59 Porosity

Porosity consists of cavity-type discontinuities formed by gas entrapment during solidification. Porosity may be surface porosity or subsurface porosity.

Surface porosity consists of discrete spherical or elongated holes on the surface of the weld. It is formed if dissolved gases cannot fully escape before the weld metal solidifies. It may be caused by insufficient shielding gas coverage. It may also be caused by excessive gas flow rates that create turbulence that expose the molten weld to oxygen in the air. It may be detrimental to fatigue strength if aligned in a direction perpendicular to the direction of stresses.

Subsurface porosity consists of discrete spherical or elongated holes within the body of the weld. Subsurface porosity distribution is classified as uniformly scattered, cluster, or linear.

• Uniformly scattered porosity exhibits a uniform distribution of pores throughout the weld metal, with size varying from almost microscopic to 1/8” in diameter.
• Cluster porosity voids occur in the form of clusters separated by considerable lengths of pore-free weld metal. Cluster porosity is associated with changes in welding conditions, such as stopping or starting of the arc.
• Linear porosity is characterized by an accumulation of pores in a relatively straight line. The number and size of the pores and their linear distribution with respect to the axis of the weld usually define linear porosity. Linear porosity generally occurs in the root pass.

Primary causes of porosity are dirt, rust, and moisture on the surface of the base metal or on the welding consumables. It may also be caused by insufficient shielding gas coverage.

Porosity is usually the least harmful type of weld discontinuity. Many fabrication standards and codes provide comparison charts that show the amount of porosity that may be acceptable. When porosity exceeds the amount allowable, it must be ground out and repaired. Porosity is detected by radiographic testing (RT) for subsurface porosity and by visual examination (VT) or liquid penetrant examination (PT) for surface porosity. With RT, subsurface porosity has the appearance of sharply defined dark spots of rounded contour.

Porosity Prevention

Porosity is prevented by improving welding housekeeping conditions that can cause the porosity. Good housekeeping includes the use of clean materials and well-maintained equipment and the proper alignment of fans and drafts.

Also, avoiding the use of excessive current and arc lengths can prevent porosity. High currents and excessive arc lengths may cause high consumption of the deoxidizing elements in the covering of shielded metal arc electrodes, leaving insufficient quantities available to combine with the gases in the molten metal during cooling.

Specific methods of preventing porosity depend on the type of welding process. For example, changing welding conditions such as gas flow rate and gas purity for gas shielded processes compensates for improper arc length, welding current, or electrode manipulation. Reducing travel speed may also decrease porosity.