Russia’s new nuclear icebreaker, Leader, will be protected by an ice belt made of extra strong steel, according to Sergey Golosienko, deputy head of the research and production complex of the KM Prometey Central Research Institute.
The ship will be able to navigate ice 4 metres thick. The thickness of metal sheets for nuclear icebreakers can be up to 60mm, and in some cases 80mm for areas subjected to the most intense loads. This is the ice belt, part of the hull from the bow to approximately the middle of the ship's length on both sides. However, even high-strength ferrous metal is vulnerable in the Arctic seas.
Operation of the first nuclear-powered icebreaker, Lenin, showed that the combined effect of ice and sea water leads to corrosion-erosion of the outer layer. In the first years of operation of the icebreaker, the roughness of the steel skin increases more than tenfold before it stabilises, and this seriously reduces the speed of the ship. To protect the metal from wear and tear, the surface can be coated with a special paint, but the effect is temporary and the paint needs to be restored almost annually.
Material scientists now developed bimetal - clad steel – to solve this problem. Clad sheets are used in various industries for protection against aggressive environments. It is an alternative to single-layer, homogeneous rolled products from expensive alloy steels.
The clad steel for the ice belt of a nuclear icebreaker consists of two sheets. The main one - ordinary ferrous metal about 50mm thick - and stainless steel cladding 5mm thick, which comes in to contact with ice and water. This steel is highly resistant to wear, and the roughness changes only slightly even under the most severe conditions.
At first, as an experiment, sheets of clad steel were welded into the ice belt of the hulls of the nuclear-powered icebreakers Arktika, Siberia and Yamal as patches, in those areas where the most intense wear take place during operation. After several years of icebreaker operation, the patches remained smooth, and only the protective paint had worn away while the roughness of the metal around the patches increased markedly. It was decided to make the entire ice belt from clad steel.
The first and so far the only icebreaker with such a belt is the 50 Let Pobedy (50 Years of Victory), which has been in operation since 2007, and the ice belt is still almost as good as new.
On the Arktika type icebreakers now under construction (project 22220), the ice belt is not clad. When they were designed, the work necessary to make a decision on combining the ice belt with a new system of electrochemical cathodic protection had not been not completed. On these icebreakers, an ice-resistant epoxy-based Inerta coating is applied to the outer surface of the underwater part of the hull to maintain its integrity.
Now, the reliability of the electrochemical cathode system is beyond doubt, and it is possible to use clad steels as part of the hull of the nuclear icebreaker Leader (Project 10510). Steel sheets for the manufacture of the ice belt of the first Leader will be purchased in 2021. The maximum width of the ice belt in the hull will be more than 7m, and the length about 100m on each side. In total, more than 1000 tons of rolled metal will be needed to make the belt.
An alternative option is also being considered. Bimetallic rolled products can be replaced by homogeneous ones made of high-strength nitrogen-containing stainless steel, developed at the Prometey institute. This solution can provide high resistance to any corrosive effects and simplify welding technologies. However the cost of nitrogen-containing steel is a deterrent.
“We are working on this issue”, Golosienko noted. Russian metallurgical plants will be able to produce new steel for the ice belt of the ship - both clad and homogeneous without any problems. No technical retrofitting is required for this, but organisational problems may arise. For the plant, which ships 10 million tons of metal a year, 1000 tons is a small amount, but there will be difficulties with the order, because the material must be certified by the Maritime Register. For the manufacturer, this is an additional cost for a pilot batch of products, for complex certification tests. “On the other hand, metal production for such an icebreaker as Leader is an image project. I hope our factories will take it up,” Golosienko said. Specialists of the institute are now developing technologies for the production of arc-steels - homogeneous steels with the highest operational reliability of all ship hull materials. They can resist destruction at a temperature of –60°? and even lower.
Arc steels have not yet come into use among shipbuilders, but in future they are expected to replace many shipbuilding steels. The first commercial order in Russia for arc-steel was drawn up with the help of Rosatom. In the upper part of the icebreaker Leader hull, it will be used in the areas with the lowest operating temperatures. Arc-steels are one of the results of work in the field of precision, that is, precise, technologies for creating new materials. Until recently, any changes in steel quality indicators were solved mainly by alloying - the introduction of various chemical elements into iron. But an increase in the content of alloying elements significantly increases the cost and complicates the production process. By fine tuning the technological process, it is now possible to obtain steels with different properties from the same workpiece. For example, a change in temperature at the end of hot deformation can affect the strength and ductility of the metal.
The of KM Prometey Central Research Institute, part of the Kurchatov Institute Research Centre, is Russia’s largest interdisciplinary materials science centre, specialising in advanced materials and technologies for industries where products, structures and equipment are operated in extreme conditions. These are shipbuilding, nuclear, thermal and hydropower, gas and oil refining industries, mechanical engineering and production of military equipment. In particular, within the framework of the design and manufacture of a new generation of VVER-type reactor plants, work is underway to create an austenitic, radiation-resistant steel for elements of in-vessel structures.