Corrosion fatigue of welded and cast-steel hybrid nodes under constant and variable amplitude loading

In a joint project of a German working group supported by the ECSC large scale tubular joints welded from fine grained steels StE 355 and StE 690 and hybrid tubulars with cast steel inserts from GS-8 Mn 7 welded in StE 355 were tested under variable amplitude loading in artificial seawater. One of the welded StE 355 nodes was tested in air, several under cathodic protection. The welded nodes from the high-strength steel underwent post-weld treatments like shot peening, profile reinforcement, and TIG-dressing. The results were evaluated for the failure criteria fatigue life to crack initiation (A is equal 1mm) and through crack. For the welded nodes from StE 355 fracture mechanical models were applied with satisfactory results. Profile reinforcement with combinded TIG-dressing gave a significantly better fatigue life compared to nodes in as-welded state. However, with hybrid tubulars, where by the use of cast steel inserts the welds were removed into areas of lower stress concentration fatigue lives higher than a factor of 100 were achieved compared to the welded nodes. The results of this investigation do not only show how to improve the fatigue life by a new design using cast steel inserts but indicate also how to revise design codes from the point of damage calculation (damage sum of 0,50 for welded nodes and 0.25 for cast steel inserts instead of the conventional value of 1.00) as well as consideration of fatigue life to initiation of a technically detectable crack with a defined depth, e.g. a is equal 1 mm.