< FERROUS ALLOYS
IRON
Cast iron is free flowing when molten, and very strong, if somewhat brittle, when solidified. The various grades of common cast iron are nominally identified by the colour of an exposed TEST FRACTURE (GREY, BLACKHEART, WHITEHEART are examples). Alloying elements such as CARBON, SILICON and PHOSPHORUS impart additional strength to the iron, allowing for it’s traditional use as a structural/decorative material.
The use of cast iron in structurally oriented decoration was pronounced at the height of the industrial revolution during the mid to latter part of the nineteenth century. Some of the great masterpieces of engineering design were constructed at this time, including Sir Joseph Paxton’s (1801-1865) Crystal Palace for the Great Exhibition of 1851. A much earlier example of decorative ironwork was the railing erected around St. Pauls Cathedral, London, this was installed between 1710 and 1714. As a rule though, use of cast iron in creating fine sculptural work was, and still is, less than common. A cast iron head originating from the Ming dynastic period (China 1368-1644), is in the Fitzwilliam Museum, this item is believed to be one of the oldest examples of a cast iron artwork to be held in a British collection [ref].
Whilst the relatively high carbon content in grey cast irons improves the metal’s outright STRENGTH, carbon also reduces the ability of iron to resist impact, shock and other stresses. The application of a shock force can potentially lead to the fracturing of a cast iron design. Damage can arise from mechanical or heat sources (hammering or welding for instance), or in service (usually due to vandalism or mishandling). For this reason the MALLEABLE and DUCTILE versions of cast iron are generally preferred for producing artworks today.
Ductile or SPHEROIDAL GRAPHITE (SG) iron – so called because the carbon content of the metal chills into graphite nodules – is an important grade. Whereas most carbon in cast iron forms as ‘flake’ like structures, the addition of magnesium to the SG iron charge, combined with a low sulphur content, encourages carbon to chill in a spherical form thus imbuing this material with it’s ductile characteristics. The ability to manipulate a cast to some extent without undue risk of fracturing is an especially useful quality in art founding, where cast sculptures are usually extensively worked during the chasing/finishing stages.
Standard rutile type carbon steel MMA welding ELECTRODES can be used for joining ductile SG iron sections, these welding electrodes being far more economic than the more usual nickel bearing electrodes which are used for welding traditional cast iron grades (see below). In the US ductile cast irons are described and designated under ASTM A536-84:1984, in Britain and Europe either BS 2789:1985 or ISO 1083:1976.
The joining of cast iron sections is traditionally done with fastenings in the form of rivets or bolts, either of these methods minimise the likelihood of the cast fracturing – always a potential danger with the localised heat concentrations required for fusion welding processes. Many artists and designers working with grey cast irons use this to their advantage, and employ fastenings as an integral visual feature in the artwork itself. Welding of grey and other traditional irons can be done by MMA (STICK) welding, using a flux covered electrode containing a nickel bearing alloy (this type of electrode can be very expensive). The LOW HYDROGEN type welding electrodes and filler wires used for joining low carbon steel alloys are an alternative option, though the cast must be pre-heated before welding commences to minimise the possibility of cracking in the joint zone (SG iron excepted). Cast iron can also be joined by oxy-gas, in particular flux assisted BRONZE WELDING, which is a essentially a non-fusion brazing technique.
WROUGHT IRON >
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