Ferroalloys and steel alloying

Steel production in fire-resistant pots, i.e. crucibles, has been known since the ancient world times. It was even mentioned by Aristotle in the 4th century B.C. Crucible steel production was most spread in the countries of Ancient East: Persia, India and Syria where the steel was used for production of arm Blanche, knives and hand tools. It involved direct production of steel from ore without the iron ball production phase due to higher temperature. In some parts of Asia crucible iron (steel) production has existed up to the end of the 19th century, while in handicraft metallurgic industry it is still in use. The rise of the highest quality crucible steel – the so called wootz or damask – happened in the 5th – 13th centuries. It is very probable that some craftsmen used additives (keeping their secrets securely) with alloying elements added to steel, however, there are no direct evidences of it.

The history of ferroalloys is relatively short if compared to that of iron and steel. Ancient iron Artefacts were of fairly pure iron containing only carbon as a significant alloying element. That was self-evident as the steel was produced via direct reduction route in bloomery-type furnaces at so low temperatures that iron was formed in so lid state and other components like manganese and silicon which are typical in modern steels were found only as slag inclusions in steel. Occasionally, iron could contain such easily reducible elements like Ni and Cu originating from ores or in the case of nickel even from meteoritic iron . When bigger shaft furnaces were developed with stronger air blasting through tuyeres, temperature in the combustion zone was increased, and iron could dissolve more carbon and melt: thus the blast furnace process was discovered. This progress took place in the late Medieval Age in Central Europe. The product was carbon-saturated cast iron which typically contained a few percent of silicon and eventually also some manganese depending on the ore composition. Pig iron from blast furnaces was used as foundry iron for castings or converted to steel by refining process. Such processes were gradually developed but bloomery steel kept its dominance until the 19th century.

Two centuries ago reverberatory furnaces and the crucible process were used for refining hot metals. In both process oxygen reacts with carbon to get low carbon steel. Principally some alloying would have been possible but there were several prerequisites before a rational alloying could be carried out. Firstly, the breakthrough inventions in chemistry in the end of 18th and early 19th century with discovery of elements (like nicke~ oxygen, manganese, chromium, molybden~ silicon from 1751 to 1824) and understanding of chemical reactions like combustion/oxidation and reduction made it possible also to recognize essential events of contemporary iron and steelmak:ing processes and to start developing new processes . Secondly, there should be some evidence of beneficial influences of additions on steel properties. This means understanding of steel microstructure and its relations to steel properties and further to influencing mechanisms of alloying elements. Thirdly, it should be possible to produce potential alloying materials at a reasonable price. During the second half of 1800s these prerequisites gradually began to be fulfilled.

Bessemer in 1855,. Thomas and Gilchrist 1878-1879 and Siemens and Martin brothers in 1860-70. developed these three different methods for refining steel. In parallel with Bessemer's process development, Scottish metallurgist Robert Mushet added manganese-containing "spiegeleisen" in liquid steel to "kill" it, to prevent "wild boiling" caused by carbon-oxygen reaction and CO gas formation. Manganese was thus used for steel deoxidation. Also the beneficial effect to avoid hot shortness by binding excess sulphur was soon recognized .Spiegeleisen containing 8-15 % Mn and ~5 % C was produced already in the 18th century in blast furnaces. Mushet was also one of those who developed first ''tool steels" with 1-2 % Mn in 1860s.Robert Hadfield invented work hardening steel in the 1880s with 11-14 %Mn and 1 %C [l]. This"Hadfield steel" has still a firm position in impact- and wear-resistance type applications.

At that time metallurgists started to consider addition of alloying elements to steel in a form of a ferroalloy(fig 1)- an alloy of iron with at least one another element except carbon. Production of ferroalloys was, in general, much easier and more economic than to make pure elements (Mn, Cr, Si, Ti, V, W ... ) but even the product was more practicable for alloying due to lower melting temperature. Small scale production of ferroalloys was started in 1860s by using crucible process. Chromium ore were reduced by coal in graphite crucibles which were heated to high temperatures to get liquid high carbon alloy with ~25 %Cr. High-Mn ferromanganese production (80% Mn and 6-7%C) was started in a French blast furnace in 1877. It was also demonstrated that FeSi could be produced in a blast furnace, as well as low content FeTi and FeV. On the other hand production of FeCr in the same way was found difficult due to high melting point of the slag formed during smelting. When electric furnace technology was introduced at the end of 1800s, electric smelting of ferroalloys gradually progressed in early 1900s and nowadays all ferroalloys which require furnace technology are being produced exclusively in electrical furnaces.

The occurrence of silicon in iron has its historical origin in blast furnace iron production.Relatively high Si contents (several percents) could be obtained in pig iron which was used in foundries as grey cast iron where Si promotes graphite formation and improves ductility. Swedish chemist Jacob Berzelius produced a kind of ferrosilicon in early 1800s by crucible reduction. He also succeeded to separate elemental silicon in 1824. Production of elemental silicon turned to be very difficult whereas ferrosilicon was easier to produce. In blast furnaces it was possible to produce hot metal up to 20 % Si in the late 19th century. The product was used for steel deoxidation and alloying. When electric furnace technology emerged it was soon applied for FeSi production too. In ferroalloys production: furnaces were designed to operate in submerged arc mode (SAF) in which the high resistivity of the charge is utilized for smelting.