11-SAW Welding 2006

CSWIP 3.1 Welding Inspection

TWI Training & Examination Services

Course Reference WIS 5

Submerged Arc WeldingSubmerged Arc Welding

• Submerged arc welding was developed in the Soviet Union during the 2nd world war for the welding of thick section steel.

• The process is normally mechanized.

• The process uses amps in the range of 100 to over 2000, which gives a very high current density in the wire producing deep penetration and high dilution welds.

• A flux is supplied separately via a flux hopper in the form of either

fused or agglomerated.

• The arc is not visible as it is submerged beneath the flux layerand no eye protection is required.

Submerged Arc Welding Introduction

SAW Principle of operation

Submerged Arc Welding

-- ++

Power Power

supplysupply

Filler wire spoolFiller wire spool

Flux hopperFlux hopper

Wire electrodeWire electrode

FluxFlux

Slide railSlide rail

Transformer/ Rectifier

Power return cable

Power control panel

Welding carriage control unit

Granulated flux

Granulated flux

Welding carriage

Electrode wire reel

SAW Basic Equipment

Principles of operation

Factors that determine whether to use SAWFactors that determine whether to use SAW chemical chemical

composition and mechanical properties required for the weld composition and mechanical properties required for the weld

depositdeposit

•• thickness of base metal to be weldedthickness of base metal to be welded

•• joint accessibilityjoint accessibility

•• position in which the weld is to be madeposition in which the weld is to be made

•• frequency or volume of welding to be performedfrequency or volume of welding to be performed

SAW methodsSAW methods

SemiautomaticSemiautomatic MechanisedMechanised AutomaticAutomatic

SAW equipment

Power sources can be:Power sources can be:

•• transformers for ACtransformers for AC

•• transformertransformer--rectifiers for DCrectifiers for DC

Static characteristic can be:Static characteristic can be:

•• Constant Voltage (flat) Constant Voltage (flat) -- most of the power sourcesmost of the power sources

•• Constant Current (drooping)Constant Current (drooping)

SAW equipment

Constant Voltage (Flat Characteristic) power sources:Constant Voltage (Flat Characteristic) power sources:

• most commonly used supplies for SAW

• can be used for both semiautomatic and automatic welding

• self-regulating arc

• simple wire feed speed control

• wire feed speed controls the current and power supply controls the voltage

• applications for DC are limited to 1000A due to severe arc blow (also thin wires!)

ARC CHARACTERISTICS

Volts

Amps

OCV

Constant Voltage Characteristic

Small change in voltage = large change in amperage

The self adjusting arc.

Large arc gap

Small arc gap

SAW equipment

Constant Current (Drooping Characteristic) power sources:Constant Current (Drooping Characteristic) power sources:

• Over 1000A - very fast speed required - control of burn off rate and stick out length

• can be used for both semiautomatic and automatic welding

• not self-regulating arc

• must be used with a voltage-sensing variable wire feed speed

control

• more expensive due to more complex wire feed speed control

• arc voltage depends upon wire feed speed whilst the power source controls the current

• cannot be used for high-speed welding of thin steel

ARC CHARACTERISTICS

Volts

Amps

OCV

Constant Current/Amperage Characteristic

Large change in voltage = Smaller change in amperage

Welding Voltage

Large arc gap

Small arc

gap

Courtesy of ESAB ABCourtesy of ESAB AB

SAW equipment

Welding headsWelding heads

Wire feed Wire feed

motormotorFeed roll Feed roll

assemblyassembly

Torch Torch

assemblyassembly

Contact tipContact tipTracking Tracking

systemsystem

SlidesSlides

Wire reelWire reel

Flux Flux

hopperhopper

Flux recovery Flux recovery

systemsystem

SAW equipment

Welding heads can be mounted on a:Welding heads can be mounted on a:

Tractor type carriageTractor type carriage

• provides travel along straight or gently curved joints

• can ride on tracks set up along the joint (with grooved wheels) or on the workpiece itself

• can use guide wheels as tracking device

• due to their portability, are used in field welding or where the piece cannot be moved



SAW Equipment

Welding heads can be mounted Welding heads can be mounted

on a:on a:

Column / Beam carriageColumn / Beam carriage

•provides linear travel only

•are capable of linear motion in 3 axes

•because workpiece must be brought to the weld station, they are use mostly in the workshop


SAW operating variables

• welding current

• current type and polarity

• welding voltage

• travel speed

• electrode size

• electrode extension

• width and depth of the layer of flux


Welding currentWelding current

••controls depth of penetration and the amount of controls depth of penetration and the amount of

base metal melted base metal melted �� dilutiondilution


Welding currentWelding current

•too high current � excessive excess weld metal

(waste of electrode) � increase weld shrinkage and causes greater distortions

•excessively high current � digging arc, undercut, burn

through; also a high and narrow bead � solidification

cracking

•too low current � incomplete fusion or inadequate penetration

•excessively low current �unstable arc


Current type and polarityCurrent type and polarity

••Usually DCEP Usually DCEP ��

deep penetration, deep penetration,

better resistance to better resistance to

porosityporosity

••DCEN increase DCEN increase

deposition rate but deposition rate but

reduce penetration reduce penetration

(surfacing)(surfacing)

••AC used to avoid arc AC used to avoid arc

blow; can give blow; can give

unstable arcunstable arc


Welding voltageWelding voltage•welding voltage controls arc

length

•an increased voltage can increase pick-up of alloying elements from an alloy flux

•increase in voltage produce a flatter and wider bead

•increase in voltage increase flux consumption

•increase in voltage tend to reduce porosity

•an increased voltage may help

bridging an excessive root gap


Welding voltageWelding voltage

•low voltage produce a “stiffer” arc � improves

penetration in a deep weld

groove and resists arc blow

•excessive low voltage

produce a high narrow bead � difficult slag

removal


Welding voltageWelding voltage

•excessively high voltage produce a “hat-shaped” bead �

tendency to crack

•excessively high voltage increase undercut � make slag removal

difficult in groove welds

•excessively high voltage produce a concave fillet weld that

is subject to cracking


Travel speedTravel speed

•increase in travel speed decrease heat input � less filler metal applied per unit of length � less excess weld metal

� weld bead becomes smaller


Travel speedTravel speed

•excessively high speed lead to undercut, arc blow

and porosity

•excessively low speed produce “hat-shaped” beads � danger

of cracking

•excessively low speed produce rough beads and lead to slag inclusions


Electrode sizeElectrode size

•at the same current, small electrodes have higher current density � higher deposition rates


Electrode extensionElectrode extension•increased electrode extension adds resistance in the welding circuit � increase in deposition rate � decrease in penetration and bead width

•to keep a proper weld shape, when electrode extension is increased,

voltage must also be increased

•when burn-through is a problem (e.g. thin gage), increase electrode

extension

•excessive electrode extension � it is more difficult to maintain the

electrode tip in the correct position


Depth of fluxDepth of flux

•depth of flux layer influence the appearance of weld

•usually, depth of flux is 25-30 mm

•if flux layer is to deep � arc is too confined � result a rough ropelike appearing weld

•if flux layer is to deep � gases cannot escape � the surface of molten weld metal becomes irregularly distorted

•if flux layer is too shallow � flashing and spattering will occur � poor appearance and porous weld

SAW technological variables

Travel angle effect Travel angle effect -- Butt weld on platesButt weld on plates

Penetration Deep Moderate SPenetration Deep Moderate Shallowhallow

Excess weld metal Maximum Moderate MinimumExcess weld metal Maximum Moderate Minimum

Tendency to undercut Severe Moderate MinimumTendency to undercut Severe Moderate Minimum

SAW technological variablesEffect of work angle Effect of work angle -- Fillet welds on plate in the HV Fillet welds on plate in the HV

positionposition

Typical work Typical work

angle = 40angle = 40°°

Smaller work Smaller work

angles reduce angles reduce

penetrationpenetration

Larger work Larger work

angles increase angles increase

penetrationpenetration


Effect of electrode position Effect of electrode position -- Fillet welds on plate in the Fillet welds on plate in the

flat positionflat position

CorrectCorrectException Exception -- when more than usual when more than usual

amount of penetration is requiredamount of penetration is required


Effect of electrode position Effect of electrode position -- Circumferential weldsCircumferential welds

••too little displacement too little displacement �� slag spills out of the weld slag spills out of the weld

••too much displacement too much displacement �� slag runs ahead welding head slag runs ahead welding head


Earth positionEarth position ++

--

Direction of Direction of

traveltravel

•welding towards earth produces backward arc blow

•deep penetration

•convex weld profile

•little resistance to porosity


Earth positionEarth position++

--

Direction of Direction of

traveltravel

•welding away earth produces forward arc blow

•normal penetration depth

•smooth, even weld profile

•high resistance to porosity

Weld backing

Backing stripBacking strip

Backing weldBacking weld

Copper backingCopper backing

Starting/finishing the weld

SAW variants

Twin wire SAW weldingTwin wire SAW welding•two electrodes are feed into the same weld pool

•wire diameter usually 1,6 to 3,2 mm

•electrodes are connected to a single power source � a

single arc is established

•normally operate with DCEP

� arc blow

•offers increased deposition

rate by up to 80% compared to single wire SAW

SAW variants

Wires can be oriented Wires can be oriented

for maximum or for maximum or

minimum penetrationminimum penetration

SAW variants

Tandem arc SAW processTandem arc SAW process •usually DCEP on lead and

AC on trail � reduce arc

blow

•requires two separate

power sources

•the electrodes are active in

the same puddle BUT there are 2 separate arcs

•increased deposition rate

by up to 100% compared

with single wire SAW

SAW variants

SAW tandem arc with two wires


SAW variants

Single pool Single pool -- highest deposition ratehighest deposition rate

Twin pool Twin pool -- travel speed limited by undercut; travel speed limited by undercut;

very resistant to porosity and cracksvery resistant to porosity and cracks

SAW variants

Tandem arc SAW process Tandem arc SAW process -- multiple wiresmultiple wires

•only for welding thick

sections (>30 mm)

•not suitable for use in narrow weld preparations

(root passes)

•one 4 mm wire at 600 A

� 6,8 kg/hr

•tandem two 4 mm wires

at 600 A � 13,6 kg/hrCourtesy of ESAB ABCourtesy of ESAB AB

SAW variants

Strip claddingStrip cladding •requires a special welding head (sometime

problems with arc

stability)

•can be applied on

complicated shapes (e.g. dished heads)

•higher productivity and

smaller dilution than twin

arc process

•strip electrode more

expensive than wire

SAW variants

Strip claddingStrip cladding

SAW variants

Narrow groove weldingNarrow groove welding

max. 8max. 8°°


SAW variants

Narrow gap weldingNarrow gap welding•for welding thick materials

•less filler metal required

•requires special groove

preparation and special welding head

•requires special fluxes, otherwise problems with

slag removal

•defect removal is very

difficult


SAW variants

Narrow gap weldingNarrow gap welding

SAW variants

Cold wire weldingCold wire welding

•the cold wire is not connected to power source

•increase deposition rates up to 75%

•high deposition rate at fixed heat input results in lower penetration!

SAW variants

Hot wire weldingHot wire welding

•the hot wire is connected to power source � much more

efficient than cold wire (current is used entirely to heat the

wire!)

•increase deposition rates up

to 100%

•requires additional welding

equipment, additional control of variables, considerable

set-up time and closer

operator attention

SAW variants

SAW with metal powder additionSAW with metal powder addition•increased deposition rates up to 70%; increased welding speed

•gives smooth fusion, improved bead appearance, reduced penetration and dilution from parent metal �

higher impact strength

•metal powders can modify chemical composition of final

weld deposit

•does not increase risk of cracking

•do not require additional arc energy

•metal powder can be added ahead or directly into the

weld pool

SAW variants

SAW with metal powder additionSAW with metal powder addition

••forwardforward--feed powder additionfeed powder addition

SAW variants

SAW with metal powder additionSAW with metal powder addition

••magnetic attachment of powdermagnetic attachment of powder

••SAW with metal cored wiresSAW with metal cored wires

SAW variants

Storage tank SAW of circular weldsStorage tank SAW of circular welds


15%

25%60%

Flux elements Electrode Dilution

Submerged arc welds are difficult to predict as the weld is Submerged arc welds are difficult to predict as the weld is

made up of three elements. The dilution may be as much as made up of three elements. The dilution may be as much as

60% resulting in a high susceptibility to solidification crackin60% resulting in a high susceptibility to solidification crackingg

Submerged Arc Welding Process (SAW)

Advantages of SAW• high current density, high deposition rates (up to 10 times those

for MMA), high productivity

• deep penetration allowing the use of small welding grooves

• fast travel speed, less distortion

• deslagging is easier

• uniform bead appearance with good surface finish and good fatigue properties

• can be easily performed mechanised, giving a higher duty cycle and low skill level required

• provide consistent quality when performed automatic or mechanised

• Virtually assured radiographically sound welds

• arc is not visible

• little smoke/fumes are developed

Disadvantages of SAW

• limited mainly between flat and horizontal positions

• limited to carbon, low alloy, creep resisting, stainless steels and nickel alloys

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

• slag must be cleared away after welding due to the danger of slag 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 seams only) with accurate fit-up

AdvantagesAdvantages

•• Low weldLow weld--metal costmetal cost

•• Easily automatedEasily automated

•• Low levels of ozoneLow levels of ozone

•• High productivityHigh productivity

•• No visible arc lightNo visible arc light

•• Minimum cleaningMinimum cleaning

DisadvantagesDisadvantages

•• Restricted welding Restricted welding

positionspositions

•• Arc blow on DC Arc blow on DC

currentcurrent

•• Shrinkage defectsShrinkage defects

•• Difficult penetration Difficult penetration

controlcontrol

•• Limited jointsLimited joints

Submerged Arc Welding

Any QuestionsAny Questions

QU 1. State the possible problems when using damp and

contaminated fluxes when using the sub-arc process

QU 2. State the two flux types used in the sub-arc welding

process.

QU 3. Generally what power source characteristic is required

for the SAW welding process

QU 4. State three main items of sub-arc fluxes, which require

inspection

QU 5. State the advantages and disadvantages of the sub-arc

welding process

Questions

Submerged Arc Welding Process

11-SAW Welding 2006

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