Imperfect Shape and Dimensions according to BS EN ISO 6520-1

Types of Imperfect Shapes and Dimensions:

1. Undercut



An irregular groove at the toe of a run in the parent metal or previously deposited weld metal due to welding. Characterised by its depth, length and sharpness.
Causes:
• Melting of top edge due to high welding current (especially at the free edge) or high travel speed
Prevention: Reduce power input, especially approaching a free edge where overheating can occur
• Attempting a fillet weld in horizontal-vertical (PB) position with leg length >9mm
Prevention: Weld in the flat position or use multi-run techniques
• Excessive/incorrect weaving
Prevention: Reduce weaving width or switch to multiruns
• Incorrect electrode angle
Prevention: Direct arc towards thicker member
• Incorrect shielding gas selection (MAG)
Prevention: Ensure correct gas mixture for material type and thickness (MAG)
Care must be taken during weld repairs of undercut to control the heat input. If the bead of a repair weld is too small, the cooling rate following welding will be excessive and the parent metal may have an increased hardness and the weld susceptible to hydrogen cracking.


2. Excess weld metal



Excess weld metal is the extra metal that produces excessive convexity in fillet welds and a weld thickness greater than the parent metal plate in butt welds. It is regarded as an imperfection only when the height of the excess weld metal is greater than a specified limit.
Causes:
• Excess arc energy (MAG, SAW)
Prevention: Reduction of heat input
• Shallow edge preparation
Prevention: Deepen edge preparation
• Faulty electrode manipulation or build-up sequence
Prevention: Improve welder skill
• Incorrect electrode size
Prevention: Reduce electrode size
• Travel speed too slow
Prevention: Ensure correct travel speed is used
• Incorrect electrode angle
Prevention: Ensure correct electrode angle is used
• Wrong polarity used (electrode polarity DC-ve (MMA, SAW)
Prevention: Ensure correct polarity ie DC+ve Note DC-ve must be used for TIG
The term reinforcement used to designate this feature of the weld is misleading since the excess metal does not normally produce a stronger weld in a butt joint in ordinary steel. This imperfection can become a problem, as the angle of the weld toe can be sharp leading to an increased stress concentration at the toes of the weld and fatigue cracking.


3. Excess penetration



Projection of the root penetration bead beyond a specified limit, local or continuous.
Causes:
• Weld heat input too high
Prevention: Reduce arc voltage and/or welding current;increase welding speed
• Incorrect weld preparation ie excessive root gap, thin edge preparation, lack of backing
Prevention: Improve workpiece preparation
• Use of electrode unsuited to welding position
Prevention: Use correct electrode for position
• Lack of welder skill
Prevention: Retrain welder
The maintenance of a penetration bead of uniform dimensions requires a great deal of skill, particularly in pipe butt welding. This can be made more difficult if there is restricted access to the weld or a narrow preparation. Permanent or temporary backing bars can assist in the control of penetration.


4. Overlap



Imperfection at the toe of a weld caused by metal flowing on to the surface of the parent metal without fusing to it.
Causes:
• Poor electrode manipulation (MMA)
Prevention: Retrain welder
• High heat input/low travel speed causing surface flow of fillet welds
Prevention: Reduce heat input or limit leg size to 9mm maximum for single pass fillets
• Incorrect positioning of weld
Prevention: Change to flat position
• Wrong electrode coating type resulting in too high a fluidity
Prevention: Change electrode coating type to a more suitable fast freezing type which is less fluid
For a fillet weld overlap is often associated with undercut, as if the weld pool is too fluid the top of the weld will flow away to produce undercut at the top and overlap at the base. If the volume of the weld pool is too large in a fillet weld in horizontal-vertical (PB) position, weld metal will collapse due to gravity, producing both defects (undercut at the top and overlap at the base), this defect is called sagging.


5. Linear misalignment



Misalignment between two welded pieces such that while their surface planes are parallel, they are not in the required same plane.
Causes:
• Inaccuracies in assembly procedures or distortion from other welds
Prevention: Adequate checking of alignment prior to welding coupled with the use of clamps and wedges
• Excessive out of flatness in hot rolled plates or sections
Prevention: Check accuracy of rolled section prior to welding
Misalignment is not a weld imperfection but a structural preparation problem. Even a small amount of misalignment can drastically increase the local shear stress at a joint and induce bending stress.


6. Angular distortion



Misalignment between two welded pieces such that their surface planes are not parallel or at the intended angle. Causes and prevention are the same as for linear misalignment.


7. Incompletely filled groove



Continuous or intermittent channel in the weld surface due to insufficient deposition of weld filler metal.
Causes:
• Insufficient weld metal
Prevention: Increase the number of weld runs
• Irregular weld bead surface
Prevention: Retrain welder
This imperfection differs from undercut, as it reduces the load-bearing capacity of a weld, whereas undercut produces a sharp stress-raising notch at the edge of a weld.


8. Irregular width



Excessive variation in width of the weld.
Causes:
• Severe arc blow
Prevention: Switch from DC to AC, keep arc length as short as possible
• Irregular weld bead surface
Prevention: Retrain welder
Although this imperfection may not affect the integrity of the completed weld, it can affect the width of HAZ and reduce the load-carrying capacity of the joint (in fine-grained structural steels) or impair corrosion resistance (in duplex stainless steels).


9. Root concavity



A shallow groove that occurs due to shrinkage at the root of a butt weld.
Causes:
• Insufficient arc power to produce positive bead
Prevention: Raise arc energy
• Incorrect preparation/fit-up
Prevention: Work to WPS
• Excessive backing gas pressure (TIG)
Prevention: Reduce gas pressure
• Lack of welder skill
Prevention: Retrain welder
• Slag flooding in backing bar groove
Prevention: Tilt work to prevent slag flooding
A backing strip can be used to control the extent of the root bead.


10. Burn-through



A collapse of the weld pool resulting in a hole in the weld.
Causes:
• Insufficient travel speed
Prevention: Increase the travel speed
• Excessive welding current
Prevention: Reduce welding current
• Lack of welder skill
Prevention: Retrain welder
• Excessive grinding of root face
Prevention: More care taken, retrain welder
• Excessive root gap
Prevention: Ensure correct fit-up
This is a gross imperfection which occurs due to lack of welder skill but can be repaired by bridging the gap formed into the joint, but requires a great deal of attention.