Information AboutWrought Iron |
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in Troy, New York .]] Wrought iron is commercially pure Iron , having a very small Carbon content, but usually contains some slag. It is tough, malleable, ductile and can be easily Welded . However, it is too soft to make blades and swords, or at least for their cutting edges, which need to be made of Steel with a higher carbon content. Common usage is for 'wrought iron' to mean iron of this composition. However, strictly, it should be confined to iron that has been wrought (i.e. worked) into a finished product. The unwrought commodity is according to its form 'bar iron', 'rod iron', etc. Wrought iron has been used for thousands of years, and represents the "iron" that is referred to throughout history. Wrought iron was originally produced by a variety of smelters, described today as Bloomeries . Bloomeries probably existed in a number of forms 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 dissolving in it and form pig 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-based process, rahter than a continuous one. The spongy mass contains iron and also silicates (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, expeelling slag in the process. Wrought iron is rarely completely pure. It is a fibrous material with many strands of slag are mixed into the metal. These slag Inclusions give it a "grain" like wood, and distinct look when etched. Also due to the slag, it has a fibrous look when broken or bent past its failure point. The fibers of wrought iron gives it some interesting properties, however. Hammering a piece of wrought iron cold causes the fibers to become packed tighter, which makes the iron both brittle and hard. As wrought iron lacks the carbon content necessary for Tempering , it is believed that cultures that never discovered how to make steel would cold work wrought iron tools in order to harden them. 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 . The direct process was however generally replaced long before that, 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 bounbdary 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 succesor 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. A number of processes were devised in the second half of the 18th Century for making wrought iron without Charcoal . The most successful of these was the Puddling Furnace invented by Henry Cort in 1784 . The fully deveoped process involved a series of processes. 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'). The finers metal was chanrged 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 tons per square inch (317 MPa). "Treble best" could reach 28 tons 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. Ornamental Ironwork is often referred to as "wrought iron," even though today it is more likely to be made from Mild Steel . Word Origin: The word “wrought” is the old 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''. SEE ALSO |
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