< WROUGHT IRON
CARBON STEELS: (ALLOY STEELS, LOW CARBON [MILD STEEL], LOW ALLOY [HIGH TENSILE], COR-TEN®)
Steels are converted from pig iron by burning off impurities (mainly carbon), with an injection of oxygen through the iron charge. The injection of oxygen causes iron and other oxides to form, creating a SLAG (waste) that can be removed at a convenient point in the CONVERSION process. In addition to the removal of undesirable elements from the charge (the eventual carbon levels in most mild steels are typically less than 0.20%), beneficial alloying elements are added during conversion; these include manganese and silicon, which variously DEOXIDISE the steel and improve it’s working properties.
Whilst the casting of carbon steels to make industrial components is common in commercial foundries, it is somewhat unusual for an art or design work to be specified in this material. From a purely practical point of view, the higher temperatures required for melting steel, and the tendency for the alloy to CONTRACT and shrink on cooling discourages it’s use in art founding. When specified, the carbon steels most suitable for casting purposes are designated under BS3100:1991.
Despite limited use in it’s cast form, wrought carbon steels are used extensively throughout the art foundry production process. Steels feature in items as diverse as MASTER PATTERNS and ARMATURES, fabricated structural elements and fixings. Steels are available in an enormous range of convenient hot and cold formed cross sections, allowing cost effective fabrication of structural and other design features from STANDARD STOCK materials.
For most art and design applications the MILD or LOW CARBON STEEL (LCS) grade is more than adequate. If exceptional stresses are expected, the stronger LOW ALLOY STEEL (LAS) may be specified instead. LAS, which is also sometimes also known as ‘55 or HIGH TENSILE STEEL, contains a minimal amount of carbon, with additions of manganese, nickel and chromium to improve it’s tensile strength. Indistinguishable visually from mild steels, stock low alloy steel is usually identified (in the UK) by a blue paint mark on it’s surface. It is not advisable to specify low alloy tensile steel in preference to low carbon (mild) steel for fabrications without good reason. In addition to the material’s higher cost, useful working properties including ductility, are compromised making LAS more difficult to cut and shape. Furthermore, welding LAS steel sections requires far greater control than when welding mild steel sections. Welding LAS often involves the use of specialist welding consumables and specific working procedures (primarily designed to counteract HYDROGEN CRACKING in the weld and surrounding parent metal). When LAS is specified, for instance for fabricating a major support structure, the founder or fabricator will often have to submit welding procedure sheets, together with test samples of weld deposits for laboratory investigation. A work of this type is referred to as being constructed to ‘code’ or ‘approval’. CODED WELDING of special materials often involves significant additional expenditure, and these are costs not normally allowed for in a standard foundry quotation (see also MATERIALS TESTING).
Nearly all carbon steels are readily joined by FUSION welding processes (OXY-GAS, MMA and MIG), as well as BRAZING and mechanical fastening methods. The TIG welding of most carbon steels is best done using a high quality A15 SUPERSTEEL type filler rod. These DEOXIDISED rods are substantially better for use with a TIG process than the more common oxy-gas filler rods (BS 1453:A1, 2 or 3), which can introduce a great deal of gas porosity into a deposited TIG weld. The MMA (stick) welding of LCS mild steels is straightforward, and general purpose RUTILE type covered electrode is preferred for most situations. MMA Welding of LAS high strength steel is usually carried out using a low hydrogen or BASIC type covered electrode, done in combination with PRE-HEATING procedures designed to prevent crack formations – both in the weld itself, and the surrounding steel structure. Steel is readily cut by flame and plasma methods, though again, LAS does have the potential to crack in the area of a cut.
HIGH STRENGTH WEATHERING STEELS >
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