What is Welding Defect?
Welding defect can be defined as “The Irregularities formed in the weld metal due to wrong welding process, incorrect welding patterns, incorrect welding parameters or incorrect combinations of filler and parent metal”.
Any irregularity/discontinuity when surpasses some specific code limit in the weld metal then is termed as “Welding Defect”. Discontinuity/irregularity is termed as defect only when it exceeds some specific code limitations. As a result we can conclude that every defect is a discontinuity but every discontinuity can not be termed as defect.
Defects are so common that they are parts of any kind of manufacturing. Welding defects can produce different kinds of variations from the desired welded bead shape, expected quality, size & technical requirements.
Welding defects may be visible on the surface or hidden inside the weld metal. Different kinds of crack are acceptable within permissible limits while “CRACK” is the only defect that is not accepted at all.
Percentage Division of Welding Defects
Welding defects causes have been broken down according to “American Society of Mechanical Engineering” as
- Poor Process conditions 41%
- Percent Operator error 32%
- Wrong Technique 12%
- Incorrect consumables 10%
- Bad weld grooves 5%
Action Needed for the Presence of Welding Defect
If a welding defect is detected then the following list of precautions can be taken to remove or reduce the chances of occurring of defects.
- If a defect is detected in a weld then it should be temporarily rejected & put on HOLD.
- One should look for the cause and try to use a corrective action to avoid its repetition in the future.
- An authorized professional should be consulted to find out whether the defect is repairable or not if it is then with the help of which procedures Standard procedures are approved for routine applications.
Standards Used for Evaluation of Welding Defects
All discontinuities in welding are not defects. Discontinuities are acceptable if they fall under a specified limit.
Keep in mind, acceptance standards may vary with requirement of services.
Radio-graphic standards are used for the evaluation of weld defects which are given below.
- IIW (INTERNATIONAL INSTITUTE OF WELDING) standards
- ASTM (AMERICAN SOCIETY OF TESTING MATERIALS) standards
IIW consists of the following 5 standards
ASTM consists of following standards according to ASTM E-390 Vol-II
DEFECTS NOT ALLOWED
- Lack of fusion
- Burn through
- Elongated Porosity
- Slag inclusion up to level-III
- Incomplete Penetration up to level-II
- Undercut up to level- IV
>Cluster porosity up to level-III
>Fine Scattered porosity up to level-IV
>Coarse scattered porosity up to level-II
How Defects Put an Impact?
If welding defects are ignored they can become a serious threat. Defects can result in failure of different components that are under service conditions and can cause serious accidents & property loss or even can cause death sometimes.
Common Precautions for Welding Defects
Having already said that, defects are parts of welding but you can take necessary precautions to reduce the chances of defects. Few common precautions before starting welding include
Classification of Defects According to their Appearance
Welding defects have been classified into 6 groups according to IIW
All types of crack defects are included in this category like crater crack, hot cracks, and cold cracks.
All types of cavity defects are included in this category like porosities, blowholes, shrinkage & pipes.
- Incomplete Fusion & Penetration
This includes all types of lack of fusion & lack of penetration.
- Solid Inclusion
Solid inclusion includes slags, metal oxides, tungsten & wagon track, etc.
- Imperfect Shape
Imperfect shapes include undercut, underfill, overlap, excessive penetration & improper bed shape, etc.
- Miscellaneous Defects
This category includes arc strike, excessive spatter, rough spatter, uneven ripples, etc.
Categories of Welding Defects
Welding defects have been classified into two categories.
External defects consist of following common types of defects.
- Arc Blow
- Excessive Penetration
Now let’s look at each visual defect thoroughly with examples & real-life images where necessary.
The most common type of welding defects is a crack. Cracks may be of micro or macro size. They can appear almost everywhere in the weld like, on the surface, inside the weld or at heat-affected zones.
How Cracks Occur?
Weld Cracks occur due to the intense heat of welding i.e. when localized stress exceeds the limit of tensile strength. During solidification of weld metal process shrinkage occurs and these stresses are developed.
Causes & Remedies of Weld Cracks
Categories of Cracks
- Longitudinal cracks
- Transverse cracks
- Radiating cracks
- Welder crater cracks
Types of cracks
- Hot cracks
- Cold cracks
- Crater cracks
Shrinkage cracks formation during the solidification of weld metal is known as hot cracking. It is also known as hot shortness, solidification cracking, liquation cracking & hot fissuring. When the available liquid weld metal is inadequate to fill the spaces between solidifying metal then these are cracks are certain to appear.
Cold crack is generally known as spontaneous crack & occurs when the solidification process is completed & temperature is below 200oC. Cold cracking is sometimes also known as “Delayed Cracking” because it can develop hours or days after the welding has been completed. Cold cracking occurs mostly in carbon steels, low alloy steel & high alloy steels & it can be prevented only when preheating is employed.
If the arc is broken before the crater is filled, it will cause the outer edges to cool faster than the crater & will cause crater cracks. Crack is formed as a result of produced stresses. When the weld pools start cooling down and solidification occurs, it must contain adequate volume to overcome weld metal shrinkage, if not then crater cracks will be formed. Crater cracks occur at the ending point of a weld. Worst disadvantage of crater cracks is that they can cause more cracks
The kinds of welding defects that occur when gas or small bubbles are trapped in the welding zone is called as porosity. Porosity is actually tiny holes in the weld.
Porosity can cause a serious problem in STICK or MIG welding. When molten puddle cools down it releases gases of hydrogen and carbon dioxide, if these gases don’t reach the surface before solidification then they get trapped in the weld & cause weakness of joints.
Causes & Remedies of Porosity
This welding defect occurs when the base of the metal melts away from the weld zone, consequently forming a groove in the form of a notch at the toe of the weld.
Undercut reduces the CROSS-SECTIONAL thickness of the base metal, which results in the weakness of the workpiece and weld.
When more than one passes are welded on the joint, chances of undercutting increase. If you do a pass & notice undercutting, you should remove the undercut before making the next pass. Otherwise, there is a risk of trapping slag onto the welded joint. Undercutting can ruin the QUALITY of the weld. Also, undercutting can be easily spotted after weld.
Causes & Remedies of Undercut
This welding defect occurs when weld face extends more than the toe of the weld, this causes the weld metal to roll and & form an angle less than 90O.
Causes & Remedies of overlap
This welding defect occurs when small particles of the metal attach themselves to the surrounding surface of the material. This is most common in gas metal arc welding (GMAW). It can’t be completed eliminated from the welding instead it can only be minimized while grinding.
Causes & Remedies of Spatter
Arc blow is actually the deflection of electric arc from its intended path i.e. the axis of the electrode. Arc blow is similar to the wavering of flame due to the wind’s, the only difference is arc blow happens due to magnetization instead of wind.
Arc blow is mainly caused when DC arc welding is used. It happens due to the interaction between different electromagnetic fields around the welding arc.
Common Situations that can Cause Arc Blow
There are commonly two situations of interaction between electromagnetic fields
- Interaction due to current flow through the arc gap & that due to current flow through plated being welded.
- Interaction due to current flow through the arc gap & that is localized while welding near the edge of the plates.
Kinds of Arc Blow
Major Causes of Arc Blow
Remedies of Arc Blow
Excessive Penetration (Excess Penetration Bead)
The type of defect which occurs when weld metal is lying outside the plane joining the toes is called as excessive penetration. Excessive penetration produces notches which are responsible for stress concentration. It is also completely an economic waste.
Causes & Remedies of Excessive Penetration
Internal defects consist of following common types of defects.
- Slag Inclusion
- Incomplete/Lack of Fusion
- Necklace Cracking
- Incomplete/Lack of Penetration
Now let’s look at each hidden defect thoroughly with examples & real-life images where necessary.
Slag inclusion is one of those welding defects which is easily visible in the weld. Slag ruins structural performance & quality of the weld. Slag is actually the waste material which is produced during welding process. Bits of slag that contains flux, rust & tungsten material are trapped into the weld area and become source of contaminations. Slag is actually a byproduct of shielded metal arc welding, flux cored arc welding, submerged arc welding & even gas metal arc welding.
Causes & Remedies of Slag Inclusion
Incomplete/Lack of Fusion
The type of welding defect which occurs when weld beads don’t fuse together properly or there is a lack of proper fusion between the base and weld metal. As a result a gap is created & is not filled with the molten metal.
Most common type of the incomplete fusion is known as overlap & occurs at the toe of the weld.
Causes & Remedies of Incomplete Fusion
Necklace cracking occurs when EBW (Electron Beam Welding) is used. Most of the time this defect occurs when the weld doesn’t penetrate properly. In the presence of improper penetration molten metal doesn’t flow into the cavity & a cracking defect is formed which is known as Necklace Cracking.
Causes & Remedies of Necklace Cracking
Incomplete/Lack of Penetration
Incomplete penetration occurs when filler and base metals are not joined properly or metal groove is not filled completely. This welding defect mostly occurs in butt welds. A gap or some sort of a crack is formed as a result of incomplete penetration. Welds which contain incomplete penetration are weaker and are almost useless.
Causes & Remedies of Incomplete Penetration
Since welding defects can put a huge impact on the failure of any components that is why early inspection & modification is inevitable to avoid any kind of failure which may lead to greater loss of either property or human deaths.
Highly advance & sensitive techniques are necessary for the detection of any discontinuities or defects so that danger of risk can be removed earlier by repairing/maintenance where necessary.
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