'Super steel' created for tougher military vehicles

A new form of steel has been developed that is not only incredibly strong and lightweight, but also cheap and simple to manufacture.

Created in partnership with the Ministry of Defence (MoD) and the University of Cambridge, the 'super bainite' material is already being used by Tata Steel as a strong and cheap armour for front-line military vehicles.

The alloy, known commercially as Pavise, is based on a microstructure called bainite. This forms when austenite, a high temperature phase of steel, is cooled to temperatures between 250 and 500°C.

Traditionally, the structure of austenite transforms as it cools, when slender crystals of iron incorporate themselves into the structure and carbon compounds known as carbides form.

The resulting bainite structure is very hard, but the carbides make it brittle and prone to cracking.

The Cambridge team, working in collaboration with the MoD, used precise modelling to determine that there was in fact no lower limit to the temperature at which the material could be produced.

By adding elements such as silicon and molybdenum and heat-treating the bainite at temperatures around 200°C for 10 or more days, they found that a new, carbide-free form results: super bainite.

This so-called 'super material' has a tensile strength of 2.5gigapascals (just 1m2 can support a weight equivalent to that of 2.5billion apples), a higher density of interfaces than any other type of metal, and is the world's first bulk nanostructured metal.

The steel's strength derives not only from the lack of carbides, but also from the tiny size of the iron crystals within its structure.

Most types of steel are made up of very fine crystals: the smaller and finer the crystals, the stronger the resulting steel will be.

The crystals in super bainite are between 20 and 40nm thick, comparable to the width of carbon nanotubes. For comparison, the crystals in conventional bainite are between 200 and 500nm thick.

"The size of these crystals means that the steel is very difficult to deform, resulting in a more perfect structure," said Cambridge University's Professor Harry Bhadeshia. "And because of the very slow cooking process, which is actually quite simple, we can make the steel in very large quantities at low cost."

Since their discovery, and with the use of kinetic and thermodynamic modelling, the Cambridge/MoD team has been able to tailor the composition of the steel and heat-treat it at slightly higher temperatures so that it can be manufactured in hours instead of days.

Tata Steel is now manufacturing the material at its facility at Port Talbot in South Wales, which is the first time that high-carbon steel has been manufactured on a large scale in the UK for 20 years.

Laura Hopperton

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