Effect of steel grade on the fatigue strength of butt welds in rolled and cast (very) high strength steels

This paper presents fatigue experiments of single-sided V-shaped butt welds in very high strength rolled and cast steel plates in as-welded condition. The rolled plates were of grades S460, S690, S890, and S1100 (nominal yield stress between 460 MPa and 1100 MPa) and the cast plates were of grades of similar strength. The fatigue resistance of the specimens with cracks initiating at the weld root was lower than that of those with cracks initiating at a weld toe. A positive but weak correlation was observed between ultimate tensile strength and fatigue resistance. This positive correlation was further confirmed by evaluating large databases of butt welds. The evaluation also confirmed the hypothesis that a stronger correlation occurs for welds with smaller notch effects, i.e., ground flush butt welds. Some specimens with casts delivered in Quality Class II according to ASTM E 446 failed in the cast plates, and these initiated from cast imperfections such as shrinkage cavities. Additional experiments delivered in Quality Class I did not fail in the cast base metal, or failed only after repairing prior cracks that grew from the welds. Unexpectedly, some failures were observed in the rolled steel plates away from the V-shaped butt welds (and away from the grips). Metallurgical examination revealed that the cracks in these specimens initiated at ground flush welds applied in the rolling direction, in delivered condition, likely to repair rolling flaws. Therefore, repair by welding of rolling flaws should not be allowed if relying on the high fatigue resistance of the base metal.