03 welding imperfections 30-03-07[2

THE WELDING INSTITUTE

Welding Inspection of Steels WIS 5Section 03 Welding ImperfectionsRev 30-03-07 Copyright 2007 TWI Middle East

3.1

Welding Imperfections:

What are welding imperfections?

Welding imperfections are discontinuities caused by the process of welding. As allthings contain imperfections it is only when they fall outside of an applied “level ofacceptance” that they should be termed as defects, as if present they may then render theproduct defective or unfit for its purpose. The closeness of tolerance in any applied levelof acceptance depends largely upon the application and/or the level of quality required.As all fusion welds can be considered as castings they may contain imperfectionsassociated with the casting of metals, plus any other particular imperfections associatedwith the specific welding process being used. Welded components may containimperfections, which can be classified as follows:

1) Cracks 2) Gas pores, cavities, pipes3) Solid inclusions 4) Lack of fusion5) Surface and profile 6) Mechanical/Surface damage7) Misalignment

1) Cracks

Cracks sometimes occur in welded materials, and may be caused by a great number offactors. Generally, it can be stated that for any crack like imperfection to occur in amaterial, there are 3 criteria that must be fulfilled:a) A force b) Restraint c) A weakened structure

Typical types of hot and cold cracks that will be discussed later in the course are:

1) H2 Cracks 2) Solidification Cracks 3) Lamellar Tears

All cracks have sharp edges producing high stress concentrations, which generallyresults in a rapid progression, however this also depends on the properties of the metal.Cracks are classified as planar imperfections as they are 2 dimensional i.e. length anddepth. Most are classified as defects, although some standards do allow a degree of socalled ”crater, or star” cracking.

A weld metal crack in a pipe root A solidification crack in a weld face


WeSecRe

2) Gas pores, porosity, cavities and pipes

Gas poresThese are singular gas filled cavities 1.5mm diameter, created during solidification ofthe weld and the expulsion or evolution of gases from solution in solidifying weld metal.They are generally spherical in appearance though they may extend to form elongatedgas cavities, or Worm holes depending on the conditions of solidification. The term usedto describe an areas of rounded gas pores is Porosity, which may be further classified bythe number, size and grouping of the pores within the area (i.e. Fine, or coarse clusterporosity) Gases may be formed by the breakdown of paints, oil based products, corrosionor anti corrosion products that have been left on the plates to be welded. A singular gasfilled cavity of >1.5mm diameter is termed a Blow hole Porosity can occur during theMIG or TIG process by the temporary loss of gas shielding, and/or ingress of air into thearc column and may also be caused by an incorrect setting of the shielding gas flow rate.Gas pores/porosity may also break the welds surface where they are known as surfaceporosity. Porosity may be found in deep SAW or MMA welds due to damp fluxes ordamaged MMA electrode coatings. Porosity may be prevented by correct cleaning ofmaterials, correct setting and shielding when using the TIG or MIG welding processes,and using dry undamaged consumables.

Surface cluster porosity

Fine clusterporosity

Blow hole >1.5 mm Ø

Coarse cluster porosity

Shrinkage cavity

Crater pipe

lding Inspection of Steels WIS 5tion 03 Welding Imperfections

v 30-03-07 Copyright 2007 TWI Middle East

3.2

Shrinkage cavitiesThese are internal voids or cavities that are generally formlarge single welds of high depth to width ratio (d:w) as wmay be defined as hot plastic tears caused by large oppoweld and HAZ until the ductility of the hot metal isShrinkage cavities can produce high concentrations of strmay propagate cracks to the weld surface appearing around

Crater PipesCaused at the end of a weld run, where insufficient filler m

Hollow root bead (Elongated Gas Cavity)

ed during the solidification ofith SAW or MIG/MAG. Theysing contractional forces in theovercome resulting in a tear.ess at their sharp edges, whichthe weld centreline.

etal is applied to fill the crater.



3.3

3) Solid inclusions

Solid inclusions can be metallic or non-metallic that are trapped in the weld. The type ofsolid inclusion is really dependant on the welding process being used. In weldingprocesses that use fluxes to form all the required functions of shielding and chemicalcleaning, such as MMA and SAW, slag inclusions may occur. Other welding processessuch as MIG/MAG and TIG use silicon, aluminium and other elements to de-oxidise theweld. These may form silica, or alumina inclusions. Any of these non-metalliccompounds may be trapped inside a weld. This may happen when slag traps, such asundercut have been formed. Slag traps are mostly caused by incorrect welding technique.Tungsten inclusions are metallic inclusions, which may be introduced during TIGwelding by a poor welding technique, an incorrect tungsten vertex angle, or too highamperage for the diameter of tungsten being used. Copper inclusions may be causedduring MIG/MAG welding by a lack of welding skill, or incorrect settings inmechanised, or automated MIG welding. (Mainly welding aluminium alloys)

Welding phenomena such as “Arc Blow” or the movement of the electric arc bymagnetic forces can cause solid inclusions to be trapped in welds. The locations of theseinclusions may be within the centre of a deposited weld, or between welds where theresult causes “Lack of inter-run fusion”, or at the sidewall of the weld preparationcausing “Lack of side wall fusion” Generally solid internal inclusions may be caused by:

1) Lack of welder skill. (Incorrect welding technique)2) Incorrect parameter settings, i.e. voltage, amperage, speed of travel3) Magnetic arc blow4) Incorrect positional use of the process, or consumable5) Incorrect inter-run cleaning

Internal solid inclusion

Solid inclusion causing alack of sidewall fusion

Solid inclusions from base metal undercut inthe root run, or hot pass (Slag traps)

Surface breaking solid inclusion

Internal solid inclusioncausing a lack of inter-runfusion

A slag inclusion in the root of a pipe butt weld



3.4

4) Lack of fusion

Lack of fusion imperfections, are defined as a lack of union between two adjacent areasof material. This may be accompanied or caused by other imperfections as explained inthe last section. Lack of fusion can be considered a serious imperfection, as like cracks,they produce areas of high stress concentration. Lack of fusion, or overlap (a form oflack of fusion) may occur in the weld face area during positional welding caused by theaction of gravity and incorrect use of the process. Lack of fusion may be found in weldswhere processes using high currents have been used as the arc may be deviated awayfrom the fusion faces causing a lack of fusion in that area of the weld. This effect isknown as Arc blow and is caused by electro-magnetic force within the arc and material.

Lack of fusion may also be formed in the root area of the weld where it may be found onone or both plate edges. It may also be accompanied by incomplete root penetration.Lack of sidewall fusion is commonly associated with “Dip transfer MIG welding” ofmetals of over 3mm thickness, particularly in the vertical down position. This is mainlycaused by the inherent coldness of this form of metal transfer, and the action of gravity,but may also be attributed to high inductance settings and a possible lack of welder skill.

Like solid inclusions, lack of fusion imperfections may be caused by:

1) Lack of welder skill. (Incorrect welding technique)2) Incorrect parameter settings i.e. voltage, amperage, speed of travel etc3) Magnetic arc blow4) Incorrect positional use of the process, or consumable5) Insufficient inter-run cleaning

Lack of root fusion

Lack of sidewa

Lack of sidewall fusion(Also causing an Incompletely filled groove)

Lack of inter-run

Overlap

ll fusion



3.5

5) Surface and profile

Surface and profile imperfections are generally caused by poor welding techniques. Thisincludes the use of incorrect welding parameters, electrode/blowpipe sizes and/ormanipulation and joint set up. This category may be split into two further groups of weldface and weld root. These imperfections are shown pictorially in A B & C below:

A:

Spatter is not a major factor in lowering the weldments strength, though it may maskother imperfections, and should therefore be cleaned off before inspection. Spatter mayalso hinder NDT and be detrimental to coatings. It can also cause micro cracking or hardspots in some materials due to the localised heating/quenching effect.

An incompletely filled groove will bring the weld below the DTT (Design ThroatThickness) and may also cause a high stress concentration to occur. (Ref. Page 3.4)

Lack of root fusion may cause serious stress concentrations to occur in the root area ofthe weld. It may be caused by a poor welding technique, Hi – Lo, or irregular weldpreparation i.e. Changes in root face thickness.

An Incompletely filled groove

Lack of root fusion

Spatter

A


Welding Inspection of Steels WIS 5Section 03 Welding ImperfectionsRev 30-03-07 Copyright 2007 TWI Middle Ea

B:

A bulbous contour is an imperfection as it causes sharp stress concentrations at the toesof individual passes and may also contribute to overall poor toe blend.

Arc strikes, Stray-arcing, or Stray flash may cause many problems including cracks tooccur. They can also cause depressions in the plate bringing it below the DTT.Arc strikes are normally ground; then crack detected and repaired as required.

Incomplete root penetration may be caused by too small a root gap, insufficientamperage, or poor welding technique. It may be also appear in welding at the end of apoorly dressed or feathered tack weld. It produces sharp stress concentrations, and weldsoften having a lower ATT (Actual Throat Thickness) than that specified for the joint.

Bulbous contourArc Strikes

Incomplete rootpenetration bead

B

st

3.6

Poor toe blend



3.7

Effect of a Poor Toe Blend

A very poor weld toe blend angle

An improved weld toe blend angle

The excess weld metal height is within limits but the toe blend angle is unacceptable

Generally many specifications tend to quote that “The weld toes shall blend smoothly” Thisstatement can cause many problems as it is not a quantitative statement, and therefore very muchopen to individual interpretation. To help in your assessment of the acceptance of the toe blend itshould be remembered that the higher the angle at the toe then the higher is the concentration ofstresses.A poor toe blend will be present when the excess weld metal height is excessive or the weldprofile is excessively bulbous, however it may be possible that the height is within the givenlimits, yet the toe blend is not smooth, and is therefore a defect, and unacceptable. It should alsobe remembered, that a poor toe blend in the root of the weld has the same effect. It can be clearlyseen that any rapid change in the section will induce stress concentration and therefore the use ofthe term reinforcement to describe any amount of excess weld metal is very misleading andinaccurate, though this term is very often used in many application standards.

6 mm

80°

3 mm

30°

3 mm

90°



3.8

C:

An irregular bead width is a surface imperfection, which is often referenced inapplication standards as. “The weld bead should be regular along its length”

Undercut

Undercut can be defined as a depression at the toe of a weld in a previous deposited weldor base metal caused by welding. Undercut is principally caused by an incorrect weldingtechnique, including to high a welding current, to slow a travel speed in conjunction withthe welding position i.e. 2F/2G or PB/PC. It is often found in the top toe of fillet weldswhen attempting to produce a leg length >9mm in one run. Undercut can be considered aserious imperfection, particularly if sharp as again it causes high stress concentrations. Itis gauged in its severity by length, depth and sharpness.

Base metal, “top toe” undercut

Base metal, surface undercut


Welding InspecSection 03 WelRev 30-03-07 C

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Shritheoften

tion of Steels Wding Imperfeopyright 20

nkage grooves

nkage grooves may occur in the root area and are caused by contractional forces inweld metal pulling on the hot plastic base metal in the root area. This condition is

colloquially termed as root undercut.

Weld metal, surface undercut

Shrinkage grooves

Root Run or “Hot Pass” undercut

IS 5ctions07 TWI Middle East

3.9



3.10

Root concavity. (Suck back)

This may be caused when using too high a gas backing pressure in purging. It may alsobe produced when welding with too large a root gap and depositing too thin a root bead,or too large a hot pass which may pull back the root bead through contractional stresses.

Excess root penetrationMay be caused by using too high a welding current, and/or, slow travel speed, a large rootgap, and/or a small root face. It is often accompanied by burn through, or a local collapseof the weld puddle causing a hole in the weld root bead. Penetration is only excessivewhen it exceeds the allowable limit, as laid down in the applied application standard.

Root oxidationRoot oxidation may take place when welding re-active metals such as stainless steels withcontaminated, or inadequate purging gas flow.

Incompletely fused Tack WeldsIt is often a procedural requirement for tack welds to be feathered (Lightly ground andblended) prior to welding. This requirement is mainly dependent upon the class of work.Feathering should enable the tack welds to be more easily blended and any failure toachieve this correctly may result in a degree of lack of root fusion/penetration occurringin the weld root run.

Un-featheredroot tack weld

Adjacent weld area showinga lack of root fusion and/orroot penetration(See also page 3.6)

Root concavity

Pipe Plate


WeldSectiRev 3

Root oxidation in

ing Inspection of Steels WIS 5on 03 Welding Imperfections0-03-07 Copyright 2007 TWI Middle East

3.11

A Burn Through may be caused by a severely excessfollowed by local collapse of the weld root in the effe

It may be generally caused by a combination of the fo

a) > welding currentb) > root gapc) < root faced) < speed of travel

Its occurrence is also very dependent upon the welding

Excess root penetration bead(Beyond an acceptance limit)

Stainless Steel

Burn through

This may lead to a burn throughA local collapse of the weld pool
leaving a hole in the root area
ive root penetration beadcted area.

llowing factors:

position and the effect of gravity.



3.12

To summarise, surface/profile welding imperfections are as follows:

1) Incompletely filled groove/lack of fusion

2) Spatter

3) Arc strikes. (Stray arcs)

4) Incomplete root penetration

5) Lack of root fusion

6) Bulbous, or irregular contour

7) Poor toe blend

8) Irregular bead width

9) Undercut. (Weld and Base metal)

10) Root concavity. Root shrinkage grooves

11) Excess penetration. Burn through(Comparatively measured as radiographic density in some line pipe standards)

12) Root oxidation

Surface and profile imperfections are mainly caused by a lack of applied welding skill.

6) Mechanical/Surface damage

Mechanical/Surface damageThis can be defined as any material surface damage caused during the manufacturing orhandling process, or in-service conditions. This can include damage caused by:

1) Grinding 2) Chipping3) Hammering 4) Removal of welded attachments by hammering5) Chiselling 6) Using needle guns to compress weld capping runs7) Corrosion (Not caused through welding, but is considered during inspection)

As with the stray arcing, the above imperfections can be detrimental as they reduce thethrough thickness dimension of the plate in that area. They can cause local stressconcentrations and should be repaired prior to completing the job.

Chisel Marks Pitting Corrosion Grinding Marks Surface Scale



3.13

7) Misalignment

There are 2 main forms of misalignment in plate materials, which are termed:

1) Linear misalignment 2) Angular misalignment/distortion

Linear misalignment: can be controlled by the correct use/control of the weld set uptechnique i.e. tacking, bridging, clamping etc. Excess weld metal height and the rootpenetration bead are always measured from the lowest plate to the highest point ofthe weld metal, as shown below.

Angular misalignment: may be controlled by the correct application of distortioncontrol techniques, i.e. balanced welding, offsetting, or use of jigs, fixtures, clamps, etc.

Hi-Lo is a term that is generally used to describe the unevenness across the root facesbetween pipes found during set up and prior to welding. This unevenness is often causedby an un-matching and/or irregular wall thickness, or between pipes having any degreeof ovality.

Angular misalignment/distortion measured in degrees

15

Hi-Lo

Linear misalignment measured in mm

3 mm

Excess weld metal height



3.14

Summary of Welding Imperfections:

Group Type Causes/Location

Centreline Weld MetalH2 Weld Metal & HAZ

1) Cracks

Lamellar Tears Base metalPorosity

Gas pore 1.5mm

Blow hole > 1.5mm

Damp electrodesUn-cleaned plates/pipes

Loss of gas shield2) Porosity/Cavities

Shrinkage cavity Weld metal (high d:w)Slag MMA/SAW

Silica TIG/MAG (Fe steels)Poor Inter-run cleaning

Undercut in hot pass. Arc blow

Tungsten TIG Dipping tungsten in weld pool3) Solid Inclusions

Copper (MIG/MAG) Dipping tip in weld poolLack of side wall fusion

(Can be surface breaking)Arc Blow

Incorrect welding techniqueLack of root fusion Non feathering of tack welds

4) Lack of Fusion

Cold lap/overlap Positional welding techniquePoor toe blend Incorrect welding technique

Arc Strikes Poor welding techniqueIncomplete penetration < Root gap/Amps. > Root face

Incompletely filled groove Incorrect welding techniqueSpatter Damp consumables

Bulbous contour Incorrect welding techniqueUndercut:

Surface and internalToo high an amperage

Poor welding techniqueShrinkage groove (Root) Contractional stress

Root concavity Too high gas pressureExcess Penetration

Burn throughToo large root gap/amps

Too small a root face

5) Surface & Profile

Crater Pipes (Mainly TIG) Incorrect current slope-out6) Mechanical damage Hammer/Grinding marks etc. Poor workmanship

Angular Misalignment () Poor fit-up. Distortion

Linear Misalignment (mm) Poor fit-up7) Misalignment

Hi-Lo (mm) Irregular pipe wall or ovality

Notes:

The causes given in the above table should not be considered as the only possiblecauses of the imperfection given, but as an example of a probable cause.

Good working practices and correct welder training will minimise the occurrence ofunacceptable welding imperfections. (Welding defects)


Welding ISection 0Rev 30-03

WIS 5 Section 3 Exercises:

Observe the following photographs and identify any Welding Imperfections:(As indicated within the ovals)

A A

A A

A

A

A A

1) 2)

3) 4)

6)

Plate. Butt Weld Face

Pipe. Butt Weld Root

Plate. Butt Weld Root


Pipe. Butt Weld Root5) Pipe. Butt Weld Root

nspection of Steels WIS 53 Welding Imperfections-07 Copyright 2007 TWI Middle East

3.15

A

A

A

A


Welding ISection 0Rev 30-03

8)7) Pipe. Butt Weld Root Plate. Fillet Weld Face

nspection of Steels WIS 53 Welding Imperfections-07 Copyright 2007 TWI Middle East

3.16

A A

A A

A

A

A

A

7

9) 10)

A

Plate. Butt Weld Root12)11)

A

Plate. Fillet Weld Face Plate. Butt Weld Face


A

A


WeldinSectionRev 30

A A

B B

A A

B B

A

A

B

15)

A

B

13)

A

B

14)

16

17) 18)

16)

A

B

Plate. Butt Weld Root Plate. Butt Weld Face

Plate. Butt Weld Face Plate. Butt Weld Face

Pipe. Butt Weld Root Plate. Butt Weld Root

g Inspection of Steels WIS 503 Welding Imperfections

-03-07 Copyright 2007 TWI Middle East

3.17

A

B

B

A

A

B

A

B


Welding InspeSection 03 WRev 30-03-07

Record all welding imperfections that can be observed in photographs 19-24:

19) Pipe. Butt Weld Face

ction of Steels WIS 5elding ImperfectionsCopyright 2007 TWI Middle East

3.18

20) Pipe. Butt Weld Root



21) Plate. Butt Weld Face

e
ction of Steels WIS 5lding Imperfections
Copyright 2007 TWI Middle East

3.19

22) Plate. Butt Weld Root



23) Plate. Butt Weld Face

ction of Steels WIS 5elding ImperfectionsCopyright 2007 TWI Middle East

3.20

24) Plate. Butt Weld Root

03 welding imperfections 30-03-07[2

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