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Weld-porosity,Know Where it Comes From.SOLUTIONS with Effective, Practical Advice
Holes and Pores can be found in Fusion Welding and also in Resistance Spot or Seam Welding. Porosity is characterized by form, by location, by importance, by orientation, by origin, and by method of detection. Discontinuity, as defined in AWS A3.0, is an interruption of the typical structure of a material in its mechanical, metallurgical or physical characteristics.
Typically porosity is a limited space devoid of metal where gas was trapped during solidification. Weld-porosity consists in spherical or elongated cavities or voids, normally described by the following types. Scattered porosity occurs where several pores of different sizes are distributed irregularly in the volume of the weld metal. The cause is generally attributed
Cluster porosity has pores grouped locally with irregular distribution, deriving frequently from faulty start or stop of welding arc, or from arc blow. Piping porosity is described as cavities longer than their width, generally at right angle from the weld face. If it is present in the volume, it may or may not appear on the surface. It is caused by too rapid solidification. Elongated porosity is similar to the one just here above but its orientation is parallel to the weld axis. Finally Aligned or Linear porosity, shows pores more or less along a line, at certain locations within a weld or along a weld interface or boundary. Besides these types also elongated pores or Wormholes are described, indicating a large amount of gas being locked in the solidified weld metal. Weld-porosity in weld metals is caused by gases entrapped in the solidifying weld metal. The origin can be contamination or poor protection of the molten solidifying metal. Gases like hydrogen, oxygen and nitrogen have a larger solubility in molten metal than in the solid phase. Therefore during weld cooling, gases are expelled from the solidifying melt. They become trapped if solidification occurs before they reach the surface. The gas Carbon monoxide (CO), a reaction product of oxygen and carbon is more likely than oxygen to cause Weld-porosity. Moisture from inadequately dried electrodes, fluxes or work piece surface, and organic products like grease and oil on work piece surfaces are the main sources of hydrogen, principal cause of Weld-porosity in Al Cu, Ti, Mg. In steel welds, hydrogen is more likely to cause cracking (see Hydrogen Embrittlement) rather than porosity. Nitrogen from air causes Weld-porosity in carbon steel, stainless steel and nickel in GMAW (Mig) welding.
Dryness and cleanliness are the most important precautions to be always adopted. Poor (contaminated) shielding gas quality, insufficient or excessive flow dragging air, may contribute to weld oxidation and Weld-porosity, as well as any poor maintenance condition of welding torch, gun, or hoses. Finally one should monitor welders' skill and technique. Improper manipulation of electrode, torch or gun, and excessive arc length, electrode extension, or travel speed, may all increase Weld-porosity. The importance of the Weld-porosity actually present in welded work pieces will determine the acceptability of welded fabrications in accordance with the specific requirements. Porosity visible on the surface by visual examination is often indicative of a larger presence thereof below the surface.
An Article on Weld Quality Monitoring was published (3) in Issue 137 of Practical Welding Letter for January 2015. The nondestructive test methods most used for finding and evaluating the presence of Weld-porosity are volume scanning techniques, mainly ultrasonic and radiographic testing (see Welding Inspection). Two Resource Bulletins were dedicated to these subjects: Bulletin 6 of Mid October 2006 on Radiography Bulletin 49 of Mid May 2010 on Ultrasonic NDT The evaluation of the level of the discontinuities is based on comparison with Reference Blocks (for Ultrasonics) and with Reference Radiographs (for Radiography). In the common industrial practice the facilities and the inspectors for examining and approving welding productions, must be certified by recognized authorities as being competent and adequate to their responsible task For additional insight see the following Article:
ASTM E390-11
ASTM RRE0390CS [Complete Set]
ASTM RRE039001
ASTM RRE039002
ASTM RRE039003
To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.
Welding Standards
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