Austenite (γ-iron; hard) Bainite Martensite Cementite (iron carbide; Fe3C) Ledeburite (ferrite - cementite eutectic, 4.3% carbon) Ferrite (α-iron, δ-iron; soft) Pearlite (88% ferrite, 12% cementite) Spheroidite
Plain-carbon steel (up to 2.1% carbon) Stainless steel (alloy with chromium) HSLA steel (high strength low alloy) Tool steel (very hard; heat-treated) Cast iron (>2.1% carbon)
Wrought iron (almost no carbon) Ductile iron 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.Terminology
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 or bent to the point of failure.
Wrought iron has been almost totally replaced by mild steel. It is not produced at all today for commercial use, although one company in the U.K. is known to reprocess scrap, antique wrought iron into stock for commercial sale. It was used when a tough material was required, in applications such as rivets, chains, 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, etc. References relating to wrought iron may occasionally still be found in engineering literature.
Ornamental ironwork utilises the great malleability of wrought iron, and is still often referred to as "wrought iron work" even though today it is more likely to be made from mild steel.
Overview
History
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 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; near Garstang in England until about 1770; and was still in use with hot blast in New York State in the 1880s.
Bloomery process
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 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.
Indirect processes
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.
Puddling and faggoting
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. Aston's process
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 Modern production
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.
Refining
