A general review of geometric shape imperfections - types and causesJob KnowledgePart 1. IntroductionIn the job knowledge series welding imperfections such as cracks, lack of fusion, penetration and porosity have been discussed. This article looks at those imperfections related to poor geometric shape and will concentrate on the following: Excess weld metal Undercut Overlap Linear misalignment Incompletely filled grooveSuch imperfections might be considered as anomalies in the joint and they will always be present to some degree so that it becomes necessary to separate the acceptable from the unacceptable. This is done by following guidance given by the application standard, which was the basis for the component design, and/or by direction, as set out in the job contract. Examples of standards that might be referred to are: PD 5500 Specification for unfired fusion welded pressure vessels. BS EN ISO 5817 Welding. Fusion-welded joints in steel, nickel, titanium and their alloys (beam welding excluded). Quality levels for imperfections AWS D1.1 Structural welding code - SteelExcess weld metal(also called cap height, overfill or reinforcement)

Fig.1. Excess weld metalThis is weld metal lying outside the plane joining the weld toes. Note that the term 'reinforcement', although used extensively in the ASME/AWS specifications is avoided in Europe as it implies it adds strength to the welded joint, which is rarely the case.Common causesThis imperfection is formed when excessive weld metal is added to the joint, which is usually a result of poor welder technique for manual processes but may be due to poor parameter selection when the process is mechanised. That is, too much filler metal for the travel speed used. In multi-run welding a poor selection of individual bead sizes can result in a bead build-up pattern that overfills the joint. Different processes and parameters (egvoltage) can result in different excess weld metal shapes.AcceptanceThe acceptability of this imperfection is very dependent on the application in which the product will be used. Most standards have limit, related to material thickness (eg10%), but also have a maximum upper limits. Both the ratio and the maximum may be related to the severity of service that the component is expected to see. The following table gives examples taken from BS EN ISO 5817.Excess weld metal limits for quality levels:

Severity of serviceModerate, DStringent, B

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

Maximum10 mm5 mm

Transition requiredsmoothsmooth

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

An important reason for limiting the height of excess weld metal is that it represents a non-value added cost. However, it must be remembered that the height of the weld cap influences the resultant toe blend. A sharp transition causes a local stress concentration that can contribute to loss of strength, which is particularly important in fatigue situations. As a result most specifications state that 'smooth transition is required'.AvoidanceIf the imperfection is a result of welder technique then welder retraining is required. For mechanised techniques an increase in travel speed or voltage will help to reduce cap height.Undercut

Fig.2. UndercutThis is an irregular groove at the toe of a run in the parent metal.The figure shows undercut at surface of a completed joint but it may also be found at the toes of each pass of a multi-run weld. The latter can result in slag becoming trapped in the undercut region.Common causesWhen arc and gas welding, undercut is probably the most common shape imperfection. With single-sided pipe welds it may also be found at the bore surface. It may also be seen on the vertical face of fillet welds made in the horizontal vertical position.A wide spreading arc (high arc voltage) with insufficient fill (low current or high travel speed) is the usual cause. However, welder technique, especially when weaving, and the way the welding torch is angled can both cause and be used to overcome undercutting (ieangled to push the weld metal to fill the melted groove). High welding current will also cause undercut - this is generally associated with the need for a high travel speed to avoid overfilling of the joint.AcceptanceLargely because this imperfection is widespread, most standards permit some level of undercut although they do require that a 'smooth transition is required. The limits in BS EN ISO 5817 range from 0.5mm (stringent) to 1mm (moderate) for thickness (t) greater than 3mm (more stringent limits are required for t 0.5 to 3mm), while AWS D1.1 has a limit of 1mm.Measuring undercut can be a problem because of the small size of the imperfection compared with the general environment where there can be mill scale, irregularities in the surface and spatter.In critical applications the imperfection can be 'corrected' by blend grinding or by depositing an additional weld bead.AvoidanceThis imperfection may be avoided by reducing travel speed and/or the welding current and by maintaining the correct arc length.Overlap (cold lapping)

Fig.3. OverlapThis is an imperfection at a toe or root of a weld caused by metal flowing on to the surface of the parent metal without fusing to it. It may occur in both fillet and butt welds.Common causesThis is often caused by poor manipulation of the electrode or welding gun, especially when the weld pool is large and 'cold', where the welder allows gravity to influence the weld shape before solidification. Tightly adherent oxides or scale on the metal surface can also prevent the weld metal fusing with the parent metal to cause the overlap imperfection.AvoidanceAvoidance is achieved through an acceptable level of welder skill and a reduction in weld pool size (obtained by reducing current or increasing travel speed). Adequate cleaning of the parent plate is also important.AcceptanceStandards rarely allow the presence of this imperfection, unless the length is short (egBS EN ISO 5817 for moderate quality level D). Overlap can be very difficult to detect, especially if it is extremely small.Linear misalignment

Fig.4 Linear misalignment(Also known in the USA as high-low).This imperfection relates to deviations from the correct position/alignment of the joint.Common causesThis is primarily a result of poor component fit-up before welding, which can be compounded by variations in the shape and thickness of components (egout of roundness of pipe). Tacks that break during welding may allow the components to move relative to one another, again resulting in misalignment.AcceptanceThe acceptability of this defect is related to the design function of the structure or pipe line either in terms of the ability to take load across the misalignment or because such a step impedes the flow of fluid.Acceptance varies with the application:BS EN ISO 5817 relates misalignment to wall thickness but sets maximum limits (egfor material thickness t>3mm and moderate limits of imperfections D, = 0.25 x t, with a maximum of 5mm).AWS D1.1 allows 10% of the wall thickness up to a maximum of 3mm.The consequence of linear misalignment can, when welding is carried out from one side, be lack of root or sidewall fusion to give a sharp continuous imperfection along the higher weld face toe. In some situations linear misalignment in the bore of a pipe can lead to in-service problems where turbulence of the carrier fluid in the pipe creates subsequent erosion.Incomplete filled groove

Incomplete filled grooveThis is a continuous, or intermittent, channel in the surface of a weld, running along its length, due to insufficient weld metal.Common causesThis problem arises when there has been insufficient filler metal (current or wire feed too low or too high a travel speed) so that the joint has not been sufficiently filled. The result is that the thickness of weldment is less than that specified in the design, which could lead to failure.AcceptanceMost standards will not accept this type of imperfection, except perhaps over short lengths and even then a smooth transition is required. The designer expects the joint to be adequately filled, but not too much so (see excess weld metal).Often the presence of this imperfection is an indication of poor workmanship and could suggest that further training is required.ContinuationPart 2looks at shape imperfections such as excess penetration and root concavity and highlights shape imperfections related to fillet welded joints.A general review of the causes and acceptance of shape imperfections - Part 2Job KnowledgeClickherefor Part 1.This second article on shape imperfections refers mostly to fillet welds but there are two additional butt weld imperfections that require some comment.Excessive penetration (Excess penetration bead)

Fig.1. Excess penetrationExcess weld metal protruding through the root of a fusion (butt) weld made from one side only.With pipe welding this type of imperfection may cause effects in the fluid flow that can cause erosion and/or corrosion problems.Common causesPenetration becomes excessive when the joint gap is too large, the root faces are too small, the heat input to the joint is too high or a combination of these causes.AcceptanceThe criteria which sets the level of acceptable penetration depends primarily on the application code or specification.BS 2971 (Class 2 arc welding) requires that the 'penetration bead shall not exceed 3mm for pipes up to and including 150mm bore or 6mm for pipes over 150mm bore'.BS 2633 (Class 1 arc welding) gives specific limits for smaller diameters pipes, eg for pipe size 25-50mm the maximum allowed bore penetration is 2.5mm.ASME B31.3 bases acceptability on the nominal thickness of the weld, for instance, allowing for a thickness range of 13-25mm up to 4mm of protrusion. However, ASME notes that 'more stringent criteria may be specified in the engineering design'.BS EN ISO 5817 (Quality levels for imperfections), which supersedes BS EN 25817, relates the acceptable protrusion to the width of the under-bead as follows:Severity of serviceModerate, DStringent, B

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

Maximum5 mm3 mm

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

AvoidanceIt is important to ensure that joint fit-up is as specified in the welding procedure. If welder technique is the problem then re training is required.Root concavity (suck-back; underwashing)

Fig.2. Root concavityA shallow groove that may occur in the root of a butt weld.Common causesRoot concavity is caused by shrinkage of the weld pool in the through-thickness direction of the weld. Melting of the root pass by the second pass can also produce root concavity.This imperfection is frequently associated with TIG welding with the most common cause being poor preparation leaving the root gap either too small or, in some cases, too large. Excessively high welding speeds make the formation of root concavity more likely.AcceptanceThe root concavity may be acceptable. This will depend on the relevant standard being worked to. For example:BS 2971 requires that:a) there is complete root fusionb) the thickness of the weld is not less than the pipe thickness.ASME B31.3 requires that the 'total joint thickness, including weld reinforcement, must be greater than the weld thickness'.BS EN ISO 5817 sets upper limits related to the quality level,egfor thicknesses > 3mm Moderate, (D), h 0.2t but max 2mm for Stringent, (B), h 0.05t but max 0.5mm. Furthermore, a smooth transition is required at the weld toes.In effect the standards require that the minimum design throat thickness of the finished weldment is achieved. If the first two conditions of acceptance are met but the weld face does not have a sufficiently high cap, additional weld metal may be deposited to increase the throat.AvoidanceIt is important to ensure that joint fit-up is as specified in the welding procedure and that the defined parameters are being followed. If welder technique is the problem then retraining is required.Fillet welded jointsThis Section should be read in conjunction with Job Knowledge 66Fillet welded joints - a review of the practicalities.Excessive convexity

Fig.3. Excessive convexityThis feature is also covered by the definition for excess weld metal, seePart 1, and may be described as weld metal lying outside the plane joining the weld toes. Note that the term 'reinforcement', although used extensively in the ASME/AWS specifications is avoided in Europe as it implies that excess metal contributes to the strength of the welded joint. This is rarely the case.Common causesPoor technique and the deposition of large volumes of 'cold' weld metal.AcceptanceThe idealised design requirement of a 'mitre' fillet weld is often difficult to achieve, particularly with manual welding processes.BS EN ISO 5817 acceptance is based on a mitre fillet weld shape with a specific design throat and any excess weld metal is measured in relation to this mitre surface. The limits for this imperfection relate the height of the excess metal to the width of the bead with maximum values ranging from 3mm for a stringent quality level to 5mm for a moderate quality level. Surprisingly, there is no reference to a 'smooth transition' being required at the weld toes for such weld shape.AWS D1.1 also has limits relating width to acceptable excess as follows:Width of weld faceMaximum convexity

W 8mm2mm

W