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Schematic drawing of a puddling furnace Puddling (metallurgy)A number of processes for making wrought iron without charcoal were devised as the Industrial Revolution began during the latter half of the 18th century. The most successful of these was puddling, using a puddling furnace (a variety of the reverberatory furnace). This was invented by Henry Cort in 1784. It was later improved by others including Joseph Hall. In this type of furnace, the metal does not come into contact with the fuel, and so is not contaminated by impurities in it. The flame from the fire is reverberated or sent back down onto the metal on the fire bridge of the furnace.

Unless the raw material used is white cast iron, the pig iron or other raw material first had to be refined into refined iron or finers metal. This would be done in a refinery where raw coal is used to remove silicon and convert carbon from a graphitic form to a combined form. This metal was placed into the hearth of the puddling furnace where it was melted. The hearth was lined with oxidizing agents such as haematite and iron oxide. This mixture is subjected to a strong current of air and stirred with long bars, called puddling bars or rabbles through working doors. The air, stirring, and "boiling" action of the metal help the oxidizing agents to oxidize the impurities and carbon out of the pig iron to their maximum capability. As the impurities oxidize, the retaining material solidifies into spongy wrought iron balls, called puddle balls.

Shingling (metallurgy) There is still some slag left in the puddle balls so while they are still hot they must be shingled to remove the remaining slag and cinder. It may be achieved by forging the balls under a power hammer or by squeezing the bloom in a machine. The material obtained at the end of shingling is known as bloom and it is still red-hot.[25] The blooms are not useful in this form so they must be rolled into a final product.

Sometimes European ironworks would skip this step completely and roll the puddle balls. The only drawback to this is that the edges of the rough bars are not as well compressed. When the rough bar is reheated, the edges may separate and be lost into the furnace.

Rolling mill The bloom is passed through grooved rollers and flat bars were produced. These bars of wrought iron were of poor quality, called muck bars or puddle bars. To improve the quality of wrought iron, these bars were cut up, piled and tied together by wires, a process known as faggoting or piling. They were then reheated and rolled again in merchant rolls. This process may be repeated several times to get wrought iron of desired quality. Wrought iron that has been rolled multiple times is called merchant bar or merchant iron

Lancashire process The advantage of puddling was that it used coal, not charcoal as fuel. However this was little advantage in Sweden, which lacks coal. Gustaf Ekman observed charcoal fineries at Ulverstone, which were quite different from any in Sweden. After his return to Sweden in the 1830s, he experimented and developed a process similar to puddling but using forewood and charcoal, which was widely adopted in the Bergslagen in the following decades.

The Aston process In 1925, James Aston of the United States developed a process for manufacturing wrought iron quickly and economically. It involves taking molten steel from a Bessemer converter and pouring it into cooler liquid slag. The temperature of the steel is about 1500 °C and the liquid slag is maintained at approximately 1200 °C. The molten steel contains a large amount of dissolved gases so when the liquid steel hits the cooler surfaces of the liquid slag the gases are liberated. The molten steel then freezes to yield a spongy mass having a temperature of about 1370 °C. This spongy mass must then be finished by being shingled and rolled as described under puddling (above). Three to four tons can be converted per batch with this method.

Wrought iron is no longer commercially produced. The last wrought iron facility shut down in 1969. In the 1960s the price of steel production was dropping due to recycling and even using the Aston process wrought iron production was a labor intensive process. It has been estimated that the production of wrought iron costs approximately twice as much as the production of low carbon steel.

Properties The microstructure of wrought iron, showing dark slag inclusions in ferrite (iron) The slag inclusions in wrought iron give it properties not found in other forms of ferrous metal. There are approximately 250,000 inclusions per square inch. A fresh fracture shows a clear bluish color with a high silky luster and fibrous appearance.

Wrought iron lacks the carbon content necessary for hardening through heat treatment, but in areas where steel was uncommon or unknown, tools were sometimes cold-worked (hence cold iron) in order to harden them. An advantage of its low carbon content is its excellent weldability. Furthermore, sheet wrought iron cannot bend as much as steel sheet metal (when cold worked).

Wrought iron can be cast, however there is no engineering advantage as compared to cast iron; cast iron is much easier to produce and thus cheaper, so it is exclusively chosen over wrought iron.

Due to the variations in iron ore origin and iron manufacture, wrought iron can be inferior or superior in corrosion resistance compared to other iron alloys. There are many mechanisms behind this corrosion resistance. Chilton and Evans found that nickel enrichment bands reduce corrosion. They also found that in puddled and forged and piled the working over of the iron spread out copper, nickel and tin impurities, which produce electrochemical conditions that slow down corrosion.

The slag inclusions have been shown to disperse corrosion in to an even film to resist pitting. Another study has shown that slag inclusions are pathways to corrosion. Other studies show that sulfur impurities in the wrought iron decrease corrosion resistance, but phosphorus increase corrosion resistance. Environments with a high concentration of chlorine ions also decreases wrought iron's corrosion resistance.

Wrought iron has a rough surface so it can hold platings and coatings better. For instance, a galvanic zinc finish is approximately 25–40% thicker than the same finish on steel. In Table 1, the chemical composition of wrought iron is compared to that of pig iron and carbon steel. Although it appears that wrought iron and plain carbon steel have similar chemical compositions, this is deceiving. Most of the manganese, sulfur, phosphorus, and silicon are incorporated into the slag fibers present in the wrought iron, so wrought iron really is purer than plain carbon steel.

Table 1: Chemical composition comparison of pig iron, plain carbon steel, and wrought iron[25] Material Iron Carbon Manganese Sulfur Phosphorus Silicon Pig iron 91–94 3.5–4.5 0.5–2.5 0.018–0.1 0.03–0.1 0.25–3.5 Carbon steel 98.1–99.5 0.07–1.3 0.3–1.0 0.02–0.06 0.002–0.1 0.005–0.5 Wrought iron 99–99.8 0.05–0.25 0.01–0.1 0.02–0.1 0.05–0.2 0.02–0.2 All units are percent weight

Table 2: Properties of wrought iron Property Value Ultimate tensile strength [psi (MPa)][40] 34,000–54,000 (234–372) Ultimate compression strength [psi (MPa)][40] 34,000–54,000 (234–372) Ultimate shear strength [psi (MPa)][40] 28,000–45,000 (193–310) Yield point [psi (MPa)][40] 23,000–32,000 (159–221) Modulus of elasticity (in tension) [psi (MPa)][40] 28,000,000 (193,100) Melting point [°F (°C)][41] 2,800 (1,540) Specific gravity 7.6–7.9 7.5–7.8 Amongst its other properties, wrought iron becomes soft at red heat, and can be easily forged and forge welded. It can be used to form temporary magnets, but cannot be magnetized permanently, and is ductile, malleable and tough.

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