HSLA STEELS WELD WELL UNDER PRESSURE.

The welding laboratories experimented with hyperbaric autogenous gas tungsten arc welding in a vessel under air pressures ranging from 1 atmosphere (103 kPa) to 165 psia (1138 kPa). The experimenters fed shielding gas (H, Ar, or N//2) through the welding torch at pressure slightly higher than surrounding pressure. The experimenters tested ASTM A633 and API X-80, both HSLA steels. Metal welded under pressure shows higher ductility than the same metal welded at atmospheric pressure. The hyperbaric process also constricts the arc and stirs the weld puddle. Dendrites form during solidifation, so the stirring breaks off dendrites and pieces of dendrites. The result is smaller grains and a more random orientation. The high gas velocity cools the weld puddle, further contributing to fine grain size. All of these counter the tendency to grain growth. The microstructures of these welds prove that dendrite width decreases as the surrounding pressure increases. High pressure welding allows less alloy metal loss.