Autumn 2002 PW 5 - Cranfield University

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8 FEATURE I magine the scene – it’s a warm summer evening in the year AD50. You finish ripping the flesh off the bones of your dinner and walk down to the stream. Suddenly the evening sun flickers on a huge shimmering wall of terror, like nothing you have seen before. You drop your water and run, hearing the screaming and clash of weapons above your desperate heartbeat. But it is too late, the battle is lost. The Romans are here to civilise you… © Picture courtesy of Legio Secunda Augusta, Portsmouth
www. cranfield.ac.uk/alumni ’Ware T H E To a hapless barbarian, the sight of a Roman Legion in full polished body armour must, indeed, have been terrifying. The Roman Empire was built on such armour and the fighting prowess of its legionary soldiers is legendary. But how much physical protection would this amour have actually afforded? Neil Morgan, of the Department of Materials and Medical Sciences at Shrivenham, has been trying to find out. By good fortune, Neil met Dr David Sim – a blacksmith and part-time research fellow who, as a specialist in Roman Metallurgy at Reading University, has appeared on numerous television programmes including What the Romans Did For Us. Neil explained: “For some time David had wondered if the Roman armour known as Lorica Segmentata may have been constructed in a much more advanced way than we’d thought. Through the body in charge of the excavation of a Roman fort near Hadrian’s Wall, the Vindolanda Trust, David was able to get hold of a small fragment of metal for analysis at Cranfield’s Shrivenham facility.” Neil carried out an extensive metallographic survey of the fragment’s structure and fabrication, with remarkable findings. The scanning electron microscope revealed a distinct interface along the centre of the fragment from one end to the other – appearing to confirm David’s hypothesis that Lorica Segmentata was made from layers of sheet material joined together using a technique known as ‘fire-welding’. This involves clamping together the separate sheets and heating them to approximately 1200° to 1300°C, a process which causes the separate layers to diffuse across the interface boundary and consolidate into one piece – ‘diffusion bonding’ as we know it. “The fragment revealed other interesting features,” said Neil. “It was generally assumed that Roman armour was made from low-carbon ‘bloomery’ iron – or wrought iron. The fragment, however, is an iron with approximately 0.4 to 0.6 weight percent carbon – or what we would call a plain carbon steel. 9 FEATURE Romans! © Picture courtesy of Legio Secunda Augusta, Portsmouth “In addition, a number of micro-hardness surveys showed a steadily increasing hardness from one face to the other – raising the extraordinary possibility that the Romans had developed a technique for producing a ‘face-hardened’ armour system.” Neil continued: “These findings have already started a lot of discussion among archaeologists and classicists who are surprised by the microstructure and construction of the armour. “The whole ethos of this project, using our unique combination of research and teaching interests, is perfect for Cranfield University. Defence technology, manufacturing, materials science, military science, forensic engineering and welding technology are all represented in the work. And the collaboration with Reading’s archaeology department is vital, both for access to armour samples and because it is only they who are able to place the armour within its historical context.” As well as the obvious historical interest of this work, Neil believes the microstructure may also hold interesting possibilities for modern armour systems. The structure of the different phases in the steel is unusual, and early simulations indicate that it may be possible to produce a steel with a significantly increased toughness compared to that usually associated with an air cooled, plain carbon steel. Subsequently, many other fragments have been analysed, a number of which have a layered microstructure similar to the original sample. One in particular reveals an interesting combination of large-grained, almost pure iron bonded to a much finer-grained low-carbon steel. The mechanical properties of these layers would be quite different and it is still not clear how this may have been employed. Mechanical testing of similar microstructures has also yielded interesting results. The unusual morphology of iron carbides observed within the Vindolanda samples was simulated using modern plain carbon steels, and subjected to various tests. The simulations proved that a carefully controlled heat treatment was required to reproduce the structures seen in the Roman fragments, and that flexibility and fracture toughness were both substantially increased as a result. Despite these exhaustive tests, one question still remains – are these complicated structures and mechanical properties an artefact of Roman metal working techniques; or a result of intentional processing based on hundreds of years of empirical research and development?
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Autumn 2002 PW 5 - Cranfield University

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