Improvement opportunities on fatigue and corrosion behaviors in offshore fastener threads combining a maraging steel skin with a class 10.9 32CRB4 core

The new generation of large offshore wind turbines (+15 MW) will require an improvement in the bolted joints used in these turbines. From the material perspective, this improvement will involve the development of fasteners that combine both corrosion-resistant properties and enhanced high-cycle fatigue performance, reducing the frequency of maintenance in the installations. This work explores the possibility of using L-DED (laser-directed energy deposition) technology to create hybrid fasteners consisting of a high-toughness 10.9 class carbon steel core and a 1.5 mm maraging steel coating that provides good resistance to marine corrosion. C300, 15–5 PH, and 17–4 PH maraging steel grades were tested. The results show that, in addition to corrosion protection, the 15–5 PH and 17–4 PH coatings improve high cycle fatigue performance compared to the fastener fully made in 32CrB4 used as a reference. Fifty percent of the coated fasteners tested in this work were fatigue runouts after 5 × 10⁶ cycles, while none of the reference specimens reached such condition. The average fatigue life in the coated specimens was around 3.5 × 10⁶ cycles, 3 times higher than the reference (average fatigue life slightly higher than 1 × 10⁶ cycles). Another advantage to consider is that the application of the coating via L-DED does not impose limitations on the subsequent turning and thread-rolling operations, nor does it require additional finishing steps.

Acknowledgements: The authors would like to thank ERREKA Smart Fastening for enabling the machining and thread cold-rolling operations to be carried out at their facilities and supplying personnel for the related trials.

Funding: This research is supported by the Department of Economic Development, Sustainability and Environment of the Basque Government (HAZITEK 2022 Programme).