welding imperfection part 1

7/25/2019 Welding Imperfection part 1

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A general review of geometric shape imperfections -

types and causesJob Knowledge

Part 1. IntroductionIn the job knowledge series welding imperfections such as cracks, lack of fusion, penetration and

porosity have been discussed. This article looks at those imperfections related to poor geometric shape

and will concentrate on the following:

Excess weld metal

Undercut

Overlap

Linear misalignment

Incompletely filled groove

Such imperfections might be considered as anomalies in the joint and they willalways be present to

some degree so that it becomes necessary to separate the acceptable from the unacceptable. This is

done by following guidance given by the application standard, which was the basis for the component

design, and/or by direction, as setout in the job contract. Examples of standards that might be referred

to are:

PD 5500 Specification for unfired fusion welded pressure vessels.

BS EN ISO 5817 Welding. Fusion-welded joints in steel, nickel, titanium and their alloys (beam

welding excluded). Quality levels for imperfections AWS D1.1 Structural welding code - Steel

Excess weld metal

(also called cap height, overfill or reinforcement)

Fig.1. Excess weld metal

This is weld metal lying outside the plane joining the weld toes. Note that the term 'reinforcement',

although used extensively in the ASME/AWS specifications is avoided in Europe as it implies it adds

strength to the welded joint, which is rarely the case.

Common causes


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This imperfection is formed when excessive weld metal is added to the joint, which is usually a result of

poor welder technique for manual processes but may be due to poor parameter selection when the

process is mechanised. That is, too much filler metal for the travel speed used. In multi-run welding a

poor selection of individual bead sizes can result in a bead build-up pattern that overfills the joint.Different processes and parameters (eg voltage) can result in different excess weld metal shapes.

Acceptance

The acceptability of this imperfection is very dependent on the application in which the product will be

used. Most standards have limit, related to material thickness (eg 10%), but also have a maximum upper

limits. Both the ratio and the maximum may be related to the severity of service that the component is

expected to see. The following table gives examples taken from BS EN ISO 5817.

Excess weld metal limits for quality levels:

Severity of service Moderate, D Stringent, B

Limit (up to maximum) h = 1mm + 0.25 b h = 1mm + 0.1 b

Maximum 10 mm 5 mm

Transition required smooth smooth

Where: h = height of excess & b = width of bead (seefigure 1)

An important reason for limiting the height of excess weld metal is that it represents a non-value added

cost. However, it must be remembered that the height of the weld cap influences the resultant toe

blend. A sharp transition causes a local stress concentration that can contribute to loss of strength,

which is particularly important in fatigue situations. As a result most specifications state that 'smooth

transition is required'.

Avoidance

If the imperfection is a result of welder technique then welder retraining is required. For mechanised

techniques an increase in travel speed or voltage will help to reduce cap height.

Undercut

Fig.2. Undercut

This is an irregular groove at the toe of a run in the parent metal.

The figure shows undercut at surface of a completed joint but it may also be found at the toes of each

pass of a multi-run weld. The latter can result in slag becoming trapped in the undercut region.


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Common causes

When arc and gas welding, undercut is probably the most common shape imperfection. With single-

sided pipe welds it may also be found at the bore surface. It may also be seen on the vertical face of

fillet welds made in the horizontal vertical position.

A wide spreading arc (high arc voltage) with insufficient fill (low current or high travel speed) is the usual

cause. However, welder technique, especially when weaving, and the way the welding torch is angled

can both cause and be used to overcome undercutting (ie angled to push the weld metal to fill the

melted groove). High welding current will also cause undercut - this is generally associated with the

need for a high travel speed to avoid overfilling of the joint.

Acceptance

Largely because this imperfection is widespread, most standards permit some level of undercutalthough they do require that a 'smooth transition is required. The limits in BS EN ISO 5817 range from

0.5mm (stringent) to 1mm (moderate) for thickness (t) greater than 3mm (more stringent limits are

required for t 0.5 to 3mm), while AWS D1.1 has a limit of 1mm.

Measuring undercut can be a problem because of the small size of the imperfection compared with the

general environment where there can be mill scale, irregularities in the surface and spatter.

In critical applications the imperfection can be 'corrected' by blend grinding or by depositing an

additional weld bead.

AvoidanceThis imperfection may be avoided by reducing travel speed and/or the welding current and by

maintaining the correct arc length.

Overlap (cold lapping)

Fig.3. Overlap

This is an imperfection at a toe or root of a weld caused by metal flowing on to the surface of the parent

metal without fusing to it. It may occur in both fillet and butt welds.

Common causes

This is often caused by poor manipulation of the electrode or welding gun, especially when the weld

pool is large and 'cold', where the welder allows gravity to influence the weld shape before


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solidification. Tightly adherent oxides or scale on the metal surface can also prevent the weld metal

fusing with the parent metal to cause the overlap imperfection.

AvoidanceAvoidance is achieved through an acceptable level of welder skill and a reduction in weld pool size

(obtained by reducing current or increasing travel speed). Adequate cleaning of the parent plate is also

important.

Acceptance

Standards rarely allow the presence of this imperfection, unless the length is short (eg BS EN ISO 5817

for moderate quality level D). Overlap can be very difficult to detect, especially if it is extremely small.

Linear misalignment

Fig.4 Linear misalignment

(Also known in the USA as high-low).

This imperfection relates to deviations from the correct position/alignment of the joint.

Common causes

This is primarily a result of poor component fit-up before welding, which can be compounded by

variations in the shape and thickness of components (eg out of roundness of pipe). Tacks that break

during welding may allow the components to move relative to one another, again resulting in

misalignment.

AcceptanceThe acceptability of this defect is related to the design function of the structure or pipe line either in

terms of the ability to take load across the misalignment or because such a step impedes the flow of

fluid.

Acceptance varies with the application:

BS EN ISO 5817 relates misalignment to wall thickness but sets maximum limits (eg for material

thickness t>3mm and moderate limits of imperfections D, = 0.25 x t, with a maximum of 5mm).

AWS D1.1 allows 10% of the wall thickness up to a maximum of 3mm.


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The consequence of linear misalignment can, when welding is carried out from one side, be lack of root

or sidewall fusion to give a sharp continuous imperfection along the higher weld face toe. In some

situations linear misalignment in the bore of a pipe can lead to in-service problems where turbulence of

the carrier fluid in the pipe creates subsequent erosion.

Incomplete filled groove

Incomplete filled groove

This is a continuous, or intermittent, channel in the surface of a weld, running along its length, due to

insufficient weld metal.

Common causes

This problem arises when there has been insufficient filler metal (current or wire feed too low or too

high a travel speed) so that the joint has not been sufficiently filled. The result is that the thickness of

weldment is less than that specified in the design, which could lead to failure.

AcceptanceMost standards will not accept this type of imperfection, except perhaps over short lengths and even

then a smooth transition is required. The designer expects the joint to be adequately filled, but not too

much so (see excess weld metal).

Often the presence of this imperfection is an indication of poor workmanship and could suggest that

further training is required.

Continuation

Part 2 looks at shape imperfections such as excess penetration and root concavity and highlights shapeimperfections related to fillet welded joints.

A general review of the causes and acceptance of

shape imperfections - Part 2

Job Knowledge


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Click here for Part 1.

This second article on shape imperfections refers mostly to fillet welds but there are two additional butt

weld imperfections that require some comment.

Excessive penetration (Excess penetration bead)

Fig.1. Excess penetration

Excess weld metal protruding through the root of a fusion (butt) weld made from one side only.

With pipe welding this type of imperfection may cause effects in the fluid flow that can cause erosion

and/or corrosion problems.

Common causes

Penetration becomes excessive when the joint gap is too large, the root faces are too small, the heat

input to the joint is too high or a combination of these causes.

Acceptance

The criteria which sets the level of acceptable penetration depends primarily on the application code or

specification.

BS 2971 (Class 2 arc welding) requires that the 'penetration bead shall not exceed 3mm for pipes up to

and including 150mm bore or 6mm for pipes over 150mm bore'.

BS 2633 (Class 1 arc welding) gives specific limits for smaller diameters pipes, eg for pipe size 25-50mm

the maximum allowed bore penetration is 2.5mm.

ASME B31.3 bases acceptability on the nominal thickness of the weld, for instance, allowing for athickness range of 13-25mm up to 4mm of protrusion. However, ASME notes that 'more stringent

criteria may be specified in the engineering design'.

BS EN ISO 5817 (Quality levels for imperfections), which supersedes BS EN 25817, relates the acceptable

protrusion to the width of the under-bead as follows:

Severity of service Moderate, D Stringent, B

Limit (up to maximum) h 1mm + 1.0 b h 1mm + 0.2 b

Maximum 5 mm 3 mm


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For thicknesses > 3mm where: h = height of excess & b = width of root (see Fig.1)

Avoidance

It is important to ensure that joint fit-up is as specified in the welding procedure. If welder technique is

the problem then re training is required.

Root concavity (suck-back; underwashing)

Fig.2. Root concavity

A shallow groove that may occur in the root of a butt weld.

Common causes

Root concavity is caused by shrinkage of the weld pool in the through-thickness direction of the weld.

Melting of the root pass by the second pass can also produce root concavity.

This imperfection is frequently associated with TIG welding with the most common cause being poor

preparation leaving the root gap either too small or, in some cases, too large. Excessively high weldingspeeds make the formation of root concavity more likely.

Acceptance

The root concavity may be acceptable. This will depend on the relevant standard being worked to. For

example:

BS 2971 requires that:

a) there is complete root fusion

b) the thickness of the weld is not less than the pipe thickness.

ASME B31.3 requires that the 'total joint thickness, including weld reinforcement, must be greater than

the weld thickness'.

BS EN ISO 5817 sets upper limits related to the quality level, eg for thicknesses > 3mm Moderate, (D), h

0.2t but max 2mm for Stringent, (B), h 0.05t but max 0.5mm. Furthermore, a smooth transition is

required at the weld toes.

In effect the standards require that the minimum design throat thickness of the finished weldment is

achieved. If the first two conditions of acceptance are met but the weld face does not have a sufficiently

high cap, additional weld metal may be deposited to increase the throat.


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Avoidance

It is important to ensure that joint fit-up is as specified in the welding procedure and that the defined

parameters are being followed. If welder technique is the problem then retraining is required.

Fillet welded jointsThis Section should be read in conjunction with Job Knowledge 66 Fillet welded joints - a review of the

practicalities.

Excessive convexity

Fig.3. Excessive convexity

This feature is also covered by the definition for excess weld metal, see Part 1, and may be described as

weld metal lying outside the plane joining the weld toes. Note that the term 'reinforcement', althoughused extensively in the ASME/AWS specifications is avoided in Europe as it implies that excess metal

contributes to the strength of the welded joint. This is rarely the case.

Common causes

Poor technique and the deposition of large volumes of 'cold' weld metal.

Acceptance

The idealised design requirement of a 'mitre' fillet weld is often difficult to achieve, particularly with

manual welding processes.

BS EN ISO 5817 acceptance is based on a mitre fillet weld shape with a specific design throat and any

excess weld metal is measured in relation to this mitre surface. The limits for this imperfection relate the

height of the excess metal to the width of the bead with maximum values ranging from 3mm for a

stringent quality level to 5mm for a moderate quality level. Surprisingly, there is no reference to a

'smooth transition' being required at the weld toes for such weld shape.

AWS D1.1 also has limits relating width to acceptable excess as follows:

Width of weld face Maximum convexity


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W 8mm 2mm

W


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Undersized fillet welds (fillet welds smaller than those

specified)

Fig.5. Undersized fillet weld

Common causes

The welding related causes are associated with high welding speeds and low welding currents.

Acceptance

Therefore, it is normally assumed that fillet welds will be at least of the size specified. BS EN SIO 5817

states that limits to insufficient throat thickness are not applicable to processes with proof of greater

depth of penetration, therefore a fillet weld with an apparent throat thickness smaller that thatprescribed should not be regarded as being imperfect if the actual throat thickness with a compensating

greater depth of penetration complies with the nominal value. That is if we can be sure there is good

penetration the smaller fillet may be acceptable, however, this should be discussed with the designer of

the fabrication. The limits set by the standard.

Relying upon deep penetration to provide the required minimum design throat thickness can be difficult

to justify. Penetration is a weld characteristic that is hard to measure directly and reliance must be

placed on the stringent control of both the welding process and the welder. Manual welding can rarely

be relied upon to provide the required consistency but it is an option with mechanised welding systems.

Imperfection: fillet weld

having a throat

thickness smaller than

the nominal value

Quality levels

Moderate D Intermediate C Stringent B

Long imperfections NOT permitted NOT permitted

Short imperfections (see Fig.5) h 0.3mm+ 0.1 a

max 2mm max 1mm

Avoidance

Adhere to the specified welding procedure and parameters. Use sufficient current and appropriate

travel speed. Where possible mechanise the welding operation.


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Asymmetric fillet weld (a fillet weld where the legs are of

unequal length)

Fig.6. Asymmetric fillet weld

Common causes

Due to incorrect electrode positioning or to gravity pulling the molten pool towards one face of the

joint. It is an mainly a problem with fillet welds made in the horizontal/vertical (PB) position.

Acceptance

There are instances where asymmetry may be specified (eg to place the toe stress concentration in a

particular region).

BS EN ISO 5817 would, for a 10mm leg length fillet weld (ie 7.1mm throat) allow a difference in leg

lengths of about 2.5mm at the stringent quality level and 3.4mm at the moderate quality level.

Acceptance is related to the throat thickness.

The consequence of this imperfection is a significant increase in weld volume. Provided the leg length

requirement is achieved there would not be a loss of strength. Perhaps this is why, in other standards, a

requirement is not specified and the acceptability is left to the inspection personnel to make the

'engineering judgement'!

Poor fit-up


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Fig.7. Poor fit-up

The most common imperfection is an excessive gap between the mating faces of the materials.

Common causes

Poor workshop practice, poor dimensioning and tolerance dimensions on drawings.

Acceptance

A major problem with fillet welds is ensuring the gap between the components is within defined limits.

BS EN ISO 5817 specifies the acceptance criteria as follows:

Quality levels

Moderate D Intermediate C Stringent B

h 1mm + 0.3 a h 0.5mm + 0.2 a h 0.5mm + 0.1 a

max 4mm max 3mm max 2mm

Where h = fit-up gap and a = fillet weld design throat

Figure 7shows that the gap results in a reduction in the leg length on the vertical plate and this, in turn,

results in a reduction in the throat thickness of the joint. A 10mm leg length fillet with a root gap of

3mm gives an effective leg of 7mm (a throat of 4.9mm instead of the expected 7mm).

When the application of BS EN ISO 5817 is not required, the guidance of BS EN 1011-2 can be followed,

which recommends a maximum gap of 3mm. This standard also states that the size of the fillet weld can

be increased to compensate for a large gap.

This discrepancy is addressed within AWS D1.1. which permits a root gap of up to 5mm for material

thickness up to 75mm. However, 'if the (joint) separation is greater than 2mm the leg of the fillet weld

shall be increased by the amount of the root opening, or the contractor shall demonstrate that the

effective throat has been obtained'.

This Job Knowledge article was originally published in Connect, January 2004. It has been updated so the

web page no longer reflects exactly the printed version.


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Fillet welded joints - a review of the practicalities

Job KnowledgeFillet welded joints such as tee, lap and corner joints are the most common connection in welded

fabrication. In total they probably account for around 80% of all joints made by arc welding.

It is likely that a high percentage of other joining techniques also use some form of a fillet welded joint

including non-fusion processes such as brazing, braze welding and soldering. The latter techniques are

outside the scope of this article.

Although the fillet weld is so common, there are a number of aspects to be considered before producing

such a weld. This article will review a number of topics that relate to fillet welded joints and it is hoped

that even the most seasoned fabricator or welding person will gain from this article in some way.

Common joint designs for fillet welds are shown below in Fig.1.

Fig 1. Common joint designs for fillet welds

Fillet weld featuresISO 2553 (EN 22553) uses the following notation as Figs.2 and 3 show.

a = throat thickness

z = leg length

s = deep penetration throat thickness

l = length of intermittent fillet


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Fig. 2. Mitre fillet

Fig 3. Deep penetration fillet

Fillet weld shapes

Over specified fillet welds or oversized fillet welds


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Fig 4. Weld sizes in relation to the required leg lengths or throat thickness

One of the greatest problems associated with fillet welded joints is achieving the correct weld size in

relation to the required leg lengths or throat thickness (Fig.4).

The designer may calculate the size and allow a 'safety factor' so that the weld specified on the

fabrication drawing is larger than is required by design considerations.

The weld size is communicated by using an appropriate weld symbol.

In the UK the weld size is frequently specified by referring to the leg length 'z' in ISO 2553 where the

number gives the weld size in millimetres as shown in Fig.5.

Fig 5. Weld size specification (UK)

In Europe, it is more common to find the design throat thickness, 'a' specified ( Fig.6).

Fig 6. Weld size specification (Europe)

Once the drawing has been issued to the shop floor, it is usual to find an additional safety factor alsobeing applied on by the welder or inspector. It is also common to hear 'add a bit more it will make it

stronger'.

The outcome is an oversized weld with perhaps an 8mm leg length rather than the 6mm specified by the

designer. This extra 2mm constitutes an increase in weld volume of over 80%.

This coupled with the already over specified weld size from the designer's 'safety factor' may lead to a

weld that is twice the volume of a correctly sized fillet weld.

By keeping the weld to the size specified by the drawing office, faster welding speeds can be achieved,

therefore increasing productivity, reducing overall product weight, consumable consumption and

consumable cost.

The other benefit is that, in the case of most arc welding processes, a slight increase in travel speed

would in most cases see an increase in root penetration so that the actual throat thickness is increased:

An oversized weld is therefore very costly to produce, may not have 'better strength' and is wasteful of

welding consumables and may see other fabrication problems including excessive distortion.

Lap joints welded with fillet welds.


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As discussed earlier, oversized welds are commonplace and the lap joint is no exception. The designer

may specify a leg length that is equal to the material thickness as in Fig.7.

Fig 7. Lap joint - leg length specification

Strength considerations may mean that the fillet weld size need not be anywhere near the plate

thickness. In practice the weld may also be deficient in other ways for example:

Fig 8. Example showing an undersized fillet weld

Due to melting away of the corner of the upper plate (Fig.8), the vertical leg length is reduced meaning

that the design throat has also been reduced; therefore an undersized weld has been created. Care is

therefore needed to ensure that the corner of the upper plate is not melted away. Ideally the weld

should be some 0.5-1mm clear of the top corner (Fig.9).

Fig 9. Ideally the weld should be 0.5-1mm clear of the top corner

It may be the designer may therefore specify a slightly smaller leg length compared to the thickness of

the component.

To compensate for this reduction in throat thickness it may be necessary to specify a deep penetrationfillet weld. This amount of additional penetration would need to be confirmed by suitable weld tests.

Additional controls may also be needed during production welding to ensure that this additional

penetration is being achieved consistently.

In addition to the reduction in throat thickness there is the potential for additional problems such as

overlap at the weld toe due to the larger weld pool size (Fig.10) or an excessively convex weld face and

consequential sharp notches at the weld toe (Fig.11).


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Fig 10. Overlap at the weld toe due to the larger weld pool size

Fig 11. Excessively convex weldface and consequential sharp notches at the weld toe

Both the potential problems shown in Figs.10 and 11 could adversely influence the fatigue life of the

welded joint due to the increased toe angle, which acts as a greater stress concentration.

Poor fit-up can also reduce the throat thickness as in Fig.12. The corner of the vertical component has

been bevelled in the sketch in an exaggerated manner to illustrate the point.

Fig 12. Throat thickness may be reduced by poor fit-up

SummaryFillet welded joints are not only the most frequently used weld joints but are also one of the most

difficult to weld with any real degree of consistency. Fillet welds require a higher heat input than a butt

joint of the same thickness and, with less skilled welders this can lead to lack of penetration and/or

fusion defects that cannot be detected by visual examination and other NDT techniques.

Fillet welded joints are not always open to NDT or are indeed time consuming to many non-destructively

testing techniques such as radiography or ultrasonic testing and the results are often difficult tointerpret. Inspection methods such as visual inspection, magnetic particle inspection and penetrant

inspection are surface examination techniques only and with visual inspection, much of the effort is

expended in measuring the size of the weld rather than identifying other quality aspects.

Fillet welded joints are therefore much more difficult to weld and inspect. Often the welds that are

produced are larger than they need to be or they may be of a poor shape which can adversely influence

their service performance.


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To overcome these difficulties, designers need to specify accurately the most appropriate throat size

and welding personnel should strive to achieve the specified design size. Welders also need to be

adequately trained and sufficiently skilled to be capable of maintaining an acceptable weld quality.

This article was written by Mark Cozens of Weld-Class Solutions. Any enquiries regarding the content of

the article should be addressed initially to the Editor, Connect.

A review of the application of weld symbols on

drawings - Part 1

Job Knowledge

Weld symbols have been used for many years and are a simple way of communicating design office

details to a number of different industrial shop floor personnel such as welders, supervisors, and

inspectors. Subcontractors are often required to interpret weld symbols on engineering drawings, from

perhaps the main contractor or client. It is essential that everyone should have a full understanding of

weld symbol requirements to ensure that the initial design requirement is met.

There are a number of standards which relate to weld symbols including British, European, International

and American (American Welding Society) standards. Most of the details are often similar or indeed, the

same, but it is essential that everyone concerned knows the standard to be used. One of the first

requirements therefore is:

Which standard?The UK has traditionally used BS 499 Part 2. This standard has now been superseded by BS EN 22553,

however in many welding and fabrication organisations there will be old drawings used that make

reference to out of date standards such as BS 499 Pt 2.

BS EN 22553 is almost identical to the original ISO 2553 standard on which it was based. Therefore we

can say, for at least this article's scope, there are no significant differences, but it is essential that the

reader consults the specific standard. The American system is also similar in many respects but will not

be covered here.

Basic requirementsAll the standards have the same requirements in relation to the following items:

Arrow line and arrow head

Reference line

The arrow line can be at any angle (except 180 degrees) and can point up or down. The arrow head must

touch the surfaces of the components to be joined and the location of the weld. Any intended edge

preparation or weldment is not shown as an actual cross sectional representation, but is replaced by a

line. The arrow also points to the component to be prepared with single prepared components. See Figs.

1-4.

welding imperfection part 1

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