Steel-good factor - super bainitic steel armour
The role of armour in protecting military personnel is critical. Paul A Davies from Tata Steel looks at the developments of super bainitic steel armour produced in the UK.
Armour steel grades have not been manufactured in the UK in significant volumes for more than 25 years. Instead, the manufacture and fabrication of military platforms, including armoured fighting vehicles – not to mention the varied civil applications requiring ballistic protection – has relied on imported armour steel. The UK Aerospace, Defence and Security (ADS) industry association indicates that the defence sector represents 10% of UK high technology manufacturing, employing more than 300,000 people and requiring a significant volume of steel products.
The UK is the leading exporter of defence equipment and services in Europe, and is only exceeded globally by the USA. The sector generates more than £35bln per year for the UK economy.
The requirement for specialist armour steels was highlighted by the UK Defence Technology Strategy, published in 2006 by the Ministry of Defence (MoD). The need was identified for an increase in manufacturing capability – effectively armour steel on demand – for which there is no British-based supplier. Therefore, a research programme was initiated to understand the viability of specialist steel manufacture for armour and consider the opportunities for collaboration. In 2008, the situation in the USA was similar, with limited domestic production capacity of quenched and tempered wrought armour plate, despite the demands of new military vehicle programmes. In particular, the ‘Berry amendment’ legislation placed restrictions on the procurement and import of specialty metals. This led to a potential shortfall of metals, including specialist perforated steel armour, for the fabrication of the Mine Resistant Ambush Protected (MRAP), which is a matter of highest national defence urgency. Changes in armour specifications allowed the production of thinner armour plate from steel coil versus reversing mill plate, which brought US specifications in line with Europe’s. The new military specifications include ultra high-hardness armour steel (600 Brinell) and perforated armour plate, which made it possible to expedite the procurement of armour for the MRAP vehicles.
New steel genre
The development of bainitic steels with super fine microstructures, has a long history. The concept was discovered more than 10 years ago at the University of Cambridge, UK, in partnership with the Ministry of Defence, during an unintentionally long heat-treatment cycle. The unique properties of super bainite steel were quickly recognised to have military applications by the Ministry of Defence’s Defence Science and Technology Laboratory (Dstl). These include use as ultra high-hardness armour steel (600 Brinell) suitable for appliqué armour, which is often used to increase the protection of the basic hull on military vehicles.
The steel is based on a new metallurgical concept developed from a detailed understanding of the thermodynamics and related transformation characteristics of the steel. This has proved that uniquely valuable properties can be obtained by a previously unrecognised route. The concept is based on high-carbon steel with alloying additions of silicon, chromium and molybdenum, and a heat treatment called isothermal hardening.
Conventional martensitic-type armour steels are typically supplied as quenched and tempered plates in the hardened condition. Super bainite represents a new genre of steel with a unique nano-scale structure produced by heat treatment, which creates remarkable physical properties without having to use expensive alloying additions. The hot rolled and annealed steel is profiled by laser cutting and perforated by either drilling or mechanically punching before heat treatment. The heat treatment process uses standard austempering equipment and involves heating to around 900°C, quenching in air or a salt bath and then holding at about 250°C for an extended period. The austenite phase progressively transforms into a nanofine bainitic microstructure with time, producing the final properties, including ultra high-hardness (more than 600 Brinell), providing exceptional ballistics performance, especially against armour piercing bullets.
Mr Alun Thomas, Manager of Process Technology Primary Processes at Tata explains, ‘Super bainite is a grade unlike any other manufactured by Tata Steel in Port Talbot. The chemistry had never been manufactured in volume before through a wide continuous strip slab caster. The elemental additions significantly lowered the through process temperatures and this, combined with its high strength, presented some unique operational and technical challenges for the steelmaking and casting teams, which pushed the boundaries of what was considered possible.’
Dstl own the patents relating to super bainite and granted Tata Steel a production licence in June 2011, permitting manufacture and processing in the UK, and global export. Under the agreement a significant volume of steel strip has been manufactured (circa 500 tonnes), ready for processing into perforated armour plates for use in front line armoured vehicles.
Perforated steel provides a strike face, disrupting the flight path of high velocity projectiles. As the projectile strikes an edge or the steel web directly, the high tensile strength, ultra high-hardness and inherent stiffness of the steel mean the interaction fractures both the projectile’s solid core and armour steel in a localised area. Pieces of projectile then topple or turn in conjunction with a burst of steel fragments in a conic section. The drift gap allows space and time to transfer the energy of the projectile over a larger area, which are absorbed by a ductile or composite armour material, preventing penetration of the hull. The holes have a secondary role in blunting the propagation of cracks through the steel and therefore limiting the damage around the impact site.
Therefore, perforated steel designs should be regarded as an array of edges rather than holes. However, the bigger the hole, the more susceptible it is to small calibre threats. This can be overcome by up-armouring from a known base level of protection. The ballistic performance of perforated super bainite steel armour is at least twice that of conventional rolled homogenous steel armour in terms of ballistic mass efficiency, allowing a reduction in gauge for the same protection level. Weight reduction is particularly important when armoured military vehicles traverse foreign landscapes. Such environments typically include poor road infrastructure, especially weak bridges and transport by air using both transport helicopters and fixed wing aircraft, such as the new Airbus A400M.
There is a trend in the use of lightweight structural composite materials to build military vehicles, highlighted by the construction of the Foxhound light protected patrol vehicle built by Force Protection Europe in UK. Spaced armour systems, incorporating a perforated steel strike face (disruptor) complement the use of composite absorber material, such as glass reinforced plastic (GRP), aramids (Kevlar) or high-performance polyethylene (HPPE), such as Dyneema. The current project status is focused on optimising the perforated design (hole shape and size) and gauge (areal density), in conjunction with a selection of composite materials to provide lightweight system that must comply with international ballistic standards. For example, the challenge to reduce weight prompted the US Defence Advanced Research Projects Agency to set a target a total area density of less than 73kg/m2 capable of defeating 7.62mm (0.30 calibre) APM2 armour piercing rounds.
The ultimate aim of the project is to provide a costeffective perforated steel strike face component, as an alternative to ceramic tiles in a lightweight armour system.
Thanks to Professor Peter Brown, Dstl, Professor Harry Bhadeshia, Tata Steel Professor of Metallurgy at Cambridge University, and Dr Carlos Garcia-Mateo, the National Centre for Metallurgical Research in Madrid.