Fatigue behaviour of butt welded joints in a high strength steel

Modern technological processes make it possible to produce high strength steel with up to 1200 MPa yield strength. This high yield strength gives potential for considerable improvements in performance and reduction in weight, which are of increasing importance in the transport sector and in vehicles used in the construction industry. However, data found in literature show that welded structural parts using these high and ultra-high strength steels have a fatigue resistance not very different in comparison with those obtained in conventional steels with a much lower yield stress.In this work a fatigue study was performed in a high strength steel in order to evaluate the influence on fatigue strength of the following factors: stress concentration at weld toe, internal defects and welding process method. The material used in this study was a steel sheet called DOMEX 600 DC from SSAB with 5 mm of thickness. This is a high strength steel with a yield stress of 670 MPa and a tensile strength of about 750 MPa.Butt welded specimens using MAG welding process were submitted to cyclic loading in servo hydraulic machine. Three welding conditions of transverse butt joints and base material were investigated in this fatigue study: as welded; welds overfill removed by grinding; welds overfill and first weld root removed by grinding.The overfill removing by grinding promotes a significant improvement of the fatigue resistance in comparison with the as welded condition. The removing of the root of the first weld by disc grinding improves significantly the weld quality by reducing the level of internal defects, leading to a further increasing on fatigue strength close to the parent material.The comparison of characteristic curves obtained for each welding condition and the fatigue class (FAT) indicates that, in general, fatigue strength of DOMEX 600 DC steel is significantly higher than fatigue strength recommended by IIW for conventional steels. The higher difference was found for welds with overfill removed and the first weld root submitted to a disc grinding operation.