Engineering

Part 2 - Iron and steel

The Iron Age began in Syria and Egypt sometime around 5000 years ago. Despite its variable quality, iron was harder than bronze and was preferred for tools and weapons. As soon as it was hot, the furnace was closed to restrict the amount of air and force the burning charcoal to extract the oxygen from the iron oxide ore. The furnace was not hot enough to melt the iron so, as the oxygen was removed, iron particles collected at the bottom of the furnace and these were welded together by hammering (hence wrought iron).

About 2000 years ago, to obtain higher temperatures, the Chinese developed blast furnaces using water wheel driven piston pumps that forced air into the furnaces. But, at very high temperatures, pure iron absorbs carbon, producing cast iron (2.5 to 4.5% carbon) which has a lower melting point and is more easily cast. Cast iron farm tools and weapons were common in China when the Chinese discovered how to make quench-hardenable steel, with a carbon content between 0.1 and 2.1 %. Metal workers melted low carbon wrought iron with high carbon cast iron to make steel. They did not have ways to accurately determine temperatures, or the percentage of alloying elements or impurities, but experimented with recipes that got the desire results.

(Cast iron melts around 1200 °C while steel has a melting point between 1370 and 1540 °C and is up to 1000 times harder than wrought (pure) iron. Wrought iron melts between 1500 and 1600 °C).

During the Middle Ages in Europe, the blast furnace idea was picked up with the use of bellows. These were made with two pieces of wood hinged together and used to compress a leather bag with a nozzle at the hinged edge. Two leather flap valves allowed air into the bag when the bellows was raised and forced it out through the nozzle when the bag was compressed. By attaching a second bellows on top of the first and using a weight to open the first bellows the blacksmith could pump air alternately from each bellows by stepping on a lever that closed the first bellows and opened the other. Removing his foot allowed the weight to force air from the second bellows while sucking fresh air into the first., raised the weight and opened the second bellows.

Mild steel, with a carbon content between 0.05% and 0.25%, is relatively soft and cannot be hardened by heat treatment. 

Carbon steel has a carbon content typically between 0.7 and 2.1%. It can be hardened by heating and quenching (rapid cooling) in water or oil which leaves it much harder and stronger but also more brittle. A secondary process of annealing (heating to a lower temperature and again quenching) substantially reduces brittleness, permitting the steel to be used for impact tools like hammers and chisels.

Blacksmiths in India and Sri Lanka were making high carbon steels (Wootz or Damascus steels, famous for durable swords with the property of keeping a sharp edge) around 2300 years ago. This technology migrated through the Arab world and Persia (Iran). 

In Europe, blister steel was developed as away to increased the amount of carbon in wrought iron. The steel was more workable than iron, allowing it to be pressed or rolled. The process, described in Prague in 1574, used long stone pots filled with iron bars and charcoal packed in alternating layers. The pots were sealed with clay, to make them airtight, and heated in a brick furnace for up to four weeks. Bars were examined regularly and when the iron had gained about 1% in weight, by absorbing carbon from the charcoal, the pots were left to cool for about fourteen days. The bars were then cut, heated and forge-welded together three or four times to make a more homogeneous, higher quality steel known as shear steel. (Forge-welding involved hammering together red hot bars of metal together until they became one piece).Or, they could be cut up and melted with a flux in a crucible inside a furnace to become crucible steel or cast steel, a process devised by Benjamin Huntsman in Sheffield, Britain, in the 1740s. By 1631, Swedish iron ore was found to be the best raw material as it contained to least amount of impurities.

In Britain, until about 1700, iron was made with large quantities of charcoal (carbon), produced by baking wood in a wood fired kiln to drive off the hydrogen. (Burning a slice of toast does the same thing). And, as forests were cut down, wood became more expensive and iron masters started using coke made from coal. Initially, the outer layer of a heap of coal was burned and the heat drove out the liquids and gases from the remaining coal leaving the interior of the heap virtually pure carbon. The process was very inefficient as only about 30% of the coal was converted into coke.

In 1709, Abraham Darby built a coke-fired blast furnace to produce cast iron and in 1768, John Wilkinson built a more efficient oven to convert coal into coke. Brick beehive kilns (ovens) allowed more control of the process so that more than 60% of the coal was converted into coke.

By 1870, there were 14,000 beehive ovens, on the British West Durham coalfields, producing about 4 million tons of coke, about 1 million tons being use by the iron industry while the other 3 million tons were used for railway locomotives, industrial steam engines and for heating and cooking. Coke was the normal fuel used in steam railway locomotives during the early years and steam engines were ubiquitous in the textile and other industries. Iron production increased markedly after 1750 when steam engines were used to blow air into blast furnaces thereby increasing furnace temperatures.

In 1784, Henry Cort patented a version of the puddling process that was improved by ironmasters Crawshay and Homfray at Merthyr Tydfil, Wales, Britain. This produced high quality wrought iron by extracting carbon from cast iron. Puddlers stirred the molten cast iron in the puddling furnace with long, oar-shaped tools, so that oxygen from the air combined with the carbon in the cast iron. As the melting point of iron was higher than that of cast iron, a mass of iron accumulated in the furnace and this was removed and worked with steam powered forge hammers before being rolled into sheets or rails. By 1860, there were more than 3000 puddling furnaces in Britain.

In 1856, Henry Bessemer developed a brick-lined cylindrical receptacle, (or converter) that allowed oxygen to be blown through molten cast iron. The oxygen reacted with the carbon, releasing carbon dioxide gas and producing a more pure iron. The carbon and silicon could be removed in minutes but unfortunately some of the oxygen and unwanted phosphorus remained in the iron.

British metallurgist Robert Mushet knew that manganese would remove oxygen from molten iron and this solved Bessemer's problem as long as he used phosphorus-free ore from Sweden or Wales. In 1876, Sidney Thomas solved the phosphorus problem by adding limestone (a basic flux) to remove the phosphorus, permitting the use of iron ore from anywhere in the world.

As steel production costs decreased, steel rail prices dropped more than 80% between 1867 and 1884.