Metallurgical defects-gas porosity, Mechanical Engineering

Gas Porosity

Porosity is the presence of gas pockets or voids caused by the entrapment of gas evolved during weld metal solidification. This Porosity is caused due to improper shielding, rust on the weld face, too much generation of gas in weld pool, fast cooling of slag, etc. Sometimes elongated tubular gas pockets are called worm holes. It is seen by testing that porosities reducing the cross sectional areas of the weld upto 5 % have no significant effect on the strength, ductility or toughness. But, when porosities are present near the surface, it could lead to failure on bend test. Porosity is considered to be the least dangerous defect but is often an indication of damp electrodes having been used.

Pores in welds in mild steel are therefore normally regarded as harmless as regards the reliability of a welded joint provided that the weld is not working under corrosive conditions and when the thinner weld material at the pores can cause leaks after a short time.

Porosity in weld in high tensile steels is a warning of hydrogen cracks in the weld metal and the heat affected zone. Pores are often linked together by a hydrogen crack.

In static test, porosity will affect fillet welds much more adversely than butt welds. The different types of porosity are

  1. Random porosity (Less Severe)
  2. Linear or aligned porosity (more severe)
  3. Cluster porosity (Moderate / More severe)
  4. Blow holes (size is greater than 1/6" )
  5. Worm holes / spring
  6. Micro porosity - Seen under microscopes only
  7. Starting porosity - formed during starting of welding
  8. Scattered porosity
  9. Pin holes

Porosity is easily detected by radiography. Surface pores are usually detected by LPI.

Posted Date: 9/18/2012 12:42:22 AM | Location : United States

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