Puddling (metallurgy)
[3] Sulfur impurities from the coke made it 'red short', or brittle when heated, and so the finery process was unworkable for it.
It was not until around 1750, when steam powered blowing increased furnace temperatures enough to allow sufficient lime to be added to remove the sulfur, that coke pig iron began to be adopted.
[5] The Cranage brothers, also working alongside the River Severn, allegedly achieved this experimentally by using a coal-fired reverbatory furnace, in which the iron and the sulphurous coal could be kept separate but it was never used commercially.
Although they were unaware of the necessary effects of the oxygen supplied by the air, they had at least abandoned the previous misapprehension that mixture with materials from the fuel were needed.
[5] Cort's process consisted of stirring molten pig iron in a reverberatory furnace in an oxidising atmosphere, thus decarburising it.
[6] Cort's efforts to license this process were unsuccessful as it only worked with charcoal smelted pig iron.
Modifications were made by Richard Crawshay at his ironworks at Cyfarthfa in Merthyr Tydfil, which incorporated an initial refining process developed at their neighbours at Dowlais.
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.
This problem was resolved probably at Merthyr Tydfil by combining puddling with one element of a slightly earlier process.
The slag separated, and floated on the molten iron, and was removed by lowering a dam at the end of the trough.
Hall substituted roasted tap cinder for the bed, which cut this waste to 8%, declining to 5% by the end of the century.
The production of mild steel in the puddling furnace was achieved circa 1850[citation needed] in Westphalia, Germany and was patented in Great Britain on behalf of Lohage, Bremme and Lehrkind.
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.
Fettling is the process of painting the grate and walls around it with iron oxides, typically hematite;[11] this acts as a protective coating keeping the melted metal from burning through the furnace.
This mixture is then heated until the top melts, allowing for the oxides to begin mixing; this usually takes 30 minutes.
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),[14][15] so the furnace has to be continually fed during this process.
[18] Before the development of the basic refractory lining (with magnesium oxide, MgO) and the wide-scale adoption of the Gilchrist–Thomas process ca.
1880 it complemented acidic Bessemer converters (with a refractory material made of SiO2) and open hearths because unlike them, the puddling furnace could utilize phosphorous ores abundant in Continental Europe.
