Vee Angle-Related Defects in High Frequency Welding of Tube & Pipe

The Vee Angle affects the weld quality in high frequency welding of tube & pipe.  The following weld defects can be related with it.

(1) Entrapments (Black Penetrators).  This type of defect is usually a metal oxide that has been trapped on the bond plane instead of being squeezed out with the molten metal.  These oxides are formed  on the surface of the molten metal edges in the vee.  When the vee angle is too small, the strip edges can be over heated, producing too much molten metal mixed with oxides.  The normal squeeze out does not completely remove the larger than normal liquid volume and an entrapment results.

(2) Pre-Arcs. When the vee angle is too small, a pre-arc can occur when the HF current jumps across the vee ahead of the apex, usually as a result of a silver or bit of scale falling across the vee.  The short-circuit diverts the current momentarily, robbing the vee of heat.  The very short duration of the diverted current leaves only a short defect, often no longer than the wall thickness.  It is easily observed when the weld is broken open  and has a flat, shiny surface surrounded by the woody fracture of the rest of the weld area.

(3) Open Seam.  When the vee angle is too big, the strip edges might not get sufficient weld heat.  The edges usually show a blue heat tint suggesting that some heat was input.  However, the edge face remains flat and smooth, showing no signs of having been molten.

As a conclusion, the Vee Angle may not be too small or too big.  When it stays within an appropriate range, It becomes much easier to achieve a consistent and solid weld.






References
Robert K. Nichlos, Comman HF Welding Defects, Thermatool Corp.