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Saving a national treasure

Sweden's historic Vasa ship is under threat from iron leakage from its steel bolts. In 2011, the Vasa Museum joined forces with Sandvik in a research project to preserve the wooden warship for future generations.

Vasa, a world-famous wooden warship that sank on its maiden voyage in 1628 and was recovered in 1961, needs work if it is to survive into the future. Built from more than a thousand oak trees, Vasa's hull was originally held together with 5,500 low allow steel bolts. However, during the 333 years the ship spent at the bottom of the Baltic Sea, the bolts corroded and so were replaced during the boat's restoration in the 1960s. Now, after 50 years in the humid environment of Stockholm's purpose-built Vasa Museum, Vasa's oak hull is suffering from iron leakage from the replacement bolts.

It was important to find a new material that would not compress or damage the hull. In cooperation with Sandvik, Anders Ahlgren, an engineer at the Vasa Museum, decided to replace all of Vasa's 5,500 bolts with specially designed duplex stainless steel bolts. Before undertaking this massive project, museum officials decided on a trial replacement of 1,000 bolts. The bolts were installed in 2011 and monitored for a year using sophisticated instruments that measured movement and pressure on the ship to an accuracy of a hundredth of a millimeter.

"The evaluations show that we are on the right track and can continue to switch the bolts," Ahlgren says. "In collaboration with Sandvik, we have managed to cope with the delicate balancing act of finding the smallest possible pressure from the bolts so that they hold the hull together without damaging it."

Sandvik has used two of its strongest and most corrosion-resistant alloys, Sandvik SAF 2507 and Sandvik SAF 2707 HD, for the bolts. These materials are often used in extremely tough environments, such as in the oil and gas industry. All 5,500 bolts are scheduled to be replaced by 2017 and are expected to hold Vasa together for at least 150 years

The world's most powerful warship
Vasa, planned to be the world's most powerful warship and the pride of Gustav II Adolf, was completed in Stockholm in 1628. On her maiden voyage, she sank after sailing less than one nautical mile. The shipwreck was brought to the surface in 1961 and restored to its full glory. Today Vasa resides in the Vasa Museum, a maritime museum in Stockholm that was constructed around the ship. More than 1 million people visit Vasa each year.

Read more and watch film about the collaboration between Sandvik and the Vasa Museum

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