Hyperbaric welding was performed in an unmanned chamber at pressures up to 30 bar (300 m depth). A flux-cored and a solid wire were used as consumables; two typical offshore steels served as base materials. The study, which mainly focussed on the weld metal properties, included chemical analysis, detailed microstructural characterization, tensile and charpy impact testing, and for selected conditions a fracture mechanics analysis. When using argon as pressurizing gas, the impact toughness decreases only slightly at higher pressures «15%); the room temperature tensile properties and COD values for crack growth initiation show no pressure dependence. The solid wire has slightly lower impact toughness values than the flux-cored wire. An attempt is made to explain the mechanical properties on the basis of the observed microstructures.
For more than a decade extensive efforts have been made in the development and application of underwater welding /1–11/. This is mainly due to the repair and construction work which has to be carried out on underwater parts of pipelines or platforms installed by the offshore industry /12–14/. Various methods are used depending on the water depth and the type of work; recent results have been summarized in numerous reports eg /2,3,5/. Underwater wet welding, for example, is characterized by the welder being in the water and the weld region being either wet or covered by a small gas filled chamber. Dry, or hyperbaric, welding is performed with the welder and the weld area in a gas filled chamber, which is held at the pressure of the surrounding water. In addition, 1 bar.
Welding is possible by keeping the pressure in such a chamber at 1 bar. There are two important factors which complicate underwater welding, namely the pressure and - especially for wet welding the water. These can, for example, cause changes in the arc geometry, temperature and stability /2,3,15/, variations in the metallurgical reactions /1–6,12,16,17/, strong effects on the cooling times /4-7,9–11/, and increased hydrogen concentrations in the weld /13/. Although code quality underwater welding is possible today to a limited extent, in most of the studies a loss of toughness has been observed with increasing water depth /1,4,5/; a complete understanding of this embitterment has not yet been achieved.
The GKSS research center has started an extensive marine technology program, which includes underwater welding and which is focussed on both technological development and more fundamental research. As part of this underwater program, a series of tests with dry hyperbaric MIG welding will be described in this paper. The tests were carried out at pressures from 1 bar to 30 bar. The materials selected were a pipeline steel StE 445.7TM (=X65TM), and a structural steel StE36 (500) which is used for platform components. Based on preliminary tests two commercial C-Mn-Si wires, one a flux-cored wire with basic flux, and, the other a solid wire, were chosen.
