Puddling (metallurgy)
Puddling is the process of converting pig iron to bar (wrought) iron in a coal fired reverberatory furnace. It was developed in England during the 1780s. The molten pig iron was stirred in a reverberatory furnace, in an oxidizing[citation needed] environment to burn the carbon, resulting in wrought iron. It was one of the most important processes for making the first appreciable volumes of valuable and useful bar iron (malleable wrought iron) without the use of charcoal. Eventually, the furnace would be used to make small quantities of specialty steels.
Though it was not the first process to produce bar iron without charcoal, puddling was by far the most successful, and replaced the earlier potting and stamping processes, as well as the much older charcoal finery and bloomery processes. This enabled a great expansion of iron production to take place in Great Britain, and shortly afterwards, in North America. That expansion constitutes the beginnings of the Industrial Revolution so far as the iron industry is concerned. Most 19th century applications of wrought iron, including the Eiffel Tower, bridges, and the original framework of the Statue of Liberty, used puddled iron.
History
Modern puddling was one of several processes developed in the second half of the 18th century in Great Britain for producing
The need for puddling
Pig iron contains much free carbon and is brittle. Before it can be used, and before it can be worked by a blacksmith, it must be converted to a more malleable form as bar iron, the early stage of wrought iron.
Invention
In 1783,
Ninety years after Cort's invention, an American labor newspaper recalled the advantages of his system:
"When iron is simply melted and run into any mold, its texture is granular, and it is so brittle as to be quite unreliable for any use requiring much
tensile strength. The process of puddling consisted in stirring the molten iron run out in a puddle, and had the effect of so changing its anotomic arrangement as to render the process of rolling more efficacious."[6]
Cort's process (as patented) only worked for white cast iron, not grey cast iron, which was the usual feedstock for forges of the period. This problem was resolved probably at Merthyr Tydfil by combining puddling with one element of a slightly earlier process. This involved another kind of hearth known as a 'refinery' or 'running out fire'.[7] The pig iron was melted in this and run out into a trough. The slag separated, and floated on the molten iron, and was removed by lowering a dam at the end of the trough. The effect of this process was to desiliconise the metal, leaving a white brittle metal, known as 'finers metal'. This was the ideal material to charge to the puddling furnace. This version of the process was known as 'dry puddling' and continued in use in some places as late as 1890.
An additional development in refining gray iron was known as 'wet puddling', also known as 'boiling' or 'pig boiling'. This was invented by a puddler named
One big problem with puddling was that up to 15% of the iron was drawn off with the slag because sand was used for the bed. Hall substituted roasted tap cinder for the bed, which cut this waste to 8%, declining to 5% by the end of the century.[8]
Hall subsequently became a partner in establishing the Bloomfield Iron Works at Tipton in 1830, the firm becoming Bradley, Barrows and Hall from 1834. This is the version of the process most commonly used in the mid to late 19th century. Wet puddling had the advantage that it was much more efficient than dry puddling (or any earlier process). The best yield of iron achievable from dry puddling is one ton of iron from 1.3 tons of pig iron (a yield of 77%), but the yield from wet puddling was nearly 100%.
The production of
The puddling process began to be displaced with the introduction of the
Process
The process begins by preparing the puddling furnace. This involves bringing the furnace to a low temperature and then fettling it. Fettling is the process of painting the grate and walls around it with iron oxides, typically hematite;[10] this acts as a protective coating keeping the melted metal from burning through the furnace. Sometimes finely pounded cinder was used instead of hematite. In this case the furnace must be heated for 4–5 hours to melt the cinder and then cooled before charging.
Either white cast iron or
More fuel is then added and the temperature is raised. The iron completely melts and the carbon starts to burn off. When wet puddling, the formation of carbon monoxide (CO) and carbon dioxide (CO2) due to reactions with the added iron oxide will cause bubbles to form that cause the mass to appear to boil. This process causes the slag to puff up on top, giving the rabbler a visual indication of the progress of the combustion. As the carbon burns off, the melting temperature of the mixture rises from 1,150 to 1,540 °C (2,100 to 2,800 °F),[13][14] so the furnace has to be continually fed during this process. The melting point increases since the carbon atoms within the mixture act as a solute in solution which lowers the melting point of the iron mixture (like road salt on ice).
Working as a two-man crew, a puddler and helper could produce about 1500 kg of iron in a 12-hour shift.[15] The strenuous labour, heat and fumes caused puddlers to have a very short life expectancy, with most dying in their 30s.[16] Puddling was never able to be automated because the puddler had to sense when the balls had "come to nature".
Puddled steel
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In the late 1840s, the German chemist
Puddling furnace
The puddling furnace is a metalmaking technology used to create wrought iron or steel from the pig iron produced in a blast furnace. The furnace is constructed to pull the hot air over the iron without the fuel coming into direct contact with the iron, a system generally known as a reverberatory furnace or open hearth furnace. The major advantage of this system is keeping the impurities of the fuel separated from the charge.
The hearth is where the iron is charged, melted and puddled. The hearth's shape is usually elliptical; 1.5–1.8 m (4.9–5.9 ft) in length and 1–1.2 m (3.3–3.9 ft) wide. If the furnace is designed to puddle white iron then the hearth depth is never more than 50 cm (20 in). If the furnace is designed to boil gray iron then the average hearth depth is 50–75 cm (20–30 in). Due to the great heat required to melt the charge the grate had to be cooled, lest it melt with the charge. This was done by running a constant flow of cool air on it, or by throwing water on the bottom of the grate.
The fireplace, where the fuel is burned, used a cast iron grate which varied in size depending on the fuel used. If bituminous coal is used then an average grate size is 60 cm × 90 cm (2.0 ft × 3.0 ft) and is loaded with 25–30 cm (9.8–11.8 in) of coal. If anthracite coal is used then the grate is 1.5 m × 1.2 m (4.9 ft × 3.9 ft) and is loaded with 50–75 cm (20–30 in) of coal.
A double puddling furnace is similar to a single puddling furnace, with the major difference being there are two work doors allowing two puddlers to work the furnace at the same time. The biggest advantage of this setup is that it produces twice as much wrought iron. It is also more economical and fuel efficient compared to a single furnace.
See also
Footnotes
- ^ Schubert (1958), p. 99.
- ^ a b Schubert (1958), p. 100.
- ISBN 978-0901462886.
- ^ a b c d Schubert (1958), p. 106.
- ^ Schubert (1958), pp. 105–106.
- ^ "The Puddling of Iron," The Workingman's Advocate [Chicago], vol. 9, no. 9 (January 25, 1873), pg. 1.
- ^ Referred to as a "finery" and "run-out fire" by Overman, but not to be confused with the finery in the finery forge.
- ^ Landes (1969), p. 33.
- ^ a b Overman, Fredrick (1854). The Manufacture of Iron, in All Its Various Branches. Philadelphia: H. C. Baird. pp. 267, 268, 287, 283, 344.
- ISBN 81-219-1960-6.
- ^ W. K. V. Gale, The Iron and Steel Industry: a Dictionary of Terms (David and Charles, Newton Abbot 1971), 165.
- ^ R. F. Tylecote, 'Iron in the Industrial Revolution' in R. F. Tylecote, The Industrial Revolution in Metals (Institute of Metals, London 1991), 236-40.
- ISBN 0-87938-186-8.
- ^ W. K. V. Gale, The British Iron and Steel Industry (David and Charles, Newton Abbot, 1967), 70–79.
- ISBN 0415147921.
- ^ Landes (1969), p. 218.
- ^ "Puddled Steel – Graces Guide".
- ^ 'The Development of the Early Steelmaking Processes – An Essay in the History of Technology' by Kenneth Charles Barraclough. Thesis submitted to the University of Sheffield for the Degree of Doctor of Philosophy, May 1981
- Sources
- Landes, David. S. (1969). The Unbound Prometheus: Technological Change and Industrial Development in Western Europe from 1750 to the Present. Cambridge, New York: Press Syndicate of the University of Cambridge. ISBN 0-521-09418-6.
- Schubert, H.R. (1958). Extraction and Production of Metals: Iron and Steel. A History of Technology IV: The Industrial Revolution. Oxford University press.
Further reading
- W. K. V. Gale, Iron and Steel (Longmans, London 1969), 55ff.
- W. K. V. Gale, The British Iron and Steel Industry: a technical history (David & Charles, Newton Abbot 1967), 62–66.
- R. A. Mott, 'Dry and Wet Puddling' Trans. Newcomen Soc. 49 (1977–78), 153–58.
- R. A. Mott (ed. P. Singer), Henry Cort: the great finer (The Metals Society, London 1983).
- K. Barraclough, Steelmaking: 1850–1900 (Institute of Materials, London 1990), 27–35.
- Overman, Fredrick (1854). The Manufacture of Iron, in All Its Various Branches. Philadelphia: H. C. Baird. pp. 259–302.
- Paul Belford's paper on N. Hingley & Sons Ltd
- The Iron Puddler: My Life in the Rolling Mills and What Came of It. by James J. Davis. New York: Grosset and Dunlap, 1922. (ghostwritten by C. L. Edson)