Why do materials sproede when they freeze

How steel becomes resistant to the cold

Steel is mainly a mixture of iron and carbon, with other elements added in many cases. The composition determines the properties. For example, the more carbon a steel contains, the harder, but also the more brittle it becomes. A low carbon content in turn ensures good flexibility, but comes at the expense of hardness. Modern manufacturing processes therefore aim to ensure the highest possible hardness with good ductility. This is a problem especially at low temperatures, such as those found in arctic waters, as steel becomes very brittle at a certain temperature, which is dependent on its composition. In this state, even the smallest cracks can break ship hulls, for example.

Steels with a high proportion of additives are more resistant to cracking, but at the same time also significantly more expensive. Behind this is a change in the microstructure of the steel, which hinders the propagation of cracks in the material. This is exactly what Yuuji Kimura from the National Institute for Materials Science in Tsukuba and his team have achieved through their purely thermal-mechanical treatment: Instead of the more round crystal grains that are created in classic steel production, the modified steel contains elongated, rice-like particles, which also are speckled with tiny carbide crystals. These grains arrange themselves in such a way that they lie on top of each other and thereby form a kind of fibrous layer structure. If a crack now appears, it can push the layers apart, but loses a large part of its energy so that it cannot reproduce any further. If the steel does crack, the crack surface looks more like a frayed piece of wood.

The treated steel has roughly the same hardness as conventional variants at normal temperatures, but remains stable for much longer in the cold. Since no expensive additives are necessary, its production should be comparatively cheap. It could therefore be used, for example, for icebreakers, pipelines or tanks for very cold liquid gases. The only restriction: Since it cannot withstand the same load in all directions due to the layer structure, there could be problems with very thin sheets that are subject to heavy loads.

Nature, online service Original work by the researchers: Yuuji Kimura (National Institute for Materials Science, Tsukuba) et al .: Science, Vol. 320, p. 1057 ddp / Wissenschaft.de? Ilka Lehnen-Beyel
May 24, 2008

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