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Wrought Iron, Cast Iron, and Steel in Two Pages

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Wrought Iron, Cast Iron, and Steel in Two Pages Historic Production of Wrought Iron, Cast Iron, and Steel (in Two Pages) (Richard Pieper, A6768, GSAPP, Columbia University) "Smelting" is the process by which a metal is extracted from ore. The smelting of iron by mankind began about 2000 BC, most likely in southwest Asia. For thousands of years iron smelting was done by interlayering charcoal (in essence, carbon, produced from wood) and iron ore (basically iron oxides and stony impurities found in the earth) in a masonry furnace, and setting fire to the charcoal. Air was provided to the furnace by a simple draft hole which admitted air at the base of the furnace, or in some cases by a simple single chamber bellows which assisted the combustion. The temperatures achieved in such simple furnaces were in the range of 800 to 1100 degrees Centigrade, and since iron melts to a liquid at a higher temperature than this, the iron thus produced was a relatively coarse material, which contained a large amount of slag (glassy silica and iron silicates from the impurities in the ore). Because of the low temperatures, carbon from the charcoal did not intermix with the iron, so that the iron contained very little carbon, and the pure (non-carbon containing) form of iron produced in this manner was quite malleable. The "blooms" of iron extracted from these "bloomery" furnaces had to be repeatedly hammered to remove the slag and to produce usable shapes, and this iron was known as "wrought" iron (meaning "worked" or "hammered" iron). This bloomery smelting process is known to metallurgists as the "direct reduction" process, in that the iron ore was "reduced" directly from iron oxide to pure iron. In summary, while wrought iron has pockets or layers of slag impurities mixed in with it, it is the purest form of iron in that it has very little carbon alloyed with it. Wrought iron is malleable hot or cold, has good tensile strength, and is moderately corrosion resistant. Until about 1500 AD this was the only type of iron produced in Europe. Wrought iron was also produced in North America in bloomery furnaces from the time of earliest settlement (1630) to the early 1800's. The introduction of the "blast furnace", an improvement in smelting (about 1500 AD in Europe, by 1780 in North America, many sources say) led to significantly increased production of iron. The use of water power and improved bellows increased the amount of air that could be pumped into the smelting furnace, raising the temperature in the furnace above that needed to liquefy the iron in the presence of excess carbon (about 1150 degrees centigrade). These blast furnaces (for the blast of air that was injected) produced liquid iron that drained from the base of the furnace. The iron produced by the iron ore/charcoal mix in this case, however, was actually an alloy of iron and 3 to 4.5% carbon (from the charcoal) and had radically different properties than the wrought iron produced by the bloomery process. This iron/carbon alloy was known as "cast iron", and was brittle (had little tensile strength) and not at all malleable (could not be hammered hot or cold). This brittleness is due to the presence and configuration of the carbon, which occurs as flakes of graphite in the material. (As it turns out, the flakes of graphite also contribute remarkable corrosion resistance, so "cast iron' will rust superficially, but generally not to great depth.) In the production of cast iron in blast furnaces, molten iron was allowed to drain into a sand mold that had a central trough flanked by a number of smaller lateral troughs, and because this somewhat resembled a sow suckling a litter of piglets, the iron ingots cast in this manner also came to be called "pig iron". Cast iron was useful for fireplace backs, or architectural columns, or any other use needing heat resistance and compressive strength; however, it was not suitable by itself for any use requiring tensile strength, such as floor beams, or for shaping after casting. Liquid cast iron could be converted to wrought iron, however, by stirring it while it was molten to burn off the excess carbon. This process, which was called "puddling", was laborious and time consuming, and resisted efforts at mechanization. Production of wrought iron in this manner, by first producing cast iron, and then puddling it, is known by metallurgists as the "indirect reduction" process. "Puddled" wrought iron, while easier to make than "bloomery" wrought iron, remained relatively expensive. Thus, even in the 1850's in the US, we find floor beams which use inexpensive cast iron for the body of the beam and small rods of wrought iron at their base to provide tensile strength. Steel is another alloy of iron and carbon. The amount of carbon in steel is much less than in cast iron (mild steel used for building typically has only .25 % carbon, while tool steel in chisels and such may have as much as 2%). Another important aspect of the carbon in steel is that it is entirely chemically combined with the iron as an iron carbide (Fe3C, known as "cementite") and does not occur as free flakes of carbon. Steel is less malleable than wrought iron, and can be heat treated to make it harder as well as less brittle, and thus is preferable to wrought iron for edge tools and strength. Railroad rails made of steel required replacement much less frequently than those made of wrought iron, and this was an important early use of steel. Up until the mid-nineteenth century, steel was most commonly produced by recombining small amounts of carbon with wrought iron. This may be done in many ways, but one typical way was to place pieces of wrought iron and powdered charcoal in a sealed box or crucible, and heat the crucible. Steel produced in this manner was obviously expensive, so even in the second quarter of the 19th century it was common to find edge tools which had small pieces of steel forge-welded onto less expensive wrought iron blades to serve as the cutting edge of the tool. Mass production of steel became possible only at mid-century, with the introduction of the Bessemer Process circa 1856. Basically, this technical advancement involved blowing air through molten cast iron to burn off the excess carbon. The process is much more complicated than this, of course, and involves the addition of other materials to assure that enough carbon is left behind, and not too much oxygen is introduced. When perfected, the process could convert an entire mass of molten cast iron into steel in a matter of minutes without any additional fuel. Puddling, by comparison, would take days of labor and tons of additional fuel to convert a similar amount of cast iron into wrought iron. Improvements on the Bessemer Process in the 1860's and 1870's to make it more suitable for all types of iron ores led to it's adoption in the US, and resulted in dramatic increases in the amounts of steel being produced. Prices charged for wrought iron and steel railroad rails illustrate the changes in the industry: In 1867 wrought iron rails were selling for $83 per ton, while steel rails sold for $170 per ton. In 1884 production of iron rails had essentially ceased, and steel rails were priced at $32 per ton. By 1885, steel had essentially replaced wrought iron in the US. .
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