Wrought iron

WROUGHT IRON

Wrought iron is an iron alloy with a very low carbon (less than 0.08%) content in contrast to cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which gives it a "grain" resembling wood that is visible when it is etched or bent to the point of failure. Wrought iron is tough, malleable, ductile, corrosion-resistant and easily welded. Before the development of effective methods of steelmaking and the availability of large quantities of steel, wrought iron was the most common form of malleable iron. It was given the name wrought because it was hammered, rolled or otherwise worked while hot enough to expel molten slag. The modern functional equivalent of wrought iron is mild or low carbon steel. Neither wrought iron nor mild steel contains enough carbon to be hardenable by heating and quenching.

Refining and carburizing wrought iron

Historically, a modest amount of wrought iron was refined into steel, which was used mainly to produce swords, cutlery, chisels, axes and other edged tools as well as springs and files. The demand for wrought iron reached its peak in the 1860s, being in high demand for ironclad warships and railway use. However, as properties such as brittleness of mild steel improved with better ferrous metallurgy and as steel became less costly to make thanks to the Bessemer process and the Siemens-Martin process, the use of wrought iron declined.

Wrought iron forging

Many items, before they came to be made of mild steel, were produced from wrought iron, including rivets, nails, wire, chains, rails, railway couplings, water and steam pipes, nuts, bolts, horseshoes, handrails, wagon tires, straps for timber roof trusses, and ornamental ironwork, among many other things.

Forming wrought iron

Wrought iron is no longer produced on a commercial scale. Many products described as wrought iron, such as guard rails, garden furniture and gates, are actually made of mild steel. They retain that description because they are made to resemble objects which in the past were wrought (worked) by hand by a blacksmith (although many decorative iron objects, including fences and gates, were often cast rather than wrought).

Iron wrought gate

The word "wrought" is an archaic past participle of the verb "to work," and so "wrought iron" literally means "worked iron". Wrought iron is a general term for the commodity, but is also used more specifically for finished iron goods, as manufactured by a blacksmith. It was used in that narrower sense in British Customs records, such manufactured iron was subject to a higher rate of duty than what might be called "unwrought" iron. Cast iron, unlike wrought iron, is brittle and cannot be worked either hot or cold. Cast iron can break if struck with a hammer.

In the 17th, 18th, and 19th centuries, wrought iron went by a wide variety of terms according to its form, origin, or quality. While the bloomery process produced wrought iron directly from ore, cast iron or pig iron were the starting materials used in the finery forge and puddling furnace. Pig iron and cast iron have higher carbon content than wrought iron, but have a lower melting point than iron or steel. Cast and especially pig iron have excess slag which must be at least partially removed to produce quality wrought iron. At foundries it was common to blend scrap wrought iron with cast iron to improve the physical properties of castings.

Iron wrought gate

For several years after the introduction of Bessemer and open hearth steel, there were different opinions as to what differentiated iron from steel; some believed it was the chemical composition and others that it was whether the iron heated sufficiently to melt and "fuse". Fusion eventually became generally accepted as relatively more important than composition below a given low carbon concentration. Another difference is that steel can be hardened by heat treating. Historically, wrought iron was known as "commercially pure iron", however, it no longer qualifies because current standards for commercially pure iron require a carbon content of less than 0.008 wt%.

Source: https://en.wikipedia.org/wiki/Wrought_iron

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