intro welding tech

8/11/2019 Intro Welding Tech

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Introduction to

Welding Technology

CONSULTANT ENGINEERS - METALLURGY AND WELDING

The WeldNet


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Welding processes

Fusion welding

Involves melting & solidification

Solid phase welding

Explosive bonding

Diffusion welding

Friction welding


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Fusion welding

Most commonly used processes

Heat sourceelectric arc, gas flame, laser

Filler metal

From electrode, rod, wires, powder, fluxes

Independently added filler

No filler (autogenous welding)


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Weld

The AWS definition for a welding processis A materials joining process which producescoalescence of materials by heating them to

suitable temperatures with or without theapplication of pressure or by the application ofpressure alone and with or without the use offiller material".

Filler (if used) has a melting temperaturesimilar to the parts being joined


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Weldability

The capacity of a materialto be welded

under the imposed fabrication

conditionsinto a specific, suitablydesigned structureand to perform

satisfactorily in intended service.

(ANSI / AWS A3.0)


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Factors affecting weldability

Weldability is often considered to be a

material property.

However the effect of other variables should not beignored.

Weldability is also affected by:

Design of a weld

Service conditions

Choice of welding process


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Design

Weld joint design and execution

Thickness, location, access, environment

Restraint

Weldment size, assembly sequence

Service stresses

Safety factor for welds


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Physical properties

Melting and vaporisation temperatures

Electrical and thermal properties

Conductivity, expansion coefficient, thermalcapacity, latent heat

Ionisation potential of electrode

Magnetic susceptibility

Reflectivity


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Solidification of weld metal

Dendritic or cellular growth

Segregation

Depends on composition

Cooling rate

Can lead to solidification cracking


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Dilution

Proportion of weld metal that comes from the

base material

Must be considered for each weld runAffects composition, properties, risk of defects

Greatest effect when filler composition is

different to either or both base metals100% for autogenous welds


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Chemical properties

Affinity of weld metal for O, N and H

Susceptibility to porosity, embrittlement

Presence of a surface film on base metal Oxide films

Paint or metallic surface coating

Fluxing / De-oxidising properties of a slag


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Contaminant gases

Nitrogen and oxygen from air

Hydrogen from

Moisture in air

Moisture in consumables or surface

contaminants

Organic materials (grease, oil, paint etc)


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Gas-metal reactions

Liquid metal may react with air or other gases

Depends on

Liquid metal composition Gas composition

Consequences

Porosity - gas released on solidification

Formation of compounds

Embrittlement


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Metallurgical properties

Strengthening mechanism of base material

Weld versus base material strength

Freezing range Susceptibility to solidification cracking

Susceptibility to detrimental phases forming

during welding

Embrittlement or corrosion


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Service environment

Extreme environments

Corrosive

Low temperature (brittle failure)

High temperature (oxidation, creep, embrittlement) Others (wear, fatigue, nuclear)

The more extreme the environment

The more difficult it is to find suitable materials

The more restricted the welding procedure

becomes to avoid service failure (arc energy)


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Welding variables

Arc energy (heat input)

Preheat and interpass temperature

Filler metal composition


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Arc energy

v

IxEQ 06.0

Q = arc energy in kJ/mmI = welding currentE = arc voltagev = travel speed in mm/min

Low arc energy Small weld pool size Incomplete fusion High cooling rate

Martensite and hydrogen cracking

High arc energy Large weld pool size Low cooling rate Increased solidification

cracking risk Low ductility and strength Precipitation of unwanted particles

(corrosion and ductility)


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Preheat and interpass

Preheat is applied independently

Gas torches

Gas radiant heaters Electric resistance heaters

Interpass temperature

Temperature before next pass is added

Controlled by a cooling time, or air or water cooling


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Raising PH/IP temperature

Slows cooling rate

Reduces HICC in steels

Can increase risk of solidification cracks

Can increase tendency to embrittlement

Improves fusion

Reduces temperature gradient

Minimises distortion and residual stress


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Fusion weld structure

Composite Weldmetal

Unmixed fusedbase metal

HAZ

PartiallyMeltedZone Fusion Line


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Thermal gradients in HAZ

Time

Temperature Fusion lineFusion line + 2mmFusion line + 5 mm


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Thermal HAZ regions


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HAZ Structure

Grain refining

Weld Coarse grain regionDisturbed microstructure

Original base material


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Weld positions and

joints


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Welding positions - plate

Flat 1G Horizontal2G

Vertical3G

Up or Down

Overhead4G


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Welding positions - pipe

Axis vertical2G

Axis horizontal5G

Axis inclined 456G


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Weld joints

Cruciform

LapCorner

Butt Tee


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Weld Types


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Weld types

Butt weld

Between mating members

Best quality High weld preparation cost

Fillet weld

Easy preparation

Asymmetric loads, lower design loads


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Butt welds

Joint types:

Double welded butt

Permanent or temporary backing

Single welded butt

Lower stress concentration

Easier ultrasonic testing or radiography

Expensive preparation


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Butt weld types

Single veecan be single

or double welded

Single bevel Double vee

Backed butt (permanent or temporary)


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Butt weld terms

Root faceRootgap

Fusion face

Included angle

Bevel angle

Cap / Reinforcement

Root run


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J Preparations

Land

Root radiusSingle U preparation

Double U butt


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Fillet welds

Simple to assemble and weld

Stress concentrations at toes and root

Notch at root (fatigue, toughness)Critical dimension is throat thickness

Root gap affects throat thickness

Radiography and ultrasonic testing is oflimited use

Large fillets are uneconomic


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Fillet weld terms

Root

Toe

Weld face

Toe

Throatthickness

Leg length

Gaps shall be taken into account for minimum leg length


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Weld preparation dimensions

Standard preparations

AS/NZS1554, AS/NZS:3992

AWS D1.1, ASME B31.3

Non Standard (Compromise at fabricators risk)

Weld cross sectional area

Cost Ease of welding (risk of defects)


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Weld Defects and

Discontinuities


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Welding discontinuitiesDiscontinuities are essentially defects that fall within the

limitations of the welding standard requirements

Cracks Never a discontinuity !!

Porosity Most common complying weld defect

Incomplete fusion / Inclusions Some allowed by most welding standards

Defective profile Under-weld, over-weld, lack of root bead, burn through, undercut,spatter etc.

Most client specifications limit these types


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Welding defects- Causes Cracks

HACC / HICC, solidification, liquation causes

Porosity Gas entrapment / ejection, poor shielding

Incomplete fusion Sidewall, inter run, root pass, weld toes ( cold lap )

Electrode angle implicated or poor joint profile

Inclusions Slag, oxide, tungsten

Usually operator induced

Defective weld profile / finish Under-weld, over-weld, lack of root bead, burn through, undercut

Usually operator induced


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Some weld defects

Incomplete penetration

Cold lapUndercut

Incomplete sidewall fusion Incomplete root fusion

Slag inclusion


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Solidification cracking Low melting point constituents

Sulphur, Phosphorus, Tin, Lead, Niobium

Undesirable eutectics

Grain boundary segregation Segregation of sulphides etc.

Lowering ductility and raising crack sensitivity

Strains arising during solidification Solidification range

Material types, contamination

Base material dilution, lowering weld strength

Expansion coefficient Differing between base material and weld material

Clad materials

Weld pool shape and size Depth-to-width ratio

Surface concavity

Arc energy


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Solidification cracks

Crater crack

Longitudinal crack Centreline Crack


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Weldability of structural steel

Benchmark against which other materials

are judged

Risk of hydrogen induced cold cracking.Only occurs in ferritic, bainitic or martensitic

steel


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Hydrogen induced cold cracks

HACCHydrogen assisted Presence of hydrogen

Susceptible microstructure

Tensile Stress Temperature

Below ~ 100C

HICCHydrogen induced

Hydrogen embrittlement Susceptible microstructure / stress not always

required


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Susceptible microstructure

Weld metal or HAZ

Martensite or upper bainite

Composition

Hardenability and hardness - carbon equivalent

TTT diagramsCooling rates

Cooling time between 500C and 300C Section thickness

Preheat temperature


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Sources of tensile stress

Residual stress Restraint

Through thickness in thick sections

Applied stress Excessive peening

Lifting

Presetting

Fairing and straightening operations


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Hydrogen From consumables

Moisture absorption

Potential hydrogen test

Basic consumables have lower potential hydrogen

From joint contamination Fabrication practices

Environment

Machinery

Temperature and time dependent

> 150C lower riskdiffusion of hydrogen < 150C to ambient - if susceptible, cracking will occour


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Methods of control

Preheat

Slow down cooling rate between 800C and

500C Remove hydrogen before weld cools

below 150C

Stress relief immediately after weldingLow temp temperature heat treatment (150C

to 250C, known as out-gassing)


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HAZ Cracking

All these approaches are based on studies of the risk ofHAZ cracking.

Weld metal cracking is less understood.

Weld metal cracking is more likely in

Alloy steel weld metals of over 500 MPa yieldstrength

Submerged arc welds (Chevron cracks)


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Lamellar tearing

Pull-out crack (obsolete)


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Lamellar tearing

Separation or cracking along planes

parallel to the principal plane of

deformation. Occurs in rolled sections mainly but can

also occur in extrusions and forgings.

Does not occur in castings Not to be confused with plate lamination.


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Lamellar tearing


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Appearance

Woody looking or stepped crack

Parallel to rolling direction (in rolled

sections) Sometimes associated with HACC / HICC

in the HAZ.


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Factors affecting risk

Material

Through-thickness properties

DesignThrough thickness strains and restraint

Fabricator

Over-welding


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Design approach

Consider corner, tee and cruciform joints a

risk

Thicker members are at risk (morerestrained)

Consider joint details with lower risk

Specify material with adequate throughthickness ductility (testedZ grade)


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Joint details with lower risk

Reduce weld size

Diffuse through thickness strains with joint

design Minimise restraint

Balance weld detail

Avoid welds intersecting in a corner


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Joint detail comparison

Poor details Improved details


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Fabrication practices

Carefully sequence fabrication to minimise

restraint

Choose rolling direction perpendicular toweld axis

Test cold formed materials for tearing

Ultrasonically inspect weld areas forlaminations before fit-up


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Welding practices

Do not over weld

Follow practices that minimise stress and

distortion Buttering can be used to avoid lamellar

tearing but is expensive.


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Residual stress and

distortion


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Residual stress sources

Uneven plastic deformation

Hot or cold forming (rolling, pressing, bending,

shot blasting)Cutting (machining, shearing)

Uneven heating and cooling

Welding, flame cutting, flame straightening Uneven solid phase change

Quenching steelmicrostructure expansion


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0

50

-50

-100

-150

TEMPERATURE IN MIDDLE BAR Deg C

0 100 200 300 400 500 600

100

150

200

-200STR

ESSIN

MIDDLEBA

R

MPa

Heating a restrained barMiddle baris heated to600C andallowed tocool

B

A

C

D

Compression

Tension

EF


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XX XX

Residual stress in a butt weld

ssx

ssy

ssx

0 TensionCompression

XX XX

sy Tension

Compression


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Possible consequences

Distortion

Weld cracking

Brittle failure

Fatigue

Stress corrosion cracking


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Distortion

Angular

Longitudinal Transverse


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Minimising distortion

Avoid over-welding

Use a planned welding sequence

Restrain the weldmentPreset to allow for distortion

Welding techniques

Fast high power techniques, back-stepping,

preheat

Preheatto maximise area of shrinkage


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End of presentation

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