Wrought iron
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| Iron alloy phases |
|---|
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Austenite (γ-iron; hard) |
| Types of Steel |
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Plain-carbon steel (up to 2.1% carbon) |
| Other Iron-based materials |
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Cast iron (>2.1% carbon) |
Wrought iron is commercially pure iron, having a very small carbon content (not more than 0.15 percent), but usually containing some slag. It is tough, malleable, and ductile and is easily welded. However, it is too soft for blades and swords, at least for their cutting edges, which are usually made of steel with a higher carbon content.
Wrought iron is so named because it is "wrought" or worked from a "bloom" of porous iron mixed with slag and other impurities. The word "wrought" is an archaic past tense 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.
Wrought iron has been used for thousands of years, and represents the "iron" that is referred to throughout western history. It is a fibrous material with many strands of slag mixed into the metal. These slag inclusions give it a "grain" resembling wood, with distinct appearance when etched. Also due to the slag, it has a fibrous look when broken or bent past its failure point.
Wrought iron has been replaced to a very great extent by mild steel. It is, therefore, hardly produced at all today. It was used where a tough material is required. Wrought iron, at present, is used for rivets, chains, ornamental iron work, railway couplings, water and steam pipes, raw material for manufacturing of steel, bolts and nuts, horse shoe bars, handrails, straps for timber roof trusses, boiler tubes, roofing sheets, etc.
Ornamental ironwork is often referred to as "wrought iron," even though today it is more likely to be made from mild steel.
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[edit] History
[edit] Bloomery process
Wrought iron was originally produced by a variety of smelters, 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.
During the Middle Ages, water-power was applied to the process, probably initially only 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 the Spain and southern France as Catalan Forges to the mid 19th century, in Austria as the stuckofen to 1775; near Garstang in England until about 1770; and was still in use with hot blast in New York State in the 1880s.
[edit] Indirect processes
The direct process was largely replaced during the Middle Ages with an indirect smelting process, involving a blast furnace and then one of a succession of a further processes, including the finery forge, and later the puddling furnace.
Examples of the blast furnace have been discovered from the Middle Ages at Lapphyttan, Sweden and in Germany. This was combined with a further process making osmond iron, balls of wrought iron.
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. The product of a blast furnace, pig iron, had a high carbon content and was brittle. In order to use it in ironmongery, this had to be converted to wrought iron. This was the function of the finery forge and successor processes. These remelted the pig iron and (in effect) burnt out the carbon, producing a bloom, which was then forged into a bar. If rod iron was required a slitting mill was used.
The introduction of coke for use in the blast furnace by Abraham Darby in 1709 (or perhaps others a littler earlier) changed ironmaking and eventually replaced charcoal. Not only was the fuel much cheaper, but it is also less friable, allowing the furnaces to be much larger. However, charcoal continued to be the fuel for the finery.
[edit] Puddling and faggoting
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 the puddling furnace invented by Henry Cort in 1784. The fully developed process involved a series of stages. First the iron was melted in a "refinery" or "running out fire". The iron was run out into a trough whose dam was lowered enough to run off the slag, thus reducing the silicon content. This produced a brittle white metal ("finers metal"), which was charged to the puddling furnace, where it was melted and stirred. The resultant puddled ball was "shingled" with a hammer and then rolled in a rolling mill to produce "muck bar". This would be broken up and faggotted. Wrought iron which had been faggoted twice was referred to as "Best"; if faggoted again it would become "Best Best", then "Treble best", etc.
Faggoting resulted in impurities within the metal ending up as long thin inclusions, creating a grain within the metal. "Best" bars would have a tensile strength along the grain of about 23 short tons-force per square inch (317 MPa). "Treble best" could reach 28 short tons-force per square inch (386 MPa). The strengths across the grain would be about 15% lower. This grain makes wrought iron especially tricky to smith, as it behaves much like wood grain—prone to spontaneous splitting along the grain. In old, very rusted pieces of wrought iron, the grain is revealed, making the iron bear a striking resemblance to reddish-brown wood.
[edit] Aston's process
In 1925, James Aston of the United States developed a wholly mechanical process for manufacturing wrought iron quickly and economically. It is carried out as follows:
- Molten steel from a Bessemer converter is poured into cooler liquid slag. Temperature of molten steel is about 1500 °C and that of liquid slag is about 1200 °C.
- Molten steel contains large amounts of dissolved gases. These gases are liberated when it strikes the slag.
- Molten steel freezes to yield a spongy mass having a temperature of about 1370 °C.
- This spongy mass is then shingled and rolled as described below.
[edit] Modern production
Wrought iron is relatively pure, and normally contains less than .15% carbon and other impurities. But the process of its manufacture is laborious and tedious. Following are the four distinct operations involved in its manufacture:
- Refining
- Puddling
- Shingling
- Rolling
[edit] Refining
Pig iron is melted and a strong current of air is directed over it. It is being well agitated or stirred when the current of air is passing over. It is thus thoroughly oxidized. It is then cast into moulds. It is cooled suddenly so as to make it brittle. This is known as "refined pig iron". It has also been known as finers metal and as refined iron.
[edit] Puddling
Conversion of pig iron into wrought iron by stirring in a molten state is known as puddling. It is carried out in a reverberatory furnace. In this type of furnace, the metal does not come into contact with the fuel, and flame from the fire is reverted or sent back on the metal in the hearth.
A reverberatory furnace, which is of rectangular shape, is built with refractory materials such as firebricks. The combustion chamber and the chimney are situated at opposite ends. Grating is provided in the combustion chamber to collect ash in an ash pit. Next to the combustion chamber is the hearth portion with shallow depth. Hearth lining consists of molten slag or rich iron ore. It is supported on steel plates, which in turn are supported on dwarf brick walls. Water jackets are provided for circulation of water to cool the furnace. Various doors or openings for fuel feeding, working and slag removal are provided. The roof is given a peculiar shape so that flames of gas produced are concentrated on hearth.
The refined pig iron is broken into lumps and is melted in the hearth of the reverberatory furnace. The hearth lining acts as an oxidizing agent and, in addition, oxidizing substances such as haematite ore, oxide of iron, etc., are added to the refined pig iron. It is subjected to intense heat and a strong current of air. It is kept well stirred by long bars through working doors.
During the process of puddling, most of the carbon content and other impurities of the pig iron are oxidized. Slag formed is removed through a slag removal door. The purified iron becomes thick and assumes the form of white spongy iron balls, known as puddle balls, the weight of which is about 50 to 70 kg.
[edit] Shingling
By this operation, the slag contained is removed. It may be achieved by forging the balls under a power hammer or by passing the balls through a squeezing machine. In the case of the power hammer, the balls are placed on an anvil and a falling hammer forges them. A squeezing machine consists of two cylinders, which are placed one inside the other. The smaller cylinder has corrugations on its outer surface and the larger cylinder has corrugations on its inner surface. The balls are placed in between the cylinders and then the inner cylinder is rotated.
Shingling also helps in binding or welding the particles of puddle balls. The material obtained at the end of shingling is known as bloom and it is still in red-hot condition.
[edit] Rolling
The bloom is passed through grooved rollers and flat bars of sizes such as about 4 m × 10 cm × 25 mm are obtained. These bars of wrought iron of poor quality are called muck bars. To improve the quality of wrought iron, these bars are tied together by wires, a process known as faggoting and they are heated and rolled again. This process may be repeated several times to get wrought iron of desired quality.
[edit] Properties
The fibers in wrought iron give it properties not found in other forms of ferrous metal. Hammering a piece of wrought iron cold causes the fibers to become packed tighter, which makes the iron both brittle and hard. Wrought iron lacks the carbon content necessary for tempering, but in areas where steel was uncommon or unknown, tools were sometimes cold-worked (hence "cold iron") in order to harden them. Furthermore, wrought iron cannot be bent as sharply as steel, for the fibers can spread and weaken the finished work.
Other properties of wrought iron include the following:
- It becomes soft at white heat and it can be easily forged and welded.
- It can be used to form temporary magnets, but cannot be magnetized permanently.
- It fuses with difficulty. It cannot, therefore, be adopted for making castings.
- It is ductile, malleable and tough.
- It is moderately elastic.
- It is less affected by saline water than steel, and resists corrosion better.
- Its fresh fracture shows clear bluish colour with a high silky luster and fibrous appearance.
- Its melting point is about 1500 °C.
- Its specific gravity is about 7.8.
- Its ultimate compressive strength is about 2000 kgf/cm² (200 MPa).
- Its ultimate tensile strength is about 4000 kgf/cm² (400 MPa).
[edit] Defects
Wrought iron is defective in quality if it is either coldshort or redshort.
Coldshort (or "bloodshot") wrought iron is very brittle when it is cold. It cracks if bent. It may, however, be worked at high temperature. This defect occurs when phosphorus is present in excess quantity. Historically, coldshort iron was considered good enough for nails.
Redshort wrought iron possesses sufficient tenacity when cold, but it cracks when bent or finished at a red heat. It is, therefore, useless for welding or forging; indeed, for almost any purpose. This defect occurs when sulphur is present in excess quantity.
[edit] Notes
Bealer, Alex W. (1995). The Art of Blacksmithing. Edison, NJ: Castle Books, 28-45. ISBN 0785803955.
