A mechanical process for removing a specimen from a welded seam so that the weld metal may be examined. The specimen, or plug, should include the entire width of the weld seam and enough of the adjacent parent metal to allow observation of the degree of fusion. The plug is cut with a circular saw which has a pilot (drill) in the center. An electric drill, pneumatic motor, or drill press may be used to drive the trepan.
The trepanned plug is usually polished to a bright, smooth finish and then etched. A satisfactory etching reagent for steel specimens is a 50% solution of hydrochloric acid used at a temperature: of 70°C (160°F). At this temperature, the solution is active but there is negligible change in concentration because of evaporation.
The principal defects which may appear in the etched specimen (as noted in both the API and ASME codes) are lack of fusion, slag inclusions, gas pockets, cracks and undercutting.