We continue with Alan Banks’ fascinating article about the Bessemer process:
Bessemer first started working with an ordinary reverbatory furnace but during a test a couple of pig ingots got off to the side of ladle and were sitting above it in the hot air of the furnace. When Bessemer went to push them into the ladle he found that they were steel shells: the hot air alone had converted the outer parts of the ingots to steel. This crucial discovery led him to completely redesign his furnace so that it would force high-pressure air through the molten iron using special air pumps. Intuitively this would seem to be folly because it would cool the iron, but due to exothermic oxidation both the silicon and carbon react with the excess oxygen leaving the surrounding molten iron even hotter, facilitating the conversion to steel.
Bessemer licenced the patent for his process to five ironmasters, for a total of £27,000, but the licences failed to produce the quality of steel he had promised and he later bought them back for £32,500. He realised the problem was due to impurities in the iron and concluded that the solution lay in knowing when to turn off the flow of air in his process; so that the impurities had been burnt off, but just the right amount of carbon remained. However, despite spending tens of thousands of pounds on experiments, he could not find the answer
The simple, but elegant, solution was first discovered by English metallurgist Robert Forester Mushet, who had carried out thousands of scientifically valid experiments in the Forest of Dean. His method was to first burn off, as far as possible, all the impurities and carbon, then reintroduce carbon and manganese by adding an exact amount of spiegeleisen.
This had the effect of improving the quality of the finished product, increasing its malleability – its ability to withstand rolling and forging at high temperatures and making it more suitable for a vast array of uses.
The Dowlais Iron Company was the first licensee of the Bessemer process, constructing the world’s most powerful rolling mill in 1857, and producing its first Bessemer steel in 1865.
The Bessemer process revolutionized steel manufacture by decreasing its cost, from £40 per long ton to £6-7 per long ton during its introduction, along with greatly increasing the scale and speed of production of this vital raw material. The process also decreased the labour requirements for steelmaking.
Prior to its introduction, steel was far too expensive to make bridges or the framework for buildings and thus wrought iron had been used throughout the Industrial Revolution. After the introduction of the Bessemer process, steel and wrought iron became similarly priced, and most manufacturers turned to steel. The availability of cheap steel allowed large bridges to be built and enabled the construction of railways, skyscrapers, and large ships.
Other important steel products were steel cable, steel rod and sheet steel which enabled large, high-pressure boilers and high-tensile strength steel for machinery which enabled much more powerful engines, gears and axles than were possible previously. With large amounts of steel it became possible to build much more powerful guns and carriages, tanks, armored fighting vehicles and naval ships. Industrial steel also made possible the building of giant turbines and generators thus making the harnessing of water and steam power possible. The introduction of the large scale steel production process perfected by the Englishman Henry Bessemer paved the way to mass industrialisation as observed in the 19th-20th centuries.
Obsolescence
Commercial steel production using this method stopped in Workington in 1974. It was replaced by processes such as the basic oxygen (Linz-Donawitz) process, which offered better control of final chemistry.
The Bessemer process was so fast (10–20 minutes for a heat) that it allowed little time for chemical analysis or adjustment of the alloying elements in the steel. Bessemer converters did not remove phosphorus efficiently from the molten steel; as low-phosphorus ores became more expensive, conversion costs increased. The process permitted only limited amount of scrap steel to be charged, further increasing costs, especially when scrap was inexpensive. Use of electric arc furnace technology competed favourably with the Bessemer process resulting in its obsolescence.
Many thanks to the Wirral Model Engineering Society for this article
http://www.wirralmodelengineeringsociety.co.uk/index.html