Interpretation of Weld Radiographs, 1. Introduction Assessment and interpretation of radiographic images is widely used in industry for the quality control 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 by various conditions. The standards usually quoted for eyesight require that personnel are able to read a minimum of the J2 level on the Jaeger eyesight chart with the chart at positioned a distance of 30.5 centimetres. 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 was created and be aware of the radiographic technique used. The interpreter should have details of the weld 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 entering a darkened room from bright sunlight some time should be spent under darkroom conditions prior to commencing interpretation in order that eyesight can adjust to the low light level. Viewer screens should 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 the edges to prevent fingerprints and damage to the film surfaces. Soft cotton gloves are often used by interpreters 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 the interpreter. The film viewer can be activated by a foot switch when the film to be examined is in position. 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 of artefacts that may interfere with the assessment. Where film quality is unacceptable the area of weld covered by the film should be re-radiographed. 2.1 Identification Manufacturers may have a method of radiographic identification which is linked to a quality system but the following is a guide to the normal requirements for details appearing on the radiograph. The identification should include the manufacturer’s symbol, the component/item/weld number as appropriate, the location within the weld (such as location markers 1 to 2, B to C etc) and the date radiography was carried out. The identification details usually appear in the image but sometimes a system of “flashing” the details on to the film before exposure is used. In all cases location markers which indicate the diagnostic length (extent of the weld on the film to be examined) must appear as radiographic images. The repair 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 and heat 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 range of density values. A more accurate method is to use an electronic device known as a film transmission densitometer. This device simply measures the logarithmic ratio of incident to transmitted light through the image from the viewer. Film density is therefore a number which will vary from 0 (film totally transparent) to about 5 (film virtually opaque). In general, densities above 4 are only used for special applications. The densitometer must be regularly calibrated for accuracy throughout its range and must be set to zero on the illuminated viewer immediately before use. Film density influences the contrast and hence the visibility of defects on a radiograph. Film contrast is the difference in density between adjacent areas on the radiograph, the greater the density difference the higher the contrast. In addition, radiographic film characteristics are such that contrast increases with film density. For this reason a minimum film density on the area being examined is required 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 ray techniques. 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. These minimum figures for film density apply to the area of interest (the diagnostic length of the weld) on the radiograph. 2.3 Radiographic sensitivity The ability of a radiograph to reveal internal defects is determined by the quality or sensitivity of the image produced. In addition it should be noted that planar weld defects such as cracks or lack of sidewall fusion may appear faint or even be invisible if they are unfavourably orientated with the direction of the radiation beam. The sensitivity of the radiograph produced is affected by many factors but basically, the higher the contrast and definition (sharpness) of the image the more sensitive the technique will be for detecting imperfections in the object being examined. Image Quality Indicators (IQIs) are used in order to demonstrate that adequate radiographic sensitivity has been achieved. An image quality indicator is a device placed on the surface of the component prior to radiography. The indicator provides a comparative measure of the definition and contrast 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 varying diameters. The wallet is placed in the area of interest with the wires positioned across the weld. The radiographic sensitivity can be given a numerical value by dividing the diameter of the smallest wire visible on the radiograph by the thickness of the component and expressing it as a percentage. For example 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 calculation ASME (American Society of Mechanical Engineers) and EN standards provide reference tables defining the smallest wire which should be visible for acceptable sensitivity according to the component thickness and the radiographic technique used. In the European standard there are nineteen wire diameters in the range from 3.2 mm to 0.05 mm covering component thicknesses from over 380 mm down to 1.5 mm. Each IQI has seven wires and there are four models covering the range with overlap between models. The IQI wire material is available 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 are eight different material groups, three for the light metals including aluminium and five for the heavy metals including steel. 2.3.2 ASTM Penetrameters The ASTM plate/hole type IQI (known as a penetrameter) is a small piece of material radiographically similar to the component and is placed beside the weld to be radiographed. Its thickness is typically 2% of the component thickness and there are three holes, 1T, 2T and 4T in diameter where ‘T’ is the thickness of the penetrameter. There are eight different material models covering the light and heavy metal groups. The ASME Code Section V Article 2 details the appropriate penetrameter selection and defines the ‘essential’ hole in the penetrameter which must be visible on the radiograph to establish that satisfactory image quality has been achieved. For weld radiography, metal shims are used to build up the material under the penetrameter so that it rests on a thickness equivalent to the weld and weld cap reinforcement. 2.3.3 IQI Placement For meaningful results it is necessary that the indicator be placed on the surface of the component facing the radiation (source side) and preferably towards the edge of the field of view (the least favourable position). In certain situations, for reasons of access, it is permitted to place the IQI between the film and the object being radiographed. This is necessary for instance when radiographing a pipe butt weld where there is no access to the bore or where there is gas or fluid in the pipe. With the IQI in contact with the film a much clearer image of the IQI will be produced giving a false (favourable) indication of sensitivity. To compensate for this, the codes or and standards define a more stringent requirement as to which IQI wire must be visible or which designation of penetrameter should be used. When film side 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 the image and reduce radiographic sensitivity. Code requirements specify that a lead letter “B” must be attached to the back of the film cassette before exposure. During interpretation if the interpreter can discern a faint light image of the lead letter on the radiograph then this would indicate that excessive backscatter had been present during exposure and that the radiograph is unacceptable. The absence of 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 image quality indicators such as the ISO type DIN (Deutsches Institute fur Normung e.V.) or British Standard. 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 were the same. The British Standards IQI differed from the EN type in that the wire numbering was reversed (higher numbers 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 being radiographed. Artefacts can be produced by mechanical or chemical damage to the film before or after processing and by damaged or dirty intensifying screens. Artefacts are cause for rejection of the 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 from film scratches can usually be identified by viewing the film in reflected light and should be visible on one 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 to identify 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 of automatic film processors and streakiness or mottling which can be due to insufficient agitation during manual development. Under or over development usually leads to a mottled effect on the finished 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 dry or partially dry film being wetted locally either by splashing or by water running down from a film hanger clip. Static Electricity In very dry conditions static charge can build up on the film in the plastic film cassette or when removed from the film storage box. This may discharge when the film is removed for processing or loading. The discharge sparks cause dark marks in the image due to the exposure to light. The marks 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 the possible defects which may occur, the equipment and access available, the material and the shape of the item. 3.1 Single Wall Single Image Radiography is usually carried out by the single wall, single image (SWSI) technique which requires access to both surfaces of the object to be radiographed. The source of radiation is placed on one side 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 the panoramic technique where the X-ray head or gamma radiation source is placed at the centre of the vessel or pipe and film is placed around the outer circumference of the weld. The complete weld can be radiographed in a single exposure with this technique. The resulting image may be on one single length of film covering the entire weld length or on a series of overlapping films with location markers. 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 the requirement for access to both surfaces of the item to be inspected. This occurs with radiography of pipe butt welds for example where access along the pipe is restricted by size or bends or where the pipework is in service. In these situations techniques are used which involve having the radiation source and film on opposite sides external to the pipe and passing the radiation beam through both pipe 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 wall double image (DWDI) technique. It can be applied where the radiation source is in line with the plane of the weld producing a radiograph where the upper and lower weld images are superimposed or by offsetting the source so that the upper and lower regions of the weld are separated in the image. For complete coverage of the weld using the superimposed technique it is necessary to produce three separate 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 the pipe 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 90 mm diameter. The film is wrapped around the pipe and the exposure made by passing radiation through both pipe walls. Only the image from the weld section closest to the film will be suitable for examination since the side furthest from the film will produce a blurred and distorted image. For complete coverage of the weld it is necessary to make several separate overlapping exposures at positions around the pipe. The number of exposures required is dependent on the diameter and wall thickness of the pipe. The relevant standards give guidance on establishing the required number of exposures. Since access to the pipe bore is usually restricted film side IQIs are permitted for this technique. 4. Weld Quality Following the review of film quality, radiographs should be examined for the presence of defects in the weld and adjacent material. Examination should be carried out even if the film quality is unacceptable since gross defects may be visible and the component could be rejected without the need for further radiography. Defects visible should be noted and the component sentenced according to the applicable acceptance criteria. Where there is doubt whether an image is due to an internal 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 radiographic images. The severity of weld defects such as excessive penetration or undercutting is difficult to judge using radiographic evidence alone. Wherever possible defects of this type should be judged for acceptability 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 the radiograph 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 in the radiograph as a series of dark areas along the centre of the weld varying in density according to the 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, due to insufficient weld material. The channel may be along the centre or along one or both edges of the weld. It produces an image in the radiograph of a dark band or dark patches within the image of the weld. Where this occurs at the edge of the weld cap it is distinguished from undercutting by the straight 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. It appears in the radiograph as a dark / irregular /intermittent band along the edge of either the cap or root bead or between adjacent capping runs. It may therefore appear inside or outside the weld image 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 for assessment. Suspected defects which appear to be surface breaking should be confirmed by visual or NDE surface inspection techniques. 4.2.1 Cracks Cracks due to welding may occur at the point of solidification, during the deposition of subsequent welding runs or at a time after the completion of welding. Cracks may occur either in the weld deposit or in the parent material. Cracks are usually parallel to the welding direction but can also occur 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 orientation relative to the direction of the radiation. Even a slight deviation from the optimum orientation will greatly reduce the chances of detection. When they are detected they appear in the radiograph as dark, 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 or between successive layers of weld material. The ability of radiographic techniques to detect lack of fusion is strongly dependent on the orientation of the defect with respect to the incident beam of radiation. Lack of fusion with the parent material will appear as a fine dark straight line which may be continuous or intermittent. Unfavourably orientated lack of fusion with the parent material may sometimes be detected due to the presence of associated slag inclusions or porosity: a slag inclusion with a straight edge normally indicates lack of fusion. 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, the edges of which will be straight. Where welds are deposited without a root gap, lack of penetration may appear as a single continuous or intermittent straight dark line. Root gaps frequently close during welding and even in cases where there should have been a root gap the lack of penetration may 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 will appear 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 less absorbing 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 during welding, the materials usually coming from the welding equipment in use. Tungsten inclusions are associated 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 always appear as bright images which tend to be sharp and angular. They are usually small – typically 0.5 to 1 mm. Copper inclusions can occur with submerged-arc or other welding processes where the consumable electrode is fed through a copper contact. If the copper touches the weld pool it will melt and become included in the weld. Copper is radiographically more absorbing than steel so the inclusions 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 radiation and the gas is many times less dense than the surrounding material. Gas pores appear on a radiograph as sharply defined dark circular spots. They may be isolated, grouped or evenly distributed. 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 the radiograph they have an appearance similar to that of slag. The radiographic indication usually has rounded 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 of the defect. If the worm hole is in line with the radiation a very dark rounded shadow is formed. If the wormhole is not directly in line with the radiation beam then the dark spot has a faint tail. Where a lamination in the parent material or a lack of fusion is the source of wormholes they are often apparent 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 a star like appearance in the radiograph and their radiographic image rarely measures more than 3 or 4 mm. Crater pipes appear on the radiograph with an image similar to that of an isolated wormhole and may be associated with cracking.
Posted on: Wed, 28 Aug 2013 23:30:58 +0000
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