In this thesis I focus on extending the previous researches on ultra high strength steels and their weldability especially considering the stress ratio and it’s effects on the fatigue strength of welded joints. Basis for this Master’s Thesis is formed by the research results of UL-TRASTEEL-project conducted by Professor Timo Björk at Lappeenranta University of Technology and Bachelor’s Thesis of Tuomas Skriko.

In this research I tested non-load-carrying cruciform fillet welded joints (12 specimen) under constant amplitude tensile loading and came into a conclusion that increasing the stress ratio decreases the average FAT-values. The connection between stress ratio and FAT-results is clear and the decrease of FAT-values is especially notable after stress ratio values of 0.35 and higher.

The FAT-values of post-treated welded joints were better than non-post-treated. With non-post-treated joints it also possible to reach the average workshop FAT-value of 100 MPa and above it with mechanized MAG-welding.