WELDING DEFECTS, CAUSES AND PREVENTION

Texts : Shikh Ahmad Hanif / Amir Azli Mohd Al-Kharid

What is welding defects? Welding defects may be define as a discontinuity or discontinuities that failed to meet the minimum acceptance per standard or specifications. Defects may occurred in excessive conditions, outside the acceptance limits which risks to compromise the stability or the functionality of the welded structure. They are also acceptable discontinuities, the same type of discontinuity of a lesser degree, might be considered harmless and acceptable.

Several commons defect found on welding are as follow:

1. POROSITY

It is a cavity type discontinuity (void) formed by gas entrapment during solidification.

Gases present in weld environment H2, N2, O2, CO, CO2 etc from flux covered electrodes.

These gases trapped during solidification and cause porosity.

Porosity on weldment

Porosity as seen on RT film

Factors promoting porosity:

• Improper coating on the electrode.

• Longer arcs

• Faster arc travel speeds.

• Too low and too high arc currents

• Incorrect welding technique (stringer beads are more apt  than moderately weaved beads.

• Unclean job surface

• Improper base metal composition (e.g. high S & C content will lead to porosity) 

Porosity remedies:

• Use low hydrogen welding process, filler metals high in deoxidizers for excessive H2,N2 atmosphere. 

• Use preheat or increase heat input to combat high solidification rate.

• Clean joint surfaces and adjacent surfaces.

• For reducing excessive moisture use recommended procedure for baking and storing electrodes

2. CRACKS

A crack is a fissure produced in a metal by tearing action due to in-adequate pre-heat or fast cooling problem. Cracks may also caused by shrinkage stresses in high constraint areas.

Crack on weldment

Cracks further classified into:

1. Direction with respect to longitudinal axis of the weld
2. Longitudinal crack:Those lying in a direction parallel to the longitudinal weld axis.
3. Transverse crack: Those lying in a direction perpendicular to the longitudinal weld axis. 

Hot Cracks
Cracks formed immediately after welding. Also called centerline or solidification crack.

Centerline crack

Hot Cracking Causes:

• Imposition of excessive strains in restrained welds

• Parent metal composition with excess sulphur,phosphorus

• Base metal contamination

• Inadequate filling of crater ( crater cracking)

Hot Cracking Remedies:

• Plan welding procedures which induce less thermal strain

• Use low sulphur, phosphorus containing base metal for welding

• Avoid contamination of base metal

Cold Crack

Delayed crack

They are also called as underbead crack or  delayed crack. Generally occurs in HAZ (Heat Affected Zone). They lie in directly adjacent to weld bead in haz area, subsurface and difficult to detect. However if the cold crack propagate to surface, the crack can be seen visually. Cold cracking may due to metal brittleness combined with a tensile stress exceeding the fracture stress.Brittleness is the result of :

(a)Solidification of low melting constituents. 

(b)Phase changes (e.g. formation of martensite) during cooling.

 
Factors promoting cold crack:

• Joint restraint and high thermal severity

• Hydrogen in the weld metal.

• Presence of impurities.

• Embrittlement of HAZ in low alloy steels.

• Weld of insufficient sectional area.

• High welding speeds and low current density.

• Cold cracking occurs in both weld metal and HAZ.

Cold craking remedies:

• Welding consumables must be hydrogen controlled

• Bake electrodes as recommended by electrode manufacturer

• Preheat as per Welding Procedure Specification

• Heat input to be maintained even for tack welding

 
3. INCOMPLETE JOINT PENETRATION

When the weld metal does not extend to the required depth into the joint root

Causes of Incomplete joint penetration:

• Improper Joints( U joint gives better than J butt joint)

• Too large root face.

• Root gap too small.

• Less arc current.

• Faster arc travel speed.

• Too large electrode diameter.

• Joint misalignment.

     
Remedies of Incomplete penetration:

• Use proper joint geometry.

• Increase root opening or Use small electrode in root.

• Follow correct WPS (Welding Procedure Specification).

4. LACK OF FUSION (LOF)

Weld discontinuity in which fusion did not occur between weld metal and fusion faces joining or adjoining weld beads.

Factors promoting LOF:

• Improper manipulation of welding electrode.

• Weld joint design.

• Improper heat input.

• Surface contamination which leads slag formation and prevents fusion

LOF Remedies:

• Minimum heat input to be maintained

• Correct Electrode angle

• Avoid molten pool flooding the arc

• Proper cleaning of oxides slag

5. INCLUSIONS

• Inclusion is entrapped foreign solid material, such as slag,flux,tungsten or oxide. 
• Associated with Lack of Fusion(LOF). 
• Only occurs when any welding technique uses Flux shielding. 
• Due to improper manipulation of welding electrode and improper cleaning between multi run

Inclusions found in weldment

Inclusions as seen on RT film

Remedies for inclusion

• Better inter-pass cleaning

• Remove heavy convex beads   

• Do not use electrodes with damaged coating

6. UNDERCUT & UNDERFILL

Undercut

Undercut is a surface discontinuity which occurs in the base metal directly adjacent to the weld.

It is a condition in which the base metal has been melted away during the welding operation and there was insufficient filler metal deposited

This result is a linear groove in the base metal which may have a relatively sharp configuration.

Cause: Improper welding technique, Too high welding heat

Underfill

Underfill is a condition in which the weld face or root surface extends below the adjacent surface of the base metal.

Underfill at the root weld of Pipes are termed as INTERNAL CONCAVITY or SUCK BACK

It can be cause by excessive heating and melting of root pass during deposition of second pass.

Cause: Excessive travel speed

7. MISALIGNMENT

Offset or mismatch occurs where two pieces welded together are not properly aligned. It occurs because of carelessness and  also due to joining different thicknesses (transition thickness).

Sample of misalignment

Misalignment as seen on RT film

8. SPATTER


AWS A 3.0 describes spatter as “metal particles expelled during fusion welding that do not form a part of weld. Small particles of weld metal expelled from the welding operation which adhere to the base metal surface. The cause for this discontinuity are: High welding current which can cause excessive turbulence in the weld zone,long arc length, severe electrode angles, high amperages. Its presence gives difficulty in UT,MT,PT

Spatters on weldment

Excessive turbulence in the weld zone

 
*Writer is student of UniKL Malaysia France Institute in Bachelor of Welding and Quality Inspection. He just completed 3 months industrial training  and can be contacted via email sahanif89@gmail.com