where, E = Arc voltage; I = welding current and V = Welding speed
Heat input per unit length of the
weld
Q = EI/V
LBW  High reflectivity of metal surfaces leads to low efficiency.
PAW  Reflectivity is not a problem.
GTAW  DCEN > AC > DCEP (Polarity).
GMAW, SMAW  Heat transfer to the electrode can be transferred back to
the work piece through metal droplets.
SAW  Arc is covered with a thermally insulating blanket of molten slag and
granular flux, thus reducing heat losses to the surroundings.
EBW  Keyhole in EBW acts like a “black body” trapping the energy from
the electron beam.
Welding parameters: Welding Current [I]
Current heat Melting rate
Deposition rate
(Amount of filler
metal deposited)
Fusion zone
(Increasing the
penetrating power)
Increasing current will
lead to more effect on the
fusion zone penetration
Welding parameters: Arc voltage [v]
Arc voltage α Arc length
Arc voltage
Arc length
Bead width
If arc length increases or decreases too much then arc becomes unstable.
L1
L2
Welding parameters: Speed [s]
0.5 m/min
1.0 m/min
Welding speed
Decrease in penetration
Increase in bead width
Molten metal
has low
thermal
conductivity
High productivity,
less heat input, less
distortion and
residual stress
Welding parameters: Electrode diameter
 Larger electrode diameter  current has to be
increased  high deposition rate
 Smaller electrode diameter  less deposition
rate  less diameter means higher current
density
Modes of metal transfer
Spray transfer  App. 200 drops per second
Globular transfer  less than 10 drops per second
 More current is needed.
 High energy and high speed
droplets leads to higher
penetration.
 Used for over head welding.
 No spattering.
 Less current is needed.
 Weld metal gets wasted due
to spattering and causes
porosity sometimes.
Four welding positions
1G
2G
3G
4G
Preheating in welding
(To remove moisture and lower the thermal gradient)
It lowers the cooling rate in the weld metal and base metal, producing a more
ductile metallurgical structure with greater resistant to cracking.
The slower cooling rate provides an opportunity for any hydrogen that may be
present to diffuse out harmlessly without causing cracking.
It reduces the shrinkage stresses in the weld and adjacent base metal, which is
especially important in highly precision joints.
It raises some steels above the temperature at which brittle fracture would occur
in fabrication.
 Excessive hardening in the HAZ can be avoided by preheating.
Basic Weld Joints - Types
Butt joint geometries
Cruciform form joint
Welding defects (Type, size & location)
External weld defects Internal weld defects
Lack of deposition
Lack of penetration
Over deposition
Undercut
spatters
Surface cracks
Internal cracks
Slag inclusion
Porosity
Blow holes
Lamellar tearing
Arc strike
Lack of deposition
Cross section area of weld Strength of the weld joint
Remedy: Reweld
Higher welding speed & low
melting rate of the filler
UNDERFILL
Lack of penetration
Base metal
Filler metal
Lesser penetration at the root
Greater stress
concentration (Act
as a crack)
Reason:
 Low heat input
 Higher welding speed
 Incorrect weld groove
geometry
 Heat transfer through
molten weld pool is lesser
when compared with
perpendicular direction to
welding.
Over deposition
More heat input, more HAZ
due to extra metal deposition
Remedy: Grinding
EXCESSIVE REINFORCEMENT
Arc strike
Damage on the parent material resulting from the accidental striking of an arc outside
the weld area.
Spatters (metal droplets)
Spatters have to be removed because
corrosion will start from spatters.
Porosity is possible in the weldment.
Undercut
Excessive current, causing the edges of
the joint to melt and drain into the
weld; this leaves a drain-like impression
along the length of the weld.
Surface cracks
Remedy: Identify the crack and remove it.
Adjusting the weld composition through filler
metal.
Tack weld
can avoid
this crack.
 Cracks in weld joint develop when localized
residual tensile stresses exceed the UTS of the
material.
 Cracks will lead to poor ductility.
 Due to high Sulphur and carbon contents.
 Due to martensite structure formation.
 Presence of Hydrogen.
Slag inclusion
This type of defect usually occurs in welding processes that use flux, such
as shielded metal arc welding, flux-cored arc welding, and submerged arc
welding. Poor weld bead profile resulted in pockets of
slag being trapped between the weld runs
Smooth weld bead profile allows the slag to
be readily removed between runs
 Slag may serve as a initiation for cracking.
Porosity
 Due to entrapment of gases in the solidifying weld metal.
 Gases come in the weldment from flux constituents, shielding gases, absorbed
moisture, gases dissolved in the metal itself.
 Surface contaminations.
Control of porosity:
High welding current (low cooling rate)
 Low welding speeds
 Short arc lengths
 Baking of flux and coated electrodes
 Cleaning of work pieces
Lamellar tearing
 It’s a cracking problem caused by the presence of elongated inclusions (Sulphides
of Mn and Fe), which are deformed in the direction of rolling or extrusion.
 Stresses formed during welding lead to debonding of theses inclusions from the
matrix resulting in the formation of microcracks.
 During multipass welding, microcracks leading to cracking.
 This cracking takes place only in the base metal, even away from the HAZ.
Remedy: modifying the joint design to reduce the stresses formed during welding,
reducing the Sulphur content in steel, addition of Ca modifies the composition of
inclusions and makes them more resistant to deformation.
Property: through thickness ductility.
Residual stresses causes
• Stress corrosion cracking
• Cold cracking hydrogen induced cracking
• fatigue crack
Controlling residual stress
Minimize heat input
Preheating
No of passes during welding
 Residual stresses are induced in the
metal structures during welding.
 The intensity of Residual stresses
plays very important role in the fatigue
life of the component.
 In general, The magnitude of the
welding induced residual stresses are
nearly equal to half of the times of yield
stress of the material.
 Thermal Conductivity & Coefficient
of Thermal Expansion of the material
plays vital role in distortion and residual
stresses.
Causes for distortion
• Localized heating
• Non uniform stress distribution
Distortion occurs in these forms:
Longitudinal shrinkage
Transverse shrinkage
Angular distortion
Distortion
Dimensional Inaccuracy caused by Distortion
Dimensional accuracy is very important in
welding. Heat flow in the direction
perpendicular to the weld line is more.
Transverse shrinkage > longitudinal shrinkage
Weld
direction
Transverse shrinkage
longitudinal shrinkage
Welding direction
After some distance of
moving arc, same thermal
profile will be repeated.
This is known as quasi
steady state in welding.
More distortion
Types of Distortion
 Shrinkage
 Angular distortion
 Buckling deformation
 Rotational deformation
All the distortions are caused by the
shrinkage force generated due to the
thermal loading on the structure.
A single V groove butt weld leads to more distortion than the double V groove butt
weld of same thickness plate.
Welding in neutral axis will balance the shrinkage force of one side against another
side from the neutral axis.
Welding technology by A.Vinoth Jebaraj
Basic forms of
common Lattice
structure
Ti, Zn, Mg, Cd
Fe, V, Nb, Cr, Si
Al, Ni, Ag, Cu,
Au, C, Mn, N
Ferrite : This phase has a Body Centre Cubic structure (B.C.C) which can
hold very little carbon; typically 0.0001% at room temperature. It can
exist as either: alpha or delta ferrite.
Austenite: This phase is only possible at high temperature in steels which
are containing carbon. It has a Face Centre Cubic (F.C.C) atomic structure
which can contain up to 2% carbon in solution.
Cementite: Unlike ferrite and austenite, Cementite is a very hard
intermetallic compound consisting of 6.7% carbon and the remainder iron,
its chemical symbol is Fe3C. Cementite is very hard, but when mixed with
soft ferrite layers its average hardness is reduced considerably.
Pearlite: A mixture of alternate strips of ferrite and Cementite in a single
grain. A fully pearlitic structure occurs at 0.8% Carbon. It is a lamellar
structure, which is relatively strong and ductile.
Martensite: At fast cooling rates, the austenite might not have sufficient time
to transform completely to ferrite and Pearlite and will provide a different
microstructure. In this case, some of the untransformed austenite will be
retained and the carbon is held at supersaturated state. This new structure
is called ‘martensite’.
Allotropic Transformation: Changes in phase transformation with respect
to the temperature is called as ‘allotropic transformation’.
Example: Iron has a BCC lattice structure from room temp. up to 910°C,
and from this point to 1388°C it is FCC. Above this point to melting point,
1538°C it is again BCC.
Stainless Steel TIG weld joint
Metallurgical Problems in Welding:
Burning
Segregation
Gas pockets
 Hot and cold cracking
 Dilution
 Stainless steels constitute a group of high alloy steels based on Fe – Cr, Fe – Cr – C
and Fe – Cr – Ni systems.
 It contains minimum of 10.5% chromium
 Formation of chromium oxide passive layer on the surface
 Prevents oxidation and corrosion
Types of stainless steels
 Martensitic (4xx)
 Ferritic (4xx)
 Austenitic (2xx, 3xx)
 Duplex (austenitic and ferritic)
 Precipitation hardened
Uniform corrosion or General Corrosion
 It usually occurs in acid
environments or hot alkaline
solutions.
Pitting corrosion
 It is a form of localized corrosion and is
identified by attacks at small discrete spots
on the steel surface.
 Chloride environment facilitate this local
breakdown of the passive layer, especially if
there are imperfections in the metal surface.
Crevice corrosion
It is a form of localized corrosion and occurs under the same
conditions as pitting in neutral or acidic chloride solutions. It usually
found at flange joints or at threaded connections.
Effect of Chromium
BCC structure (Ferrite)  It tends to stabilize BCC structure when dissolved
with iron.
Phase diagram for Iron and chromium
 Above 14% of chromium austenite
cannot form at all.
 Increasing Cr (30 – 60%) will
form sigma phase by prolonged
heating below 800°C.
 Carbon and Manganese also tends
to stabilize austenite thus
expanding the gamma loop.
 Silicon tends to stabilize ferrite
and reduce the gamma loop.
Effect of Carbon
Effect of carbon on the size of the gamma loop in Iron – Chromium
phase diagram
Effect of Nickel
FCC structure  tends to stabilize FCC structure when dissolved with
iron.
 In pure iron, Austenite is stable
only above 910°C.
 Increasing the Ni content allows
austenite to remain stable at lower
temperatures, open gamma loop
will form.
 To retain a complete austenite
structure, very high Ni content is
needed.
Phase diagram for Iron - Nickel
Effect of Nickel
Other Austenite & Ferrite Stabilizers
Austenite stabilizers Ferrite stabilizers
Nickel Chromium
Carbon Molybdenum
Manganese Silicon
Nitrogen Titanium
Cobalt Niobium
copper Vanadium
Creq = %Cr + 1.0 (%Mo) + 0.5 (%Nb + %Ta) + 1.5 (%Si) + 2 (%Ti) + (%W + %V )
Nieq = % Ni + 30 (% C) + 0.5 (% Mn) + 30 (% N) + 0.5 (%Co)
WRC – 92 Diagram
High chromium
level  ferrite
structure over the
entire temp range
Low chromium level 
form a closed gamma
loop transform to
Austenite on heating 
martensite on cooling
Chromium and
enough Nickel
Austenite
structure over the
entire temp range
Chromium with
intermediate Ni
 mixture of
ferrite and
austenite
Pitfalls in welding Stainless Steel
Sensitization  leads to local destroy of corrosion resistance due to the formation of
chromium carbides between 500 – 800°C  sensitization mainly depends on carbon
content, temperature and time of heating.
Chromium carbide formation in
the grain boundaries
Chromium carbide formation in HAZ
Effect of temperature and carbon content on the time to produce
chromium carbide
 The low carbon stainless steels 304L, 316L and 317L have a maximum carbon content of
0.03% and they can be successfully welded without sensitization for many applications.
 But for extended service at high temperatures, low carbon alone is not enough to ensure
freedom from sensitization.
Overcoming the issue of carbide precipitation
 It is common to use stabilized steels like 312 and 347. Alloying elements are titanium
and niobium.
 They are strong carbide formers which tie up the carbon preventing it from forming
carbides with chromium.
 To ensure complete stabilization titanium content must exceed five times the addition of
carbon & nitrogen and the niobium and tantalum content must exceed ten times the
carbon and nitrogen.
 Solution treatment  Treat the steel at a temperature high enough
to dissolve all the carbides.
 Heating to 1000 – 1150 °C followed by rapid cooling.
 Slow cooling leads to the risk of re-allowing carbide precipitation
through the sensitive range.
 Stress relieving treatment (800 – 900°C) will not work.
Solution Annealing treatment
Stress Corrosion Cracking (SCC)
Stress (Residual stress) + Corrosion  Stress Corrosion
Cracking
SCC happens on the metal surface by the combined effect of corrosion and mechanical
stress in particularly at chloride containing marine environments.
Higher ductility offered by the Austenitic grades of stainless steels leads to SCC
attack in the ASS grades like SS 304L and SS 316L. SCC attack takes the form of
thin branched cracks.
How to avoid SCC attack?
Mechanical Properties of Austenitic stainless steel
 Higher work hardening capacity
 Excellent low temperature toughness
 Higher creep strength
Solidification modes in Austenitic stainless steel
Pseudo binary section of the Fe–Cr–Ni
ternary diagram at 70% Fe
Weld solidification cracking or Hot cracking
1. Presence of impurities such as sulphur and phosphorous
Sulphur and phosphorous are rejected to the weld liquid due to the low solubility in
primary austenite structure. This liquid is getting trapped in the grain boundaries as a
low melting point segregate.
2. Primary structure formed on solidification
If the primary solidification phase is ferrite, then the rejection of sulphur does not occur
and the weld is crack free.
Note: Hydrogen induced cracking is not a problem with austenitic grades unless
martensitic structure is present. Because hydrogen has higher solubility in FCC
structure at high temperature.
Effect of composition on Hot cracking resistance
 Primary ferritic solidification mode greatly reduces the hot cracking during
welding.
 Delta ferrite, has higher solubility for impurity elements, which reduces the
concentration of P and S at the grain boundaries.
 3 – 8% delta ferrite at room temperature, is composed of ferrite – austenite
interfaces leads to difficult situation for hot cracking.
 WRC – 92 diagram must be used to ensure that primary ferritic solidification
occurs, to avoid hot cracking.
HAZ cracking in Austenitic stainless steel
 HAZ cracking has been observed in austenitic steels in thick section welds,
particularly in the stabilized austenitic grades like SS 347.
 This is due to the formation of low melting point films in the grain boundaries of
HAZ.
 Titanium and niobium along with impurities such as phosphorous and silicon
seem to be the main contributors for this type of cracking.
Embrittlement in stainless steel welds
 Delta ferrite  sigma, chi and chromium enriched alpha.
 Increasing Cr and Mo content, leads to the formation of sigma phases.
 Mo has greater effect (4 times greater) than the Cr in sigma formation.
 Nitrogen additions have a retarding effect on sigma phase formation.
Sigma phase formation
1. Type ‘A’ Solidification
2. Type ‘AF’ Solidification
3. Type ‘FA’ Solidification
4. Type ‘F’ Solidification
Type ‘A’ Solidification Type ‘AF’ Solidification
Type ‘FA’ Solidification
If Creq / Nieq Austenite
When cooling rates
are moderate and
Creq / Nieq is low
but still within the
FA range
Type ‘F’ Solidification
At low Creq / Nieq
ratio within the
ferrite range an
acicular ferrite
structure will form
316L BM
Equiaxed austenite grains
316L WM
Austenite mixed with Vermicular ferrite
Microstructure of ASS base metal & weld
Usually 2 to 3% of delta ferrite will be present in the ASS microstructure. This is a
preferential site for carbide (M23C6) and sigma phases.
HAZ of Austenitic Stainless Steel
Nature of the HAZ  depends upon the composition and microstructure of the base
metal
Grain growth  Increase in heat input results in grain coarsening
Ferrite formation  formation of coarser ferrite grains if the primary solidification
structure is ferrite.
Precipitation in HAZ  precipitation of carbides and nitrides
Grain boundary liquation segregation of impurity elements (Ti & Nb)  HAZ
Liquation cracking  segregation of phosphorous and sulphur also promote liquation
cracking
Effect of Creq / Nieq ratio on solidification cracking
 Presence of two phase microstructure at the end of solidification is the primary
reason for high resistance.
Thermal Properties of Austenitic stainless steel
Metal Thermal
conductivity
W/m-k
Melting point Coefficient of thermal
expansion/°C
SS 304 16.2 @ 100°C
21.5 @ 500°C
1425°C 17.6×10-6
Carbon Steel 46 @ 100°C 1510°C 11.7×10-6
 Less heat is required
to make the same size
weld.
 But increased
distortion and risk of
buckling in Austenitic
grades of SS.
Autogenous welding of stainless steel
Without filler metal addition  flux coating  can be weld up to 6 mm thick plate 
improve penetration depth
Welding of Ferritic Stainless Steel
 BCC Crystal Structure  Absence of Nickel  High SCC resistance  undergo Ductile Brittle
transition when the temperature is lowered.
 Chromium exceeding 12% (upper limit of Chromium is 36%) and carbon limited to 0.1%.
 Higher Chromium contents confer greater corrosion resistance. But they result in
embrittlement and an enhanced tendency to form intermetallic phases such as σ on thermal
exposure.
 Greater yield strength , reduction in ductility and impact resistance
 single phase microstructure, greater atomic mobility, grain coarsening, impact transition is
high due to embrittling effect of Cr dissolved in ferrite.
 475° embrittlement  heating temperature range 400 - 550°C  precipitation of Cr rich alpha
Welding of Cast Iron
 Iron with 1.7 to 4.5% carbon and 0.5 to 3% silicon.
 Lower melting point (1150 to 1200 °C) and better cast ability.
 Types of Cast iron
 Grey cast iron - carbon as graphite
 White cast iron - carbides, often alloyed
 Ductile cast iron - Nodular, spheroidal graphite
 Malleable cast iron
 Effect of cooling rate
 Slow cooling favours the formation of graphite & low hardness.
 Rapid cooling promotes carbides with high hardness.
 Thick sections cool slowly, while thin sections cool quickly.
 Sand moulds cool slowly, but metal chills can be used to increase cooling rate &
promote white iron.
Since the compositions of
most cast irons are around
the eutectic point of the
iron carbon system, the
melting temperature
which is
about 300°C lower than
the melting point of pure
iron.
White Cast Iron
 If White, crystalline crack surface
observed when a casting fractures, then it
is called as white cast iron.
 Due to the absence of ductility, the base
metal and HAZ are susceptible to cracking
during cooling after welding.
 If cooling is rapid, then the excess carbon
remains in the metastable form of iron
carbide (also called as cementite).
 It contains 2.5 to 3.8% C, 0.2 to 2.8% Si,
and as much as 5.5% Ni, 30% Cr, 6.5%
Mo, and 30% Mn, if designed for wear
resistance.
 Tensile strengths can range from 160 to
620 MPa
Because of its extreme hardness and brittleness,
white cast iron is considered unweldable. Used for
abrasion resistance.
Almost all the carbon is in the combined
form.
Grey Cast Iron
 Formation of iron carbide during solidification
is suppressed entirely.
 Graphite precipitates directly from the melt as
elongated and curved flakes in an iron matrix
saturated with carbon.
 When a grey iron casting fractures, the crack
path follows these graphite flakes
 The gray cast iron has a very low ability to
bend and low ductility.
 Possibly a maximum of 2% ductility will be
obtained in the extreme low carbon range. The
low ductility is due to the presence of the
graphite flakes which act as discontinuities.
Full graphitization  The majority of the carbon dissolved in the iron at high temperatures is deposited as
graphite on the existing flakes during cooling. The structure then consists of graphite flakes in a ferrite
matrix, referred to as ferritic grey cast iron.
If graphitization of the carbon dissolved in the iron at high temperatures is prevented during cooling, iron
carbide precipitates out and the matrix is Pearlitic (referred to as Pearlitic grey cast iron).
Between 2.5% and 4% carbon and between 1% and 3%
silicon
MOST WIDELY
USED CAST IRON
Ductile Iron or Spheroidal Graphite Iron or Nodular Iron
 Free graphite in these alloys precipitates from
the melt as spherical particles rather than
flakes.
 This is accomplished through the addition of
small amounts of magnesium or cerium to the
ladle just before casting.
 The spherical graphite particles do not
disrupt the continuity of the matrix to the
same extent as graphite flakes, resulting in
higher strength and toughness.
Malleable Cast Iron
Heat treatment of white cast iron  Iron carbide decompose into iron and carbon  The
structure is changed to Pearlitic or ferritic which increases its ductility.
This reaction is favoured by high temperatures, slow cooling rates, high carbon & silicon
contents.
Ferritic Malleable Cast Iron
Carbon nodules in ferrite matrix is known as ferritic
malleable cast iron.
Iron carbides present are dissociated during a high-
temperature anneal above 870 °C for more than 60 h.
Pearlitic Malleable Cast Iron
If full graphitization is prevented and a controlled
amount of carbon remains in the iron during cooling,
finely distributed iron carbide plates nucleate in the
iron at lower temperatures.
 Graphite dissolves and precipitates iron carbide in HAZ  loss in ductility
 Ferritic malleable grades display the best weldability of the malleable cast irons.
 Pearlitic malleable irons, because of their higher combined carbon content, have lower impact
strength and higher crack susceptibility when welded.
 Post weld annealing  softens the hardened zone and regains the minimal ductility
(For small welded parts  550°C & For heavy sections 200°C)
 MMA welding cast iron, using low-carbon steel and low-hydrogen electrodes at low currents,
produces satisfactory welds in malleable iron.
 If low-carbon steel electrodes are used, the part should be annealed to reduce the hardness in
the weld (due to carbon pickup) and in the HAZ.
Grey cast iron
During welding, graphite in flake form, carbon can readily be introduced into the weld
pool, causing weld metal embrittlement. (Base metal dilution)
Grey cast iron welds are susceptible to the formation of porosity. (controlled by lowering
the amount of dilution with the base metal, or by slowing the cooling rate so that gas has
time to escape.)
Ductile Iron
Pearlitic ductile iron produces a larger amount of Martensite in the HAZ than ferritic
ductile iron and is generally more susceptible to cracking.
Ductile cast irons are generally more weldable than grey cast irons.
Practical considerations in welding Cast Iron
Base metal preparation: dye penetrant inspection for defects  formation of porosity due
to the presence of oil or grease in environment Degassing of casting remove residual
surface graphite prior to welding
Joint design modifications:
Reduces the risk of cracking by deflecting the
path of a crack.
Welding induced cracking in Cast Iron
Formation of Iron carbide & high carbon Martensite leads to cracking.
 To prevent the formation of Martensite
Multi pass welding, preheating, Interpass temperature, PWHT tempers the
Martensite in HAZ
 To reduce the size of the HAZ
Reduction of heat input, use of small diameter electrodes, use of low melting
point welding fillers, use of lower preheat temperatures
Aluminium Alloys
Non Heat Treatable Alloys Heat Treatable Alloys
 Strength depends upon the hardening
effects of elements
(Mn, Si, Fe & Mg)
 These alloys are work hardenable.
Strengthening is possible by cold
working. They incapable of forming
second phase precipitates for improved
strength.
 1xxx, 3xxx, 4xxx, 5xxxx series of alloys.
Example:
Solution strengthened AA 5083 σy = 230
MPa
 Initial Strength depends upon the hardening effects of
elements
(Cu, Mg, Zn & Si)
 These elements is various combinations show increasing
solid solubility with increasing temperature. Thus, heat
treatment will impart pronounced strengthening.
 2xxx, 6xxx, 7xxx series of alloys.
Example:
AA 7075 σy = 505 MPa
Work hardenable alloys
Pure Al – 1xxx
Al-Mn – 3xxx
Al – Si – 4xxx
Al – Mg – 5xxx
Al – Fe – 8xxx
Al – Fe – Ni – 8xxx
Precipitation hardenable alloys
Al – Cu – 2xxx
Al – Cu – Mg – 2xxx
Al – Cu – Li – 2xxx
Al – Mg – Si – 6xxx
Al – Zn – 7xxx
Al – Zn – Mg – 7xxx
Al – Zn – Mg – Cu – 7xxx
Al – Li – Cu – Mg – 8xxx
Effect of Alloying Elements
1xxx series  Aluminium of 99% or higher purity  used where thermal/electrical conduction or
corrosion resistance becomes dominant over strength in design considerations.
2xxx series  Copper (2 to 10% Cu) is the principal alloying element with magnesium as secondary
addition  Heat treatment increases yield strength with loss in elongations  Comparatively low
corrosion resistance than other Al alloy grades. unweldable because the formation of aluminium-
copper intermetallics in weld metal renders them
brittle.
3xxx series  Manganese (5 to 50 ppm) is the major alloying element  Non heat treatable  only
limited percentage of Mn (1.5%) can be effectively added to aluminium.
4xxx series  Silicon is the major alloying element  added up to 12% which cause lowering of the
melting point used in welding filler wire.
Effect of Alloying Elements
5xxx series  Magnesium is the major alloying element  high strength  good
resistance to corrosion in marine environment. Care must be taken during processing to
avoid the formation of Mg3Al2.
6xxx series  Contain silicon and magnesium to form magnesium silicone  heat
treatable  possess good formability and corrosion resistance with medium strength
7xxx series  Zinc is the major alloying element (1 to 8%) coupled with Mg and Cu 
heat treatable alloys of high strength  used in air frame structures and for highly
stressed parts (7075, 7050 and 7049 high strength alloys)
Welding technology by A.Vinoth Jebaraj
Heat treatment to increase the strength of aluminium alloys is a three step process.
 Solution heat treatment  dissolution of soluble phases
(Allow the maximum amounts of soluble hardening elements in the alloy)
 Quenching  development of super saturation
(To produce supersaturated solution)
 Age hardening  Precipitation of solute atoms either at room temperature
(natural aging) or elevated temperature (artificial aging or precipitation heat
treatment)
The major alloy systems with precipitation hardening in Aluminium Alloy include:
1. Aluminium – Copper systems with strengthening from CuAl2
2. Aluminium – Copper – Magnesium systems (Mg intensifies precipitation)
3. Aluminium – magnesium – silicon systems with strengthening from Mg2 Si
4. Aluminium – Zinc – magnesium systems with strengthening from MgZn2
5. Aluminium – Zinc – magnesium – copper systems
2xxx series
6xxx series
7xxx series
Heat Treatable Aluminium Alloys
Age Hardening
Natural Aging Artificial Aging
 Sufficient precipitation occurs in few days at room
temperature.
 Precipitation hardening results from natural aging
leads to high ratios of tensile to yield strength, high
fracture toughness, high resistance to fatigue.
 Example: stable condition reached after few days in
2xxx series alloys.
 6xxx series and 7xxx series alloys are less stable at
room temperature and continue to exhibit notable
changes for many years.
 Sufficient precipitation occurs in the
temperature range from 115 to 190°C;
times vary from 5 to 48 hrs.
 Example: some 7xxx series alloys
Properties Influencing Joining:
 Aluminium Oxide formation  Melting point of Al2O3 is App. 1926°C (3 times greater than Al)  thick
layer Al2O3 of will absorb moisture from the air  Oxide film must be removed from surface prior to
welding
 Moisture is a source of hydrogen rapid cooling rate leads to retained free hydrogen within the weld
and causes porosity decrease weld strength and ductility.
 High thermal conductivity Al conducts heat 5 times greater than Steel more heat input is needed
even though the melting point is half that of steel  weld solidify quickly  surface tension hold the
weld metal in position and makes all position welding practical.
Fusion welding of Aluminium Alloy
 Electrical Conductivity Aluminium possess high electrical conductivity 
resistance heating of the electrode does not occur
 High coefficient of thermal expansion  decrease about 6% in volume when
solidifying  This change in dimension leads to distortion.
 The absence of color change as temperature approaches the melting point.
Filler alloy selection criteria:
Ease of welding, tensile or shear strength of the weld, weld ductility, service
temperature, corrosion resistance, color match, sensitivity to weld cracking.
Weldability of Non-Heat Treatable Aluminium Alloy
 They are incapable of forming second phase precipitates for improved strength.
 Positive Attribute: Many of the alloying elements needed for precipitation hardening (Cu+Mg and
Mg+Si) can lead to hot cracking during welding.
 HAZ is not compromised by coarsening or dissolution of precipitates. Thus, leads to higher
efficiency. Loss in strength in the HAZ is not nearly as severe as that experienced in heat treatable
alloys.
Therefore, reasonable joint strength can be obtained in the as welded condition without the need for
post weld heat treatment.
 Hot cracking: EBW & LBW result in cracking when Mg is boiled off.
Hot cracking sensitivity Vs. Magnesium content
To avoid hot cracking, use of high magnesium filler alloy is recommended.
 Porosity: All Aluminium alloys are susceptible to Hydrogen Induced Cracking. Due to
abrupt drop in hydrogen solubility when going from liquid to solid.
 Remedy: Minimize Hydrogen pickup during welding, use of high grade shielding gas
and careful storage of filler wire.
Weld Properties of Non-Heat Treatable Aluminium Alloy
Loss in strength in the HAZ:
 HAZ of Non heat treatable alloys is limited to recrystallization and grain growth in the
absence of strengthening precipitates which may coarsen in Heat treatable alloys.
 Therefore, 5xxx series alloys are popular in welded pressure vessels.
Weld metal microstructure:
 Weld metal is the weakest part of the joint.
 Weld metal microstructure consists of columnar dendritic substructure that has
interdendritic eutectic constituents.
(Fe, Mn)Al6  For 1xxx and 3xxx alloys
Mg3Al2  For 5xxx alloys
Welding of Heat Treatable Aluminium Alloy
 Precipitation hardening mechanism of alloys requires alloying elements with appreciable solid
solubility in aluminium at elevated temperatures, but limited solubility at low temperature.
 Fusion welding redistributes the hardening constituents in the HAZ which locally reduces
material strength.
 High strength alloys (e.g. 7010 and 7050) and most of the 2xxx series are not recommended for
fusion welding because they are prone to liquation and solidification cracking.
 The technique of Friction Stir Welding is particularly suited to this kind of aluminium alloys.
It is capable of producing sound welds in many alloys, particularly those heat treatable alloys
which are prone to hot cracking during fusion welding.
Welding process for Aluminium Alloy
SMAW: Insufficient degree of cleaning  gas produced by flux coating not enough to
obtain the defect free weld. Deposit rate is limited due to changing the electrode.
GMAW: There is no slag formation  possible to perform weld in all positions  rate
deposition is roughly two times than SMAW  weld quality is good
Argon  low thermal conductivity, more stable arc
Helium  High thermal conductivity, erratic arc
 Titanium alloys are widely used in Aerospace industries due to its
lightness and high strength. Used in connecting rods of Automobiles
for better fuel efficiency. (Example: Ferrari and Porsche)
 Density 4200 kg/m3 . It is having higher resistance to heat. Its
melting point is 1668°C which is higher than that of steel.
 Due to its non-toxic characteristics, it is also used as a bio material.
Also suitable for high temperature applications.
Welding technology by A.Vinoth Jebaraj
Types of Titanium Alloys
Based on the applications, it can be divided as:
 Corrosion resistant alloys
 High strength alloys
 High temperature alloys
Based on the crystal structure, it can be divided as:
 Alpha alloys
 Alpha – Beta alloys
 Beta alloys
Why Dissimilar welding is needed?
[For process system operate at different service conditions]
More resistible to
corrosion
More easy to process
and inexpensive
Wrought
structure
Wrought
structure
Cast structure
Issues in Dissimilar welding
Welding of similar alloys Welding of dissimilar alloys
The shape of the weld is
symmetric
Different solidification
microstructure and alloy
segregation
Responsible Factors for Dissimilar weld
failure
 General alloying problems (Brittle phase formation and
mutual solubility) of two metals.
 Widely differing melting points
 Differences in coefficients of thermal expansion
 Differences in thermal conductivity
 Carbon migration
 Corrosion/Oxidation
Austenitic Stainless Steel Vs. Carbon Steel
 Selecting proper filler metal for dissimilar welding estimating the probable weld
microstructure diluted by both the base metals.
Selection of Filler Metal
 Filler metal must provide the joint design requirements, such as mechanical
properties and corrosion resistance.
 Filler metal must fulfill the weldability criteria with respect to dilution, melting
temperature, and other physical property requirements of the weldment.
Element (SS 309) Content (%) (SS 308) Content (%)
Iron, Fe 60 66
Chromium, Cr 23 20
Nickel, Ni 14 11
Manganese, Mn 2.0 2.0
Silicon, Si 1.0 1.0
Carbon, C 0.20 0.080
Phosphorous, P 0.045 0.045
Sulfur, S 0.030 0.030
Chemical composition of Filler Metal
SS/CS Welded Piping
Factors Influencing Joint Integrity
Weld metal
 Weld metal composition and its properties  Non uniform composition adjacent to the base
metal.
 Solidification characteristics of the weld metal are influenced by dilution and composition
gradients near each base metal.
 It is important to investigate the phase diagram of two metals involved.
 If there is no or little solubility between the two metals, the joint will not be successful.
 Intermetallic compounds form between dissimilar welds must be investigated.
 Sometimes, it is necessary to use a third metal that is soluble in each metal in order to obtain a
successful joint.
Dilution
Factors Influencing Joint Integrity
 Filler metal must be able to accept dilution (alloying) by the base metals without
producing crack sensitive microstructure.
 Weld must be stronger than the weaker of the two metals being joined, i.e. possessing
sufficient tensile strength and ductility so that the joint will not fail.
The Average percentage of a specific alloying element in the diluted weld metal can be
calculated using the equation developed by AWS:
XW = (DA)(XA) + (DB)(XB) + (1 – DT)(XF)
XW = Avg. percentage of element X in the weld metal
XA = Avg. percentage of element X in the base metal A
XB = Avg. percentage of element X in the base metal B
XF = Avg. percentage of element X in the filler metal F
DA = percent dilution by base metal A
DB = percent dilution by base metal B
DT = percent total dilution by the base metals A and B.
Calculation weldment composition
Consider SS 316 is welded to a 2.25Cr – 1Mo low alloy ferritic pressure vessel
steel with a Ni – Cr alloy filler metal ER Ni Cr – 3.
Alloy Cr Ni Mo Fe
2.25Cr – 1Mo 2.5 - 1.0 95.5
SS 316 17.0 12.0 2.5 63.0
ER Ni Cr – 3 20.0 72.0 - 3.0
Assuming that the total dilution is 35%, 15% by Cr – Mo alloy steel and 20% from SS316.
Therefore, the Avg. percentage of Cr, Ni and Mo in the weld metal are calculated as
follows:
Cr, % = 0.15(2.5) + 0.20(17.0) + 0.65(20.0) = ?
Ni, % = 0.20(12.0) + 0.65(72.0) = ?
Mo, % = 0.15(1.0) + 0.20(2.5) = ?
Causes of more Dilution
 High Travel Speed. Too much heat applied to parent metal instead of on filler metal.
 High welding Current. High current welding processes, such as Submerged Arc
Welding can cause high dilution.
 Thin Material. Thin sheet TIG welded can give rise to high dilution levels.
 Joint Preparation. Square preps generate very high dilution. This can be reduced by
carefully buttering the joint face with high alloy filler metal.
If dilution from one base metal is less detrimental than from the other, the arc should be
directed towards that metal.
Non uniform dilution may produce inconsistent joint properties.
Control of Dilution
 Amperage: Increased current density increases dilution.
 Polarity: DCEN gives more penetration leads to more dilution.
 Electrode size: Smaller electrode, lower the amperage, lower the dilution
 Travel speed: Decrease in travel speed decreases the amount of base metal melted and
increases proportionally the amount of filler metal melted.
 Oscillation: Greater width of electrode oscillation reduces dilution.
 Arc shielding: shielding medium affects dilution.
Granular flux without alloy addition > He, Ar, Co2 > granular flux with alloy addition
(additional filler in the form of powder, wire, strip)
Oxidation
 Compositional variations at the interfaces in the weld result in oxidation when
operating at high temperatures in air and formation of notches at these location.
 Such notches are potential stress raisers in the joint.
 It cause oxidation failure along the weld interface under cyclic thermal conditions.
Melting Temperature
 Differences in melting point of the two base metals can result in rupture of the
metal which is having low melting point.
 Solidification and contraction of the high melting point material will induce
residual stress in the other metal when it is in partially solidified condition.
 Remedy: Buttering  Depositing one of more layers of filler metal of intermediate
melting temperature on the face of the base metal with higher melting
temperature.
 Buttering layer provides barrier layer that will slow the undesirable migration of
elements from the base metal to the weldment during PWHT and service at high
temperature.
EDGE PREPARED FOR BUTTERING. FACE BUTTERED WITH FILLER METAL. BUTTERED FACE PREPARED FOR WELDING.
JOINT ALIGNED FOR WELDING.
JOINT WELDED WITH STAINLESS STEEL FILLER METAL.
BUTTERING TECHNIQUE
Thermal Conductivity
 Welding heat source must be directed at the metal having the higher thermal
conductivity to obtain the proper heat balance.
 Preheating the metal having high thermal conductivity heat loss to the base metals
can be balanced.
 Dilution is more uniform with balanced heat input.
 Preheating the base metal of higher thermal conductivity also reduces the cooling
rate of the weld metal and HAZ.
 Preheating also reduce the heat needed to melt the base metal
Coefficient of Thermal Expansion
 Large differences in CTE leads to tensile residual stress in one metal (may hot
cracking during welding and cold cracking during service) and compressive residual
stress in the other.
 CTE of the weld metal should be intermediate between those of the base metals.
 If CTE difference is small, then weld metal may have a CTE equivalent to that of one
of the base metals.
Carbon Migration
 Chromium in steel has greater affinity for carbon than iron.
 In ASS/CS weld using austenitic filler, carbon can diffuse from the base metal into
the weld metal at temperature above 425°C and more rapidly at above 595°C.
 Carbon migration takes place during PWHT and high temperature service
applications. Excessive carbon migration into SS weld weakens the HAZ of carbon
steel.
 During cyclic temperature service, HAZ will be subjected to varying shear stresses
because of the differences in CTE of base and weldments. These stresses lead to
fatigue failure.
Why not possible for Aluminium Vs. SS?
 Fe and Al are not compatible materials.
 Fe melting point = 1538°C, Al melting point = 660°C
 Both metals have no solubility for the other in the solid state.
(Intermetallic phases FeAl2, Fe2Al5 and FeAl3)
 High residual stresses due large variation in CTE, thermal conductivity.
 Al can be joined to SS or CS by solid state welding process and also by EBW.
Most welded dissimilar combinations
 Stainless steels to Carbon or low alloy steels
 Nickel base alloys to Steels
 Cobalt base alloys to steels
 Copper base alloys to steels
Definition: Residual stresses are self balancing internal system of stresses arising from
non-uniform mechanical or thermal straining with some measure of plastic flow.
Residual stresses are elastic in nature exist in a body when all external forces are
removed.
Residual stresses Vs. Distortion
Development of Residual Stresses
Temperature change Vs. Residual stresses
Basic Mechanism of Residual Stress
Types of Residual Stresses
They are commonly classified into two groups:
Macro Residual stresses (Residual stresses of the first kind)
[Measured over a gauge length that encompasses several grains]
Micro Residual stresses (Residual stresses of the second kind)
[Measured within a single grain or a particular set of grains
Both types can contributing SCC or fatigue initiation depending upon the situation.
Residual Stresses distribution
Factors Influencing Residual stresses
 Materials Properties
 Specimen Dimension
 Welding Processes
 Welding Sequence
Sources of Residual Stresses
 Residual stresses owing to the shrinkage process of the Seam
and HAZ.
 Residual stresses owing to the more rapid cooling of the surface
[quenching residual stresses]
 Residual stresses owing to a phase transformation

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Welding technology by A.Vinoth Jebaraj

  • 2. Solid state welding Fusion Arc Welding Gas Welding Laser Welding
  • 3.  Welding metallurgy deals with the interaction of different metals (similar & dissimilar) and interaction of metals with atmospheric gases within a short period of time (i.e. fraction of seconds). Solubility of atmospheric gases and the effect of shielding gases with molten weldment Solid state transformation during cooling after welding Microstructural changes in weldment and HAZ after welding Influence of welding parameters on welding process Effect of impurities in the weld Changes in Mechanical & Corrosion properties
  • 4. Molten weld pool  semi solid weld  fully solidified weldment What will happen, when the weld metal is in hot liquid state ? 1. No distinct structure 2. No orderliness in the arrangement of atoms 3. High degree of mobility between atoms due to heat energy involved in welding. When molten weld cools, atoms loose their energy and their mobility and formed into a definite patterns. These patterns are arranged in a three dimensional form and forms a crystalline solid.
  • 5. Classification of commercial welding processes Gas Welding Electric Arc welding High density beam welding Oxyacetylene welding SMAW GTAW, PAW GMAW, FCAW SAW, ESW EBW LBW All the welding processes involves these operations Liquid/ Solid interface Solid/Solid interface FSW & FW & RSW
  • 6. 10 W/cm2 10000 W/cm2 App. 6000°C App. 3300°C App. 20000 to 30000°C 50 W/cm2
  • 7. Factors affecting the heat input needed for welding Dimensions of the parent metal (Thickness) Thermal conductivity Preheating temperature of the base metal Melting point Rate of heat input Electrode angle with respect to welding direction
  • 8. Types of welding processes High Arc Energy Welding Processes Gas Tungsten Arc Welding (GTAW) TIG welding Setup Weld Bead using TIG welding Position of electrode and filler metal during welding  Low deposition rate and slow speed welding. But clean weld.
  • 9. Three different Polarities in welding DCEN  straight polarity  more power (about two-thirds) is located at the work end of the arc and less (about one-third) at the electrode End  narrow and deep weld
  • 10. DCEP  reverse polarity  heating effect of electrons is now at the tungsten electrode  shallow weld  used for welding thin sheets of strong oxide-forming materials  The positive ions of the shielding gas bombard the work piece, knocking off oxide films and producing a clean weld surface AC  Good penetration and oxide cleaning action both can be obtained
  • 11. Plasma Arc welding (PAW): Similar to GTAW Orifice gas as well as shielding gas  converging action of orifice gas nozzle  arc expands only slightly with increasing arc length Comparison between a plasma arc and a gas tungsten arc 1. Gas plasma, 2. Nozzle protection, 3. Shield Gas, 4. Electrode, 5. Nozzle constriction, 6. Electric arc
  • 12. Gas Metal Arc Welding (GMAW) or Metal Inert Gas Welding (MIG) Most clean weld process & High deposition rate & high productivity Weld Bead using MIG welding MIG Welding Setup
  • 13. Shielded Metal Arc Welding (SMAW) or MMAW or Stick welding The flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination. Protection  Deoxidation  Arc stabilization  Metal addition
  • 14. • Submerged Arc Welding: Arc welding process that uses a continuous, consumable bare wire electrode, arc shielding is provided by a cover of granular flux. Used for welding thick plates due to high current input – Arc is not visible – Arc efficiency is high
  • 15. Flux Cored Arc Welding (FCAW) Similar to GMAW  Both shielding gas and flux coated electrode are used in FCAW
  • 16. Laser Beam Welding (LBW) Laser weld in 13- mm-thick A633 steel Beam Temperature: 20000°C
  • 17. Power density  1010 W/m2 Beam diameter  0.3 – 0.8 mm Beam Temperature: 30000°C
  • 20. Applications: welding of ship hulls, storage tanks, and bridges. Extremely high deposition rate. One single pass is enough for even high thickness. Weld heat input is very high.  weld can be done only in vertical position. large heat input leads to low toughness in the weld because of coarser grain size in the weldment and HAZ.
  • 22. Neutral flame oxygen and acetylene are mixed in equal amounts primary combustion (Chemical reaction between oxygen and acetylene in the inner cone)  products of primary combustion (CO and H2) react with O2 and forms CO2 and H2O  secondary combustion area (protection envelop) preventing oxidation. Reducing flame  excessive acetylene  greenish acetylene feather between inner and outer envelop  used for welding aluminum alloys and carbon steel Oxidizing flame  excessive oxygen  presence of unconsumed oxygen  used for welding brass  because copper oxide covers the weldment and prevents zinc evaporation from the weldment.
  • 23. Welding process Filler metal Nature of Shielding Heat Input GTAW Non consumable tugnsten electrode & Filler metal Argon/ Helium High GMAW Consumable filler wire Argon/ Helium/ CO2 High SMAW Consumable flux coated electrode Flux coated electrode High FCAW Consumable flux coated electrode Flux coated electrode High SAW Consumable bare electrode Granular Flux High LBW Autogenous or Consumable Argon/ Helium Low EBW Autogenous or Consumable No shielding / vacuum Low FSW Autogenous welding No shielding Solid state process SW Autogenous welding No shielding Solid state process Gas Welding Consumable electrode No shielding Very High
  • 24. Welding process Activation energy Power density Temperature GTAW Electric Arc Transfer Medium 6000°C GMAW Electric Arc Transfer Medium 6000°C SMAW Electric Arc Transfer Medium 6000°C FCAW Electric Arc Transfer Medium 6000°C SAW Electric Arc Transfer High 6000°C LBW Radiation Transfer High 10000°C EBW Radiation Transfer High 20000°C PAW Gas Transfer High 6000°C SW Electric current Solid state welding -- Gas Welding Gas Transfer Low 3000°C
  • 25. Efficiency in welding and Heat input: Where , Q = Heat transfer rate from the heat source to the work piece Qnominal = Nominal power of the heat source Always efficiency is less than one [η˂1] due to the lose of heat to the surroundings during welding. Where, E = Arc voltage; I = welding current and V = Welding speed Heat input per unit length of the weld Q = EI/V
  • 26. LBW  High reflectivity of metal surfaces leads to low efficiency. PAW  Reflectivity is not a problem. GTAW  DCEN > AC > DCEP (Polarity). GMAW, SMAW  Heat transfer to the electrode can be transferred back to the work piece through metal droplets. SAW  Arc is covered with a thermally insulating blanket of molten slag and granular flux, thus reducing heat losses to the surroundings. EBW  Keyhole in EBW acts like a “black body” trapping the energy from the electron beam.
  • 27. Welding parameters: Welding Current [I] Current heat Melting rate Deposition rate (Amount of filler metal deposited) Fusion zone (Increasing the penetrating power) Increasing current will lead to more effect on the fusion zone penetration
  • 28. Welding parameters: Arc voltage [v] Arc voltage α Arc length Arc voltage Arc length Bead width If arc length increases or decreases too much then arc becomes unstable. L1 L2
  • 29. Welding parameters: Speed [s] 0.5 m/min 1.0 m/min Welding speed Decrease in penetration Increase in bead width Molten metal has low thermal conductivity High productivity, less heat input, less distortion and residual stress
  • 30. Welding parameters: Electrode diameter  Larger electrode diameter  current has to be increased  high deposition rate  Smaller electrode diameter  less deposition rate  less diameter means higher current density
  • 31. Modes of metal transfer Spray transfer  App. 200 drops per second Globular transfer  less than 10 drops per second  More current is needed.  High energy and high speed droplets leads to higher penetration.  Used for over head welding.  No spattering.  Less current is needed.  Weld metal gets wasted due to spattering and causes porosity sometimes.
  • 33. Preheating in welding (To remove moisture and lower the thermal gradient) It lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistant to cracking. The slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking. It reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly precision joints. It raises some steels above the temperature at which brittle fracture would occur in fabrication.  Excessive hardening in the HAZ can be avoided by preheating.
  • 34. Basic Weld Joints - Types
  • 36. Welding defects (Type, size & location) External weld defects Internal weld defects Lack of deposition Lack of penetration Over deposition Undercut spatters Surface cracks Internal cracks Slag inclusion Porosity Blow holes Lamellar tearing Arc strike
  • 37. Lack of deposition Cross section area of weld Strength of the weld joint Remedy: Reweld Higher welding speed & low melting rate of the filler UNDERFILL
  • 38. Lack of penetration Base metal Filler metal Lesser penetration at the root Greater stress concentration (Act as a crack) Reason:  Low heat input  Higher welding speed  Incorrect weld groove geometry  Heat transfer through molten weld pool is lesser when compared with perpendicular direction to welding.
  • 39. Over deposition More heat input, more HAZ due to extra metal deposition Remedy: Grinding EXCESSIVE REINFORCEMENT Arc strike Damage on the parent material resulting from the accidental striking of an arc outside the weld area.
  • 40. Spatters (metal droplets) Spatters have to be removed because corrosion will start from spatters. Porosity is possible in the weldment. Undercut Excessive current, causing the edges of the joint to melt and drain into the weld; this leaves a drain-like impression along the length of the weld.
  • 41. Surface cracks Remedy: Identify the crack and remove it. Adjusting the weld composition through filler metal. Tack weld can avoid this crack.  Cracks in weld joint develop when localized residual tensile stresses exceed the UTS of the material.  Cracks will lead to poor ductility.  Due to high Sulphur and carbon contents.  Due to martensite structure formation.  Presence of Hydrogen.
  • 42. Slag inclusion This type of defect usually occurs in welding processes that use flux, such as shielded metal arc welding, flux-cored arc welding, and submerged arc welding. Poor weld bead profile resulted in pockets of slag being trapped between the weld runs Smooth weld bead profile allows the slag to be readily removed between runs  Slag may serve as a initiation for cracking.
  • 43. Porosity  Due to entrapment of gases in the solidifying weld metal.  Gases come in the weldment from flux constituents, shielding gases, absorbed moisture, gases dissolved in the metal itself.  Surface contaminations. Control of porosity: High welding current (low cooling rate)  Low welding speeds  Short arc lengths  Baking of flux and coated electrodes  Cleaning of work pieces
  • 44. Lamellar tearing  It’s a cracking problem caused by the presence of elongated inclusions (Sulphides of Mn and Fe), which are deformed in the direction of rolling or extrusion.  Stresses formed during welding lead to debonding of theses inclusions from the matrix resulting in the formation of microcracks.  During multipass welding, microcracks leading to cracking.  This cracking takes place only in the base metal, even away from the HAZ. Remedy: modifying the joint design to reduce the stresses formed during welding, reducing the Sulphur content in steel, addition of Ca modifies the composition of inclusions and makes them more resistant to deformation. Property: through thickness ductility.
  • 45. Residual stresses causes • Stress corrosion cracking • Cold cracking hydrogen induced cracking • fatigue crack Controlling residual stress Minimize heat input Preheating No of passes during welding  Residual stresses are induced in the metal structures during welding.  The intensity of Residual stresses plays very important role in the fatigue life of the component.  In general, The magnitude of the welding induced residual stresses are nearly equal to half of the times of yield stress of the material.  Thermal Conductivity & Coefficient of Thermal Expansion of the material plays vital role in distortion and residual stresses.
  • 46. Causes for distortion • Localized heating • Non uniform stress distribution Distortion occurs in these forms: Longitudinal shrinkage Transverse shrinkage Angular distortion Distortion
  • 47. Dimensional Inaccuracy caused by Distortion Dimensional accuracy is very important in welding. Heat flow in the direction perpendicular to the weld line is more. Transverse shrinkage > longitudinal shrinkage Weld direction Transverse shrinkage longitudinal shrinkage
  • 48. Welding direction After some distance of moving arc, same thermal profile will be repeated. This is known as quasi steady state in welding. More distortion
  • 49. Types of Distortion  Shrinkage  Angular distortion  Buckling deformation  Rotational deformation All the distortions are caused by the shrinkage force generated due to the thermal loading on the structure. A single V groove butt weld leads to more distortion than the double V groove butt weld of same thickness plate.
  • 50. Welding in neutral axis will balance the shrinkage force of one side against another side from the neutral axis.
  • 52. Basic forms of common Lattice structure Ti, Zn, Mg, Cd Fe, V, Nb, Cr, Si Al, Ni, Ag, Cu, Au, C, Mn, N
  • 53. Ferrite : This phase has a Body Centre Cubic structure (B.C.C) which can hold very little carbon; typically 0.0001% at room temperature. It can exist as either: alpha or delta ferrite. Austenite: This phase is only possible at high temperature in steels which are containing carbon. It has a Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbon in solution. Cementite: Unlike ferrite and austenite, Cementite is a very hard intermetallic compound consisting of 6.7% carbon and the remainder iron, its chemical symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite layers its average hardness is reduced considerably.
  • 54. Pearlite: A mixture of alternate strips of ferrite and Cementite in a single grain. A fully pearlitic structure occurs at 0.8% Carbon. It is a lamellar structure, which is relatively strong and ductile. Martensite: At fast cooling rates, the austenite might not have sufficient time to transform completely to ferrite and Pearlite and will provide a different microstructure. In this case, some of the untransformed austenite will be retained and the carbon is held at supersaturated state. This new structure is called ‘martensite’.
  • 55. Allotropic Transformation: Changes in phase transformation with respect to the temperature is called as ‘allotropic transformation’. Example: Iron has a BCC lattice structure from room temp. up to 910°C, and from this point to 1388°C it is FCC. Above this point to melting point, 1538°C it is again BCC.
  • 56. Stainless Steel TIG weld joint
  • 57. Metallurgical Problems in Welding: Burning Segregation Gas pockets  Hot and cold cracking  Dilution
  • 58.  Stainless steels constitute a group of high alloy steels based on Fe – Cr, Fe – Cr – C and Fe – Cr – Ni systems.  It contains minimum of 10.5% chromium  Formation of chromium oxide passive layer on the surface  Prevents oxidation and corrosion Types of stainless steels  Martensitic (4xx)  Ferritic (4xx)  Austenitic (2xx, 3xx)  Duplex (austenitic and ferritic)  Precipitation hardened
  • 59. Uniform corrosion or General Corrosion  It usually occurs in acid environments or hot alkaline solutions. Pitting corrosion  It is a form of localized corrosion and is identified by attacks at small discrete spots on the steel surface.  Chloride environment facilitate this local breakdown of the passive layer, especially if there are imperfections in the metal surface.
  • 60. Crevice corrosion It is a form of localized corrosion and occurs under the same conditions as pitting in neutral or acidic chloride solutions. It usually found at flange joints or at threaded connections.
  • 61. Effect of Chromium BCC structure (Ferrite)  It tends to stabilize BCC structure when dissolved with iron. Phase diagram for Iron and chromium  Above 14% of chromium austenite cannot form at all.  Increasing Cr (30 – 60%) will form sigma phase by prolonged heating below 800°C.  Carbon and Manganese also tends to stabilize austenite thus expanding the gamma loop.  Silicon tends to stabilize ferrite and reduce the gamma loop.
  • 62. Effect of Carbon Effect of carbon on the size of the gamma loop in Iron – Chromium phase diagram
  • 63. Effect of Nickel FCC structure  tends to stabilize FCC structure when dissolved with iron.  In pure iron, Austenite is stable only above 910°C.  Increasing the Ni content allows austenite to remain stable at lower temperatures, open gamma loop will form.  To retain a complete austenite structure, very high Ni content is needed. Phase diagram for Iron - Nickel
  • 64. Effect of Nickel Other Austenite & Ferrite Stabilizers Austenite stabilizers Ferrite stabilizers Nickel Chromium Carbon Molybdenum Manganese Silicon Nitrogen Titanium Cobalt Niobium copper Vanadium
  • 65. Creq = %Cr + 1.0 (%Mo) + 0.5 (%Nb + %Ta) + 1.5 (%Si) + 2 (%Ti) + (%W + %V ) Nieq = % Ni + 30 (% C) + 0.5 (% Mn) + 30 (% N) + 0.5 (%Co) WRC – 92 Diagram
  • 66. High chromium level  ferrite structure over the entire temp range Low chromium level  form a closed gamma loop transform to Austenite on heating  martensite on cooling Chromium and enough Nickel Austenite structure over the entire temp range Chromium with intermediate Ni  mixture of ferrite and austenite
  • 67. Pitfalls in welding Stainless Steel Sensitization  leads to local destroy of corrosion resistance due to the formation of chromium carbides between 500 – 800°C  sensitization mainly depends on carbon content, temperature and time of heating. Chromium carbide formation in the grain boundaries Chromium carbide formation in HAZ
  • 68. Effect of temperature and carbon content on the time to produce chromium carbide
  • 69.  The low carbon stainless steels 304L, 316L and 317L have a maximum carbon content of 0.03% and they can be successfully welded without sensitization for many applications.  But for extended service at high temperatures, low carbon alone is not enough to ensure freedom from sensitization. Overcoming the issue of carbide precipitation  It is common to use stabilized steels like 312 and 347. Alloying elements are titanium and niobium.  They are strong carbide formers which tie up the carbon preventing it from forming carbides with chromium.  To ensure complete stabilization titanium content must exceed five times the addition of carbon & nitrogen and the niobium and tantalum content must exceed ten times the carbon and nitrogen.
  • 70.  Solution treatment  Treat the steel at a temperature high enough to dissolve all the carbides.  Heating to 1000 – 1150 °C followed by rapid cooling.  Slow cooling leads to the risk of re-allowing carbide precipitation through the sensitive range.  Stress relieving treatment (800 – 900°C) will not work. Solution Annealing treatment
  • 71. Stress Corrosion Cracking (SCC) Stress (Residual stress) + Corrosion  Stress Corrosion Cracking SCC happens on the metal surface by the combined effect of corrosion and mechanical stress in particularly at chloride containing marine environments. Higher ductility offered by the Austenitic grades of stainless steels leads to SCC attack in the ASS grades like SS 304L and SS 316L. SCC attack takes the form of thin branched cracks. How to avoid SCC attack?
  • 72. Mechanical Properties of Austenitic stainless steel  Higher work hardening capacity  Excellent low temperature toughness  Higher creep strength
  • 73. Solidification modes in Austenitic stainless steel Pseudo binary section of the Fe–Cr–Ni ternary diagram at 70% Fe
  • 74. Weld solidification cracking or Hot cracking 1. Presence of impurities such as sulphur and phosphorous Sulphur and phosphorous are rejected to the weld liquid due to the low solubility in primary austenite structure. This liquid is getting trapped in the grain boundaries as a low melting point segregate. 2. Primary structure formed on solidification If the primary solidification phase is ferrite, then the rejection of sulphur does not occur and the weld is crack free. Note: Hydrogen induced cracking is not a problem with austenitic grades unless martensitic structure is present. Because hydrogen has higher solubility in FCC structure at high temperature.
  • 75. Effect of composition on Hot cracking resistance  Primary ferritic solidification mode greatly reduces the hot cracking during welding.  Delta ferrite, has higher solubility for impurity elements, which reduces the concentration of P and S at the grain boundaries.  3 – 8% delta ferrite at room temperature, is composed of ferrite – austenite interfaces leads to difficult situation for hot cracking.  WRC – 92 diagram must be used to ensure that primary ferritic solidification occurs, to avoid hot cracking.
  • 76. HAZ cracking in Austenitic stainless steel  HAZ cracking has been observed in austenitic steels in thick section welds, particularly in the stabilized austenitic grades like SS 347.  This is due to the formation of low melting point films in the grain boundaries of HAZ.  Titanium and niobium along with impurities such as phosphorous and silicon seem to be the main contributors for this type of cracking.
  • 77. Embrittlement in stainless steel welds  Delta ferrite  sigma, chi and chromium enriched alpha.  Increasing Cr and Mo content, leads to the formation of sigma phases.  Mo has greater effect (4 times greater) than the Cr in sigma formation.  Nitrogen additions have a retarding effect on sigma phase formation. Sigma phase formation
  • 78. 1. Type ‘A’ Solidification 2. Type ‘AF’ Solidification 3. Type ‘FA’ Solidification 4. Type ‘F’ Solidification
  • 79. Type ‘A’ Solidification Type ‘AF’ Solidification Type ‘FA’ Solidification If Creq / Nieq Austenite When cooling rates are moderate and Creq / Nieq is low but still within the FA range
  • 80. Type ‘F’ Solidification At low Creq / Nieq ratio within the ferrite range an acicular ferrite structure will form
  • 81. 316L BM Equiaxed austenite grains 316L WM Austenite mixed with Vermicular ferrite Microstructure of ASS base metal & weld Usually 2 to 3% of delta ferrite will be present in the ASS microstructure. This is a preferential site for carbide (M23C6) and sigma phases.
  • 82. HAZ of Austenitic Stainless Steel Nature of the HAZ  depends upon the composition and microstructure of the base metal Grain growth  Increase in heat input results in grain coarsening Ferrite formation  formation of coarser ferrite grains if the primary solidification structure is ferrite. Precipitation in HAZ  precipitation of carbides and nitrides Grain boundary liquation segregation of impurity elements (Ti & Nb)  HAZ Liquation cracking  segregation of phosphorous and sulphur also promote liquation cracking
  • 83. Effect of Creq / Nieq ratio on solidification cracking  Presence of two phase microstructure at the end of solidification is the primary reason for high resistance.
  • 84. Thermal Properties of Austenitic stainless steel Metal Thermal conductivity W/m-k Melting point Coefficient of thermal expansion/°C SS 304 16.2 @ 100°C 21.5 @ 500°C 1425°C 17.6×10-6 Carbon Steel 46 @ 100°C 1510°C 11.7×10-6  Less heat is required to make the same size weld.  But increased distortion and risk of buckling in Austenitic grades of SS.
  • 85. Autogenous welding of stainless steel Without filler metal addition  flux coating  can be weld up to 6 mm thick plate  improve penetration depth
  • 86. Welding of Ferritic Stainless Steel  BCC Crystal Structure  Absence of Nickel  High SCC resistance  undergo Ductile Brittle transition when the temperature is lowered.  Chromium exceeding 12% (upper limit of Chromium is 36%) and carbon limited to 0.1%.  Higher Chromium contents confer greater corrosion resistance. But they result in embrittlement and an enhanced tendency to form intermetallic phases such as σ on thermal exposure.  Greater yield strength , reduction in ductility and impact resistance  single phase microstructure, greater atomic mobility, grain coarsening, impact transition is high due to embrittling effect of Cr dissolved in ferrite.  475° embrittlement  heating temperature range 400 - 550°C  precipitation of Cr rich alpha
  • 87. Welding of Cast Iron  Iron with 1.7 to 4.5% carbon and 0.5 to 3% silicon.  Lower melting point (1150 to 1200 °C) and better cast ability.  Types of Cast iron  Grey cast iron - carbon as graphite  White cast iron - carbides, often alloyed  Ductile cast iron - Nodular, spheroidal graphite  Malleable cast iron  Effect of cooling rate  Slow cooling favours the formation of graphite & low hardness.  Rapid cooling promotes carbides with high hardness.  Thick sections cool slowly, while thin sections cool quickly.  Sand moulds cool slowly, but metal chills can be used to increase cooling rate & promote white iron. Since the compositions of most cast irons are around the eutectic point of the iron carbon system, the melting temperature which is about 300°C lower than the melting point of pure iron.
  • 88. White Cast Iron  If White, crystalline crack surface observed when a casting fractures, then it is called as white cast iron.  Due to the absence of ductility, the base metal and HAZ are susceptible to cracking during cooling after welding.  If cooling is rapid, then the excess carbon remains in the metastable form of iron carbide (also called as cementite).  It contains 2.5 to 3.8% C, 0.2 to 2.8% Si, and as much as 5.5% Ni, 30% Cr, 6.5% Mo, and 30% Mn, if designed for wear resistance.  Tensile strengths can range from 160 to 620 MPa Because of its extreme hardness and brittleness, white cast iron is considered unweldable. Used for abrasion resistance. Almost all the carbon is in the combined form.
  • 89. Grey Cast Iron  Formation of iron carbide during solidification is suppressed entirely.  Graphite precipitates directly from the melt as elongated and curved flakes in an iron matrix saturated with carbon.  When a grey iron casting fractures, the crack path follows these graphite flakes  The gray cast iron has a very low ability to bend and low ductility.  Possibly a maximum of 2% ductility will be obtained in the extreme low carbon range. The low ductility is due to the presence of the graphite flakes which act as discontinuities. Full graphitization  The majority of the carbon dissolved in the iron at high temperatures is deposited as graphite on the existing flakes during cooling. The structure then consists of graphite flakes in a ferrite matrix, referred to as ferritic grey cast iron. If graphitization of the carbon dissolved in the iron at high temperatures is prevented during cooling, iron carbide precipitates out and the matrix is Pearlitic (referred to as Pearlitic grey cast iron). Between 2.5% and 4% carbon and between 1% and 3% silicon MOST WIDELY USED CAST IRON
  • 90. Ductile Iron or Spheroidal Graphite Iron or Nodular Iron  Free graphite in these alloys precipitates from the melt as spherical particles rather than flakes.  This is accomplished through the addition of small amounts of magnesium or cerium to the ladle just before casting.  The spherical graphite particles do not disrupt the continuity of the matrix to the same extent as graphite flakes, resulting in higher strength and toughness.
  • 91. Malleable Cast Iron Heat treatment of white cast iron  Iron carbide decompose into iron and carbon  The structure is changed to Pearlitic or ferritic which increases its ductility. This reaction is favoured by high temperatures, slow cooling rates, high carbon & silicon contents. Ferritic Malleable Cast Iron Carbon nodules in ferrite matrix is known as ferritic malleable cast iron. Iron carbides present are dissociated during a high- temperature anneal above 870 °C for more than 60 h. Pearlitic Malleable Cast Iron If full graphitization is prevented and a controlled amount of carbon remains in the iron during cooling, finely distributed iron carbide plates nucleate in the iron at lower temperatures.
  • 92.  Graphite dissolves and precipitates iron carbide in HAZ  loss in ductility  Ferritic malleable grades display the best weldability of the malleable cast irons.  Pearlitic malleable irons, because of their higher combined carbon content, have lower impact strength and higher crack susceptibility when welded.  Post weld annealing  softens the hardened zone and regains the minimal ductility (For small welded parts  550°C & For heavy sections 200°C)  MMA welding cast iron, using low-carbon steel and low-hydrogen electrodes at low currents, produces satisfactory welds in malleable iron.  If low-carbon steel electrodes are used, the part should be annealed to reduce the hardness in the weld (due to carbon pickup) and in the HAZ.
  • 93. Grey cast iron During welding, graphite in flake form, carbon can readily be introduced into the weld pool, causing weld metal embrittlement. (Base metal dilution) Grey cast iron welds are susceptible to the formation of porosity. (controlled by lowering the amount of dilution with the base metal, or by slowing the cooling rate so that gas has time to escape.) Ductile Iron Pearlitic ductile iron produces a larger amount of Martensite in the HAZ than ferritic ductile iron and is generally more susceptible to cracking. Ductile cast irons are generally more weldable than grey cast irons.
  • 94. Practical considerations in welding Cast Iron Base metal preparation: dye penetrant inspection for defects  formation of porosity due to the presence of oil or grease in environment Degassing of casting remove residual surface graphite prior to welding Joint design modifications: Reduces the risk of cracking by deflecting the path of a crack.
  • 95. Welding induced cracking in Cast Iron Formation of Iron carbide & high carbon Martensite leads to cracking.  To prevent the formation of Martensite Multi pass welding, preheating, Interpass temperature, PWHT tempers the Martensite in HAZ  To reduce the size of the HAZ Reduction of heat input, use of small diameter electrodes, use of low melting point welding fillers, use of lower preheat temperatures
  • 96. Aluminium Alloys Non Heat Treatable Alloys Heat Treatable Alloys  Strength depends upon the hardening effects of elements (Mn, Si, Fe & Mg)  These alloys are work hardenable. Strengthening is possible by cold working. They incapable of forming second phase precipitates for improved strength.  1xxx, 3xxx, 4xxx, 5xxxx series of alloys. Example: Solution strengthened AA 5083 σy = 230 MPa  Initial Strength depends upon the hardening effects of elements (Cu, Mg, Zn & Si)  These elements is various combinations show increasing solid solubility with increasing temperature. Thus, heat treatment will impart pronounced strengthening.  2xxx, 6xxx, 7xxx series of alloys. Example: AA 7075 σy = 505 MPa
  • 97. Work hardenable alloys Pure Al – 1xxx Al-Mn – 3xxx Al – Si – 4xxx Al – Mg – 5xxx Al – Fe – 8xxx Al – Fe – Ni – 8xxx Precipitation hardenable alloys Al – Cu – 2xxx Al – Cu – Mg – 2xxx Al – Cu – Li – 2xxx Al – Mg – Si – 6xxx Al – Zn – 7xxx Al – Zn – Mg – 7xxx Al – Zn – Mg – Cu – 7xxx Al – Li – Cu – Mg – 8xxx
  • 98. Effect of Alloying Elements 1xxx series  Aluminium of 99% or higher purity  used where thermal/electrical conduction or corrosion resistance becomes dominant over strength in design considerations. 2xxx series  Copper (2 to 10% Cu) is the principal alloying element with magnesium as secondary addition  Heat treatment increases yield strength with loss in elongations  Comparatively low corrosion resistance than other Al alloy grades. unweldable because the formation of aluminium- copper intermetallics in weld metal renders them brittle. 3xxx series  Manganese (5 to 50 ppm) is the major alloying element  Non heat treatable  only limited percentage of Mn (1.5%) can be effectively added to aluminium. 4xxx series  Silicon is the major alloying element  added up to 12% which cause lowering of the melting point used in welding filler wire.
  • 99. Effect of Alloying Elements 5xxx series  Magnesium is the major alloying element  high strength  good resistance to corrosion in marine environment. Care must be taken during processing to avoid the formation of Mg3Al2. 6xxx series  Contain silicon and magnesium to form magnesium silicone  heat treatable  possess good formability and corrosion resistance with medium strength 7xxx series  Zinc is the major alloying element (1 to 8%) coupled with Mg and Cu  heat treatable alloys of high strength  used in air frame structures and for highly stressed parts (7075, 7050 and 7049 high strength alloys)
  • 101. Heat treatment to increase the strength of aluminium alloys is a three step process.  Solution heat treatment  dissolution of soluble phases (Allow the maximum amounts of soluble hardening elements in the alloy)  Quenching  development of super saturation (To produce supersaturated solution)  Age hardening  Precipitation of solute atoms either at room temperature (natural aging) or elevated temperature (artificial aging or precipitation heat treatment) The major alloy systems with precipitation hardening in Aluminium Alloy include: 1. Aluminium – Copper systems with strengthening from CuAl2 2. Aluminium – Copper – Magnesium systems (Mg intensifies precipitation) 3. Aluminium – magnesium – silicon systems with strengthening from Mg2 Si 4. Aluminium – Zinc – magnesium systems with strengthening from MgZn2 5. Aluminium – Zinc – magnesium – copper systems 2xxx series 6xxx series 7xxx series Heat Treatable Aluminium Alloys
  • 102. Age Hardening Natural Aging Artificial Aging  Sufficient precipitation occurs in few days at room temperature.  Precipitation hardening results from natural aging leads to high ratios of tensile to yield strength, high fracture toughness, high resistance to fatigue.  Example: stable condition reached after few days in 2xxx series alloys.  6xxx series and 7xxx series alloys are less stable at room temperature and continue to exhibit notable changes for many years.  Sufficient precipitation occurs in the temperature range from 115 to 190°C; times vary from 5 to 48 hrs.  Example: some 7xxx series alloys
  • 103. Properties Influencing Joining:  Aluminium Oxide formation  Melting point of Al2O3 is App. 1926°C (3 times greater than Al)  thick layer Al2O3 of will absorb moisture from the air  Oxide film must be removed from surface prior to welding  Moisture is a source of hydrogen rapid cooling rate leads to retained free hydrogen within the weld and causes porosity decrease weld strength and ductility.  High thermal conductivity Al conducts heat 5 times greater than Steel more heat input is needed even though the melting point is half that of steel  weld solidify quickly  surface tension hold the weld metal in position and makes all position welding practical. Fusion welding of Aluminium Alloy
  • 104.  Electrical Conductivity Aluminium possess high electrical conductivity  resistance heating of the electrode does not occur  High coefficient of thermal expansion  decrease about 6% in volume when solidifying  This change in dimension leads to distortion.  The absence of color change as temperature approaches the melting point. Filler alloy selection criteria: Ease of welding, tensile or shear strength of the weld, weld ductility, service temperature, corrosion resistance, color match, sensitivity to weld cracking.
  • 105. Weldability of Non-Heat Treatable Aluminium Alloy  They are incapable of forming second phase precipitates for improved strength.  Positive Attribute: Many of the alloying elements needed for precipitation hardening (Cu+Mg and Mg+Si) can lead to hot cracking during welding.  HAZ is not compromised by coarsening or dissolution of precipitates. Thus, leads to higher efficiency. Loss in strength in the HAZ is not nearly as severe as that experienced in heat treatable alloys. Therefore, reasonable joint strength can be obtained in the as welded condition without the need for post weld heat treatment.  Hot cracking: EBW & LBW result in cracking when Mg is boiled off.
  • 106. Hot cracking sensitivity Vs. Magnesium content To avoid hot cracking, use of high magnesium filler alloy is recommended.  Porosity: All Aluminium alloys are susceptible to Hydrogen Induced Cracking. Due to abrupt drop in hydrogen solubility when going from liquid to solid.  Remedy: Minimize Hydrogen pickup during welding, use of high grade shielding gas and careful storage of filler wire.
  • 107. Weld Properties of Non-Heat Treatable Aluminium Alloy Loss in strength in the HAZ:  HAZ of Non heat treatable alloys is limited to recrystallization and grain growth in the absence of strengthening precipitates which may coarsen in Heat treatable alloys.  Therefore, 5xxx series alloys are popular in welded pressure vessels. Weld metal microstructure:  Weld metal is the weakest part of the joint.  Weld metal microstructure consists of columnar dendritic substructure that has interdendritic eutectic constituents. (Fe, Mn)Al6  For 1xxx and 3xxx alloys Mg3Al2  For 5xxx alloys
  • 108. Welding of Heat Treatable Aluminium Alloy  Precipitation hardening mechanism of alloys requires alloying elements with appreciable solid solubility in aluminium at elevated temperatures, but limited solubility at low temperature.  Fusion welding redistributes the hardening constituents in the HAZ which locally reduces material strength.  High strength alloys (e.g. 7010 and 7050) and most of the 2xxx series are not recommended for fusion welding because they are prone to liquation and solidification cracking.  The technique of Friction Stir Welding is particularly suited to this kind of aluminium alloys. It is capable of producing sound welds in many alloys, particularly those heat treatable alloys which are prone to hot cracking during fusion welding.
  • 109. Welding process for Aluminium Alloy SMAW: Insufficient degree of cleaning  gas produced by flux coating not enough to obtain the defect free weld. Deposit rate is limited due to changing the electrode. GMAW: There is no slag formation  possible to perform weld in all positions  rate deposition is roughly two times than SMAW  weld quality is good Argon  low thermal conductivity, more stable arc Helium  High thermal conductivity, erratic arc
  • 110.  Titanium alloys are widely used in Aerospace industries due to its lightness and high strength. Used in connecting rods of Automobiles for better fuel efficiency. (Example: Ferrari and Porsche)  Density 4200 kg/m3 . It is having higher resistance to heat. Its melting point is 1668°C which is higher than that of steel.  Due to its non-toxic characteristics, it is also used as a bio material. Also suitable for high temperature applications.
  • 112. Types of Titanium Alloys Based on the applications, it can be divided as:  Corrosion resistant alloys  High strength alloys  High temperature alloys Based on the crystal structure, it can be divided as:  Alpha alloys  Alpha – Beta alloys  Beta alloys
  • 113. Why Dissimilar welding is needed? [For process system operate at different service conditions] More resistible to corrosion More easy to process and inexpensive Wrought structure Wrought structure Cast structure
  • 114. Issues in Dissimilar welding Welding of similar alloys Welding of dissimilar alloys The shape of the weld is symmetric Different solidification microstructure and alloy segregation
  • 115. Responsible Factors for Dissimilar weld failure  General alloying problems (Brittle phase formation and mutual solubility) of two metals.  Widely differing melting points  Differences in coefficients of thermal expansion  Differences in thermal conductivity  Carbon migration  Corrosion/Oxidation
  • 116. Austenitic Stainless Steel Vs. Carbon Steel  Selecting proper filler metal for dissimilar welding estimating the probable weld microstructure diluted by both the base metals.
  • 117. Selection of Filler Metal  Filler metal must provide the joint design requirements, such as mechanical properties and corrosion resistance.  Filler metal must fulfill the weldability criteria with respect to dilution, melting temperature, and other physical property requirements of the weldment.
  • 118. Element (SS 309) Content (%) (SS 308) Content (%) Iron, Fe 60 66 Chromium, Cr 23 20 Nickel, Ni 14 11 Manganese, Mn 2.0 2.0 Silicon, Si 1.0 1.0 Carbon, C 0.20 0.080 Phosphorous, P 0.045 0.045 Sulfur, S 0.030 0.030 Chemical composition of Filler Metal SS/CS Welded Piping
  • 119. Factors Influencing Joint Integrity Weld metal  Weld metal composition and its properties  Non uniform composition adjacent to the base metal.  Solidification characteristics of the weld metal are influenced by dilution and composition gradients near each base metal.  It is important to investigate the phase diagram of two metals involved.  If there is no or little solubility between the two metals, the joint will not be successful.  Intermetallic compounds form between dissimilar welds must be investigated.  Sometimes, it is necessary to use a third metal that is soluble in each metal in order to obtain a successful joint.
  • 120. Dilution Factors Influencing Joint Integrity  Filler metal must be able to accept dilution (alloying) by the base metals without producing crack sensitive microstructure.  Weld must be stronger than the weaker of the two metals being joined, i.e. possessing sufficient tensile strength and ductility so that the joint will not fail. The Average percentage of a specific alloying element in the diluted weld metal can be calculated using the equation developed by AWS: XW = (DA)(XA) + (DB)(XB) + (1 – DT)(XF) XW = Avg. percentage of element X in the weld metal XA = Avg. percentage of element X in the base metal A XB = Avg. percentage of element X in the base metal B XF = Avg. percentage of element X in the filler metal F DA = percent dilution by base metal A DB = percent dilution by base metal B DT = percent total dilution by the base metals A and B.
  • 121. Calculation weldment composition Consider SS 316 is welded to a 2.25Cr – 1Mo low alloy ferritic pressure vessel steel with a Ni – Cr alloy filler metal ER Ni Cr – 3. Alloy Cr Ni Mo Fe 2.25Cr – 1Mo 2.5 - 1.0 95.5 SS 316 17.0 12.0 2.5 63.0 ER Ni Cr – 3 20.0 72.0 - 3.0 Assuming that the total dilution is 35%, 15% by Cr – Mo alloy steel and 20% from SS316. Therefore, the Avg. percentage of Cr, Ni and Mo in the weld metal are calculated as follows: Cr, % = 0.15(2.5) + 0.20(17.0) + 0.65(20.0) = ? Ni, % = 0.20(12.0) + 0.65(72.0) = ? Mo, % = 0.15(1.0) + 0.20(2.5) = ?
  • 122. Causes of more Dilution  High Travel Speed. Too much heat applied to parent metal instead of on filler metal.  High welding Current. High current welding processes, such as Submerged Arc Welding can cause high dilution.  Thin Material. Thin sheet TIG welded can give rise to high dilution levels.  Joint Preparation. Square preps generate very high dilution. This can be reduced by carefully buttering the joint face with high alloy filler metal. If dilution from one base metal is less detrimental than from the other, the arc should be directed towards that metal. Non uniform dilution may produce inconsistent joint properties.
  • 123. Control of Dilution  Amperage: Increased current density increases dilution.  Polarity: DCEN gives more penetration leads to more dilution.  Electrode size: Smaller electrode, lower the amperage, lower the dilution  Travel speed: Decrease in travel speed decreases the amount of base metal melted and increases proportionally the amount of filler metal melted.  Oscillation: Greater width of electrode oscillation reduces dilution.  Arc shielding: shielding medium affects dilution. Granular flux without alloy addition > He, Ar, Co2 > granular flux with alloy addition (additional filler in the form of powder, wire, strip)
  • 124. Oxidation  Compositional variations at the interfaces in the weld result in oxidation when operating at high temperatures in air and formation of notches at these location.  Such notches are potential stress raisers in the joint.  It cause oxidation failure along the weld interface under cyclic thermal conditions.
  • 125. Melting Temperature  Differences in melting point of the two base metals can result in rupture of the metal which is having low melting point.  Solidification and contraction of the high melting point material will induce residual stress in the other metal when it is in partially solidified condition.  Remedy: Buttering  Depositing one of more layers of filler metal of intermediate melting temperature on the face of the base metal with higher melting temperature.  Buttering layer provides barrier layer that will slow the undesirable migration of elements from the base metal to the weldment during PWHT and service at high temperature.
  • 126. EDGE PREPARED FOR BUTTERING. FACE BUTTERED WITH FILLER METAL. BUTTERED FACE PREPARED FOR WELDING. JOINT ALIGNED FOR WELDING. JOINT WELDED WITH STAINLESS STEEL FILLER METAL. BUTTERING TECHNIQUE
  • 127. Thermal Conductivity  Welding heat source must be directed at the metal having the higher thermal conductivity to obtain the proper heat balance.  Preheating the metal having high thermal conductivity heat loss to the base metals can be balanced.  Dilution is more uniform with balanced heat input.  Preheating the base metal of higher thermal conductivity also reduces the cooling rate of the weld metal and HAZ.  Preheating also reduce the heat needed to melt the base metal
  • 128. Coefficient of Thermal Expansion  Large differences in CTE leads to tensile residual stress in one metal (may hot cracking during welding and cold cracking during service) and compressive residual stress in the other.  CTE of the weld metal should be intermediate between those of the base metals.  If CTE difference is small, then weld metal may have a CTE equivalent to that of one of the base metals.
  • 129. Carbon Migration  Chromium in steel has greater affinity for carbon than iron.  In ASS/CS weld using austenitic filler, carbon can diffuse from the base metal into the weld metal at temperature above 425°C and more rapidly at above 595°C.  Carbon migration takes place during PWHT and high temperature service applications. Excessive carbon migration into SS weld weakens the HAZ of carbon steel.  During cyclic temperature service, HAZ will be subjected to varying shear stresses because of the differences in CTE of base and weldments. These stresses lead to fatigue failure.
  • 130. Why not possible for Aluminium Vs. SS?  Fe and Al are not compatible materials.  Fe melting point = 1538°C, Al melting point = 660°C  Both metals have no solubility for the other in the solid state. (Intermetallic phases FeAl2, Fe2Al5 and FeAl3)  High residual stresses due large variation in CTE, thermal conductivity.  Al can be joined to SS or CS by solid state welding process and also by EBW.
  • 131. Most welded dissimilar combinations  Stainless steels to Carbon or low alloy steels  Nickel base alloys to Steels  Cobalt base alloys to steels  Copper base alloys to steels
  • 132. Definition: Residual stresses are self balancing internal system of stresses arising from non-uniform mechanical or thermal straining with some measure of plastic flow. Residual stresses are elastic in nature exist in a body when all external forces are removed. Residual stresses Vs. Distortion
  • 134. Temperature change Vs. Residual stresses
  • 135. Basic Mechanism of Residual Stress
  • 136. Types of Residual Stresses They are commonly classified into two groups: Macro Residual stresses (Residual stresses of the first kind) [Measured over a gauge length that encompasses several grains] Micro Residual stresses (Residual stresses of the second kind) [Measured within a single grain or a particular set of grains Both types can contributing SCC or fatigue initiation depending upon the situation.
  • 138. Factors Influencing Residual stresses  Materials Properties  Specimen Dimension  Welding Processes  Welding Sequence
  • 139. Sources of Residual Stresses  Residual stresses owing to the shrinkage process of the Seam and HAZ.  Residual stresses owing to the more rapid cooling of the surface [quenching residual stresses]  Residual stresses owing to a phase transformation