To use S355 steel plate for offshore applications, a full qualification is needed according to the appropriate standard EN 10225:2009: Weldable structural steels for fixed offshore structures – Technical delivery conditions.
This standard specifies a whole host of investigations to be performed, from welding with pre-defined heat input levels – using submerged arc welding (SAW) and flux cored arc welding (FCAW) – and full testing of the welds to specific tests to assess the hardenability and cold cracking sensitivity.
Metal Structures Centre performed qualifications in this field for 30 mm and 50 mm thick plates. For this qualification, welding procedure development was performed using the five-wire submerged arc welding facility as well as the MIG/MAG welding robots. After procedure development and welding of test plates, full testing was performed in-house, including cross-sectioning and metallography, hardness testing, bend and tensile testing and Charpy impact and Crack Tip Opening Displacement (CTOD) testing. In addition, TIG welding was used for the hardenability evaluation and manual metal arc welding for cold cracking tests.
Based on the results, the evaluated S355 plate steel can be sold for and applied in offshore applications.
The fracture surface analysis showed a brittle failure initiated on an intergranular failure. Characterisation of the bulk material demonstrated that the toughness of the material was insufficient.
Cold cracking (CTS) test
Cross-section of weld