A phenomenon which causes liquids in contact with their own vapors to reduce to minimum area, as if covered by an invisible membrane. This effect is attributed to forces that arise across the surface of the liquid because the atoms or molecules at the exposed surface are subject to interatomic forces from within the liquid. Surface tension is measured in ergs/cm2.

In welding, the phenomenon of surface tension comes into play when filler metal and slag globules come close to or in contact with the molten base metal in the weld crater, with or without the aid of gravity.

Surface tension not only attracts the liquid filler metal and slag globules into the liquid crater, but makes it possible to deposit weld metal in a horizontal, vertical or overhead position. At the same time, the surface tension determines the shape of weld contours. If it were not for surface tension, it would be impossible to deposit weld metal in any position other than flat. See OVERHEAD WELDING POSITION and GLOBULAR TRANSFER.

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