Super bainite steel – perfection in impenetrability
While ceramics, smart materials and additive manufacturing have dominated recent military material innovation headlines, metal alloys have been quietly evolving to deliver unprecedented new protective properties. A new form of steel alloy called super bainite has been licensed to Tata Steel for use in its vehicle development projects – and it performs even better with holes in.
Super bainite sounds like something out of a comic book, and indeed given its strength and armouring properties Wolverine might like to consider it as an upgrade to his adamantium-enhanced skeleton and claws. But even Wolverine would think twice about taking on a tank armoured with super bainite, which offers a new type of super-strong, affordable ballistic protection for armoured vehicles.
Before its super successor came along, the common-or-garden bainite phase of steel was named after Edgar C. Bain, who with E. S. Davenport first described it in the 1920s. Without going too deeply into the materials science, it is created when a high-temperature phase of steel known as austenite is cooled to temperatures between 250 and 500° and microstructures of iron crystals and carbides form in it.
Bainite has traditionally been used in gun barrel manufacture where a steel barrel is quenched in a high temperature salt bath. This makes it very hard, but unsuitable for armour as in its original state the carbides and the size of the iron crystals in it make it brittle and prone to cracking.
Strength through heat treating
When University of Cambridge researchers set out to develop new steel alloys with military applications for the Ministry of Defence's Defence Science and Technology Laboratory (Dstl), they figured that if bainite's brittleness could be reduced, its hardness would make it an ideal starting point. Working alongside the Dstl's Professor Peter Brown, Professor Harry Bhadeshia and Dr Francisca Caballero at Cambridge's Department of Materials Science and Metallurgy started modelling how bainite would respond to different manufacturing conditions.
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They found that by applying a heat treatment process called isothermal hardening, holding the steel at about 200oC over several days, they could create ultra-hard and strong steel without expensive added alloy metals. However, this lengthy process would have made it unsuitable and uneconomical for mass production, so by upping the heat treatment temperature to 250oC, they reduced the treatment time to just eight hours.
This super bainitic steel - or super bainite - process produces steel crystals just one tenth the size of those in standard steel, making it much stronger. And by carefully adjusting the quantities of the alloying elements including manganese, silicon, chromium and molybdenum, the level of carbides could be reduced, making it less prone to cracking.
The new derivative was found to have a tensile strength of around 2.5 gigapascals, much stronger than similar metals used in military applications, absorbing a large amount of energy by deformation without allowing ingress of projectiles.
Furthermore, research at Dstl's Porton Down facility discovered that a carefully calculated pattern of round holes or narrow slots drilled into sheets of the material made it twice as resistant to ballistic impact as a monolithic steel sheet. While this sounds counterintuitive, it works by creating thousands of edges which interrupt the path of an incoming projectile and reduce it to blunt fragments. Its performance was confirmed in ballistic testing at Porton Down's firing range.
Tata Steel and the Pavise tradename
In 2011, the Ministry of Defence signed a licensing agreement with Tata Steel to manufacture super bainite at its Port Talbot facility for use as lightweight appliqué armour for the hull or turret armoured vehicles. The deal was facilitated through the Dstl's Ploughshare Innovations subsidiary, which manages the commercial licensing to industry of defence technology intellectual property.
According to Tata, who market it under the trademarked name Pavise, incorporating it into an armour system as a perforated strike face brings the total ballistic mass efficiency of the armour up to 2.5. This means that when fitted to an armoured fighting vehicle it will be able to withstand strikes by weapons which the original vehicle armour would not be able to. Tata sell it in 4mm and 10mm gauge plates that in different combinations offer NATO STANAG L2 to L4 protection. It can be laser-profiled and paint adheres to it easily so it can be painted to match its host vehicle.
Unlike some rival armour materials that have a complex manufacturing process which makes for a long lead time, Pavise is made in a similar way to Tata's other steel products, so it can be produced flexibly on demand and production ramped-up when required.
Later in 2011, super bainite was unveiled as a new armouring solution at the Defence Security and Equipment International exhibition in London. At the event, the then minister for defence equipment, support and technology, Peter Luff, said: "This cutting-edge UK invention and the manufacturing agreement mean that the UK now has its own onshore supply of high-performance armour steel. Super bainite has both military and civilian applications, providing Tata Steel with important export opportunities."
However, like all superheroes it has an inherent weakness; it is so strong it can't currently be welded, so it can't be made into large structural pieces to actually constitute an armoured vehicle, for example.
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The process developed by Tata Steel and the Dstl and Cambridge researchers is unique, but other manufacturers do manufacture their own versions of bainitic high strength steel - the European demand alone is as much as 400,000 tonnes a year. While the military applications for such a highly resilient, lightweight material are obvious, it also offers several potential civilian applications.
Super bainite could be used to give increased stiffness to roll cages within car manufacture while reducing weight, for example, to add strength and support in civil engineering projects or even to protect future space craft from impact by orbiting debris. High-carbon steel is also used for hand tools, springs and bearings.
The researchers and Tata Steel are unwilling to comment on super bainite, or its alter ego Pavise, at the moment as no publicity is currently being planned, so any new developments remain under wraps for the time being. However, while new materials may be stealing the march over traditional metals, the likes of ceramics and smart materials should steel themselves for the ongoing march of super bainite.