State of the Art
Highly loaded parts in the structure of large ships (e.g. window corners in cruise ships) are currently strengthened with thick sections of conventional steel, or the design (e.g. radius of the notches) needs to be modified. High tensile steels (HTS) can offer significant weight saving (5-20%, case-dependant), improved strength and more design freedom. While feasibility of HTS in shipbuilding has been shown in previous projects; joining processes and joint properties are currently weakening HTS structures and decreasing practical use.
Objective
Demonstration of the effectiveness of High Strength Low Alloy (HSLA) for improving the mechanical performance (static and fatigue) of the marine structures (incl. welded joints) by means of test campaign and using appropriate design tools (e.g. development numerical simulation and statistical methods).
Solution
Welding procedures and weld post-processing techniques (friction stir processing, over-lamination) are systematically investigated and tested to improve quality and increase fatigue life of welds in design details made of different HTS types, e.g. for longitudinal bulkheads. Knowledge was enhanced related to the effects of process parameters, design variations and post-processing on the fatigue performance and corrosion behaviour of typical complex HTS details in maritime structures. Failure mechanisms and approval criteria was also defined. Thanks to numerical simulation and statistical models, process reliability was verified thus to obtain pre-approval for similar applications. Smaller, real-scale specimens, sufficient to achieve fully valuable SN-curves, were produced in realistic production environments.
The specimens for the laboratory tests were cut from welded coupons and welded using the FCAW (Flux-cored arc welding) method. Qualification tests were performed on four different welded material combinations in order to assure the quality of welded joint whose fatigue and fracture behaviour was evaluated in successive tests. The qualification test consists of macro hardness, tensile, charpy, bending, and NDT test. Besides that, the corrosion, fracture toughness and fatigue test also have been done to find the best material combination for the post processing method which involves the friction stir welding and composite overlamination. The result of fatigue tests demonstrated the HSLA steels welded in shipyard conditions have fatigue performances which fulfil the requirements of the rules.
Potential
Process parameters and post-treatment methods developed in the demo are applicable for a wide variety of large structures in ships and offshore structures, including renewable energy devices. Further application is possible in land-based steel structures, like bridges or buildings.