A weld flaw is any indication on a weld that can compromise the quality of the weld. A weld flaw can be accepted or rejected depending on three factors: type, size and location of the flaw/discontinuity. All welds contain features or discontinuities. It is only when a discontinuity exceeds the relevant acceptance standard that is becoming a defect.
Difference Between a Weld Discontinuity And a Defect
A weld discontinuity (also known as weld imperfection) is any interruption in the normal flow of the structure in a weldment present. This could be in either the weld metal or adjacent parent metal. The interruption can be found in the physical, mechanical or metallurgical characteristics of the material or weldment.
Discontinuities can be defined as the irregularities formed in the given weld metal due to wrong or incorrect welding patterns, etc. The discontinuity may differ from the desired weld bead shape, size, and intended quality. They may occur either outside or inside the weld metal. Some discontinuities may not cause rejection if they are under permissible limits stated in the applicable code or standard.
Once a discontinuity or a group of discontinuities exceeds the limits stated in the applicable code or standard it becomes a weld defect. Upon discovering a welding defect, there must be appropriate rectification.
Categorisation of Discontinuities
A discontinuity can be categorised as internal or external based on their location on the weld. Also, they can be classified as either volumetric or planar according to their size, shape and orientation.
These are discontinuities which are located inside the weldment and not open to the surface of the weld. These discontinuities cannot be found with visual examination and some types of non-destructive examinations such as dye penetrant. Defects such as Solid inclusions, internal cavities and lack of fusion are under this category. These discontinuities can only be detected by NDT methods such as Radiographic Testing(RT) and Ultrasonic Testing(UT).
As the name implies, these discontinuities are on the surface of the weldment which can be detected by visual inspection and/or by other NDT methods such as Dye Liquid Penetrants (DPI) and Magnetic particle inspection (MPI).
These are three dimensional (they have length, width and thickness) such as slag inclusions and porosity.
These are two dimensional, that is, they lie on one plane, such as lack of fusion and cracks.
Types Of Flaws
There are several types of weld defects which may fall under different classifications depending on their location, size and shape in a particular consideration. We can break down weld defects into the following main categories;
- Lack of fusion
- Excess reinforcement and excess penetration
- Mechanical damage
An inclusion is a solid foreign matter that is entrapped during welding. It can be a metallic inclusion such as tungsten, copper or other metal or a slag inclusion which may be linear, isolated or grouped. It can also be a non-metallic inclusion such as sulphide and oxide which are a product of chemical reactions, physical effects and contamination which occurs during welding. Inclusion defects are usually internal and volumetric in nature. They are most commonly caused by incorrect welding parameters, incorrect manipulation of the electrode by the operator, incorrect inter-run cleaning or poor storage of consumables.
Lack Of Fusion And Incomplete Penetration
Lack of fusion is another serious weld defect which can occur as a result of;
- A lack of fusion between the weld metal and the parent metal at the root of the weld when complete penetration is required
- A lack of sidewall fusion which occurs between weld metal and parent metal at a side weld outside of the root of the weld
- A lack of inter-run fusion which occurs between adjacent layers of weld metal on a multi-run welds
These are normally detected in welds where improper welding variables have been used and where there are improper manipulations of the electrode from the welding operator. Incorrect joint design and fit-up can also lead to lack of fusion issues.
Porosity and other cavities such as wormholes and blowholes are caused by entrapment of gases in the weldment. These are classified as internal and volumetric defects.
Porosity can be;
- Linear – A line of gas pores substantially parallel to the axis of the weld
- Localised – An isolated group of gas pores
- Surface pores – A gas pore/s which breaks the surface of the weld, uniformly distributed porosity
- A number of gas pores distributed in a substantially random but uniform manner throughout the weld metal or a wormhole
- An elongated or tubular gas cavity in the weld metal
- Caused by insufficient or excessive shielding gas and contamination of the weld joint caused by oils, paints and rust
Undercut is a local reduction in a section of the parent metal alongside the weld deposit. This occurs at the toe of the weld or in the fusion face of multi-run welds. These are external and can be continuous or intermittent. Generally caused by welding current that is too high, travel speed that is too high or an incorrect operator technique.
Under-fill is a longitudinal continuous or intermittent area in the surface of a weld that is below the adjacent surface of the parent metal due to insufficient deposition of weld metal. This is external and can easily be detected by visual inspection. High welding travel speeds and high heat inputs can cause under-fill.
These are the most serious of weld defects because they can easily cause the failure of the welded structure. Depending on the orientation of the crack in the weld, it can be classified as longitudinal or transverse. Longitudinal when the direction is parallel to the weld and transverse when the direction is across the weld or at 90 degrees. Another type of crack is crater crack, which may be star-like in shape. This usually originates at the termination of a weld bead. Cracks are planar and can be internal or external. Cracks can have various causes depending on the type of crack. Crater cracks can be caused by incorrect termination of the arc and high welding currents, whereas centerline cracks can be caused by excessive joint restraint, depth to width ratio of runs or incorrect consumable selection.
Excess Reinforcement Or Penetration
For butt welds – is excess weld metal above the height of the parent metal or excess weld metal protruding through the root side of a weld made from one side of the joint.
For fillets – is excess weld metal above the specified fillet size including throat thickness.
Excess reinforcement/penetration can be caused by excessive current, too slow travel speeds and incorrect joint fit-up.
Over-Roll / Overlap
This is weld metal at the toe of the weld that covers the parent metal surface but has not fused to it. Generally caused by slow travel speed and wrong torch angle.
Mechanical damage is an indentation in the surface of the parent metal or weld caused by damage during preparation, welding, dressing or handling. These could be caused by the incorrect use of grinders, hammers, chipping hammers etc.
How To Check Welding Defects
The easiest way to make sure that your welding is of an acceptable standard is to visually inspect the welding defect. If you notice any of the issues described above, you can take steps to rectify the issue or assess if they are not going to risk the quality of your workmanship.
As a welder, it’s important to pay close attention to potential defects in your weld so as to not compromise your work on the larger project at hand. Most welding defects can be corrected with the right expertise, but the first step is identifying the issue. If you need professional help with your commercial welding project, contact us at Technoweld on 1300 00 WELD or visit our contact page.