Significance of defects in welds, Mechanical Engineering

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SIGNIFICANCE OF DEFECTS IN WELDS

 

In the correct sense of the word, a defect is a rejectable discontinuity or a flaw of rejectable nature. Certain flaws acceptable in one type of product need not be of acceptable nature in another product. A defect is definitely a discontinuity but a discontinuity need not necessarily be a defect. Acceptance or rejection of flaws is based on different factors and to mention a vital few are:

  • Stresses to which the parts will be subjected during service.
  • Type of material used.
  • The temperature and pressure to which the parts will be stressed.
  • Its thickness.
  • The environment (corrosive or non corrosive).
  • Safety.
  • Consequences of failure.
  • Cost and accessibility for repair, etc.

 


Acceptance standards dictate the type of inspection and testing the weld is subjected to before giving a judgement. The quality control incharge shall analyse whether the flaw is critical, major or minor and whether the flaws are inherent because of the process or are due to the processing or service conditions. It is of immense importance to see that the base material used for fabrication shall be of good quality and attested materials are demanded in the manufacture of space vehicles and ships, submarines, pressure vessels, power boiler components, heavy duty cranes, structures, bridges, etc., wherein the failure of weld will lead to loss of life, money and reputation.

The weld defects can be broadly classified into two types. They are:

  • Planar defects or two dimensional defects.
  • Voluminar defects or three dimensional defects.

 


Planar defects such as crack, lack of fusion, lack of penetration, severe undercut are critical in nature and  involve lack of bonding and are not tolerated to any extent. Voluminar defects such as slag inclusion, cavities, pores, etc. are tolerated to a certain extent depending on the product class. Geometric defect such as excess reinforcement, underfill or underflush, root concavity are also permitted to a certain extent. If they form sharp notches, they are smoothened out wherever accessible to avoid stress concentration.


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