WELDED TUBE JOINTS
For highly reliable leak tightness, many nuclear and fossil plant owners are turning to welded tube joints. Since titanium cannot be successfully welded to other materials, a titanium tube sheet, which can be either solid or explosively bonded titanium, is required. Cladding at least 3/16" thick is preferred.

The tube should be roller expanded before welding. Either a light tack roll or a full expansion can be performed, at the technician's choice. The expansion is required to keep air from the back face from contaminating the weld and to prevent vibration of the tube which could result in weld cracking. Both the tube holes and the tube ends should be cleaned with a solvent such as acetone before welding. Methanol or chlorinated solvents such as trichlorethylene should not be used.

The welding should be done using an automatic tool such as the Kynex Titanium Welding Gun. This gun applies an inert gas to the metal surfaces during the heating, welding and cooling process and has a plug to keep the gas inside. If the metal temperature is above about 800 degrees F when the shielding is removed, the weld may become discolored indicating contamination. Dark blue- purple oxide surfaces should be removed by grinding and rewelding. Light straw colored welds have an oxide film which is formed below 700 degrees F. This color is not indicative of weld contamination and can be removed by wire brushing with stainless steel brushes.