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 thearrangement of atoms

3. High degree of mobility betweenatoms due to heat energyinvolved 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

LBWAll 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 DeoxidationArc 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 usedin FCAW

Laser Beam Welding (LBW)

Laser weld in 13-mm-thickA633 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 highthickness.

Weld heat input is very high.

weld can be done only in verticalposition.

large heat input leads to low toughness inthe weld because of coarser grain size inthe 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 αArc lengthArc voltage

Arc length

Bead width

If arc length increases or decreases too much then arc becomes unstable.

L1L2

Welding parameters: Speed [s]

0.5 m/min

1.0 m/min

Welding speedDecrease in penetration

Increase in bead widthMolten 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 beincreased high deposition rate

Smaller electrode diameter less depositionrate less diameter means higher currentdensity

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 groovegeometry

Heat transfer throughmolten weld pool is lesserwhen compared withperpendicular direction towelding.

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 becausecorrosion will start from spatters.

Porosity is possible in the weldment.

Undercut

Excessive current, causing the edges ofthe joint to melt and drain into theweld; this leaves a drain-like impressionalong the length of the weld.

Surface cracks

Remedy: Identify the crack and remove it.Adjusting the weld composition through fillermetal.

Tack weld can avoid this crack.

Cracks in weld joint develop when localizedresidual tensile stresses exceed the UTS of thematerial.

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 ofslag being trapped between the weld runs

Smooth weld bead profile allows the slag tobe 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, absorbedmoisture, 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 inputPreheating No of passes during welding

Residual stresses are induced in themetal structures during welding.

The intensity of Residual stressesplays very important role in the fatiguelife of the component.

In general, The magnitude of thewelding induced residual stresses arenearly equal to half of the times of yieldstress of the material.

Thermal Conductivity & Coefficientof Thermal Expansion of the materialplays vital role in distortion and residualstresses.

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 shrinkageWeld

direction

Transverse shrinkage

longitudinal shrinkage

Welding direction

After some distance ofmoving arc, same thermalprofile will be repeated.This is known as quasisteady 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 buttweld of same thickness plate.

Welding in neutral axis will balance the shrinkage force of one side against anotherside from the neutral axis.

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 – Cand 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 ChromiumBCC structure (Ferrite) It tends to stabilize BCC structure when dissolvedwith 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 withiron.

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 stabilizersNickel Chromium

Carbon MolybdenumManganese Silicon

Nitrogen TitaniumCobalt 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–Niternary 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’ SolidificationType ‘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 / Nieqratio within the ferrite range an acicular ferrite

structure will form

316L BMEquiaxed austenite grains

316L WMAustenite 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 Thermalconductivity

W/m-k

Melting point Coefficient of thermal expansion/°C

SS 304 16.2 @ 100°C21.5 @ 500°C

1425°C 17.6×10-6

Carbon Steel 46 @ 100°C 1510°C 11.7×10-6

Less heat is requiredto make the same sizeweld.

But increaseddistortion and risk ofbuckling in Austeniticgrades 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 surfaceobserved when a casting fractures, then itis called as white cast iron.

Due to the absence of ductility, the basemetal and HAZ are susceptible to crackingduring cooling after welding.

If cooling is rapid, then the excess carbonremains in the metastable form of ironcarbide (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 wearresistance.

Tensile strengths can range from 160 to620 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 solidificationis suppressed entirely.

Graphite precipitates directly from the melt aselongated and curved flakes in an iron matrixsaturated with carbon.

When a grey iron casting fractures, the crackpath follows these graphite flakes

The gray cast iron has a very low ability tobend and low ductility.

Possibly a maximum of 2% ductility will beobtained in the extreme low carbon range. Thelow ductility is due to the presence of thegraphite flakes which act as discontinuities.

Full graphitization The majority of the carbon dissolved in the iron at high temperatures is deposited asgraphite on the existing flakes during cooling. The structure then consists of graphite flakes in a ferritematrix, referred to as ferritic grey cast iron.

If graphitization of the carbon dissolved in the iron at high temperatures is prevented during cooling, ironcarbide 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 & siliconcontents.

Ferritic Malleable Cast Iron

Carbon nodules in ferrite matrix is known as ferriticmalleable 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 controlledamount of carbon remains in the iron during cooling,finely distributed iron carbide plates nucleate in theiron 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 sections200°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 dueto the presence of oil or grease in environmentDegassing of casting remove residualsurface 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 – 1xxxAl-Mn – 3xxx Al – Si – 4xxx

Al – Mg – 5xxx Al – Fe – 8xxx

Al – Fe – Ni – 8xxx

Precipitation hardenable alloysAl – Cu – 2xxx

Al – Cu – Mg – 2xxxAl – 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 seriesManganese (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 pointused 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)

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 heattreatment)

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 formationMelting 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 porositydecrease 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.

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 andmutual 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 weldmicrostructure 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 withoutproducing crack sensitive microstructure.

Weld must be stronger than the weaker of the two metals being joined, i.e. possessingsufficient 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 becalculated using the equation developed by AWS:

XW = (DA)(XA) + (DB)(XB) + (1 – DT)(XF)

XW = Avg. percentage of element X in the weld metalXA = Avg. percentage of element X in the base metal AXB = Avg. percentage of element X in the base metal BXF = Avg. percentage of element X in the filler metal FDA = percent dilution by base metal ADB = percent dilution by base metal BDT = 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 vesselsteel 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 asfollows:

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 ArcWelding 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 bycarefully 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 bedirected 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 whenoperating 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 conductivityheat 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