7 Most Common Defects In Welding and its causes

Table of Contents

7 Most Common Defects In Welding and its causes

Welding Defects

The defects in the weld can be defined as irregularities in the weld metal produced due to incorrect welding parameters or wrong welding procedures or wrong combination of filler metal and parent metal.

Weld defect may be in the form of variations from the intended weld bead shape, size and desired quality. Defects may be on the surface or inside the weld metal. Certain defects such as cracks are never tolerated but other defects may be acceptable within permissible limits. Welding defects may result into the failure of components under service condition, leading to serious accidents and causing the loss of property and sometimes also life.

Different Types of Welding Defects

1. Cracks

2. Distortion

3. Gas inclusion

4. Inclusions

5. Lack of fusion and incomplete penetration

6. Lamellar tearing

7. Undercut

1. Cracks :

Cracks may be of micro or macro size and may appear in the weld metal or base metal or base metal and weld metal boundary. Different categories of cracks are longitudinal cracks, transverse cracks or radiating/star cracks and cracks in the weld crater. Cracks occur when localized stresses exceed the ultimate tensile strength of material. These stresses are developed due to shrinkage during solidification of weld metal.

Causes : Cracks may be developed due to,

poor ductility of base metal,
high sulpher and carbon contents,
high arc travel speeds i.e. fast cooling rates,
too concave or convex weld bead
high hydrogen contents in the weld metal.

2. Distortion/ Imperfect Shape

Welding methods that involve the melting of metal at the site of the joint necessarily are prone to shrinkage as the heated metal cools. Shrinkage then introduces residual stresses and distortion. Distortion can pose a major problem, since the final product is not the desired shape.

Excessive reinforcement is formed if high currents, low voltages, slow travel speeds and large size electrodes are used. Excessive root penetration and sag occur if excessive high currents and slow travel speeds are used for relatively thinner members.

Distortion is caused because of shrinkage occurring due to large heat input during welding.

3. Gas inclusion

Gas inclusions is a wide variety of defects that includes porosity, blow holes, and pipes (or wormholes). The underlying cause for gas inclusions is the entrapment of gas within the solidified weld. Gas formation can be from any of the following causes: high sulphur content in the workpiece or electrode, excessive moisture from the electrode or workpiece, too short of an arc, or wrong welding current or polarity.

4. Inclusions

There are two types of inclusions: linear inclusions and rounded inclusions.

Linear inclusions occur when there is slag or flux in the weld. Slag forms from the use of a flux, which is why this type of defect usually occurs in welding processes that use flux, such as shielded metal arc welding, flux-cored arc welding, and submerged arc welding, but it can also occur in gas metal arc welding.

Causes: This defect usually occurs in welds that require multiple passes and there is poor overlap between the welds. The poor overlap does not allow the slag from the previous weld to melt out and rise to the top of the new weld bead.

5. Lack of fusion and incomplete penetration

Lack of fusion is the failure to fuse together either the base metal and weld metal or subsequent beads in multipass welding because of failure to raise the temperature of base metal or previously deposited weld layer to melting point during welding. Lack of fusion can be avoided by properly cleaning of surfaces to be welded, selecting proper current, proper welding technique and correct size of electrode.

6. Lamellar tearing;

Lamellar tearing is a type of welding defect that occurs in rolled steel plates that have been welded together due to shrinkage forces perpendicular to the faces of the plates.
Lamellar tearing is caused mainly by sulfurous inclusions in the material. Other causes include an excess of hydrogen in the alloy. This defect can be mitigated by keeping the amount of sulfur in the steel alloy below 0.005%
Modifying the construction process to use casted or forged parts in place of welded parts can eliminate this problem, as Lamellar tearing only occurs in welded parts.

7.UnderCutting :

Undercutting is when the weld reduces the cross-sectional thickness of the base metal and which reduces the strength of the weld and workpieces. One reason for this type of defect is excessive current, causing the edges of the joint to melt and drain into the weld; this leaves a drain-like impression along the length of the weld. Another reason is if a poor technique is used that does not deposit enough filler metal along the edges of the weld. A third reason is using an incorrect filler metal, because it will create greater temperature gradients between the center of the weld and the edges