The properties of cast iron, wrought iron and steel
Cast iron has good compressive strength but relatively poor tensile strength.
Because of the impurities in cast iron and its crystalline structure, although it is a strong material in compression, it is weak in tension and is very brittle. As a result when it failed it did so in an explosive manner with little warning. It can be subject to blemishes and flaws, hence the warning made by a 19th century ironwork contractor: "It is not the masses of metal that constitute strength, but the judicious proportions and forms of the casting".
The casting process enables the production of complex shapes.
Because it is produced by casting, ie. made by pouring into a mould, it is well suited to the production of intricate shapes, both structural and decorative. For structural use, cast iron was best suited to elements where the predominant load caused compression; and so was most commonly used for columns.
Wrought iron has better tensile strength than cast iron and is more ductile.
Wrought iron has better properties than cast iron for structural use. Because of its low carbon content wrought iron is ductile, 'tough' and has good resistance to corrosion. It is more fibrous in texture, highly ductile, and is strong in both tension and compression.
The cross-sectional shapes of cast and wrought iron were generally different.
Due to the nature of cast iron, the beams produced were usually of a non-symmetric cross section, the tensile flange (nearly always the lower flange) being much larger than the compressive flange
In the final stage of production wrought iron is hammered or rolled so, in its killed (as opposed to forged) form, it usually occurs in simple shapes of constant section such as plates, tees, angles and bars.
Because of the nature of the puddling process that was used to produce the material, wrought iron was produced in small volumes.
The puddling process involved remelting pig iron and then mixing in air to oxidise the carbon. The material so produced would typically have very low carbon contents of around 0.05%. Some of the impurities remaining in the material exist as pockets of slag which are stretched out into long threads during the hammering or rolling processes that were used to form it into structural sections. These long threads give wrought iron the characteristic fibrous appearance mentioned above.
Wrought iron was more expensive than cast iron and could not be moulded into the kind of ornate shapes characteristic of cast iron.
The properties of wrought iron made it better for structural purposes than cast iron but the structural elements produced were small in both length and cross section. Thus beams in wrought iron generally spanned relatively short distances. In addition, sections in wrought iron were often compound, being built up by rivetting together (sometimes many) smaller sections.
The Bessemer process for converting iron into steel was patented in 1855.
Although the Bessemer process was patented in 1855, for the next 15-20 years, wrought and cast iron continued to satisfy most of the demand for iron-based building products, due mainly to problems of producing steel from alkaline ores. These problems, caused principally by the presence of phosphorous, were solved by Gilchrist and Thomas in 1879. Cheap steels, similar to those used today were available from about 1870 (though cast and wrought iron continued to dominate sales until around 1880). These steels had high tensile and compressive strengths plus good ductility.
The railway industry was responsible for the initial growth in the steel industry where it was used for rails (1860s), wheels and axles (around 1870) and boilers (1872).
The use of steel for structural purposes was initially slow.
For 25 years after the introduction of the Bessemer process in 1855 the use of steel in structures was tentative. It was not until 1880 that an era of construction based on reliable mild steel began. By that date the quality of steels being produced had become reasonably consistent so engineers began to discuss what permissible stresses were appropriate for the different structural uses to which this new material might be put.
As the rolling process has evolved the efficiencies of the cross sectional shapes have improved.
The common cross sectional shapes used in structures have remained very similar for many years (the I section, the angle and the tee). Early I section beams in the late 1800s had to be rolled with tapered flanges, and it was not until 1959 that Dorman Long introduced the rolling of sections with parallel flanges and sharp corners.
Parallel flanges gave two beneficial characteristics:
· more material was pushed into the flanges where it was most effective in helping to carry bending stresses and to resist deflection.
· tapered washers were no longer needed for the inside flange face of bolt holes