Puddling (metallurgy)

  Puddling was an Industrial Revolution means of making iron and steel. In the original puddling technique, molten iron in a reverberatory furnace was stirred with rods, which were consumed in the process. Later, it was also used to produce a good-quality steel with the correct amount of carbon; this was a highly skilled art, but both high-carbon and low-carbon steels were successfully produced on a small scale, particularly for swords and other weapons.

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History

Puddling was the first true industrial process to make steel from pig iron. A primitive version of the process was known in China already in the 3rd century. The pig iron tapped off the blast furnace was puddled with iron bars, bringing it into it contact with oxygen in the air and burning off any surplus carbon.

In Europe, the process was one of several that were developed in the second half of the 18th century for producing bar iron from pig iron without the use of charcoal. It was invented by Henry Cort at Fontley in Hampshire in 1783–4 and patented in 1784. A superficially similar (but probably less effective) process was patented the previous year by Peter Onions. Cort's process consisted of stirring molten pig iron in a reverberatory furnace in an oxidising atmosphere, thus decarburising it. When the iron 'came to nature', that is, to a pasty consistency, it was gathered into a puddled ball, shingled, and rolled (as described above). This application of the rolling mill was also Cort's invention.

Unfortunately, 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'. 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.

The alternative was known as 'wet puddling'. This was invented by a puddler called Joseph Hall at Tipton. He began adding scrap iron to the charge. Later he tried adding iron scale (in effect, rust). The result was spectacular in that the furnace boiled violently. This was in fact a chemical reaction between the oxidised iron in the scale and the carbon dissolved in the pig iron. Again to his surprise, the resultant puddle-ball produced good iron. Hall subsequently became a partner in establishing the Bloomfield ironworks 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 and that described above under 'process'. 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 a ton of iron from 1.3 tons of pig iron, but the yield from wet puddling was close to 1.0.

The production of mild steel in the puddling furnace was only achieved in about 1850 in Westphalia in Germany and was patented in Great Britain on behalf of Lohage, Bremme and Lehrkind. It only worked with pig iron made from certain kinds of ore. The cast iron had to be melted quickly and the slag to be rich in manganese. When the metal came to nature, it had to be removed quickly and shingled before further carburisation occurred. The process was taken up at the Low Moor Ironworks at Bradford in Yorkshire (England) in 1851 and in the Loire valley in France in 1855. It was widely used.

Puddling furnace

The puddling furnace is a metalmaking technology to create wrought iron from the pig iron produced in a blast furnace. Pig iron contains high amounts of carbon and other impurities, making it brittle. The puddling furnace burns off these impurities to produce a malleable low-carbon steel or wrought iron.

The furnace was constructed to pull the hot air over the iron without it coming into direct contact with the fuel, a system generally known as a reverberatory furnace or open-hearth process. After lighting and being brought to a low temperature, the furnace is prepared for use by "fettling"; painting the grate and walls around it with iron oxides, typically hematite. Iron is then placed on the grate, normally about 600 lbs, and allowed to melt on top, mixing with the oxides. The mixture is then stirred vigorously with a "rabbling-bar", a long iron rod with a hook formed into one end. This causes the oxygen from the oxides to react with impurities in the pig iron, notably silicon, manganese (to form slag) and to some degree sulfur and phosphorus, which form gases and are removed out the chimney.

More fuel is then added and the temperature raised. The iron completely melts and the carbon starts to burn off as well. The carbon dioxide formed in 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, so the furnace has to be continually fed during this process. Eventually the carbon is mostly burned off and the iron 'comes to nature', forming into a spongy plastic material, indicating that the process is complete, and the material can be removed.

The hook on the end of the bar is then used to pull out large "puddle-balls" of the material, about 40 kg each. These are then hammered ('shingled') using a powered hammer, latterly steam hammer, to expel slag and weld shut internal cracks, while breaking off chunks of impurities. The iron is then re-heated and rolled out into flat bars or round rods. For this, grooved rollers were used, the grooves being of successively decreasing size so that the bar was progressively reduced to the desired dimensions. The quality of this may be improved by faggoting.

The puddling furnace began to be displaced with the introduction of the Bessemer Process, which produced mild steel or wrought iron for a fraction of the cost and time. For comparison, an average size charge for a puddling furnace was 600 lb (270 kg), for a Bessemer converter it is 15 short tons (13 600 kg). The puddling process could not be scaled up, being limited by the amount that the puddler could handle. It could only be expanded by building more furnaces.

Impact

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 process. This enabled a great expansion of iron production in Great Britain to take place. That expansion constitutes the Industrial Revolution so far as the iron industry is concerned.

Precursor in China

By the Han dynasty (2nd century BCE-3rd century CE), Chinese metallurgists had discovered how to puddle molten pig iron, using blast furnaces and stirring it in the open air until it lost its carbon and became wrought iron. In Chinese, the process was called chao, literally, stir frying.^{[citation needed]}

See also

• Crucible steel
• Steelmaking
• Decarburization

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 (1977-8), 153-8.
• 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.