13 December 2017

Welding distortion

One of the major problems found with weldments is called distortion. Distortion is caused mainly because of the shrinkage that takes place in weldments. The shrinkage taking place is a weldment depends upon the geometry and type of the weld. 

There are three types of distortions possible in welding :

  • Transverse shrinkage occurring perpendicular to the weld line.
  • Longitudinal shrinkage occurring parallel to the weld line, which is very small of the order of about 0.1 % of the weld length and hence can be neglected.
  • Angular change as a rotation about the weld line.
Now we can check the methods for reducing all three types of shrinkage :

The methods for reducing the transverse shrinkage are :
  • Decrease the total weight of the weld metal. 
  • Increase the metal deposited in the first pass.
Some other factors which influence the transverse shrinkage are :
  • Root opening.
  • Joint design.
  • Electrode diameter.
  • Degree of constraint.
The methods for reducing the angular shrinkage are :
  • Fillet welds in a structure are affected by the way the structure is designed and the type of restraint provided.
Control of distortion :

As we saw above, distortions are inevitable in a welding operation. It is, therefore, necessary to find ways by which these can be minimized to prepare to satisfactory weldment.
  • One of the important ways to control the distortions is a good design of the product with a minimum number of joints.
  • Proper design of the joint helps in reducing the magnitude of the problem.
In spite of this, there still will be distortion and therefore during the assembly process care should be taken to see that the distortions are controlled. 

There are two possibilities one is to present the members to compensate for this distortion another one is to assemble parts correctly and then apply a proper restraint to minimize the distortion during the welding process. 

The most generally preferred method in the industry is using restraint. There are many ways used for restraining such as clamps, fixtures and even tack welds. Through this method reduces the distortion, it causes high residual stresses, which may lead to cracking. Hence it is necessary to carefully apply restraint without causing too high a magnitude of harmful residual stresses. 

Another method available for reducing the distortions is the preheating of the members of the weldment such that the heat of welding would be properly balanced.