Bessemer process - Bessemer converter, Predecessor processes, Importance, Obsolescence

A process for converting pig iron (high carbon iron from the blast furnace) into steel (low carbon iron alloy). Air is blown through the molten iron; the oxygen of the air converts the carbon in the iron into carbon dioxide, which escapes. This reaction produces heat which keeps the iron molten. The principle was known to Chinese and Japanese ironmasters by the 17th-c, and to US ironmaster William Kelly (1811–88) as early as 1846, but the process is named after Henry Bessemer, who introduced it successfully into Britain in 1856. It was an invention of the greatest importance, because it led to many industrial processes being greatly changed or inaugurated by the easy availability of steel. It did not work well with some ores, for which the open-hearth process was devised.

The Bessemer process was the first inexpensive industrial process for the mass-production of steel from molten pig iron. The process is named after its inventor, Henry Bessemer, who took out a patent on the process in 1855.

Bessemer converter

The process is carried on in a large ovoid steel container lined with clay or dolomite called the Bessemer converter. The converter is pivoted on trunnions so that it can be rotated to receive the charge, turned upright during conversion, and then rotated again for pouring out the molten steel at the end. In order to give the steel the desired properties, other substances could be added to the molten steel when conversion was complete, such as spiegeleisen (an iron-carbon-manganese alloy). When the required steel had been formed, it was poured out into ladles and then transferred into moulds and the lighter slag is left behind.

Predecessor processes

Before the Bessemer process Britain had no practical method of reducing the carbon content of pig iron. Steel was manufactured by the reverse process of adding carbon to carbon-free wrought iron, usually imported from Sweden. The manufacturing process, called cementation process, consisted of heating bars of wrought iron together with charcoal for periods of up to a week in a long stone box. Up to 3 tons of expensive coke was burnt for each ton of steel produced. Such steel when rolled into bars was sold at £50 to £60 a long ton.

This process was refined in the 1700s with the introduction of Benjamin Huntsman's crucible steel making technique, which added an additional three hours firing time, and additional massive quantities of coke. In making crucible steel, the blister steel bars were broken into pieces and melted in small crucibles each containing 20 kg or so. This produced higher quality crucible steel, and increased the cost. The Bessemer process reduced to about ½ hour the time to make steel of this quality, while requiring only the coke needed initially to melt the pig iron. The earliest Bessemer converters produced steel for £7 a long ton, although they priced it initially at around £40 a ton. Sheffield has an international reputation for steel-making, which dates from 1740, when Benjamin Huntsman discovered the crucible technique for steel manufacture, at his workshop in the district of Handsworth. This process had an enormous impact on the quantity and quality of steel producton and was only made obsolete, a century later, in 1856 by Henry Bessemer's invention of the Bessemer converter which allowed the true mass production of steel. Bessemer had moved his Bessemer Steel Company to Sheffield to be at the heart of the industry.

Importance

The Bessemer process revolutionized the world. Prior to its widespread use steel was far too expensive to use in most applications, and wrought iron was used throughout the Industrial Revolution. After its introduction steel and wrought iron were similarly priced, and all manufacture turned to steel.

Obsolescence

In the U.S. commercial steel production using this method stopped in 1968. The Bessemer process was so fast (20 minutes or less 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 phosphorous efficiently from the molten steel; The process only permitted a limited amount of scrap steel to be charged, further increasing costs, especially when scrap was inexpensive. Certain grades of steel were sensitive to the nitrogen which was part of the air blast passing through the steel.