Abstract: An elastomeric mandrel has a shallow groove around the outer diameter. A woven sleeve of ceramic fiber surrounds the mandrel; which in turn is covered with a woven sleeve of fiberglass. The mandrel with coverings is inserted into tubing to be joined by welding with the shallow groove positioned under the joint. The mandrel is diametrically expanded by means located outside of the tubing to press the coverings against the inside of the tubing during time of welding, and the height of the weld underbead is controlled by the groove in the mandrel acting as a mold.