CARBON STEELS: ALLOY STEELS, LOW CARBON [MILD], LOW ALLOY [HIGH TENSILE]
Steels are created by conversion from pig IRON - burning off impurities such as excess carbon. This is achieved by forcing oxygen through the molten iron charge. The injection of oxygen causes the formation of iron and other oxides which combine as a SLAG (waste), later removed at a convenient point in the CONVERSION process. Following conversion the carbon remaining in the average mild steel grade is in the region of 0.20%. Beneficial alloying elements are also 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 heavily CONTRACT and shrink on cooling discourages it’s use in art founding. When requested, the carbon steels most suited to casting 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 for the cost effective fabrication of structural and other design features from STANDARD STOCK materials.
For most art and design applications a LOW CARBON STEEL [LCS] ('MILD') grade is more than adequate. If exceptional stresses and loads 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 along with additions of manganese, nickel and chromium to improve 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 problematic to cut and shape. Furthermore, welding LAS steel sections requires far greater control than when welding mild steel sections - this 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 example when fabricating a major support structure, the founder or fabricator will often have to submit welding procedure sheets along with test weld samples 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, these are costs not normally allowed for in a standard foundry quotation (see also MATERIALS TESTING).
Nearly all carbon steels are easily joined using one of the FUSION welding processes (OXY-GAS, MMA and MIG), in addition to BRAZING and mechanical fastening methods (bolts etc). TIG welding of most carbon steels is best done with 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. MMA (stick) welding of LCS mild steels is straightforward, the general purpose RUTILE type covered electrode is preferred for most situations. MMA Welding of LAS high strength steel is usually carried out with low hydrogen or BASIC type covered electrodes 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, PLASMA, LASER, WATER-JET and other similar methods, though like welding, LAS does have the potential to crack in the region of a cut.
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