3 May 2017

Defects in welding

In view of the severe thermal regime through which the welding process proceeds the weldments are likely to be affected and if proper care is not taken they are likely to end up with certain defects.

Distortions have been discussed in greater detail earlier, and we will see the other defects here. The likely defects are :

  • Undercut
  • Incomplete fusion
  • Porosity
  • Slag inclusion 
  • Hot cracking 
  • Cold cracking 
  • Lamellar tearing 
  • Lack of penetration or Excess penetration
Now we can see the above defects in details :

  • Undercut : 
The undercut is an extremely common welding defect it appears like a small notch in the weld surface.
It is generally attributed to the improper welding technique or excessive welding current.

Undercut defect

  • Incomplete fusion :
It occurs when individual weld beads don't fuse together or don't fuse properly to the base metal that you are welding. This will be seen as a discontinuity in the weld zone. 
The main cause for this defect is improper penetration of the joint and wrong design of the joint or incorrect welding technique including the wrong choice of the welding parameters.

Incomplete fusion defect
  • Porosity :
It is caused by the presence of gases which get entrapped during the solidification process. The main gases that cause porosity are :
  • Hydrogen
  • Oxygen 
  • Nitrogen 
There may be also some other gas that causes porosity like helium, argon and carbon dioxide that are also present in weld pool but in view of their insolubility, they do not cause porosity.

Porosity if present in large would reduce the strength of the joint.

Make sure all material are clean before start welding and try to using low hydrogen electrodes that may reduce the chances of porosity.

Porosity defect
  • Slag inclusion :
Slag is formed by the reaction with the fluxes. It is generally lighter. In view of its low density, it will float atop of the weld pool and would be chipped off after solidification.

Some of the factors that cause slag inclusion are :
  • The high viscosity of weld metal 
  • Rapid solidification 
  • Insufficient welding heat
  • Improper manipulation of the electrode
  • Undercut on the previous pass
Also, in multi-pass welding the slag solidified in the previous pass is not cleaned before depositing the next bead, which may cause slag inclusion.
Slag inclusion is like porosity, weakens the metal by providing discontinuities.

  • Hot cracking :
It generally occurs at high temperature and the size can be very small to visible.
The crack in most parts is intergranular and its magnitude depends upon the strains involved in solidification. 
It generally happens only in steels and its caused by deformities in the structure of the steel. They are more likely to form during the root pass when the mass of the base metal is very large compared to the weld metal deposited.
It can be prevented by preheating the base metal, increasing the cross-sectional area of the root bead, or by changing the contour or composition of the weld bead.

  • Cold cracking :
Cold cracking occurs at room temperature after the weld is completely cooled. It can be seen in the heat-affected zone. 
Mainly cause for cold cracking is :

  • Excessive restraint of the joint which induces very high residual stresses.
  • Martensitic transformation making the metal very hard as a result of rapid cooling.
Pre and post-heating of the weldment help in reducing the cooling rates and the consequent locking of the stresses.

  • Lamellar Tearing :
It is generally seen at the edge of the heat-affected zone. It appears as a long and continuous visual separation line between the base metal and heat-affected zone.
This is caused by the presence of the elongated inclusions such as Mn Fe and S in the base metal. 
Lamellar tearing can also be caused by the weld configuration which gives rise to high residual tensile stresses in the transverse direction.

  • Lack of penetration :
In complete penetration happens when your filler metal and base metal are not joined properly and the result is a gap or crack appears.
Lake of penetration defect
Welds that suffer from incomplete penetration are weak at best, and they will likely fail if you apply much force on them.