interpretation of weld radiographs | the ndt guy

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Interpretation of Weld Radiographs | The NDT Guy

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    Interpretation of Weld Radiographsby admin

    1. Introduction

    Assessment and interpretation of radiographic images is widely used in industry for the qualitycontrol of weldments and castings. The requirements for satisfactory interpretation are that the interpreter must have adequate eyesight,whether corrected or uncorrected, and have the ability to recognise features in the image caused byvarious conditions. The standards usually quoted for eyesight require that personnel are able to reada minimum of the J2 level on the Jaeger eyesight chart with the chart at positioned a distance of 30.5centimetres. Ability to recognise the features on a radiograph comes largely with experience.

    Before viewing a radiograph the interpreter should have a basic knowledge of how the image wascreated and be aware of the radiographic technique used. The interpreter should have details of theweld configuration and should have some knowledge of the welding procedure used.Viewing of radiographs should be carried out using a film viewer in a darkened room. When enteringa darkened room from bright sunlight some time should be spent under darkroom conditions prior tocommencing interpretation in order that eyesight can adjust to the low light level. Viewer screensshould be cleaned before viewing and care must be taken to avoid marking or damaging the film. The area where films are viewed should be clean, work surfaces dry and the films handled by theedges to prevent fingerprints and damage to the film surfaces. Soft cotton gloves are often used byinterpreters to limit the possibility of film damage.

    Each radiograph is masked on the viewer so that stray light from around the film does not blind theinterpreter. The film viewer can be activated by a foot switch when the film to be examined is inposition. A dim side light can be used in order that notes can be made during the work.

    2. Film quality

    Radiographs should be reviewed for film quality prior to interpreting the image for possible defects. Radiographs should be checked for identification, density and sensitivity and also for the presence ofartefacts that may interfere with the assessment. Where film quality is unacceptable the area of weldcovered by the film should be re-radiographed.

    2.1 Identification

    Manufacturers may have a method of radiographic identification which is linked to a quality systembut the following is a guide to the normal requirements for details appearing on the radiograph.

    The identification should include the manufacturers symbol, the component/item/weld number asappropriate, the location within the weld (such as location markers 1 to 2, B to C etc) and the dateradiography was carried out.

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  • The identification details usually appear in the image but sometimes a system of flashing the detailson to the film before exposure is used. In all cases location markers which indicate the diagnosticlength (extent of the weld on the film to be examined) must appear as radiographic images. Therepair status of the weld should also be shown, usually by markers R1(repair), R2(second repair)etc. Identification details must not encroach on the weld area of interest the length of weld andheat affected zone between the length markers.

    2.2 Film density

    Radiographic images are viewed by transmitted light with the film placed on a light box or viewer. The blackness or density of the image can be assessed by comparison with a film strip having a rangeof density values. A more accurate method is to use an electronic device known as a filmtransmission densitometer. This device simply measures the logarithmic ratio of incident totransmitted light through the image from the viewer. Film density is therefore a number which willvary from 0 (film totally transparent) to about 5 (film virtually opaque). In general, densities above 4are only used for special applications.

    The densitometer must be regularly calibrated for accuracy throughout its range and must be set tozero on the illuminated viewer immediately before use.

    Film density influences the contrast and hence the visibility of defects on a radiograph. Film contrastis the difference in density between adjacent areas on the radiograph, the greater the densitydifference the higher the contrast. In addition, radiographic film characteristics are such that contrastincreases with film density. For this reason a minimum film density on the area being examined isrequired by most codes and standards.

    ASME V Article 2 requires a minimum of 1.8 for x-ray techniques and minimum of 2.0 for gamma raytechniques. BS/EN standards require a minimum density of 2.0 (2.3 for high sensitivity techniques)for both X and gamma rays. Other codes such as JIS will accept a minimum density of 1.5. Theseminimum figures for film density apply to the area of interest (the diagnostic length of the weld) onthe radiograph.

    2.3 Radiographic sensitivity

    The ability of a radiograph to reveal internal defects is determined by the quality or sensitivity of theimage produced. In addition it should be noted that planar weld defects such as cracks or lack ofsidewall fusion may appear faint or even be invisible if they are unfavourably orientated with thedirection of the radiation beam.

    The sensitivity of the radiograph produced is affected by many factors but basically, the higher thecontrast and definition (sharpness) of the image the more sensitive the technique will be for detectingimperfections in the object being examined.

    Image Quality Indicators (IQIs) are used in order to demonstrate that adequate radiographicsensitivity has been achieved. An image quality indicator is a device placed on the surface of thecomponent prior to radiography. The indicator provides a comparative measure of the definition andcontrast achieved on the radiograph and at least one IQI should appear on each individual radiograph.

    Two types of indicator are in common use the wire type and the plate/hole type.

    2.3.1 Wire Type IQI

    The wire type IQI consists of a thin plastic wallet containing a series of wires of progressively varyingdiameters. The wallet is placed in the area of interest with the wires positioned across the weld. Theradiographic sensitivity can be given a numerical value by dividing the diameter of the smallest wirevisible on the radiograph by the thickness of the component and expressing it as a percentage. Forexample a 0.25 mm diameter wire visible on a weld of 25 mm thick would be a sensitivity of 1%. Acceptable percentage sensitivity varies with weld thickness and to remove the need for calculationASME (American Society of Mechanical Engineers) and EN standards provide reference tables definingthe smallest wire which should be visible for acceptable sensitivity according to the componentthickness and the radiographic technique used.

    In the European standard there are nineteen wire diameters in the range from 3.2 mm to 0.05 mmcovering component thicknesses from over 380 mm down to 1.5 mm. Each IQI has seven wires andthere are four models covering the range with overlap between models. The IQI wire material isavailable in copper, steel, titanium and aluminium.

    The ASTM (American Society for Testing and Materials) wire type IQI is available in four models with

  • six wires in each model covering the range of 21 wire diameters from 0.08 mm to 8mm. There areeight different material groups, three for the light metals including aluminium and five for the heavymetals including steel.

    2.3.2 ASTM Penetrameters

    The ASTM plate/hole type IQI (known as a penetrameter) is a small piece of material radiographicallysimilar to the component and is placed beside the weld to be radiographed. Its thickness is typically2% of the component thickness and there are three holes, 1T, 2T and 4T in diameter where T is thethickness of the penetrameter. There are eight different material models covering the light and heavymetal groups.

    The ASME Code Section V Article 2 details the appropriate penetrameter selection and defines theessential hole in the penetrameter which must be visible on the radiograph to establish thatsatisfactory image quality has been achieved. For weld radiography, metal shims are used to build upthe material under the penetrameter so that it rests on a thickness equivalent to the weld and weldcap reinforcement.

    2.3.3 IQI Placement

    For meaningful results it is necessary that the indicator be placed on the surface of the componentfacing the radiation (source side) and preferably towards the edge of the field of view (the leastfavourable position).

    In certain situations, for reasons of access, it is permitted to place the IQI between the film and theobject being radiographed. This is necessary for instance when radiographing a pipe butt weld wherethere is no access to the bore or where there is gas or fluid in the pipe. With the IQI in contact withthe film a much clearer image of the IQI will be produced giving a false (favourable) indication ofsensitivity. To compensate for this, the codes or and standards define a more stringent requirementas to which IQI wire must be visible or which designation of penetrameter should be used. When filmside IQI are used a lead letter F should be placed beside the IQI to indicate the positioning.

    2.3.4 Back scatter

    Back scattered radiation from surfaces and objects behind the film during exposure can degrade theimage and reduce radiographic sensitivity. Code requirements specify that a lead letter B must beattached to the back of the film cassette before exposure. During interpretation if the interpreter candiscern a faint light image of the lead letter on the radiograph then this would indicate that excessivebackscatter had been present during exposure and that the radiograph is unacceptable. The absenceof the lead letter image indicates that acceptably low scatter levels have been achieved.

    2.3.5 Obsolete wire type IQI

    Archived radiographs which pre-date the European EN and BS EN standards may include other imagequality indicators such as the ISO type DIN (Deutsches Institute fur Normung e.V.) or BritishStandard. Many of the radiograph images in this training programme have these types of IQI.

    The DIN IQI was very similar to the EN type and the wire numbering system and wire diameters werethe same.

    The British Standards IQI differed from the EN type in that the wire numbering was reversed (highernumbers being thicker wires) and the wire grouping of the models was different.

    2.4 Artefacts

    An artefact on a radiograph is any image on the film which is not related to the object beingradiographed. Artefacts can be produced by mechanical or chemical damage to the film before orafter processing and by damaged or dirty intensifying screens. Artefacts are cause for rejection ofthe film only if they interfere with the image in the area of interest of the weld being examined. Some examples of artefacts are described below.

    Scratches

    A scratch on the film can appear as a dark or light image in the radiograph. Images resulting fromfilm scratches can usually be identified by viewing the film in reflected light and should be visible onone side of the film only. Scratches on lead intensifying screens may appear in the radiographic

  • image as either light or dark lines which cannot be seen in reflected light. These are more difficult toidentify and in case of doubt it may be necessary to repeat the radiograph using different screens. Intensifying screens should be regularly inspected and should be discarded if damaged.

    Processing marks

    Examples of processing marks include roller marks which are caused by poor maintenance ofautomatic film processors and streakiness or mottling which can be due to insufficient agitationduring manual development. Under or over development usually leads to a mottled effect on thefinished radiograph. A similar effect is produced by exhausted developer.

    Water marks

    These are easily seen on the radiograph both by transmitted and reflected light and are due to a dryor partially dry film being wetted locally either by splashing or by water running down from a filmhanger clip.

    Static Electricity

    In very dry conditions static charge can build up on the film in the plastic film cassette or whenremoved from the film storage box. This may discharge when the film is removed for processing orloading. The discharge sparks cause dark marks in the image due to the exposure to light. Themarks can appear as dark star shapes or fine branching dark lines.

    3. Radiographic Techniques

    The technique applied to inspect a particular component or weld is selected by reference to thepossible defects which may occur, the equipment and access available, the material and the shape ofthe item.

    3.1 Single Wall Single Image

    Radiography is usually carried out by the single wall, single image (SWSI) technique which requiresaccess to both surfaces of the object to be radiographed. The source of radiation is placed on oneside of the item and film on the opposite side.

    3.2 Panoramic

    An arrangement of SWSI used for vessel girth welds or for large diameter pipe butt welds is thepanoramic technique where the X-ray head or gamma radiation source is placed at the centre of thevessel or pipe and film is placed around the outer circumference of the weld. The complete weld canbe radiographed in a single exposure with this technique. The resulting image may be on one singlelength of film covering the entire weld length or on a series of overlapping films with locationmarkers. Location markers must be attached to the component and not to the film cassette.

    3.3 Double Wall Techniques

    There are many instances where radiography by SWSI techniques is not possible due to therequirement for access to both surfaces of the item to be inspected. This occurs with radiography ofpipe butt welds for example where access along the pipe is restricted by size or bends or where thepipework is in service. In these situations techniques are used which involve having the radiationsource and film on opposite sides external to the pipe and passing the radiation beam through bothpipe walls to produce an image of part of the weld circumference on the film.

    3.4 Double Wall Double Image

    Small diameter pipe welds up to about 90 mm diameter can be radiographed by the double walldouble image (DWDI) technique. It can be applied where the radiation source is in line with the planeof the weld producing a radiograph where the upper and lower weld images are superimposed or byoffsetting the source so that the upper and lower regions of the weld are separated in the image. Forcomplete coverage of the weld using the superimposed technique it is necessary to produce threeseparate radiographs with the weld rotated by 120 between each. For the offset technique only two

  • radiographs with 90 rotation are required. In both cases the IQI must be positioned on top of thepipe closest to the radiation source.

    3.5 Double Wall Single Image

    The double wall single image (DWSI) technique is used on large diameter pipe welds greater than 90mm diameter. The film is wrapped around the pipe and the exposure made by passing radiationthrough both pipe walls. Only the image from the weld section closest to the film will be suitable forexamination since the side furthest from the film will produce a blurred and distorted image. Forcomplete coverage of the weld it is necessary to make several separate overlapping exposures atpositions around the pipe. The number of exposures required is dependent on the diameter and wallthickness of the pipe. The relevant standards give guidance on establishing the required number ofexposures. Since access to the pipe bore is usually restricted film side IQIs are permitted for thistechnique.

    4. Weld Quality

    Following the review of film quality, radiographs should be examined for the presence of defects in theweld and adjacent material. Examination should be carried out even if the film quality isunacceptable since gross defects may be visible and the component could be rejected without theneed for further radiography. Defects visible should be noted and the component sentencedaccording to the applicable acceptance criteria. Where there is doubt whether an image is due to aninternal defect or a surface feature the weld area should be examined visually to establish the cause.

    4.1 Weld surface features

    Listed below are some of the irregular weld surface conditions that can be seen in radiographicimages. The severity of weld defects such as excessive penetration or undercutting is difficult tojudge using radiographic evidence alone. Wherever possible defects of this type should be judged foracceptability by visual examination of the weld.

    4.1.1 Excessive root penetration

    Excess weld material protruding through the root of a fusion weld made from one side appears in theradiograph as a continuous or intermittent light irregular band within the image of the weld.

    4.1.2 Root concavity

    Root concavity is a shallow groove which may occur in the root of a single sided weld. It appears inthe radiograph as a series of dark areas along the centre of the weld varying in density according tothe depth of imperfection.

    4.1.3 Incompletely filled groove (lack of fill)

    This is a continuous or intermittent channel in the surface of the weld, running along its length, dueto insufficient weld material. The channel may be along the centre or along one or both edges of theweld. It produces an image in the radiograph of a dark band or dark patches within the image of theweld. Where this occurs at the edge of the weld cap it is distinguished from undercutting by thestraight edge of the weld preparation on the parent material.

    4.1.4 Undercutting

    An irregular groove at the toe the weld in the parent material due to burning away during welding. Itappears in the radiograph as a dark / irregular /intermittent band along the edge of either the cap orroot bead or between adjacent capping runs. It may therefore appear inside or outside the weldimage on the radiograph.

    4.1.5 Spatter

    Globules of material expelled during arc welding on to the surface of the parent material or weld. Spatter appears in the radiograph as small light spots on the weld and adjacent parent material.

  • 4.2 Weld defects

    Weld defects can occur in any position in the weld and may be visible on the radiograph forassessment. Suspected defects which appear to be surface breaking should be confirmed by visual orNDE surface inspection techniques.

    4.2.1 Cracks

    Cracks due to welding may occur at the point of solidification, during the deposition of subsequentwelding runs or at a time after the completion of welding. Cracks may occur either in the welddeposit or in the parent material. Cracks are usually parallel to the welding direction but can alsooccur in the transverse plane. Crater cracks at stop/start positions can also occur.

    The ability of the radiographic technique to detect a crack is dependent on the crack orientationrelative to the direction of the radiation. Even a slight deviation from the optimum orientation willgreatly reduce the chances of detection. When they are detected they appear in the radiograph asdark, fine and often branching lines which are usually diffuse or discontinuous.

    4.2.2 Lack of fusion

    Lack of fusion in welding can occur either between the weld deposit and the parent material orbetween successive layers of weld material.

    The ability of radiographic techniques to detect lack of fusion is strongly dependent on the orientationof the defect with respect to the incident beam of radiation. Lack of fusion with the parent materialwill appear as a fine dark straight line which may be continuous or intermittent. Unfavourablyorientated lack of fusion with the parent material may sometimes be detected due to the presence ofassociated slag inclusions or porosity: a slag inclusion with a straight edge normally indicates lack offusion. Gas escaping from an area of lack of fusion during welding may show as linear porosity.

    4.2.3 Incomplete root penetration

    Incomplete penetration appears in a radiograph as a dark continuous or intermittent linear band, theedges of which will be straight. Where welds are deposited without a root gap, lack of penetrationmay appear as a single continuous or intermittent straight dark line. Root gaps frequently closeduring welding and even in cases where there should have been a root gap the lack of penetrationmay still appear in the radiograph as a single fine dark line.

    4.2.4 Slag inclusions

    Slag inclusions are irregularly shaped, they may be either rounded/isolated or linear/elongated. Linear slag inclusions with a straight edge may indicate lack of fusion. Sometimes linear slag willappear on the radiograph as two irregular parallel lines referred to as tram lines or waggon tracks. Most weld slag and other possible sources of non-metallic inclusions are radiographically much lessabsorbing than the surrounding metallic material and appear in the radiograph as dark images.

    4.2.5 Metallic inclusions

    Materials such as tungsten or copper can be accidentally introduced into the molten weld pool duringwelding, the materials usually coming from the welding equipment in use. Tungsten inclusions areassociated with the tungsten inert-gas welding process and are caused by the break-up of the non-consumable tungsten electrode during welding. Tungsten is very dense and the inclusions alwaysappear as bright images which tend to be sharp and angular. They are usually small typically 0.5 to1 mm. Copper inclusions can occur with submerged-arc or other welding processes where theconsumable electrode is fed through a copper contact. If the copper touches the weld pool it will meltand become included in the weld. Copper is radiographically more absorbing than steel so theinclusions are bright with diffuse edges. Copper inclusions in ferritic steel welds can cause cracking.

    4.2.6 Gas porosity

    Gas pores are easily detected by radiography since they are not sensitive to the direction of radiationand the gas is many times less dense than the surrounding material. Gas pores appear on aradiograph as sharply defined dark circular spots. They may be isolated, grouped or evenlydistributed. Linear porosity is usually an indication of lack of fusion.

  • 4.2.7 Elongated Cavities (hollow bead/piping)

    These will generally only occur at the roots of welds deposited by manual metal-arc. On theradiograph they have an appearance similar to that of slag. The radiographic indication usually hasrounded ends and is usually situated above the centre of the root bead.

    4.2.8 Worm Holes

    These are gas pores which have become frozen in the weld pool while migrating towards the surface. They appear on the radiograph as a dark shadow the shape of which depends on the orientation ofthe defect. If the worm hole is in line with the radiation a very dark rounded shadow is formed. Ifthe wormhole is not directly in line with the radiation beam then the dark spot has a faint tail. Wherea lamination in the parent material or a lack of fusion is the source of wormholes they are oftenapparent in the radiograph in a herringbone shaped linear group.

    4.2.9 Crater cracks and pipes

    Crater cracks are due to shrinkage and usually occur at weld stop/start positions. They often have astar like appearance in the radiograph and their radiographic image rarely measures more than 3 or 4mm. Crater pipes appear on the radiograph with an image similar to that of an isolated wormholeand may be associated with cracking.

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