IIW Welder Module B Weld Imperfections 0 Weld
IIW Welder Module B Weld Imperfections 0
Weld defects / imperfections - incomplete root fusion or penetration The characteristic features and principal causes of incomplete root fusion are described. General guidelines on 'best practice' are given so welders can minimise the risk of introducing imperfections during fabrication. The SS Schenectady, an all welded tanker, broke in two whilst lying in dock in 1943. Principal causes of this failure were poor design and bad workmanship 1
Fabrication and service defects and imperfections As the presence of imperfections in a welded joint may not render the component defective in the sense of being unsuitable for the intended application, the preferred term is imperfection rather than defect. For this reason, production quality for a component is defined in terms of a quality level in which the limits for the imperfections are clearly defined, for example Level B, C or D in accordance with the requirements of BS EN ISO 5817. For the American standards ASME X 1 and AWS D 1. 1, the acceptance levels are contained in the standards. 2
The application code will specify the quality levels which must be achieved for the various joints. Imperfections can be broadly classified into those produced on fabrication of the component or structure and those formed as result of adverse conditions during service. The principal types of imperfections are: fabrication: - lack of fusion cracks porosity inclusions incorrect weld shape and size 3
service: - brittle fracture - stress corrosion cracking - fatigue failure Welding procedure and welder technique will have a direct effect on fabrication imperfections. Incorrect procedure or poor technique may produce imperfections leading to premature failure in service. 4
Incomplete root fusion or penetration Identification Incomplete root fusion is when the weld fails to fuse one side of the joint in the root. Incomplete root penetration occurs when both sides of the joint are unfused. Typical imperfections can arise in the following situations: - an excessively thick root face in a butt weld (Fig. 1 a) - too small a root gap (Fig. 1 b) - misplaced welds (Fig. 1 c) 5
- failure to remove sufficient metal in cutting back to sound metal in a double sided weld (Fig. 1 d) - incomplete root fusion when using too low an arc energy (heat) input (Fig. 1 e) - too small a bevel angle, - too large an electrode in MMA welding (Fig 2) 6
Figure 1: Causes of incomplete root fusion 7
Figure 2: Effect of electrode size on root fusion 8
Causes These types of imperfection are more likely in consumable electrode processes (MIG, MMA and submerged arc welding) where the weld metal is 'automatically' deposited as the arc consumes the electrode wire or rod. The welder has limited control of weld pool penetration independent of depositing weld metal. Thus, the non consumable electrode TIG process in which the welder controls the amount of filler material independent of penetration is less prone to this type of defect. In MMA welding, the risk of incomplete root fusion can be reduced by using the correct welding parameters and electrode size to give adequate arc energy input and deep penetration. 9
Electrode size is also important in that it should be small enough to give adequate access to the root, especially when using a small bevel angle (Fig 2). It is common practice to use a 3. 25 mm diameter electrode for the root so the welder can manipulate the electrode for penetration and control of the weld pool. However, for the fill passes where penetration requirements are less critical, a 4 or 5 mm diameter electrode is used to achieve higher deposition rates. In MIG welding, the correct welding parameters for the material thickness, and a short arc length, should give adequate weld bead penetration. Too low a current level for the size of root face will give inadequate weld penetration. 10
Too high a level, causing the welder to move too quickly, will result in the weld pool bridging the root without achieving adequate penetration. It is also essential that the correct root face size and bevel angles are used and that the joint gap is set accurately. To prevent the gap from closing, adequate tacking will be required. 11
Best practice in prevention The following techniques can be used to prevent lack of root fusion: - In TIG welding, do not use too large a root face and ensure the welding current is sufficient for the weld pool to penetrate fully the root - In MMA welding, use the correct current level and not too large an electrode size for the root - In MIG welding, use a sufficiently high welding current level but adjust the arc voltage to keep a short arc length - When using a joint configuration with a joint gap, make sure it is of adequate size and does not close up during welding - Do not use too high a current level causing the weld pool to bridge the gap without fully penetrating the root. 12
Acceptance standards The limits for lack of penetration are specified in BS EN ISO 5817 for the three quality levels. Lack of root penetration is not permitted for Quality Level B (stringent) and Level C (intermediate). For Quality Level (moderate) short lack of penetration imperfections are permitted. Incomplete root penetration is not permitted in the manufacture of pressure vessels but is allowable in the manufacture of pipe work depending on material and wall thickness. 13
Remedial actions If the root cannot be directly inspected, for example using a penetrant or magnetic particle inspection technique, detection is by radiography or ultrasonic inspection. Remedial action will normally require removal by gouging or grinding to sound metal, followed by re-welding in conformity with the original procedure. 14
Weld defects/imperfections in welds - lack of sidewall and inter-run fusion This article describes the characteristic features and principal causes of lack of sidewall and inter-run fusion. General guidelines on best practice are given so that welders can minimise the risk of imperfections during fabrication. Identification Lack of fusion imperfections can occur when the weld metal fails. - to fuse completely with the sidewall of the joint (Fig. 3) - to penetrate adequately the previous weld bead (Fig. 4). 15 Demagnetising a pipe
Figure 3: Lack of side wall fusion Figure 4: Lack of inter-run fusion 16
Causes The principal causes are too narrow a joint preparation, incorrect welding parameter settings, poor welder technique and magnetic arc blow. Insufficient cleaning of oily or scaled surfaces can also contribute to lack of fusion. These types of imperfection are more likely to happen when welding in the vertical position. Joint preparation Too narrow a joint preparation often causes the arc to be attracted to one of the side walls causing lack of side wall fusion on the other side of the joint or inadequate penetration into the previously deposited weld bead. 17
Too great an arc length may also increase the risk of preferential melting along one side of the joint and cause shallow penetration. In addition, a narrow joint preparation may prevent adequate access into the joint. For example, this happens in MMA welding when using a large diameter electrode, or in MIG welding where an allowance should be made for the size of the nozzle. Welding parameters It is important to use a sufficiently high current for the arc to penetrate into the joint sidewall. Consequently, too high a welding speed for the welding current will increase the risk of these imperfections. However, too high a current or too low a welding speed will cause weld pool flooding ahead of the arc resulting in poor or non-uniform penetration. 18
Welder technique Poor welder technique such as incorrect angle or manipulation of the electrode/welding gun, will prevent adequate fusion of the joint sidewall. Weaving, especially dwelling at the joint sidewall, will enable the weld pool to wash into the parent metal, greatly improving sidewall fusion. It should be noted that the amount of weaving may be restricted by the welding procedure specification limiting the arc energy input, particularly when welding alloy or high notch toughness steels. 19
Magnetic arc blow When welding ferromagnetic steels lack of fusion imperfections can be caused through uncontrolled deflection of the arc, usually termed arc blow. Arc deflection can be caused by distortion of the magnetic field produced by the arc current (Fig. 5), through: - residual magnetism in the material through using magnets for handling - earth's magnetic field, for example in pipeline welding - position of the current return 20
The effect of welding past the current return cable which is bolted to the centre of the place is shown in Fig. 6. The interaction of the magnetic field surrounding the arc and that generated by the current flow in the plate to the current return cable is sufficient to deflect the weld bead. Distortion of the arc current magnetic field can be minimised by positioning the current return so that welding is always towards or away from the clamp and, in MMA welding, by using AC instead of DC. Often the only effective means is to demagnetise the steel before welding. 21
Figure 5: Interaction of magnetic forces causing arc deflection 22
Figure 6: Weld bead deflection in DC MMA welding caused by welding past the current return connection 23
Best practice in prevention The following fabrication techniques can be used to prevent formation of lack of sidewall fusion imperfections: - use a sufficiently wide joint preparation - select welding parameters (high current level, short arc length, not too high a welding speed) to promote penetration into the joint side wall without causing flooding - ensure the electrode/gun angle and manipulation technique will give adequate side wall fusion - use weaving and dwell to improve side wall fusion providing there are no heat input restrictions - if arc blow occurs, reposition the current return, use AC (in MMA welding) or demagnetise the steel 24
Acceptance standards The limits for incomplete fusion imperfections in arc welded joints in steel are specified in BS EN ISO 5817 for the three quality levels (see Table). These types of imperfection are not permitted for Quality Level B (stringent) and C (intermediate). For Quality level D (moderate) they are only permitted providing they are intermittent and not surface breaking. For arc welded joints in aluminium, long imperfections are not permitted for all three quality levels. However, for quality levels C and D, short imperfections are permitted but the total length of the imperfections is limited depending on the butt weld or the fillet weld throat thickness. 25
Table - Acceptance limits for specific codes and application standards Application Code/Standard Acceptance limit BS EN ISO 5817: 2003 Level B and C not permitted. Level D short imperfections permitted but not surface breaking. Aluminium BS EN ISO 10042: 2005 Levels B, C, D. Long imperfections not permitted. Levels C and D. Short imperfections permitted. Pressure vessels PD 5500: 2006 Not permitted Storage tanks BS EN 14015: 2004 Not permitted Pipe work BS 2633: 1994 'l' not greater than 15 mm (depending on wall thickness) Line pipe API 1104: 2005 ‘I' not greater than 25 mm (less when weld length <300 mm) Steel 26
Detection and remedial action If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub-surface imperfections, detection is by radiography or ultrasonic inspection. Ultrasonic inspection is normally more effective than radiography in detecting lack of inter-run fusion imperfections. Remedial action will normally require their removal by localised gouging, or grinding, followed by re-welding as specified in the agreed procedure. If lack of fusion is a persistent problem, and is not caused by magnetic arc blow, the welding procedures should be amended or the welders retrained. 27
Excessive penetration (Excess penetration bead) Excess weld metal protruding through the root of a fusion (butt) weld made from one side only. With pipe welding this type of imperfection may cause effects in the fluid flow that can cause erosion and/or corrosion problems. Common causes Penetration becomes excessive when the joint gap is too large, the root faces are too small, the heat input to the joint is too high or a combination of these causes. 28
Figure 7: Excess penetration 29
Acceptance The criteria which sets the level of acceptable penetration depends primarily on the application code or specification. BS 2971 requires that the 'penetration bead shall not exceed 3 mm for pipes up to and including 150 mm bore or 6 mm for pipes over 150 mm bore'. BS 2633 gives specific limits for smaller diameters pipes, e. g. for pipe size 25 -50 mm the maximum allowed bore penetration is 2. 5 mm. ASME B 31. 3 bases acceptability on the nominal thickness of the weld, for instance, allowing for a thickness range of 13 -25 mm up to 4 mm of protrusion. However, ASME notes that 'more stringent criteria may be specified in the engineering design'. 30
BS EN 5817 relates the acceptable protrusion to the width of the under-bead as follows: Severity of service Moderate, D Stringent, B Limit (up to maximum) h < 1 mm + 1. 2 b h < 1 mm + 0. 3 b Maximum 10 mm 3 mm Where: h = height of excess & b = width of bead ( see Fig. 7) Avoidance It is important to ensure that joint fit-up is as specified in the welding procedure. If welder technique is the problem then re training is required. 31
Root concavity (suck-back; under washing) A shallow groove that may occur in the root of a butt weld. Common causes Root concavity is caused by shrinkage of the weld pool in the through-thickness direction of the weld. Melting of the root pass by the second pass can also produce root concavity. This imperfection is frequently associated with TIG welding with the most common cause being poor preparation leaving the root gap either too small or, in some cases, too large. Excessively high welding speeds make the formation of root concavity more likely. 32
Figure 8: Root concavity 33
Acceptance The root concavity may be acceptable. This will depend on the relevant standard being worked to. For example: BS 2971 requires that: a) there is complete root fusion b) the thickness of the weld is not less than the pipe thickness. ASME B 31. 3 requires that the 'total joint thickness, including weld reinforcement, must be greater than the weld thickness'. 34
BS EN 25817 sets upper limits related to the quality level, e. g. Moderate, D, h < 1. 5 mm and for Stringent, B, h < 0. 5 mm. 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 throat. 35
Avoidance It is important to ensure that joint fit-up is as specified in the welding procedure and that the defined parameters are being followed. If welder technique is the problem then retraining is required. 36
Excess convexity This 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 causes Poor technique and the deposition of large volumes of 'cold' weld metal. 37
Acceptance The idealised design requirement of a 'mitre' fillet weld is often difficult to achieve, particularly with manual welding processes. BS EN 25817 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 3 mm for a stringent quality level to 5 mm for a moderate quality level. Surprisingly, there is no reference to a 'smooth transition' being required at the weld toes for such weld shape. 38
Figure 9 – Excess convexity 39
AWS D 1. 1 also has limits relating width to acceptable excess as follows: Width of weld face Maximum convexity W < 8 mm 2 mm W < 8 to W < 25 mm 3 mm W > 25 mm Avoidance Welder technique is the major cause of this problem and training may be required. It is also important to ensure that the parameters specified in the welding procedures specification are adhered to. 40
Oversize fillet welds (welds with a throat larger than required by the design) Oversize fillet welds can represent a significant additional cost and loss of productivity. Common causes There are some welding related causes, e. g. high welding current, slow travel speeds, and some supervision related (e. g. to be safe make this fillet bigger by xmm. ) 41
Figure 10: Oversize fillet weld 42
Acceptance BS EN 25817 notes that 'for many applications a throat thickness greater than the nominal one may not be a reason for rejection'. Where called for this standard has limits related to the actual throat ( eg for stringent quality levels, the actual weld throat [a] may exceed the nominal (design) weld throat [h] by 1+0. 15 a with a maximum of 3 mm. For the moderate quality level the maximum limit for this feature is 5 mm. Avoidance Adhere to the specified welding procedure and parameters and do not add to the specified weld size. Where possible mechanise the welding operation. 43
Undersized fillet welds (fillet welds smaller than those specified) Common causes The welding related causes are associated with high welding speeds and low welding currents. Acceptance It is normally assumed that fillet welds will be at least of the size specified. BS EN 25817 states 'a fillet weld with an apparent throat thickness smaller that prescribed should not be regarded as being imperfect if the actual throat thickness with a compensating greater depth of penetration complies with the nominal value'. That is if we can be sure there is good penetration the smaller fillet may be acceptable, however, this should be discussed with the designer of the fabrication. 44
Figure 11: Undersized fillet weld 45
Quality levels Imperfection: fillet weld having a throat thickness smaller than the nominal value Moderate D Intermediate C Long imperfections NOT permitted Stringent B NOT permitted Short imperfections (see Fig. 5) h < 0. 3 mm+ 0. 1 a max 2 mm max 1 mm Relying upon deep penetration to provide the required minimum design throat thickness can be difficult to justify. Penetration is a weld characteristic that is hard to measure directly and reliance must be placed on the stringent control of both the welding process and the welder. Manual welding can rarely be relied upon to provide the required consistency but it is an option with mechanised welding systems. 46
Avoidance Adhere to the specified welding procedure and parameters. Use sufficient current and appropriate travel speed. Where possible mechanise the welding operation. Asymmetric fillet weld (a fillet weld where the legs are of unequal length) Common causes Due to incorrect electrode positioning or to gravity pulling the molten pool towards one face of the joint. It is an mainly a problem with fillet welds made in the horizontal/vertical (PB) position. 47
Figure 12: Asymmetric fillet weld 48
Acceptance There are instances where asymmetry may be specified ( eg to place the toe stress concentration in a particular region). BS EN 25817 would, for a 10 mm leg length fillet weld ( ie 7. 1 mm throat) allow a difference in leg lengths of about 2. 5 mm at the stringent quality level and 3. 4 mm at the moderate quality level. Acceptance is related to the throat thickness. The consequence of this imperfection is a significant increase in weld volume. Provided the leg length requirement is achieved there would not be a loss of strength. Perhaps this is why, in other standards, a requirement is not specified and the acceptability is left to the inspection personnel to make the 'engineering judgement'! 49
Poor fit-up The most common imperfection is an excessive gap between the mating faces of the materials. Common causes Poor workshop practice, poor dimensioning and tolerance dimensions on drawings. 50
Figure 13: Poor fit-up 51
Acceptance A major problem with fillet welds is ensuring the gap between the components is within defined limits. ISO 5817 specifies the acceptance criteria as follows: Quality levels Moderate D Intermediate C Stringent B h < 1 mm + 0. 3 a h < 0. 5 mm + 0. 2 a h < 0. 5 mm + 0. 1 a max 4 mm max 3 mm max 2 mm Where h = fit-up gap and a = fillet weld design throat 52
Figure 7 shows that the gap results in a reduction in the leg length on the vertical plate and this, in turn, results in a reduction in the throat thickness of the joint. A 10 mm leg length fillet with a root gap of 3 mm gives an effective leg of 7 mm (a throat of 4. 9 mm instead of the expected 7 mm). This discrepancy is addressed within AWS D 1. 1. which permits a root gap of up to 5 mm for material thickness up to 75 mm. However, 'if the (joint) separation is greater than 2 mm the leg of the fillet weld shall be increased by the amount of the root opening, or the contractor shall demonstrate that the effective throat has been obtained'. 53
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