Iron across the ages

Materials World magazine
,
1 Jul 2012
Eiffel Tower

From the tiny paperclip to the enormous Titanic, man’s use of metals for both purpose and pride has seen its fair share of both success and failure. Tim Carter explores how the many forms and uses of iron have shaped our metal-dependant world.

Since metallurgy brought humans out of the Stone Age, we have depended on metals as a cornerstone of our civilisation. Without metals, we would not have many basic essentials of everyday life. Once this was swords, spears and axes, with which we fought each other and killed animals for food. And we still do, albeit with more sophisticated weapons, but largely made from metal. More importantly we have cooking utensils, refrigerators, houses and aeroplanes, all of which find metal a vital part.

Iron is the most common metallic material in use today and will most probably continue to be so into the future, despite the growing application of materials such as plastics and light metals. There are several reasons for this – iron is cheap, plentiful, easy to fabricate and easy to recycle. It finds use in many applications, from jet motors and ships to the humble paperclip.

Even when other materials are used for a product, iron and its alloys play an indispensable role in manufacturing. For example, plastic components need moulds made from alloy steel for manufacture, and aluminium is produced in electrolytic production cells made from steel. Most hand and machine tools are made of iron or iron-based alloys. Iron has been used as such since prehistoric man first manipulated it to make weapons and tools more durable than the bronze items then available.

Given steel’s undoubted popularity, it does have some disadvantages. It corrodes easily without appropriate protection and is prone to embrittlement, either at low temperatures or in the presence of such elements as hydrogen. The widespread use of steel in all areas of engineering has meant that more is known about the technological properties of steel than almost any other material.

Iron in industry

One of the earliest types of iron-making furnace was found on Melville Koppie in Johannesburg, South Africa, and several other examples have been located in the region. Found by Professor Revel Mason in 1963, a replica of the furnace at Wits University was successfully operated to make iron. While the Melville furnace was relatively recent, there can be no doubt that similar furnaces were used in prehistoric times to make iron. Furnaces of this type used charcoal as fuel and could not produce molten iron. Instead, the product was a lump of sponge iron, which could then be forged into implements such as axes and spears. Originally this forging would have been carried out manually, using handheld hammers.

As the furnaces grew larger, however, the size of the sponge-iron lump produced was too large and so mechanically operated hammers, usually powered by water, were developed. These delivered a constant blow controlled by the weight of the hammer.

By the early 1700s the Darby family, of Coalbrookdale in England, and John ‘ironmad’ Wilkinson, perfected the process of making iron using coke, rather than charcoal, which produced molten iron. In the 1770s Abraham Darby III built the first iron bridge across the River Severn in the UK.

Cast iron is relatively weak in tension, but strong in compression. Careful examination of Darby’s bridge shows that each component is designed to be loaded only in compression. The bridge still exists today in a small town appropriately known as Ironbridge, where it is still in use for pedestrian traffic.

One of the most prominent uses of cast iron at this time was for the production of cylinders for the newly invented steam engine. Around 1710, Thomas Newcomen invented the ‘atmospheric’ steam engine. His design was later much improved by James Watt, and the Industrial Revolution was born.

Later, in the early 19th Century, James Nasmyth invented the steam hammer in which, unlike the water-powered helve and tilt hammers, the force of the blow could be controlled by the operator. A skilled operator could crack an egg held in a wine glass without breaking the glass, and with the next blow shake the workshop.

In 1856, Henry Bessemer first described a process for ‘The Manufacture of Iron Without Fuel’ to the British Association for the Advancement of Science. For the first time, steel was available in bulk with a controlled carbon content.

A mobile world

The expansion of the Industrial Revolution saw a massive increase in the use of steel for building ships, bridges and railways. One of the foremost engineers of the day was Isambard Kingdom Brunel, who built a series of bridges in the UK. These included the Hungerford Suspension Bridge over the Thames in London, which was later dismantled and the suspension chains used for the Clifton Suspension Bridge near Bristol, also built by Brunel. He also engineered the Saltash Bridge in Cornwall, and the Great Western Railway, the latter being the only commercially successful broad-gauge railway. Brunel was also noted for several ground-breaking ship designs, including the Great Britain – the first propeller-driven steam ship – and the Great Eastern, which was built with a double hull for safety and for many years held the record as the largest steel-built steam ship. While it was a commercial failure as a passenger carrier, the Great Eastern went on to lay the first trans-Atlantic telegraph cable.

Legend also has it that Brunel first invented the station buffet, a counter at which drinks were served, to cope with the large numbers of passengers on Great Western Railway stations.

One property that iron does not possess is the ability to float on water, which was amply demonstrated by many ships but perhaps most notably by the Titanic, which was dubbed ‘unsinkable’ by virtue of its transverse bulkheads and electrically operated, watertight connecting doors. She collided with an iceberg, fl ooded and, being made of steel, sank like so many others. The loss of ductility of steel at low temperature was not understood for many years, and was popularly believed to be the reason for the loss of the Titanic after striking an iceberg on her maiden voyage. Recent investigations of materials recovered from the ship have shown that the vessel’s hull was ruptured by rivet failure, and then broke into two at a sharp-pointed expansion joint when the ship pitched bows-down as the fore end flooded. Not a lot was known about fracture mechanics and stress concentrations then, either.

Reaching new heights

Towards the end of the 19th Century, steel construction became very popular. The Statue of Liberty, sculpted by Frédéric Auguste Barholdi, was donated to the American people by the French in 1886. While primarily made externally of copper, it was supported by an inner steel frame designed by Gustav Eiffel. In 1889 Eiffel went on to build the Eiffel Tower in Paris, giving, as he proclaimed, the French Republic the highest flagpole in the world at 324m.

Not to be outdone, the British built Lancashire’s Blackpool Tower in 1894. Sadly, they failed to recognise the failure of steel to resist a maritime environment, and in 1921 the tower was found to be so badly corroded that it had to be re-built, a process that lasted until 1924. Both towers still stand today.

The use of steel in construction was going from strength to strength. In the USA, steel-framed high buildings – popularly known as skyscrapers – were being built and many exist to this day, including the Empire State and Chrysler buildings in New York, and the Sears Tower in Chicago. Sadly, two of the tallest are no longer with us. Another failing of steel is that it loses strength with increasing temperature.

In 2001 when New York’s steel-framed Twin Towers were attacked by terrorists using jet airliners, the ensuing fires caused them to collapse. Today iron and steel are considered sophisticated and advanced materials, capable of having their technological properties tailored to almost any application. Motor vehicle external panels are made from sheet steel, which age-hardens to resist the ‘dings’ and dents popularly known as ‘car-park rash’. Interestingly, this aging takes place when the body is baked after painting. Similarly, steel wire ropes are capable of hauling economic loads in mining from depths of more than three kilometers below ground. Steel undercarriages successfully and reliably withstand having hundreds of tonnes of aircraft dropped on them at speeds of over 200km/hr-1 when the aircraft lands. And every day in every office around the world, the steel wire paperclip brings order to chaos. Without iron, the clocks of the world would stop and life as we know it would cease.