Special alloys, known as low-transformation-temperature (LTT) filler metals, alter the microstructure of the weld as it cools, thereby significantly reducing tensile residual stresses.
Source: BAM
Welds are among the most heavily stressed areas of offshore wind turbines. Researchers at the Federal Institute for Materials Research and Testing (BAM) have now demonstrated that special welding consumables can significantly increase their fatigue strength. In tests on high-strength steel components, this strength increased by 50 to 140 percent compared to conventional welds, depending on the design and seam geometry.
Offshore wind turbines consist of numerous welded components and are exposed to extreme loads from wind and waves at sea. These lead to cyclic stresses that particularly affect the weld seams. Until now, these have been considered a critical factor, especially for high-strength steels, since the welding process itself alters the material microstructure and generates harmful tensile residual stresses. For safety reasons, relevant regulations have so far only allowed the lightweight construction potential of these steels to be exploited to a limited extent.
As part of a collaborative project coordinated by BAM, novel welding consumables - so-called Low-Transformation-Temperature (LTT) filler metals - were therefore investigated. These special alloys alter the microstructure of the weld seam during cooling in such a way that tensile residual stresses are significantly reduced. In some cases, favorable compressive residual stresses even arise, which counteract the formation of fatigue cracks.
Typical high-strength steel components with additional structural reinforcement, such as those used in the towers and support structures of wind turbines, were investigated. These components are among the parts in such systems that are subjected to the highest fatigue stresses. Depending on the design and weld geometry, fatigue strength was increased by 50 to 140 percent compared to conventional welds. An additional, strategically placed LTT weld pass in the highly stressed weld zone proved particularly effective.
“Especially in offshore wind turbines, the fatigue strength of the welds is a decisive factor in determining service life,” explains Martin Hübner, lead author of the study from the Wind@BAM Competence Center. “Our results show a potential for the safe realization of higher loads and lager turbines - without time and cost-intensive post-weld treatment processes.”
A key advantage of the concept is that the desired effect is achieved directly during welding. Additional work steps are eliminated, making its application particularly attractive for industry. Looking ahead, the targeted use of LTT filler materials also opens up possibilities for repairing and upgrading existing welds.
The results are currently being discussed in expert panels and committees to explore prospects for their incorporation into regulatory standards. The findings are relevant not only for the construction of wind turbines but also for mechanical engineering, steel construction, and automotive engineering, as well as for the large specialized cranes required for the erection of future wind farms.
The project was carried out in collaboration with the Fraunhofer Institute for Mechanics of Materials (IWM) and the Research Association of the German Society for Welding and Related Processes (DVS) and was funded by the Federal Ministry for Economic Affairs and Energy as part of the Industrial Collaborative Research (IGF) program.