The close cooperation of the Finnish Maritime industry is evidenced in the highly capable icebreaking ships that it produces. Icebreakers are essential to Finland’s existence, as the specialized ships enable the country to keep its shipping lanes operational during the harshest winter months. However, traditional icebreaker design made for poor navigating vessels in open water, and thus were rendered useful during only the winter months. This was acceptable, but very expensive, as icebreakers by nature represent a significant maintenance challenge and cost. The solution: creating a new design icebreaker that could be used, profitably, year-round.
This was don by the Finnish board of Navigation, which has built at Finnyards and operated a trio of vessels that break ice in the winter, and work as supply ships in te North Sea oilfields during the summer. Bringing these two combinations of capabilities in a single vessel means advnanced hull design. To be able to carry more load, the hull had to be constructed using 500 Mpa yield strength TMCP steel with excellent impact strength properties, supplied by Rautaruukki.
The hull plating of icebreakers and other ice-going vessels is subjected to ice erosion that can remove protective paint coatings and leave the hull exposed to seawater. Under such extreme circumstances, preferential corrosion can develop in the vicinity of welded joints, e.g. the weld metal, the heat affected zone (HAZ) or fusion line (FL). The location and extent of preferential corrosion depends on the compositions of the weld metal and base plate and the heat input used in welding.
Accelerated weld metal corrosion can be prevented using a sufficient filler metal. HAZ and FL corrosion can be a problem when high strength shipbuilding steels are used, especially for low heat input welds.
Rautaruukki Steels of Finland, in cooperation with shipyards, has developed a range of RAEX POLAR steels for structures having stringent requirements on the seawater corrosion resistance of welded joints. The Finnish marine cluster is particularly renowned worldwide for its advanced products and systems born of necessity to keep its vital shipping lanes open during the harshest winter conditions. Moreover, the country’s industry traditionally works in tandem to co-develop and bring to fruition the systems, a technique that ensures that the solutions are the culmination of real working conditions.
The RAEX POLAR series of steels have been specifically developed to avoid HAZ and FL corrosion problems. The chemical composition of the steels is balanced to provide an optimum combination of corrosion resistance, strength, toughness and weldability. The corrosion resistance of all of the RAEX POLAR line is verified in specially designed corrosion tests, which lasted six months, under the auspices of the classification societies.