wis5 welding defects 02

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Copyright © 2003 TWI Ltd

Welding InspectionDefects/Repairs

Course Reference WIS 5

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A perfect butt weld joint, when subjected to an external force, provide a distribution of stress throughout its volume which is not significantly greater than parent metal.

DefinitionDefinition

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DefinitionDefinition

This is achieved as long as the following features apply :-

• Welds should consists of solid metal throughout a cross section at least equal to that of parent metal.

• All parts of a weld should be fully fused to the parent metal.

•Welds should have smoothly blended surfaces.

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Defects which may be detected by visual inspection can be grouped under five headings

Cracks

Lack of solid metals

Lack of fusion

Lack of smoothly

blended surfaces

Miscellaneous

Weld DefectsWeld Defects

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CracksCracks

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Classified by Shape Longitudinal

Transverse

Branched

Chevron

Classified by Position HAZ Centreline Crater Fusion zone Parent metal

CracksCracks Cracks that may occur in welded materials are caused generally by by many factors and may be classified by

shape and position, cracks are classed as planar.

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Process Cracks Process Cracks

HAZ hydrogen cracking

Weld metal hydrogen cracking

Solidification cracking (Hot Tearing)

Lamellar tearing

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CracksCracks

HAZ hydrogen cracking Weld metal hydrogen cracking

Solidification cracking Lamellar Tearing

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Copyright © 2003 TWI LtdTransverse crack Longitudinal crack

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LACK OF SOLID METALSLACK OF SOLID METALS

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POROSITYPOROSITYDescription : Gas pores trapped within the weld metal

Causes :

• Damp fluxes/ corroded electrode

•Grease/hydrocarbon/water contamination of prepared surface

•Air entrapment in gas shield

•Too high arc voltage/arc length

•Incorrect/insufficient deoxidant in electrode, filler or parent metal

Preventation :

• Use dry electrodes in good condition

•Optimise gas flow

•Use electrode with sufficient deoxidation activity

•Reduce arc voltage or arc length

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Gas CavitiesGas Cavities

Root pipingRoot piping

PorosityPorosity

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Cluster porosity Herring bone porosity

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Crater PipeCrater Pipe A shrinkage cavity at the end of a weld run where the arc is terminated

Causes :

•Lack of welder skill due to using processes with too high current.

•Inoperative crater filler ( GTAW )

Preventation :

• Retrain welder

•Use correct crater filling technique

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Crater Pipe/CracksCrater Pipe/Cracks

Crater pipeCrater pipe

Crater CracksCrater Cracks

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RootRoot concavityconcavity

A shallow groove, which may occur in the root of a butt weld

Causes :

•Insufficient arc power to produce positive bead

•Excessive backing pressure ( GTAW )

•Lack of welder skill

•Slag flooding in backing bar groove

Preventation :

• Raise arc energy

•Reduce gas pressure

•Retraint welder

•Tilt work to prevent slag flooding

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root concavity

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UnderfillUnderfill

A weld with thickness less than that of the parent metal

Causes :

•Insufficient weld metal

•Irregular weld bead surface

Preventation :

• Increase number of weld run

•Retrain welder

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Copyright © 2003 TWI LtdIncomplete filled groove

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Slag InclusionsSlag Inclusions Slag or other matters trapped during welding. The imperfection is of an irregular shape and thus differs in appearance from a gas pore

Causes :

•Heavy millscale/rust on work surface

•Incomplete slag removal from underlying surface of multipass weld

•Slag flooding ahead of the arc

•Entrapment of slag in work surface

•Unfused flux due to damage coating

Preventation :

• Grind surface prior welding

•Improve interun slag removal

•Position work to gain control of slag

•Dress work surface smooth

•Use electrode in good condition

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Elongated slag linesInterpass slag inclusions

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Inter- run ImperfectionsInter- run ImperfectionsIrregular along the fusion line between weld beads

Causes :

•Low arc current resulting in low fludity of weld pool

•Too high travel speed

•Inaccurate bead replacement

Preventation :

• Increase current

•Reduce travel speed

•Retrain welder

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Lack of FusionLack of Fusion

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Incomplete root penetrationIncomplete root penetrationFailure of the weld metal to extend into root of a joint

Causes :

•Excessively thick root face, insufficient root gap or failure to cut back sound metal in a ‘back gouging’ operation

•Low heat input

•Excessive inductance in GMAW dip transfer

•SMAW electrode too large( low current density )

•Use of vertical down welding

Preventation :

• Improved back gouging and edge preparation

•Increase arc power or decrease travel speed

•Improve electrical settings and possibly switch to spray transfer

•Reduce electrode size

•Switch to vertical up procedure

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Root DefectsRoot Defects

Lack of root fusion Lack of root Penetration

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Lack of root penetration Lack of root fusion

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Lack of sidewall fusionLack of sidewall fusionLack of fusion between weld metal and parent metal at one side of weld

Causes :

•Low heat input to weld

•Molten metal flowing ahead of arc

•Oxide or scale on weld preparation

•Excessive inductance in GMAW dip transfer welding

Preventation :

•Increase arc energy or increase travel speed

•Improve electrode angle and work position

•Improve edge preparation procedure

•Reduce inductance, even if this increase spatter

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Lack of Smoothly Lack of Smoothly Blended SurfacesBlended Surfaces

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Surface porositySurface porosityGas pores which break the surface of the weld

Causes :

•Damp or contaminated surface of electrode

•Low fluxing activity

•Excess sulphur ( particularly free – cutting steels) producing sulphur oxide

•Loss of gas shield gas due to long arc or high breezes ( GMAW )

Preventation :

•Clean surface and dry electrodes

•Use a high activity flux

•Use high manganese to produce MnS, note free cutting should not normally be welded

•Reduce arc length

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Excess Weld Metal ( Reinforcement )Excess Weld Metal ( Reinforcement )Reinforcement is the extra metal which produces convexity in fillet

welds and a welds thickness than the parent metal plate in butt welds.

Causes :

•Excess arc energy ( GMAW,SAW )

•Shallow edge preparation

•Faulty electrode manipulation

•Incorrect electrode size

Preventation

•Reduction of energy input

•Deepen edge preparation

•Improve welder skill

•Reduce electrode size

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Copyright © 2003 TWI LtdExcess cap reinforcement

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Excessive PenetrationExcessive PenetrationProjection of the root penetration bead beyond a specified limit

Causes :

•Weld input energy too high

•Incorrect weld preparation i.e excessive root gap, thin edge preparation, lack of backing

•Use electrode unsuited to welding position

•Lack of welder skill

Preventation

•Reduce arc power/increase weld speed

•Improve work piece preparation

•Use correct electrode position

•Retrain welder

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Excessive root penetration

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UndercutUndercutAn irregular groove at the toe of a run in the parent metal or in

previously deposited welding, cause by welding

Causes :

•Melting of top edge due to too high welding current ( especially at free edge) or high travel speed

•Attempting an HV fillet weld leg length

>9.0 MM

• Excessive/Incorrect weaving

•Incorrect electrode angle

Preventation

•Reduce power input,especially approaching a free edge where overheating can occur

•Weld in a flat position or use multirun technique

•Direct arc towards thicker member

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Cap UndercutCap Undercut

Measured in both Length & DepthMeasured in both Length & Depth

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Cap undercutRoot undercut

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OverlapOverlapAn Imperfection at the toe of a weld caused by metal flowing on to the

surface of the parent metal without fusing to it

Causes :

• Poor electrode manipulation

• High energy input/low travel speed causing surface flow of fillet weld

• Incorrect positioning of weld

• Electrode having too high a fluidity

Preventation

•Retrain welder

•Reduce the heat input or limit size of fillet weld to 9.0 mm leg by using multi run weld

•Change the flat position

•Change to less fluid weld metal

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Profile DefectsProfile Defects

Poor stop/starts

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MiscellaneousMiscellaneous

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MisalignmentMisalignmentThe non – alignment of two abutting edges in a butt joint

Causes :

• Inaccuracies in assembly procedures or distortion from other welds

• Excessive ‘out of flatness' in hot rolled plate or sections

Preventation :

•Adequate checking of alignment prior to welding coupled with the use of clamps and wedges

•Check accuracy of rolled section prior to welding

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Arc strikesArc strikesRandom areas of fused metal where the electrode, the holder, or

current return clamp accidentally touched the work and produced a short duration arc

Causes :

• Poor Access to work

• Missing insulation on electrode holder or torch

• Failure to provide an insulated resting place for the electrode holder or torch when not in use

• Loose current return clamp

Preventation :

•Improve access ( modify assembly sequence )

•Institute a regular inspection scheme for electrode holders and torches

•Provide an insulated resting place

•Regularly maintain current return clamp

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SpatterSpatterSmall droplets of electrode material can be projected clear of the weld

and may fused to the parent metal

Causes:

• High arc power

• Magnetic arc blow

• Incorrect setting for GMAW process

• Damp electrodes

Preventation :

•Reduce arc power

•Reduce arc length or swith to AC power

•Modify electrical settings ( but be careful to maintain full fusion

•Use dry electrodes

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Burn ThroughBurn Through

High Amps/volts

Small Root face

Large Root Gap

Slow Travel SpeedBurn through

A localized collapse of the weld pool due to excessive penetration resulting in a hole in the root run

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Root Coking/Oxidized Root Root Coking/Oxidized Root

Loss or insufficient back

purging gas

Most commonly occurs

when welding stainless

steels

Purging gases include

argon, helium and

occasionally nitrogen

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Mechanical DamageMechanical Damage

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Mechanical DamageMechanical Damage Mechanical damage can be defined as any surface material damage cause during the manufacturing process.This can included damage caused by:

Grinding

Hammering

Chiselling

Chipping

Breaking off welded attachments (torn surfaces)

Using needle guns to compress weld capping runs

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Parent Material DefectsParent Material Defects

A welding inspector should also inspect the parent material for any visible defects

Lamination

Mechanical damage Lap

Segregation line

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Plate LaminationPlate Lamination

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Any Questions?Any Questions?

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QuestionsQuestionsQU 1. Give two main causes for the occurrence of a burn through

QU 2. Give two main causes for the occurrence of excessive root penetration on a single-V butt weld

QU 3. Give five defects, which may occur when welding carbon steel using the MMA welding process with the current setting to low

QU 4. Give three possible causes for the occurrence of lack of side wall fusion

QU 5. Sketch the following defectsa. Lack of root wall fusion b. Lack of root penetrationc. Incomplete filled groove d. Concave root

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Weld RepairsWeld Repairs

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Welding RepairsWelding Repairs

Authorization for repair

Removal and preparation for repair

Testing of repair - visual and NDT

In the event of repair

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A weld repair may be used to improve weld profiles or extensive metal removal

Repairs to fabrication defects are generally easier than repairs to service failures because the repair procedure may be followed

The main problem with repairing a weld is the maintenance of mechanical properties

During the inspection of the removed area prior to welding the inspector must ensure that the defects have been totally removed and the original joint profile has been maintained as close as possible


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The specification or procedure will govern how the defective areas are to be removed. The method of removal may be

Grinding

Chipping

Machining

Filing

Oxy-Gas gouging

Arc air gouging


Arc air gouging

wis5 welding defects 02

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