cswip paper

8/20/2019 cswip paper

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Visual inspection of arc welds - a

guide to best practiceSection 3. Practical application of visualinspection

• Inspection details• Parent materials and edge preparations • Welding consumables

Inspection details

Introduction

There is some flexibility in applying the 'rules' for assessing weld details becausemuch depends on the type of product being examined. It appears to be a humantrait that we tend to find fault with items under scrutiny. In my many years as atrainer of inspectors, I have found that most people are likely to assess initiallyonly the negative features evident on a particular weld or product, i.e. they onlylook for, and assess, visible DEFECTS.

Frequently, there are many positive features to assess initially. By carrying out ageneral assessment beforehand, an inspector, with experience, can very often

judge the acceptability of the weld or product - based on these facts alone.

As there are many ways of applying the techniques of visual inspection, a codeof practice exists related to visual inspection practice. EN 970 provides guidanceand also details the main principles of how to carry out visual inspection, as wellas providing information on the tools that can be used and illuminationrequirements. This document provides a good first step before embarking onvisual inspection practices.

Details of the weld features to be considered during visual examination are givenbelow.


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1. Size - butt welds

Consider:

a) excess weld metal height

b) root penetration

c) weld width

d) root bead width

2. Size - fillet welds

Consider:

a) z minimum (and maximum) leg length size

b) a minimum design throat thickness


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3. Shape - butt welds

Consider:

Ideally, (a) is the most desirable but very often it may be difficult to achieve.Because of this, one should assess the excess weld height in conjunction withthe weld profile and perhaps the toe blending.

4. Shape - fillet welds

Consider:

In normal practice, (a) is the most desirable but, again, in many instances it isdifficult to achieve. Acceptance levels, therefore, allow tolerances on weld shape.


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5. Toe blend

Depending on the service conditions of the product, the toe blend may be ofgreater importance than the size and shape of the weld. A poor toe blend mayreduce service life by a considerable margin if the product is under a cyclic load.

For butt welds, consider:

For fillet welds, consider:


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6. Weld width and consistency of weld width

For butt welds and fillet welds, consider:

Imperfections have not been addressed in this section of the best practice guide.These are considered in Section 4, Imperfections - identification andinterpretation but before determining specific imperfections, assess the mainfeatures of the weld.

Other factors may be of equal importance, but the more information you haveavailable, the better.

A simple inspection procedure can be implemented which should help to ensurethat the inspector follows the same method each time. The quality controldepartment may implement such a procedure, depending on the nature of theproduct and specific company procedures.

For guidance, typical examples of visual inspection and defect assessment formsare shown below. These are of the type which could be used within astandardised procedure.


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Visual inspection assessment form

Visual Inspection of the Completed Weld. Reference No. of Weld .....

= Acceptable X = Non-Acceptable

Dimensions mm ________ l ________ w ________ t

Side 1 Side 2

Excess weld metal/Penetration bead ______________ mm ______________ mm

Weld width ______________ mm ______________ mm

Toe blend ______________ ______________

Weld consistency ______________ ______________

Arc strikes ______________ ______________

Spatter ______________ ______________Grinding marks ______________ ______________

Mechanical damage ______________ ______________

Surface colour ______________ ______________

Misalignment ______________ mm ______________

Angular distortion ______________ mm in ______________ mm

Longitudinal distortion ______________ mm in ______________ mm

Other comments:


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Defect assessment form

Defects Weld face Weld root

Mark 'A' for acceptable

Mark 'R' for rejectable

Cracks

Incomplete side fusion

Incomplete interun fusion

Incomplete root fusion

Incomplete root penetration

Undercut depth

Undercut blend

Undercut length

Excess penetration

Root concavity

Overlap

Weld face Weld root

Solid inclusions:

Slag

SilicaTungsten

Copper

Gas inclusions

Porosity

Wormholes


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Isolated pores

Crater pipes

Others:

The weld, Ref. No. ...... is/is not ...... to the standard ......

Date .......... Signature ..........

Parent materials and edge preparations

Introduction

It should be kept in mind that many problems associated with weld quality aredue to poor edge preparation of the parent materials. Before any welding iscarried out, inspection and conformance to the welding procedure requirementsat this point can prevent the formation of imperfections during manufacture.

Furthermore, it is equally important that correct welding consumables are used toensure that the required mechanical properties are met.

Parent materials

To ensure that the correct material grade or composition is used on the project, itis necessary for the inspector to check 3 main areas:

• Sizeo number of components for welding/to be joined (number off)o

thickness (abbreviated to 't')o length (abbreviated to 'l')o width (abbreviated to 'w')o diameter (abbreviated to 'ø' for pipes)


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• Typeo composition of material

or o grade of material

or o tradename of material

If the material is known, it is possible to assess the risk of cracking orother types of defect common to a particular material type. For example,the likelihood of, and susceptibility to, porosity in aluminium alloys.

• Conditiono surface condition: freedom from contaminants which may affect

welding or in-service operation, including -

- mill-scale remaining from material manufacture- protective coatings to inhibit corrosion during storage- oil, grease or paints

o distortion, due to poor storage or transportation of the material

o inherent defects, including metallurgical imperfections such assegregation bands or surface-breaking imperfections such as laps

The heat treatment condition is also important. For example, somesteels can be supplied in the normalised or quenched andtempered condition. This can have an implication for the material'smechanical properties and its weldability.


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A summary of the inspection points for parent materials is shown below.

Edge preparationsMany defects and/or imperfections may be caused by poor joint preparationwhich can be attributed to:

• method of edge preparation• finish of edge preparation• details of edge preparation

1. Method of edge preparation - and potential problems

•

thermal cuttingo increased hardnesso oxidationo poor shapeo distortion


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• machined edgeso evolution of harmful gases during weldingo poor shapeo poor finish

• sheared edgeso evolution of harmful gases during weldingo poor shapeo work-hardening

• grindingo poor shapeo poor surface finisho inaccurate features

2. Finish of edge preparation

In some cases it is necessary to specify a particular level of finish on edgepreparations, otherwise weld defects may arise. For example, TIG welding offlame-cut edges can result in serious porosity and inclusions. This is because theinherent roughness of the surface can readily trap contamination which is notremoved during welding since the TIG process does not utilise a flux or slag. Thesame may be true for ground edges which are not sufficiently smooth, and insuch cases chemical cleaning (e.g. degreasing) and mechanical cleaning will berequired - although this may be impractical in some situations.

3. Details of edge preparation

As the first weld (the root run) is often the most difficult, the edge preparationdetails must conform to the specified requirements (welding procedurespecification).


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Butt welds

Gap size too large:

a) excess penetration

b) burnthrough

c) shrinkage grooves

d) gas entrapment

Gap size too small:

1) incomplete penetration

2) incomplete root fusion

3) incomplete side wallfusion

4) slag inclusions

5) root concavity

Root face too large:

a) incomplete root

penetration

b) incomplete root fusion

Root face too small:

1) excessive penetration

2) burnthrough

3) root concavity

4) root undercut

Included angle too

large:

a) excess penetration

b) incomplete filled groove


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Included angle too

small:

1) incomplete root

penetration

2) incomplete interun fusion

3) incomplete root fusion4) incomplete sidewall fusion

5) excessive cap

6) poor toe blend

7) slag inclusions

Fillet welds

Gap size too large:

1) reduced root penetration

2) slag inclusions

3) gas inclusions4) reduced vertical leg length size

5) cracking

Poor fit-up and poor joint preparations very often account for a large proportion ofweld imperfections. (The above causes and effects are for guidance only but it

should be remembered that there is no such thing as a perfect weld.)


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Welding consumables

Introduction

The mechanical properties of a welded joint are greatly affected by the weldingconsumable used. It is, therefore, essential that the inspector confirms thecorrect consumables are to be used, or have been used.

Each welding process utilises different consumables and these are given specificterms -

• welding electrode (MMA)

• welding wire (MIG/MAG & SAW)

• welding rod (TIG & gas welding)

• welding flux (SAW & gas welding)

• shielding gas (TIG & MIG)

Type and size also have a particular meaning -

electrodes: diameter & length

wires: diameter & weight

rods: diameter & weight

flux: (SAW) diameter/size of flux particles

• size

shielding gas: cylinder size & pressure

electrodes: tradename, brand name, specification & grade offlux covering

• type

gases/fluxes/rods &wires:

tradename, brand name, specification & grade

Note: in many instances, it may also be necessary to check batch numbers of

consumables


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Condition of consumables

It is essential that every consumable is in f irst-class condition. To achieve (andmaintain) this, the following may be necessary:

• good storage consideration should be given to humidity, temperature, stacking,issue and return, date of purchase, etc

• packaging the inspector should ensure 'good housekeeping' procedures, e.g.unused consumables must be returned to their original packaging

• baking/drying moisture content of MMA electrodes and SAW fluxes must bekept low to avoid weld cracking: it may be necessary to bake/dry

these types of consumable in a purpose-made oven

Instructions provided by the consumables manufacturer should be adhered to.An in-house consumables control procedure should be used on high integrityapplications.


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Visual inspection of arc welds - a

guide to best practiceSection 2. Getting to grips with the basics

• What is visual inspection?• Terminology and definitions • Visual inspection - key principles • The welding inspector

What is visual inspection?

Introduction

Every day of our lives we rely on the practice of visual inspection - outside theworkplace as well as within it. One of the best ways of explaining the principles ofvisual inspection is to make a comparison with an everyday situation that most ofus are either familiar with, or can easily identify with.

Inspection of a motor car - an analogy

If we were to purchase a car, second-hand or new, we would apply similar visualinspection principles to those used for examining welded products - probablywithout even realising.

Examining a car before

purchase provides a useful

analogy for studying the

principles of visualinspection of welds


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The first stage of buying a car is to identify which make and model we want. Ourdecision may be based on performance in relation to top speed, acceleration,braking ability, or possibly fuel economy. Other factors considered may includethe number of passengers it can carry or, for example, its manoeuvrability forparking.

Design. We would most certainly look at the design or styling of the vehicle: willit be a sports car or coupé; an estate car; a four-wheel drive for off-road use; or ahandmade, specialist car, rather than a mass-produced vehicle?

Materials. Thought would also be given to the materials used. For example, areall car body materials suitable for our intended use of the vehicle; are the interiortrim and fittings sufficiently durable; will the materials used in the engine allow itto run on unleaded petrol?

Workmanship. How well the vehicle has been made would also be importantand this would be judged against some form of workmanship standard. Are allthe body panels a good fit; is the underside suitably protected against corrosion;have all the engine ancillaries been fitted correctly and carefully to give long andtrouble-free service?

Full inspection. It may be important to have optional extras, such as tinted glassor electrically heated rear-view mirrors. Is the vehicle's paperwork authentic andcomplete? These and other 'characteristics' would be assessed as part of the fullinspection, before purchase of the vehicle.

Testing. Before driving the vehicle for the first time, we would make sure wetested the brakes and steering and that they were both satisfactory.

Acceptance criteria. We would apply our own acceptance criteria to the vehicleas a whole before committing to the purchase.

Specification. All the key points above can be regarded as a specification. Thecar manufacturer initially provides a specification for its vehicles and this givesdetails of technical requirements.

Visual characteristics

There are similar assessments to be established for a welded construction, andbefore visual inspection can take place, it is important that the specification isknown and understood. It should, therefore, be noted that assessment of all visual characteristics should be taken on-board, so that the product (or weldment)is overall at an acceptable level of quality.


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Service conditions

In addition, it is equally important that the product's service conditions are knownso that specific characteristics are achieved, ensuring reliability and structuralintegrity in relation to service life and performance. After all, you would not expecta sports car to function reliably in off-road conditions, or a family car to performwell in a motor race!

Quality characteristics

Many products are, therefore, manufactured to specific codes of practice orspecifications and, in general, specific rules are provided in the following areas:

• design

• materials• workmanship• inspection and testing• acceptance levels

It should be appreciated that a car manufacturer will impose similar rules beforeproduction of a new vehicle.

For products not encompassed by a specification or code of practice, it may benecessary for the company's quality assurance/quality control function to specify

appropriate rules in the form of a specification, procedures and acceptance levels.

Having established the overall design specification, the next step is to consider atwhich stages visual inspection can be applied.

For many product-types or applications, it might only be necessary to applyvisual inspection on completion of the manufacturing process. This, of course,would normally be the case with visual inspection of either a new or second-handcar: we have to rely on the manufacturer to complete the before- and during-manufacture inspections.

It must be appreciated that visual inspection is not just a non-destructiveexamination method applied after welding. It can be used throughout themanufacturing process as a means of preventing quality problems, rather thanhaving to cure problems after completion.

To be effective at applying these principles in practice, it is important that thosegiven the inspection task are aware of specific items within the welding qualityrequirements, in relation to a whole range of different aspects. Much of thisphilosophy is dependent on the experience, training and qualifications of theinspector.


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Summary

In summing-up, it will be helpful to review briefly the various aspects specified incodes of practice.

Design aspects and the importance of meeting the design criterion have beenmentioned earlier. In relation to parent material requirements, you would notexpect to find the car's exhaust system made from carbon steel if thespecification for design stated stainless steel. Visually inspecting and checkingmaterials before construction can prevent such situations arising. Equally, youwould be extremely disappointed to find that due to poor workmanship, a doorwould not close properly, or that the upholstery was damaged duringmanufacture.

The above examples of production problem - and the fact that the carmanufacturer wants to sell all vehicles produced and ultimately make a profit -dictate that it is essential to have a formal inspection and testing regime which isrigorously applied throughout the manufacturing process. You may have heardthe phrase from design to delivery!

Finally, it is important to appreciate that assessing how good something lookscan be a rather subjective matter: some people like certain things which may notappeal to others. This is one of the reasons why we must have acceptance levelswhich allow everyone to apply the same standards (although no specificationallows for different interpretations of the information).

Terminology and definitions

Introduction

It is essential that inspectors report their findings using correct terminology,otherwise there may be lack of clarity, leading to misunderstanding andinefficiency.

The welding industry has a language of its own and, like any language, it takes

time to become fluent. To complicate matters, the language may be flawed byslang or dialect. It is important, therefore, that conventional welding language isadopted (and adhered to) and this can be found by reference to appropriatenational or international standards.

Furthermore, if the joint design is correct and the features (such as root gap, rootface and included angle) are correct it will improve the overall quality of theweldment and, ultimately, the product.

Below are examples of joint designs, features and weldments. Please note theappropriate terminology, which is based on British standards (where specified).


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Butt joints

Fillet joints

Completed welds show various features, which include the following -


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Butt weld features

1 = Excess weld metal height 6 = Width (w)

2 = Weld toe 7 = Length (l)

3 = Fusion boundary 8 = Weld width

4 = Heat affected zone 9 = Ripple shape

5 = Thickness (t) 10 = Weld contour

The terminology below is consistent with British Standards. However, different

standards and countries use variants of these terms, for example:

1. Excess weld metal Also termed 'reinforcement' in some codes and standards (including someBritish standards). Reinforcement is a non-preferred term because itimplies that weld strength increases with increased excess weld metalheight. This is not always the case, since the weld toe angle is alsoincreased and may cause a continuous stress concentration at the weldtoe.

Some people often refer to excess weld metal as the 'cap height'. This isregarded as a slang term and should be avoided in inspection reports.

2. Weld toe Other terms are rare in this case. It should, however, be noted that toeblending is the main aspect of assessment during visual inspection.


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3. Fusion boundary Also termed 'fusion line', which is acceptable. Many codes and standardsuse 'sidewall' as a term to report incomplete sidewall fusion. Whilst thismay appear to have little relevance to surface inspection, it is possible tofind incomplete fusion at the surface of the weld.

4. Heat affected zone This is also abbreviated to 'HAZ'. It is rarely visible on steel welds, but canoften be seen on welds made in stainless steel and titanium. With titanium,it is frequently the first assessment made during visual inspection becausethe colours demonstrate different levels of contamination of the weld,which affects overall acceptance of the completed weld.

5. Thickness

The abbreviation 't' is used in many codes and standards. It is importantthat the inspector checks the thickness of the material to ensurecompliance. This dimension is also referred to as the 'short transverse' inthe context of mechanical properties.

6. Width The inspector must ensure dimensional accuracy of the component beingwelded. Also, the inspector may report 'angular misalignment' if thecomponents have distorted during welding. This is also termed the'transverse direction' and in many cases the inspector should reporttransverse distortion, when evident.

7. Length Similarly, the inspector must check the length of the component to ensureaccuracy. This is also referred to as the 'longitudinal direction'. It may benecessary for the inspector to report longitudinal distortion, especially inthin sheet materials.

8. Weld width The weld width is an important feature that is often overlooked duringvisual inspection. Many standards require welds to have a minimum andmaximum weld width based on the original width of the fit-up of the joint.

9. Weld ripples The ripples of the weld are influenced by the travel speed used during thewelding operation. Ripples which are excessively vee-shaped can lead tosolidification problems within the weld metal, causing centreline crackingin some instances.


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10.Weld contour Most standards related to welding inspection require specific shape detailswhich should be regular and consistent. The inspector should assess thisat an early stage of final inspection to evaluate the degree of control overthe welding operation. Some standards also term this as the 'weld crown'.

The above is for guidance only; other terms are also relevant but are notincluded because this guide is not intended to be exhaustive.

Fillet welds

Fillet welds are probably the most common connection used in generalfabrication. Similar to the butt weld mentioned previously, a fillet weld hasspecific features and terminology.

Fillet weld shapes

Many specifications require fillet welds to have a certain shape, which oftentranslates to a mitre appearance. Achieving this shape consistently can bedifficult, especially with manual welding. Specifications will often allow sometolerance on the weld shape or profile, but it may be dictated by the application.

The three types of fillet weld shape shown above have advantages andlimitations. Many specifications call for a mitre shape, since this is a compromise

between the convex and concave types.

The convex profile has the advantage of an increased throat thickness, however,the resultant toe angle is much smaller, giving rise to a higher risk of failure at theweld toe(s). Conversely, the concave fillet weld has a much smoother transitionat the weld toe but the weld strength may be reduced due to a decrease in thedesign throat thickness.


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Fillet weld features

1 = Leg length (horizontal) 6 = Width (w)

2 = Leg length (vertical) 7 = Height (h)

3 = Thickness (t) 8 = Actual throat thickness

4 = Weld width 9 = Heat affected zone

5 = Length (l) 10 = Weld toe (blend)

Please note that the drawing above shows the 'actual' throat thickness (feature 8).This must not be confused with the design throat thickness, which would notnormally show penetration into the parent materials.

Visual inspection - key principles

Introduction

The methodology adopted for visual inspection is very much product-dependent.In many cases, visual inspection will be required before, during and after welding.This would be appropriate if the integrity of the product demanded that inspectionis applied throughout the entire manufacturing process. It could be that theproduct is regarded as safety-critical and therefore visual inspection is a keyactivity in ensuring that the required overall quality and integrity are met.


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Products with less demanding service requirements may only need visualinspection when welding is completed. In these cases, reliance is placed on theproduction team to apply good working practices throughout the manufacturingprocess. This approach has become more common in recent years, partly due tothe fact that inspectors are often seen as a 'roving police force'. This perceptionpromotes a negative attitude in the workplace with production and inspectionfunctions regularly in conflict and the implication that production decreases andrepairs to welds become more common.

The general procedure for carrying out visual inspection is in 4 basic parts -

1. Fact-finding Establish as many facts about the product and welding criteria as possible.Documentation pertinent to the product is usually a important source of

information and should always be carefully reviewed.

Review relevant documentation . . . Documents which relate to a particular component under construction canprovide useful facts on the characteristics of the product and help with theinspection process. For example -

• If the inspector checks the welding procedure specification beforeconstruction, it should be possible to establish the parent material'sweldability. This will tell the inspector that preheating the component iscritical or that checking of amperage, voltage and travel-speed isnecessary to ensure welding quality requirements are met.

• In addition, the welding procedure specification should state the joint type,welding process and other essential variables. All this information allowsthe inspector to think about the types of imperfection that are likely tooccur during manufacture and greatly assists the visual inspection process.


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• Also, a review of the material's 'mill sheet' could provide evidence of thecrack sensitivity of a particular material type, so that adequate precautionscan be taken to avoid cracking.

• A review of the quality plan can provide the necessary information on how,when and where visual inspection is required.

• The inspector may see from Welder Approval Certificates that a certainwelder is not approved on the type of work to be undertaken.

These are all examples of the type of information that the product documentationcan provide to help prevent problems before welding starts.

. . . and provide documentation

Documents play an important role in achieving overall integrity of the product.The inspector has to ensure that appropriate documentation is available on the

job where necessary and that all relevant documents are collected and collatedafter the manufacturing process is complete.

To help fabricators manage their welding information more efficiently, TWI hasproduced Welding Co-ordinator This is a software package which computerisespaperwork related to welding procedures, welder approval records, NDE reportsand weld datasheets.

2. Analysis of the facts

Analyse details in relation to -

• service performance of the product• drawings• code of practice• QA/QC requirements• other relevant information

3. Planning Plan the inspection sequence, identifying the critical areas and 'hold-points'related to the product (e.g. inspector signs-off acceptance of the joint fit-up

details BEFORE production welding is allowed to continue).

4. Implementation Implementation is the most difficult part, often due to the fact that the inspectionteam cannot check every inspection detail all of the time. Some reliance must,therefore, be placed on the production personnel and production supervision.


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Key questions Here are some important questions which should always be answered beforecarrying out visual inspection:

What is the nature of the product? If the product is regarded as 'safety-critical' it will be necessary to perform certaintasks at certain stages.

What operating conditions will be present? If the product is operating at low temperature, a risk of brittle fracture may exist.Therefore, some types of weld defect may increase the likelihood of failure.Alternatively, if the product is under dynamic loading, it may be important toassess the weld toe blends more critically.

What is the required quality of welding? This is an essential requirement if efficient and effective visual inspection is to beperformed. What is - and what is not - an acceptable imperfection needs to beknown in advance of the inspection.

Is there a code that relates to the product, or a standard, against which the weldsmay be inspected? The inspector must have a full understanding of the code of practice being used,and fully appreciate the technical details specified.

Is visual inspection required before during and after welding?

In certain situations, it may be only necessary to carry out inspection afterwelding. This however, relies on the skill and knowledge of the weldingsupervisor or sometimes the welders involved and their ability to follow writtenprocedures.

By finding answers to these questions, and possibly others, the Inspector willhave a good idea of the requirements and be better placed to determine whichpoints of inspection require the most emphasis.

In summary, the inspector must find out as much as possible about -

• the product• materials, welding process and consumable being used• operating conditions of the product• allowable defects (reference to the applicable code or standard)• types of joint being welded


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The product

Materials, welding process and

consumable being used


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Operating conditions of the product

Allowable defects (reference to the

applicable code or standard)


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Types of joint being welded


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The welding inspector

The diagram below gives a profile of a welding inspector. It outlinesresponsibilities and duties and the general sequences of inspection. It alsoindicates some of the important personal attributes of an effective inspector.

ChecklistAlthough the checklist below is not exhaustive, it provides a useful starting pointand covers inspection before, during and after welding.


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Before welding

a) Documentation

• code of practice• quality plan• welding procedure approvals (to code)• welder approvals (to code)• weld map or drawing, i.e. where the welds are, sizes etc, weld

identification• inspection procedures• NDE procedures• pre/post weld heat treatment procedures - methods, temperatures, times,

etc

b) Materials

• consumableso correct specificationo correct storage and bakingo controlled issue and returno batch identification

• parent materialo correct specificationo identification - transference of cast marks

• forming activity procedureso rolling, pressing

c) Weld preparations

• correct for the job - check against documentation• machined or flame cut - note any grinding requirements• sheared - thin sheet - work-hardened - any further preparation?• accurate and within tolerance• forming activity procedures


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d) Fit-up for welding

• accuracy of fit-up - misalignment• preheat and its maintenance/monitoring• tack welding - incorporated in weld?• safety• manipulators

e) Welding equipment

• maintenance and calibration• condition - in relation to safety and performance• correct consumables (wire, flux, gas) - check against welding procedure• flux re-circulation system (SAW)

During welding

a) correct parameters and monitoring equipmentb) welding procedure specification (WPS) at the work-placec) inter-run cleanlinessd) inter-run qualitye) inter-pass temperaturef) consumable controlg) maintenance of preheat?h) process control NDE regime

i) welding records and maintenance of weld maps

After welding

a) visual inspection and records

b) NDE

• working to procedure• calibrated equipment• records

c) PWHT (if required)

• furnace or local (temp gradients)• compliance with code• recording of times and temperatures• thermocouple positions


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d) Final testing (if applicable)

• hydraulic test procedure• leak test procedure• proof/load test procedure• functional test procedure• NDE

e) Cleaning, painting, preservation, packing

f) Final documentation package

• WPS, welding procedure• approval records• welder qualification records• consumables certificates• materials certificates• inspection reports• NDE reports/test reports• heat treatment records• welding records• certificate of compliance• authorised signatures

The above activities are regarded as 'surveillance inspection', as opposed to thedetails of visual inspection. However, it should be noted that visual inspectionplays an important role throughout the manufacturing process.


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Weldability of materials

SteelsIn arc welding, as the weld metal needs mechanical properties to match theparent metal, the welder must avoid forming defects in the weld. Imperfectionsare principally caused by:

• poor welder technique;• insufficient measures to accommodate the

material or welding process;• high stress in the component.

Techniques to avoid imperfections such as lack offusion and slag inclusions, which result from poorwelder techniques, are relatively well known. However,the welder should be aware that the material itselfmay be susceptible to formation of imperfectionscaused by the welding process. In the materialssection of the Job Knowledge for Welders, guidelines are given on materialweldability and precautions to be taken to avoid defects.

Material types

In terms of weldability, commonly used materials can be divided into the followingtypes:

• Steels• Stainless steels• Aluminium and its alloys• Nickel and its alloys• Copper and its alloys• Titanium and its alloys• Cast iron

Fusion welding processes can be used to weld most alloys of these materials, ina wide range of thickness. When imperfections are formed, they will be located ineither the weld metal or the parent material immediately adjacent to the weld,called the heat affected zone (HAZ). As chemical composition of the weld metaldetermines the risk of imperfections, the choice of filler metal may be crucial notonly in achieving adequate mechanical properties and corrosion resistance butalso in producing a sound weld. However, HAZ imperfections are caused by theadverse effect of the heat generated during welding and can only be avoided bystrict adherence to the welding procedure.


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This part of the materials section of Job Knowledge for Welders considers theweldability of carbon-manganese (C-Mn) steels and low alloy steels.

Imperfections in welds

Commonly used steels are considered to be readily welded. However, thesematerials can be at risk from the following types of imperfection:

• porosity;• solidification cracking;• hydrogen cracking;• reheat cracking.

Other fabrication imperfections are lamellar tearing and liquation cracking butusing modern steels and consumables, these types of defects are less likely toarise.

In discussing the main causes of imperfections, guidance is given on procedureand welder techniques for reducing the risk in arc welding.

Porosity

Porosity is formed by entrapment of discrete pockets of gas in the solidifyingweld pool. The gas may originate from poor gas shielding, surface contaminants

such as rust or grease, or insufficient deoxidants in the parent metal (autogenousweld), electrode or filler wire. A particularly severe form of porosity is 'wormholes',caused by gross surface contamination or welding with damp electrodes.

The presence of manganese and silicon in the parent metal, electrode and fillerwire is beneficial as they act as deoxidants combining with entrapped air in theweld pool to form slag. Rimming steels with a high oxygen content, can only bewelded satisfactorily with a consumable which adds aluminium to the weld pool.

To obtain sound porosity-free welds, the joint area should be cleaned anddegreased before welding. Primer coatings should be removed unlessconsidered suitable for welding by that particular process and procedure. Whenusing gas shielded processes, the material surface demands more rigorouscleaning, such as by degreasing, grinding or machining, followed by finaldegreasing, and the arc must be protected from draughts.


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Solidification cracking

Solidification cracks occur longitudinally as a result of the weld bead havinginsufficient strength to withstand the contraction stresses within the weld metal.Sulphur, phosphorus, and carbon pick up from the parent metal at high dilutionincrease the risk of weld metal (solidification) cracking especially in thick sectionand highly restrained joints. When welding high carbon and sulphur contentsteels, thin weld beads will be more susceptible to solidification cracking.However, a weld with a large depth to width ratio can also be susceptible. In thiscase, the centre of the weld, the last part to solidify, will have a highconcentration of impurities increasing the risk of cracking.

Solidification cracking is best avoided by careful attention to the choice ofconsumable, welding parameters and welder technique. To minimise the risk,consumables with low carbon and impurity levels and relatively high manganeseand silicon contents are preferred. High current density processes such assubmerged-arc and CO2, are more likely to induce cracking. The weldingparameters must produce an adequate depth to width ratio in butt welds, orthroat thickness in fillet welds. High welding speeds also increase the risk as theamount of segregation and weld stresses will increase. The welder shouldensure that there is a good joint fit-up so as to avoid bridging wide gaps. Surfacecontaminants, such as cutting oils, should be removed before welding.


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Hydrogen cracking

A characteristic feature of high carbon and low alloy steels is that the HAZimmediately adjacent to the weld hardens on welding with an attendant risk ofcold (hydrogen) cracking. Although the risk of cracking is determined by the levelof hydrogen produced by the welding process, susceptibility will also dependupon several contributory factors:

• material composition (carbon equivalent);• section thickness;• arc energy (heat) input;• degree of restraint.

The amount of hydrogen generated is determined by the electrode type and theprocess. Basic electrodes generate less hydrogen than rutile electrodes (MMA)and the gas shielded processes (MIG and TIG) produce only a small amount ofhydrogen in the weld pool. Steel composition and cooling rate determines theHAZ hardness. Chemical composition determines material hardenability, and thehigher the carbon and alloy content of the material, the greater the HAZ hardness.Section thickness and arc energy influences the cooling rate and hence, thehardness of the HAZ.

For a given situation therefore, material composition, thickness, joint type,electrode composition and arc energy input, HAZ cracking is prevented byheating the material. Using preheat which reduces the cooling rate, promotesescape of hydrogen and reduces HAZ hardness so preventing a crack-sensitivestructure being formed; the recommended levels of preheat for various practicalsituations are detailed in the appropriate standards e.g. BS 5135:1984. Ascracking only occurs at temperatures slightly above ambient, maintaining thetemperature of the weld area above the recommended level during fabrication isespecially important. If the material is allowed to cool too quickly, cracking canoccur up to several hours after welding, often termed 'delayed hydrogen cracking'.After welding, therefore, it is beneficial to maintain the heating for a given period(hold time), depending on the steel thickness, to enable the hydrogen to diffusefrom the weld area.

When welding C-Mn structural and pressure vessel steels, the measures whichare taken to prevent HAZ cracking will also be adequate to avoid hydrogencracking in the weld metal. However, with increasing alloying of the weld metale.g. when welding alloyed or quenched and tempered steels, more stringentprecautions may be necessary.

The risk of HAZ cracking is reduced by using a low hydrogen process, lowhydrogen electrodes and high arc energy, and by reducing the level of restraint.Practical precautions to avoid hydrogen cracking include drying the electrodesand cleaning the joint faces. When using a gas shielded process, a significant


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amount of hydrogen can be generated from contaminants on the surface of thecomponents and filler wire so preheat and arc energy requirements should bemaintained even for tack welds.

Reheat cracking

Reheat or stress relaxation cracking may occur in the HAZ of thick sectioncomponents, usually of greater than 50mm thickness, Fig. 4. The more likelycause of cracking is embrittlement of the HAZ during high temperature service orstress relief heat treatment.

As a coarse grained HAZ is more susceptible to cracking, low arc energy inputwelding procedures reduce the risk. Although reheat cracking occurs in sensitivematerials, avoidance of high stresses during welding and elimination of localpoints of stress concentration, e.g. by dressing the weld toes, can reduce the risk.

Weldability of steel groups

European Standard EN 287 identifies a number of steels groups which havesimilar metallurgical and welding characteristics. The main risks in welding thesegroups are:

Group W 01 low carbon unalloyed (carbon-manganese) steels and/or lowalloyed steels

For thin section, unalloyed materials, these materials are normally readilyweldable. However, when welding thicker section material with a flux process(MMA), there is a risk of HAZ cracking which will needs low hydrogen electrodes.The more highly alloyed materials also require preheat, or a low hydrogenwelding process, to avoid HAZ cracking .

Group W 02 chromium-molybdenum (CrMo) and/or chromium-molybdenum-vanadium (CrMoV) creep resisting steel

Thin section material may be welded without preheat but using a gas shielded

process (TIG and MIG); for thicker section material, and when using a fluxprocess, preheat with low hydrogen electrodes (MMA) is needed to avoid HAZand weld metal cracking. Post-weld heat treatment is used to improve HAZtoughness.

Group W 03 fine-grained structural steels and nickel steels (2% to 5%)

The weldability is similar to Group W 02 in that preheat is required for weldingthick section material with flux processes.


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Group W 04 ferritic or martensitic stainless steel, with chromium (12% to20%)

When using filler to produce matching weld metal strength, preheat is needed toavoid HAZ cracking. Post-weld heat treatment is essential to restore HAZtoughness.

An austenitic stainless steel filler can be used where it is not possible to apply apreheat and post-weld treatment.

Distortion - Prevention by fabricationtechniques

Distortion caused by welding a plate at the centre of a thin plate beforewelding into a bridge girder section. Courtesy John Allen

Assembly techniques

In general, the welder has little influence on the choice of

welding procedure but assembly techniques can often becrucial in minimising distortion. The principal assemblytechniques are:

• tack welding• back-to-back assembly• stiffening

Tack welding

Tack welds are ideal for setting and maintaining the joint gap but can also be

used to resist transverse shrinkage. To be effective, thought should be given tothe number of tack welds, their length and the distance between them. With toofew, there is the risk of the joint progressively closing up as welding proceeds. Ina long seam, using MMA or MIG, the joint edges may even overlap. It should benoted that when using the submerged arc process, the joint might open up if notadequately tacked.


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The tack welding sequence is important to maintain a uniform root gap along thelength of the joint. Three alternative tack welding sequences are shown in Fig 1:

• tack weld straight through to the end of the joint (Fig 1a). It is necessary toclamp the plates or to use wedges to maintain the joint gap during tacking

• tack weld one end and then use a back stepping technique for tacking therest of the joint (Fig 1b)

• tack weld the centre and complete the tack welding by back stepping (Fig1c).

Fig. 1 Alternative procedures used for tack welding to prevent transverseshrinkagea) tack weld straight through to end of jointb) tack weld one end, then use back-step technique for tacking the rest ofthe joint

c) tack weld the centre, then complete the tack welding by the back-steptechnique

Directional tacking is a useful technique for controlling the joint gap, for exampleclosing a joint gap which is (or has become) too wide.

When tack welding, it is important that tacks which are to be fused into the mainweld, are produced to an approved procedure using appropriately qualified

welders. The procedure may require preheat and an approved consumable asspecified for the main weld. Removal of the tacks also needs careful control toavoid causing defects in the component surface.


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Back-to-back assembly

By tack welding or clamping two identicalcomponents back-to-back, welding of bothcomponents can be balanced around the neutralaxis of the combined assembly (Fig 2a). It isrecommended that the assembly is stress relievedbefore separating the components. If stressrelieving is not done, it may be necessary to insertwedges between the components (Fig 2b) so whenthe wedges are removed, the parts will move backto the correct shape or alignment.

Fig. 2 Back-to-back assembly to control distortion when welding two identical

components

a) assemblies tacked together before welding

b) use of wedges for components that distort on separation after welding

Stiffening

Fig. 3 Longitudinal stiffeners prevent bowing in butt welded thin plate joints

Longitudinal shrinkage in butt welded seamsoften results in bowing, especially whenfabricating thin plate structures. Longitudinalstiffeners in the form of flats or angles,welded along each side of the seam (Fig 3)are effective in preventing longitudinal bowing.Stiffener location is important: they must beplaced at a sufficient distance from the jointso they do not interfere with welding, unlesslocated on the reverse side of a joint weldedfrom one side.


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Welding procedure

A suitable welding procedure is usually determined by productivity and qualityrequirements rather than the need to control distortion. Nevertheless, the weldingprocess, technique and sequence do influence the distortion level.

Welding process

General rules for selecting a welding process to prevent angular distortion are:

• deposit the weld metal as quickly as possible• use the least number of runs to fill the joint

Unfortunately, selecting a suitable welding process based on these rules mayincrease longitudinal shrinkage resulting in bowing and buckling.

In manual welding, MIG, a high deposition rate process, is preferred to MMA.Weld metal should be deposited using the largest diameter electrode (MMA), orthe highest current level (MIG), without causing lack-of-fusion imperfections. Asheating is much slower and more diffuse, gas welding normally produces moreangular distortion than the arc processes.

Mechanised techniques combining high deposition rates and high weldingspeeds have the greatest potential for preventing distortion. As the distortion is

more consistent, simple techniques such as presetting are more effective incontrolling angular distortion.

Welding technique

General rules for preventing distortion are:

• keep the weld (fillet) to the minimum specified size• use balanced welding about the neutral axis• keep the time between runs to a

minimum


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Fig. 4 Angular distortion of the joint as determined by the number of runsin the fillet weld

In the absence of restraint, angular distortion in both fillet and butt joints will be afunction of the joint geometry, weld size and the number of runs for a given crosssection. Angular distortion (measured in degrees) as a function of the number ofruns for a 10mm leg length fillet weld is shown in Fig 4.

If possible, balanced welding around the neutral axis should be done, forexample on double sided fillet joints, by two people welding simultaneously. Inbutt joints, the run order may be crucial in that balanced welding can be used to

correct angular distortion as it develops.

Fig. 5 Use of welding direction to control distortiona) Back-step weldingb) Skip welding

Welding sequence

The sequence, or direction, of welding isimportant and should be towards the free end ofthe joint. For long welds, the whole of the weldis not completed in one direction. Short runs, forexample using the back-step or skip weldingtechnique, are very effective in distortion control(Fig 5).

• Back-step welding involves depositingshort adjacent weld lengths in theopposite direction to the general progression (Fig.5a).

• Skip welding is laying short weld lengths in a predetermined, evenlyspaced, sequence along the seam (Fig 5b). Weld lengths and the spacesbetween them are generally equal to the natural run-out length of oneelectrode. The direction of deposit for each electrode is the same, but it isnot necessary for the welding direction to be opposite to the direction ofgeneral progression.


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Best practiceThe following fabrication techniques are used to control distortion:

• using tack welds to set up and maintain the joint gap• identical components welded back to back so welding can be balanced

about the neutral axis• attachment of longitudinal stiffeners to prevent longitudinal bowing in butt

welds of thin plate structures• where there is choice of welding procedure, process and technique should

aim to deposit the weld metal as quickly as possible; MIG in preference to

MMA or gas welding and mechanised rather than manual welding• in long runs, the whole weld should not be completed in one direction;

back-step or skip welding techniques should be used.


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Fillet welded joints - a review of the

practicalities

Fillet welded joints such as tee, lap and corner joints are the most common connection inwelded 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 tofillet 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

Fillet weld features

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

a = throat thicknessz = leg length

s = deep penetration throat thicknessl = length of intermittent fillet


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Fig.2 Fig.3

Fillet weld shapes

Over specified fillet welds or oversized fillet welds

Fig.4

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).


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

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

Fig.6

Once the drawing has been issued to the shop floor, it is usual to find an additional safety

factor also being 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 over80%.

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 actualthroat 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.


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Lap joints welded with fillet welds.

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

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

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

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

thickness of the component.


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To compensate for this reduction in throat thickness it may be necessary to specify a deeppenetration fillet 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 anexcessively convex weldface and consequential sharp notches at the weld toe (Fig.11).

Fig.10 Fig.11

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 greaterstress concentration.

Poor fit-up can also reduce the throat thickness as in Fig.12. The corner of the verticalcomponent has been bevelled in the sketch in an exaggerated manner to illustrate the

point.

Fig.12

Summary

Fillet 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 ahigher 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 visualexamination and other NDT techniques.


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Fillet welded joints are not always open to NDT or are indeed time consuming to manynon-destructively testing techniques such as radiography or ultrasonic testing and the

results are often difficult to interpret. Inspection methods such as visual inspection,

magnetic particle inspection and penetrant inspection are surface examination techniquesonly and with visual inspection, much of the effort is expended in measuring the size ofthe 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 shapewhich can adversely influence their service performance.

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.


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Visual inspection of arc welds - aguide to best practice

Section 4. Imperfections - identification andinterpretation

• Detectable imperfections• Practical guidance on interpretation • Imperfections associated with particular welding processes

Detectable imperfections

Introduction

It is important to keep in mind that visual assessment of a weld is, in manycircumstances, one small part of the total inspection process.

On many products, non-destructive examination (NDE) is applied, probablyconfirming 'visual uncertainties'. Also, NDE may frequently be employed to locatedefects which cannot be seen with the naked eye (although in some instanceslow-power magnification may be used).

If NDE is not being employed, it may be necessary to carry out more extensivevisual inspection which may involve longer inspection times. However, visualinspection normally requires assessment of the consistency of weld features (i.e.weld width, height and shape along entire length of weld) and of the surfacedefects present.

Many codes and standards use the term 'defect' but this word implies that theweld is substandard and therefore unacceptable. A more appropriate term is'imperfection'; this is because many weld 'defects' may prove to be adequate forspecific acceptance levels.

Defects which can be detected by visual inspection can be grouped under fiveheadings:


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• root defects• contour defects• surface irregularities• surface cracks• miscellaneous

1. Root defects

2. Contour defects

3. Surface irregularities

4. Surface cracks

5. Miscellaneous

Standard terminology for imperfections

Terminology relating to imperfections in fusion welds has been standardised inaccordance with accepted practice.


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Group 1 - Root defects

Incomplete root penetration Failure of weld metal to extend into the root of a joint

Lack of root fusion Lack of union at the root of a joint

Excess penetration bead Excess weld metal protruding through the root of a fusion

weld made from one side only

Root concavity (suck-back; underwashing - non-standard terms)

A shallow groove which may occur in the root of a buttweld, but full fusion is evident

Shrinkage groove A shallow groove caused by contraction in the metal along

each side of a penetration bead or along the weld centreline

Burnthrough (melt through)

A localised collapse of the molten pool due to excessivepenetration, resulting in a hole in the weld run

Group 2 - Contour defects


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Incompletely filled groove A continuous or intermittent channel in the surface of a weld,

running along its length, due to insufficient weld metal. Thechannel may be along the centre or along one or both edges

of the weld

Bulbous contour A non-standard term used to describe poor

appearance

Unequal legs (non standard term)

Variation of leg length on a fillet weld

Note: Unequal leg lengths may be specified aspart of the design - in which case they are not

imperfections

Group 3 - Surface irregularities

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

previously deposited weld metal

The inspector must determine if the undercut is continuous or

intermittent, or sharp or smooth


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Overlap An imperfection at the toe or root of a weld caused by metal

flowing on to the surface of the parent metal without fusing toit

Gas pore A cavity, generally under 1.5mm in diameter, formedby trapped gas during the solidification of molten

metal

Porosity A group of gas pores

Crater pipe A depression due to shrinkage at the end of a run wherethe source of heat was removed. Crater pipes may also

lead to micro-cracking


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Group 4 - Surface cracks

Crack A linear discontinuity produced by fracture

Cracks may be ...

a) ... longitudinal, in the weld metal, i.e. centreline

b) ... longitudinal, in the parent metal or heat affected

zone

c) ... transverse

d) Crater crack (star cracking)

Group 5 - Miscellaneous

Stray flash/arc burn/arc strike (stray arcing)

1. The damage on the parent material resulting from theaccidental striking of an arc away from the weld

2. The accidental striking of an arc away from the weld

Note that the same term is used for both the action and

the result


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Spatter

Globules of metal

expelled during welding

on to the surface of

parent metal or of aweld

Practical guidance on interpretation

A wide variety of imperfections can be examined and the following additionalpoints should be considered during visual inspection.

Sharp imperfections

The inspector must bear in mind that the most serious imperfections tend to bethose which are sharp. It is most important, therefore, that the inspector is able to

make a clear judgement and differentiate between sharp and smoothimperfections.

Example -

Consider sharp undercut, 0.2mm deep, compared with smooth undercut, 1.0mmdeep. It will be assumed that the acceptance levels specify an allowance of1.0mm depth of undercut.

Although the smooth undercut is on the limit of acceptance, the physicallysmooth transition of the defect means that any transverse stresses applied willtend to flow round the imperfection when the weld or product is under serviceloading.


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The depth of the sharp undercut is only 0.2mm, well under the specifiedallowance in relation to depth. However, it is likely that stresses will beconcentrated at the sharp notch where a crack might develop in service.Undetected, this crack could lead to catastrophic failure of the product.

Many sharp imperfections are often referred to as 'planar' (two-dimensional)imperfections. These generally include:

• cracks and tears

Cracking (photo

shows hydrogen-

induced cracking

in the heat

affected zone)

Lamellar tearing


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• incomplete root penetration

Weld metal has not

penetrated root gap

•

• incomplete fusion (root, inter-run and sidewall)

Lack of root

fusion

It must be appreciated that many imperfections observed also indicate thepossible presence of sub-surface defects. Non-destructive examination methods,such as radiography and ultrasonic testing, must be considered in manysituations along with visual inspection.


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Lack of sidewall fusion

Many codes will not allow ANY cracks and will restrict acceptance of other typesof planar imperfection specified in the acceptance levels to within fairly closelimits. Some acceptance levels applied to higher integrity products will not allowany such types of imperfection.

Workmanship imperfections

Experience has shown that workmanship-type imperfections - such as poorstop/starts, arc strikes and tool marks - are often not found or recorded duringvisual assessment of welded structures. In some cases, these imperfections can

lead to more serious problems developing - even catastrophic failure of theproduct due to cracks (perhaps started by an isolated arc strike).

Stray arcing or arc strikes


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There are other potential problems such as incomplete fusion and crater pipescaused by poor stop/start technique. Excessively coarse grinding marks, whilstpotentially ignored by the inspector, may set up small stress concentrationswhich could lead to failure of the product in service.

Crater pipe

Other workmanship features

We can still use the motor car analogy for visually examining a completedweldment - possibly even more so because when you assess the car there areseveral areas you would examine visually before driving the vehicle away.

The bodywork of a motor car is in many ways similar to a weldment in that it willneed to have certain visual characteristics such as the required contour (designshape).


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Shape imperfections

Overlap and sharp toe blend Other sharp imperfections include shape-type flaws such as overlap and sharptoe blend caused by excessive weld metal height on the weld face.

Overlap

Excess weld metal

Unfortunately, acceptance levels rarely provide guidance on the philosophy of

sharp versus smooth imperfections.


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Root penetration bead Excess root penetration and root concavity are other examples of shape-typeimperfections and their degree of 'sharpness' should be assessed during visualinspection.

Excess penetration bead

Root concavity


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Alignment In relation to other workmanship details which relate to construction, alignment ofcomponents and the manner in which they fit together is extremely important.Many specifications will set close limits and tolerances on both linear and angularalignment.

Dimensional checks Dimensional requirements are, of course, essential and the engineering drawingwill provide appropriate dimensions and tolerances. It may also include weldsymbols as a way of specifying the welding requirements. However, weldsymbols are not part of this best practice guide. Further details can be obtainedfrom ISO2553 or BS EN22553.

Corrosion You would certainly assess a second hand car for corrosion or rusting and this

could be sufficient reason for rejection. A similar principle can be applied towelded structures, especially if visual inspection is being employed in serviceduring the life of the component.

Colour Choice of colour may be one of the most influential factors in the purchase of amotor car. A weldment's colour is equally important as it will provide backgroundon how efficient the gas shielding was during the welding operation (TIG andMIG/MAG processes). This can be the main criterion when visually inspectingweldments in reactive materials such as titanium.

Knocks and dents This type of imperfection would certainly be assessed when visually examining amotor car and could be the reason for rejecting it. A welded structure may alsosuffer knocks and dents in the form of transportation damage, tool marks fromchipping hammers and chisels or excessive grinding marks formed during theremoval of surface contaminants.

Other points on workmanship can be found in Section 2 of this best practiceguide under Terminology and definitions .


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Inclusions

Solid inclusions Solid inclusions such as slag (from manual metal arc welding, flux cored arcwelding or submerged arc welding) are interpreted by many industrial sectors asbeing associated with lack of fusion imperfections. Furthermore, silica inclusionsassociated with the MAG welding of steel are commonly found to be the maincause of lack of fusion imperfections associated with the process.

Visual inspection cannot find solid inclusions which are entirely sub-surface -hence the need for non-destructive examination techniques such as radiographyor ultrasonic testing. On the other hand, if the inspector engaged in visualinspection were to check the cleanliness of each weld pass during welding, theproblem could, to some extent, be eliminated.

Slag inclusion

Gas inclusions Porosity and gas pores are difficult imperfections to assess by visual examinationalone. This is due to the fact that they are not always surface-breaking and when

they can be seen, the full extent of the problem is unknown until radiography orultrasonic inspection has been carried out. Another problem is that porosity mayhide other, more serious imperfections which cannot be found by any commonlyused non-destructive examination methods.


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Surface-breaking porosity

Imperfections associated with particular welding processes

Introduction

An inspector with a sound knowledge of the welding process to be used canprovide valuable information on avoiding typical defects associated with it.Process know-how will also allow the inspector to focus on particular areas afterwelding has been completed to pinpoint specific faults which could influence theacceptability of welds and the product overall.

To illustrate this point, the TIG process can give very good weld pool control -

assuming appropriate welder skill. This means that the occurrence of shape-typedefects, such as undercut or overlap, is unlikely compared with, for example,manual metal arc welding. However, tungsten inclusions and porosity are likelywith the TIG process when compared with MIG/MAG welding to which lack offusion problems have always been associated, especially when welding thick-section steel.

The manual metal arc and submerged arc processes both utilise a flux. Slag isproduced during welding which can create slag inclusions which are, therefore,associated with these process types. Also, submerged arc welding has a deeppenetration characteristic, so an experienced inspector is likely to examine the

completed weld with centreline cracking in mind because it is known that deep,narrow welds are susceptible to this type of cracking.

It may also be necessary to carry out visual inspection during the weldingoperation to ensure that specific welding parameters are met or that the numberor sequence of weld runs is in accordance with the welding procedurerequirements. In practice, many factors can apply, depending on the type ofproduct and its expected service performance. It is essential for those involved tohold a qualification in visual inspection or welding inspection such as CSWIPbefore embarking on more specialised work.


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A brief outline of the common welding processes found in industry and theirrelated, typical weld defects is given below -

TIG welding

Also known as: Tungsten Inert Gas Welding; Gas Tungsten Arc Welding (GTA orGTAW)

Process overview

Type of operation Usually manual, but can be mechanised.

Mode of operation An arc is maintained between the end of a tungsten electrode and thework. The electrode is not consumed and the current is controlled by thepower source setting. The operator must control the arc length and alsoadd filler metal if needed to obtain the correct weld. A high degree of skillis needed for best results. The arc is unstable at low currents. Specialprovision is made for starting (h.f. or surge injection) and for welding thinmaterials (pulsed TIG). In all cases the electrode and weld pool areshielded by a stream of inert gas. Filler rod is fed into the weld pool insome cases.


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Typical defects associated with process

• tungsten inclusions• lack of fusion• incomplete penetration• root concavity• undercut• porosity• burnthrough• excess penetration• oxide inclusions• unequal leg length

MMA welding

Also known as Manual Metal Arc Weldng (MMA); Shielded Metal Arc Welding(SMA or SMAW); Stick Welding

Process overview

Type of operation Manual.

Mode of operation

Arc melts parent plate and electrode to form a weld pool which isprotected by flux cover. Operator adjusts electrode feed rate, i.e. by handmovement, to keep arc length constant. Slag must be removed afterdepositing each bead. Normally a small degree of penetration, requiringplate edge preparation; but welds in thick plate or large fillets aredeposited in a number of passes. The process can also be used to depositmetal to form a surface with alternative properties.


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• overlap• porosity• slag inclusions• excessive spatter• stray flash• incomplete penetration• excess penetration• undercut• crater cracks• lack of fusion

MIG welding

Also known as: Metal Inert Gas Welding (MIG); Metal Active Gas Welding (MAG);Gas Metal Arc Welding (GMA or GMAW)

Process overview

Type of operation

Manual, mechanised or automatic.

Mode of operation An arc is maintained between the end of the bare wire electrode and thework. The wire is fed at a constant speed, selected to give the requiredcurrent, and the arc length is controlled by the power source. The operatoris not therefore concerned with controlling the arc length and canconcentrate on depositing the weld metal in the correct manner.


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The process can be operated at high currents (250-500A) when metaltransfer is in the form of a 'spray', but, except for aluminium, this techniqueis confined to welding in the flat and horizontal positions. For vertical andoverhead welding, special low-current techniques must be used, i.e. 'dip'transfer or pulsed arc. The arc and weld pool are shielded by a stream ofgas. The electrode can be solid or flux cored.


• incomplete penetration• excessive penetration• undercut• excessive spatter• cracking• porosity• lack of fusion• stray flash

Submerged Arc Welding

Also known as: SAW


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Process overview

Type of operation Mechanised, automatic or semi-automatic.

Mode of operation An arc is maintained between the end of a bare wire electrode and thework. As the electrode is melted, it is fed into the arc by a set of rolls,driven by a governed motor. Wire feed speed is automatically controlled toequal the rate at which the electrode is melted, thus arc length is constant.The arc operates under a layer of granular flux (hence 'submerged' arc).

Some of the flux melts to provide a protective blanket over the weld pool;the remainder of the flux is unaffected and can be recovered and re-usedprovided that it is dry and not contaminated.

A semi-automatic version is available in which the operator has control ofa welding gun which carries a small quantity of flux in a hopper.


• porosity• cracking• slag inclusions• incomplete penetration• excessive penetration• weld profile defects• undercut• lack of fusion•


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Oxy-acetylene Welding

Also known as: Oxy Fuel Gas Welding; Gas Welding

Process overview

Type of operation Manual.

Mode of operation A fuel gas (usually acetylene) and an oxidant gas (oxygen) are mixed andburnt. The operator must manipulate the blowpipe to give the correct weld

pool size, and also add filler metal as required. The melting is slowcompared with arc processes, limiting the speed of work. The weld pool isshielded from atmospheric contamination by the burnt gas mixture whichcan be made mildly oxidising or reducing. Requires a very high level ofoperator skill.


• unequal leg length fillet• butt weld profile too concave• butt weld profile too convex• undesirable weld profile (lap, fillet)• excessive penetration• excessive fusion of root edges• burnthrough• undercut along vertical member of fillet welded T joint• root run too large with undercut in butt joint• undercut both sides of weld face in butt joint• oxidised weld face• overheated weld• incomplete root penetration in butt joints (single V or double V)• incomplete root penetration in close square T joint


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Conclusion

Visual examination of welds is a highly skilled activity. When correctly appliedbefore, during and after welding, as both a quality control activity and a NDEtechnique, many problems associated with welding can be prevented. In contrast,non-destructive examination such as magnetic particle or penetrant inspection,radiography or ultrasonic testing may reveal imperfections AFTER welding hasbeen completed, necessitating repair or a concession which invariably increasesthe overall cost of the product and delays completion and subsequent shipment.

Formal training and qualifications in a broad range of welding technologiesgreatly contributes towards providing the skills and knowledge required to carryout effective visual inspection. This must be balanced with adequate on-the-jobexperience. Furthermore, practical experience, knowledge and correctinterpretation of appropriate codes or specifications is essential to ensureaccurate and efficient visual inspection of welded products.