Wrought iron is commercially pure iron. In contrast to steel, it has a very low carbon content. It is a fibrous material due to the slag inclusions (a normal constituent). This is also what gives it a "grain" resembling wood, which is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile and easily welded.
Examples of items that used to be produced from wrought iron include: rivets, chains, railway couplings, water and steam pipes, raw material for manufacturing of steel, nuts, bolts, horseshoes, handrails, straps for timber roof trusses, boiler tubes, and ornamental ironwork.
Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, are made of mild steel. They retain that description because they were formerly made of wrought iron or have the appearance of wrought iron. True wrought iron is occasionally required for the authentic conservation of historic structures.
Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith or other smith. It was used in this narrower sense in British Customs records, such manufactured iron being subject to a higher rate of duty than what might be called "unwrought" iron.
In the 17th, 18th and 19th centuries, wrought iron went by a wide variety of terms according to its form, origin, or quality.
The raw material produced by all indirect processes is pig iron. It has a high carbon content and as a consequence it is brittle and could not be used to make hardware. The osmond process was the first of the indirect processes, developed by 1203, but bloomery production continued in many places. The process depended on the development of the blast furnace, of which medieval examples have been discovered at Lapphyttan, Sweden and in Germany.
The bloomery and osmond processes were gradually replaced from the 15th century by finery processes, of which there were two versions, the German and Walloon. They were in turn replaced from the late 18th century by puddling, with certain variants such as the Swedish Lancashire Process. These too are now obsolete, and wrought iron is no longer manufactured commercially.
After smelting was complete, the bloom was removed, and the process could then be started again. It was thus a batch process, rather than a continuous one. The spongy mass contained iron and also silicate (slag) from the ore; this was iron bloom from which the technique got its name. The bloom had to be forged mechanically to consolidate it and shape it into a bar, expelling slag in the process.
During the Middle Ages, water-power was applied to the process, probably initially for powering bellows, and only later to hammers for forging the blooms. However, while it is certain that water-power was used, the details of this remain uncertain. This was the culmination of the direct process of ironmaking. It survived in Spain and southern France as Catalan Forges to the mid 19th century, in Austria as the stuckofen to 1775, and near Garstang in England until about 1770; it was still in use with hot blast in New York State in the 1880s.
The finery process existed in two slightly different forms. In Great Britain, France, and parts of Sweden, only the Walloon process was used. This employed two different hearths, a finery hearth for fining the iron and a chafery hearth for reheating it in the course of drawing the bloom out into a bar. The finery always burnt charcoal, but the chafery could be fired with mineral coal, since its impurities would not harm the iron when it was in the solid state. On the other hand, the German process, used in Germany, Russia, and most of Sweden used a single hearth for all stages.
The introduction of coke for use in the blast furnace by Abraham Darby in 1709 (or perhaps others a littler earlier) initially had little effect on wrought iron production. Only in the 1750s was coke pig iron used on any significant scale as the feedstock of finery forges. However, charcoal continued to be the fuel for the finery.
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.
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. 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.
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.
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.
Wrought iron is less affected by rust than most other ferrous metals due to its slag inclusions. The slag fibers tend to disperse the corrosion into an even film, thereby resisting pitting. 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|
|All units are percent weight|
|Table 2: Properties of wrought iron|
|Ultimate tensile strength [psi (MPa)]||34,000–54,000 (234–372)|
|Ultimate compression strength [psi (MPa)]||34,000–54,000 (234–372)|
|Ultimate shear strength [psi (MPa)]||28,000–45,000 (193–310)|
|Yield point [psi (MPa)]||23,000–32,000 (159–221)|
|Modulus of elasticity (in tension) [psi (MPa)]||28,000,000 (193,100)|
|Melting point [°F (°C)]||2,800 (1,540)|
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.