Wrought iron

Wrought iron, also know as rod iron or rot iron^[1], is commercially pure iron, having a low carbon content, but containing slag. It is a fibrous material due to the slag inclusions. 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, horse shoe bars, handrails, straps for timber roof trusses, boiler tubes, and ornamental ironwork.

Today, wrought iron is no longer commercially produced however, one company in the U.K. is known to reprocess scrap wrought iron commercially, mainly for the restoration of historic ironwork.[1] Many products described as wrought iron, such as guard rails, are made of mild steel. These products retain that name because they were formerly made of wrought iron. Generally, consumers asking for wrought iron are looking for hand forged items or items with the "look" of wrought iron. True wrought iron is occasionally required for the authentic conservation of historic structures, but this must be specifically requested.

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Terminology

Wrought iron is so named because it is worked from a bloom of porous iron mixed with slag and other impurities. The word "wrought" is an archaic past tense form of the verb to work. As irregular past-tense forms in English have historically been phased out over long periods of time, wrought became worked. Wrought iron literally means worked iron.

History

Overview

Wrought iron has been used for many centuries, and represents the "iron" that is referred to throughout western history. The other form of iron, "cast iron", was not introduced in the West until the 15th century and (due to its brittle character) could only be used for a limited number of purposes.

Until the Middle Ages, iron was produced by the direct reduction of ore in manually operated bloomeries. Waterpower was applied to this by 1104.^[2]

The first indirect process (where pig iron was an intermediate product) was that making osmond iron, developed by 1203, but bloomery production continued in many places. This depended on the development of the blast furnace, of which Medieval examples have been discovered at Lapphyttan, Sweden and in Germany. In any indirect process, the raw material is pig iron, the product of the blast furnace. Pig iron has a high carbon content. In consequence, it was brittle and could not be used to make ironmongery.

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 processes. 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, because wrought iron is no longer being made.

Bloomery process

Main article: Bloomery

Wrought iron was originally produced by a variety of smelting processes, all described today as bloomeries. A number of different forms of bloomery were used at different places and times. The bloomery would be charged with charcoal and iron ore (an oxide or carbonate) and lit. Air was blown in through a tuyere to heat the bloomery to a temperature somewhat below the melting point of iron. In the course of the smelt, slag would melt and run out, and carbon monoxide from the charcoal would reduce the ore to iron, which formed a spongy mass. The iron remained in the solid state. If the bloomery was allowed to become hot enough to melt the iron, carbon would dissolve into it and form "pig" or "cast" iron, but that was not the intention.

After smelting was complete, the bloom was removed, and the process can be started again. It is thus a batch process, rather than a continuous one. The spongy mass contains iron and also silicate (slag) from the ore; this is iron bloom from which the technique gets its name. The bloom then has to be forged mechanically to consolidate it and shape it into a bar, expelling slag in the process.^[3]

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.^[4] 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.^[5] near Garstang in England until about 1770;^[6] and was still in use with hot blast in New York State in the 1880s.^[7]

Osmond process

Main article: Osmond process

Osmond iron was balls of wrought iron, produced by melting pig iron and catching the droplets on a staff, which was spun in front of a blast of air so as to expose as much of it as possible to the air and oxidise its carbon content.^[8] The resultant ball was often forged into bar iron in a hammer mill.

Finery process

Main article: Finery forge

In the 15th century, the blast furnace spread into what is now Belgium and was improved. From there, it spread via the pays de Bray on the boundary of Normandy and then to the Weald in England. With it, the finery forge spread. These remelted the pig iron and (in effect) burnt out the carbon, producing a bloom, which was then forged into a bar iron. If rod iron was required a slitting mill was used.

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 the 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.^[9]

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.

Potting and Stamping

From the late 1750s, ironmasters began to develop processes for making bar iron without charcoal. There were a number of patented processes for this, which are referred to today as potting and stamping. The earliest were developed by John Wood of Wednesbury and his brother Charles Wood of Low Mill at Egremont, patented in 1763.^[10] Another was developed for the Coalbrookdale Company by the Cranage brothers.^[11] Another important one was that of John Wright and Joseph Jesson of West Bromwich.^[12]

Puddling process

Main article: 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.^[13] 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.^[14] This mixture is subjected to a strong current of air and stirred with long bars, called puddling bars or rabbles^{[15] [16]}, through working doors.^[17] 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 the retaining material solidifies into spongy wrought iron balls, called puddle balls.^[14]

Shingling

Main article: Shingling (metallurgy)

There is still some slag left in the puddle balls so while they are still hot they must be shingled.^[18] By this operation, the remaining slag and cinder is removed.^[18][14] It may be achieved by forging the balls under a power hammer or by squeezing the bloom in a squeezing machine. The material obtained at the end of shingling is known as bloom and it is still in red-hot condition.^[18] These 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 downfall to this is that the edges of the rough bars are not as well compressed so when the rough bar is reheated the edges may separate and be lost into the furnace.^[18]

Rolling

Main article: 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^[18][19] or puddle bars^[14]. 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.^[18] 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 bars or merchant iron.^[16][20]

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.^[21]

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.^[14] This spongy mass must then be finished by shingled and rolled as described under puddling (above).

3 to 4 tons can be converted per batch with this method.^[14]

Wrought iron is no longer commercially produced. The last wrought iron facility shut down in 1969.^[1] 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's been estimated that the production of wrought iron cost approximately twice as much as the production of low carbon steel.^[1]

Properties

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.^[1] A fresh fracture shows a clear bluish color with a high silky luster and fibrous appearance. Hammering a piece of wrought iron cold causes the fibers to become packed tighter, which make it both more brittle and hard.^{[citation needed]}

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.^[1] Furthermore, wrought iron cannot be bent as sharply as steel, for the fibers can spread and weaken the finished work.^{[citation needed]}

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.^[1] 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.^[1]

In Table 1 the chemical composition of wrought iron is compared to that of pig iron and plain 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.^[18]

Other properties of wrought iron include the following:

• It becomes soft at red heat and it can be easily forged and forge welded.^{[citation needed]}
• It can be used to form temporary magnets, but cannot be magnetized permanently.^{[citation needed]}
• It fuses with difficulty. It cannot, therefore, be adopted for making castings.^{[citation needed]}
• It is ductile, malleable and tough.^[18]
• It is moderately elastic.^{[citation needed]}

Defects

Wrought iron is defective in quality if it is either coldshort or redshort.

Coldshort

Coldshort (or "bloodshot") wrought iron occurs when phosphorus is present in excess quantity and is very brittle when it is cold. It cracks if bent. It may, however, be worked at high temperature. Historically, coldshort iron was considered good enough for nails.

Redshort

Redshort wrought iron possesses sulfur in excess quantity. It has sufficient tenacity when cold, but cracks when bent or finished at a red heat. It is therefore useless for welding or forging.

See also

• Pig iron
• Ornamental metal
• Wrought iron furniture
• Cold iron
• Cast iron
• Osmond process

References

• Bealer, Alex W. (1995). The Art of Blacksmithing. Edison, NJ: Castle Books, 28-45. ISBN 0785803955.