7 Joint
Cleaning Clean
surfaces are essential for good welding. All foreign material such as oil, grease,
water, paint, rust or scale
must be removed. These substances generate gas when exposed to the heat of the
welding zone. The
gas may become entrapped in the molten weld metal and cause porosity. Even a crayon
mark on the surface
can ruin an otherwise good weld. (See Fig. 31.) Result:
Weld porosity, bad weld structure.
JOINT CLEANING
CROSS SECTION
CLEANED PLATE
TOP CONTAMINATED
PLATE Result:
Clean, sound weld FIG.
31 - Effect of Plate Cleanliness on Finished Welds
