10-14. Welding Process

8/10/2019 10-14. Welding Process

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Copyright 2004, TWI Ltd World Centre for Materials Joining Technology

TWICSWIP 3.2

WIS 10SENIOR WELDING INSPECTION

WELDING PROCESS


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Welding process chart

Fusion welding

Pressurewelding

MMA TIG

MIG/MAG/FCAW

SAW EBW LBW

Arcwelding

Powerbeam

welding

Oxy fuelwelding

Electroslagwelding

Thermitwelding

Resistancewelding

Solid statewelding

PAW


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MMA - Principle of operation


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ARC CHARACTERISTICS

Volts

Amps

OCV

Constant Current/Amperage CharacteristicLarge change in voltage =

Smaller change in amperage

Welding Voltage

Large arc gap

Small arc

gap


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MMA welding

Shielding provided by decomposition of fluxcovering

Electrode consumable

Manual process

Welder controls

Main features

Arc length Angle of electrode

Speed of travel

Amperage settings


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MMA welding power source

TransformersAC only

Transformer/RectifiersAC and DCPetrol or Diesel driven GeneratorsSite workDC

InvertersAC and DC

TYPES:-

Input 240v (single phase) or415v ( 2 live phases of 3 phase supply)

Output AC (O.C.V. 80v) or DC (O.C.V. 50v) or both.

AC for Shop

DC for site work as it is SAFER (lower OCV). Also forshop work..

Current adjustment control

POWERSOURCE:-


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Equipment for MMA welding

Safety visor(with dark lens)

Electrode oven

Power return

cable

Power source.Transformer/Rectifier

Heated quiver

Inverterpower source

Power cable

Power controlpanel

Electrodes

Electrodeholder



MMA electrodes - type of covering

Acid

covering

Cellulosic

covering

contains oxides and silicates high O2

content low UTS weld

smooth profile with tendency toconcavity

slag easy to detach

limited usability

contains organic material nobacking/drying allowed

generates high level of fumes and H2cold cracking

thin slag layer, poor appearance

increased penetration



MMA electrodes - type of covering

Rutile

covering

Basic

covering

contains TiO2

stable, easy-to-use arc can operate inboth DC and AC

slag easy to detach, smooth profile

good strength weld metalcan be dried to lower H2content

contains CaF2and CaCO3

requires backing to keep H2lowvery good mechanical properties

difficult to detach slag

produce convex weld profile



3) Porosity

1) Slag inclusions

2) Arc strikes

4) Undercut

Most welding defects in MMA are caused by a lackof welder skill (not an easily controlled process),the incorrect settings of the equipment, or theincorrect use, and treatment of electrodes

5) Shape defects (overlap, excessive rootpenetration, etc.)

MMA welding typical defects



Advantages:

1) Field or shop use

2) Range of consumables

3) All positional

4) Very portable

5) Simple equipment

Disadvantages:

1) High skill factor

2) Slag inclusions

3) Low operating factor

4) High level of fume

5) Hydrogen control

MMA advantages/disadvantages



TIG - Principle of operation



Sizes/types vary depending on current/applicationTORCH

To deliver correct gas flow (velocity) depending onwelding position and joint configuration.

FLOWMETER(graduated in ltr/min)

Correct type for application.(ar, he, ar/he mixture)Argon/hydrogen for austenitic stainless steel

INERT GAS SUPPLY(Cylinder & regulator)

Note that current actually flows from this leadRETURN LEAD

Flexiblemay carry current, gas, cooling water.TORCH HOSE

Transformers for ACaluminium alloys

Rectifiers for DC - steels

Transformer/rectifier for AC/DC

Inverters for AC/DCmore portable - expensive

POWER SOURCE

440v 50Hz 3 phase or240v single phaseinput

TIG power source


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World Centre for Materials Joining Technology

Inverterpower source

Power cable

Flow-meter

Powercontrol panel

Transformer/ Rectifier

Powercontrol panel

Power return

cable

Torchassemblies

Tungstenelectrodes

Equipment for TIG




TIG torch

Ceramic

shield cup

Gas lens

Torch body

Tungstenelectrodes

Spare ceramicshielding cup

Gas diffuser

Split collet

Fitted ceramicshielding cup

Tungstenhousing

On/Off switch




Most welding defects with TIG are caused by a lack of

welder skill, or incorrect setting of the equipment. i.e.current, torch manipulation, welding speed, gas flowrate, etc.

1) Tungsten inclusions (low skill or wrong vertex angle)

2) Surface porosity (loss of gas shield mainly on site)

3) Crater pipes (bad weld finish technique i.e. slope out)

4) Oxidation of S/S weld bead, or root by poor gas cover

5) Root concavity (excess purge pressure in pipe)

6) Lack of penetration/fusion (widely on root runs)

TIG typical defects


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MIG/MAG - Principle of operation




ARC CHARACTERISTICS

Volts

Amps

OCV

Constant Voltage Characteristic

Small change in voltage =large change in amperage

The selfadjusting arc.

Large arc gap

Small arc gap


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Power returncable

Transformer/ Rectifier

Power cable& hoseassembly

Liner for wire

Welding gunassembly

External wirefeed unit

Power controlpanel

Internal wire

feed system

15kg wire spool

Equipment for MIG/MAG


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Plain top roller

Half groovedbottom roller

Wire guide

Internal wire

drive system

MIG/MAG wire drive system


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Torch body

Contact tips

Spot weldingspacer

Gas diffuser

Nozzles orshrouds

Torch head assembly(less nozzle)

On/Off switch

Hoseport

MIG/MAG torch head assembly


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MIG/MAG-methods of metal transfer

Dip transfer Transfer occur due to short circuits

between wire and weld pool, highlevel of spatter, need inductancecontrol to limit current raise

Can use pure CO2or Ar- CO2mixtures as shielding gas

Metal transfer occur when arc isextinguished

Requires low welding current/arc voltage, a lowheat input process. Resulting in low residualstress and distortion

Used for thin materials and all position welds


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Spray transfer

Transfer occur due to pinch effectNO contact between wire and weldpool!

Requires argon-rich shielding gas Metal transfer occur in small

droplets, a large volume weld pool

Requires high welding current/arc voltage, ahigh heat input process. Resulting in highresidual stress and distortion

Used for thick materials and flat/horizontalposition welds


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Globular transfer Transfer occur due to gravity or

short circuits between drops andweld pool

Requires CO2shielding gas Metal transfer occur in large drops

(diameter larger than that ofelectrode) hence severe spatter

Requires high welding current/arc voltage, ahigh heat input process. Resulting in highresidual stress and distortion

Non desired mode of transfer!


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Pulsed transfer

Controlled metal transfer, onedroplet per pulse, NO transferbetween droplet and weld pool!

Requires special power sources Metal transfer occur in small

droplets (diameter equal to thatof electrode)

Requires moderate welding current/arc voltage, areduced heat input . Resulting in smaller residualstress and distortion compared to spray transfer

Pulse frequency controls the volume of weld pool,

used for root runs and out of position welds


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3) Porosity (from loss of gas shield on site etc)

1) Silica inclusions (in Fe steels) due to poorinter-run cleaning

2) Lack of fusion (primarily with dip transfer)

Most welding imperfections in MIG/MAG are caused by lack of

welder skill, or incorrect settings of the equipment

The use of low quality wires will cause wire feed problems

Worn contact tips will cause poor power pick up, or transfer

Bad power connections will cause a loss of voltage in the arc

4) Solidification problems (cracking, centerlinepipes, crater pipes) especially on deepnarrow welds

MIG/MAG typical defects


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Disadvantages:Advantages:

1) Lower skill required

2) Easily automated

3) All positional Dip &Pulse

4) Thick/thin materials

5) Continuous electrode

1) Lack of sidewall fusion

2) Range of consumables

3) Loss of gas shield/site

4) Complex equipment

5) High ozone levels

MIG/MAG advantages/disadvantages


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Flux cored arc welding

FCAWmethods

With gasshielding -

Outershield

Without gasshielding -

Innershield

With metalpowder -

Metal core


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Outershield - principle of operation


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FCAW - differences from MIG/MAG

usually operatesin DCEP but someInnershield

wires operates inDCEN

doesn't work indip transfer mode

require knurled

feed rolls


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Advantages:

1) Field or shop use

2) High productivity

3) All positional

4) Slag supports andshapes the weld Bead

5) No need for shieldinggas

Disadvantages:


2) Slag inclusions

3) Cored wire isExpensive

4) High level of fume

(Innershield)

5) Limited to steels andnickel alloys

FCAW advantages/disadvantages


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SAW - Principle of operation


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SAW


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SAW


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Principles of operation

Factors that determine whether to use SAW

chemical composition and mechanicalproperties required for the weld deposit

thickness of base metal to be welded

joint accessibility

position in which the weld is to be madefrequency or volume of welding to beperformed


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Principles of operation

SAW methods

semiautomatic - for relatively smalldiameter electrodes and moderate travel

speed

mechanised - the majority of applications

automatic - for mass production (e.g.pipes)


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Disadvantages of SAW

limited mainly to flat/horizontal position

limited to carbon, low alloy, creep resisting, stainless steelsand nickel alloys

due to the high heat input, impact strength of weld metal/HAZmay be low; also high dilution

slag must be cleared away after welding due to the danger ofslag inclusions

need flux storage, handling and recirculation control

difficult to apply on-site due to complicated equipment

high capital costs

weld line must be regular (straight or circumferential seamsonly) with accurate fit-up


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SAW filler material

Welding wires can be used to weld:

carbon steels

low alloy steels

creep resisting steels

stainless steels

nickel-base alloys

special alloys for surfacing applications

Welding wires can be:

solid wires

metal-cored wires


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SAW FLUXES

Fused:

Baked at high temperature, glossy,hard andblack in colour, cannot add ferro-manganese,non moisture absorbent and tends to be of theacidic type

Agglomerated:

Baked at a lower temperature, dull, irregularlyshaped, friable, (easily crushed) can easily addalloying elements, moisture absorbent and tendto be of the basic type


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SAW FLUXES

Agglomerated SAW Flux


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SAW FLUXES

Fused SAW Flux


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SAW


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SAW


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SAW


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SAW

S


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SAW

Which settings do you think are incorrect

PAW P i i l f ti


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PAW - Principle of operation

PAW feat es


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PAW features

operates on DCEN

can be used for welding, spraying andcutting

Modes of

operation

Microplasma Plasma Microplasma

Plasma arcmodesNon-transferred arc Transferred arc

PAW d t /di d t


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Advantages:

1) High energyconcentration

2) High arc stability

3) Can cut stainlesssteels & aluminum

PAW advantages/disadvantages

Disadvantages:


2) Little tolerance forjoint misalignment

4) High level of UV/IRradiation, fumes

5) Torch is heavy andbulky

3) Expensive equipment

4) Wide range of

materials/thickness

5) No danger oftungsten inclusions

OAW P inciple of ope ation


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OAW - Principle of operation

Welding flame types


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Welding flame types

Carburising flame

Neutral flame

Oxidising flame

welding of Al andhigh C steels

hardfacing withCr and W carbide

welding of mostmaterials

brazing

welding of bronze

and brasses

OAW ad antages/disad antages


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OAW advantages/disadvantages

Advantages:

1) No need for powersupply

2) Low equipment cost

3) Can cut carbon andlow alloy steels

Disadvantages:


2) Wide HAZ

4) Slow process

5) Limited range ofconsumables

3) Safety issues

4) Good on thinmaterials

Choosing welding process


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Choosing welding process

Application characteristics:

Other factors to be considered:Availability and fitness for service

Skill requirements

Parent material(s) weldability

Availability of consumables

Joint design and welding position

Heat input requirements

Quality requirements

Productivity

Health and safety issues

10-14. Welding Process

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