The discovery of iron ore

The exact time at which people were able to use iron ore the first time is unknown. The oldest proof for the usage of iron originates from 4000 before Christ when the ancient Egyptians made small pieces of jewellery from ferrous meteorite stones.

It took another 1000 years until humans were able to produce wrought iron using a bloomery. The bloomery is a special furnace which contains stacked layers of charcoal and iron ore. Air is drawn into openings in the lower part of the furnace and flows through the entire furnace by natural draft. The temperatures in the bloomery are between 800 and 900°C which is lower than the melting temperature of iron. Therefore the bloomery produces a porous mixture of iron and slag called the bloom. The bloom must then be forged to remove the impurities and produce wrought iron of fairly high quality. Until it was superseded by more advanced methods in the 14th century AC the bloomery was spread all across the world.

Around the same time the advanced technique of hardening iron was discovered and originally used for the production of weapons.

A blast furnace can produce liquid iron

The first blast furnace evolved from the bloomery in the 14th century when it was enlarged and equipped with air blowers. The new furnace was able to produce liquid iron for the first time; however the pig iron contained too much carbon and was extremely brittle. Despite that it was pig iron that paved the way for the refining process and therefore allowed steelmaking.

The process in the blast furnace is still very similar but there have been many advancements and optimizations which have made modern blast furnaces far more efficient.

Steelmaking by oxidizing

Blacksmith forging iron

After the first blast furnaces started production people had to think about reducing the carbon content in the pig iron. This was achieved as well in the 14th century when they started blowing air over the molten iron which oxidized the unwanted compounds. The process is called refining. After forging they were left with malleable iron with a low degree of impurities.

In 1709 Abraham Darby started using coke in his blast furnace due to a shortage in charcoal. Doing this he enabled Henry Cort to develop a new method of steelmaking called puddling. The iron in a puddling furnace is overflown by hot combustion air while it is stirred by workers with long steel rods which are consumed in the process. This allows as much iron as possible to have contact with air.

Industrial revolution requires a large increase in steel production

After James Watt developed the steam engine in the 19th century the industrial revolution began to accelerate rapidly. The old methods of steelmaking were not efficient enough anymore.

While the industrial revolution started in England steelmaking in Germany was still in the beginnings. In 1819 Friedrich Harkort from Wetter (Ruhr) became one of the earliest steel manufacturers in Germany. Shortly after that the development began to speed up here as well.

James Beaumont Neilson increased the productivity of the blast furnace by supplying it with hot air instead of cold air. This invention significantly decreased the coke consumption. Afterwards refining the pig iron was the limiting factor in steelmaking because puddling was a very laborious procedure.

Bessemer and Siemens-Martin revolutionize steelmaking

Henry Bessemer developed the Bessemer converter in 1855. The converter was fitted with silicon stones on the inside which burn during the process and thereby allow much higher temperatures. Instead of stirring the molten iron by hand the workers could now push compressed air into the bottom of the converter. Despite the method being largely the same the Bessemer converter was far more efficient than puddling and it produced almost carbon free steel.

Bessemer converter

The main disadvantage of the Bessemer converter was that it could only be used for low phosphorous iron ore. In 1877 this limitation lead Sidney Gilchrist Thomas to invent the basic Bessemer converter. He lined the walls of the converter with dolomite to bind phosphorous.

In 1856 the brothers Friedrich and Wilhelm Siemens solved the problem of generating very high temperatures. The regenerative open hearth furnace uses the heat of its exhausts to increase the temperature of the combustion air. The open hearth furnace has lower fuel consumption and it is possible to add scrap metal to the process. The high temperature and the long duration of the process allow exact control of the properties of the product. Siemens Martin Steel was used for demanding purposes such as shipbuilding.

Wilhelm Siemens made another innovation in 1879 by developing the electric arc furnace. This new kind of furnace was able to create extremely high temperatures; however electricity was expensive at that time and the electric arc furnace was only used for high tensile steel.

Since 1990 Basic Oxygen steelmaking and its successors have replaced Siemens Martin and Bessemer steelmaking. Electric arc furnaces are often used for recycling scrap metal.