< CARBON STEELS
HIGH STRENGTH WEATHERING STEELS
One group of carbon steels of particular interest to artists and designers are the HIGH STRENGTH WEATHERING STEELS. The best known of these steels is COR-TEN® an alloy developed by the American USX Corporation, though there are a number of comprable alloys including those designated under BS 4360, ASTM A242 (T1), and in Australia and NZ AS/NZ 3678-WR350. Weathering steel is primarily used in art, design and architecture to produce constructed steel fabrications with a naturally weathered RUST patina finish. It is also worth noting that specialist founders can also usually produce a close substitute alloy suitable for castings. Cor-ten is available in either an A or B grade material – alloyed primarily with additions of carbon, silicon, manganese, phosphorus, sulphur, chromium, nickel and copper (vanadium and soluble aluminium are added to the ‘B’ grade) [ref].
As with most other alloys within the carbon steel group, Cor-ten and other weathering steels react with oxygen to produce a rust oxide on untreated surfaces. In normal carbon steels the oxidisation process can eventually lead to an oxide conversion of the entire fabric – potentially resulting in structural collapse. The alloying elements added into weathering steels (in particular the copper and chromium components), encourage a very dense oxide layer to develop on the exposed surfaces, this exceptional density effectively inhibits any deeper corrosive penetration into the fabric of the metal. Dependent upon the ambient atmosphere and conditions the steel is exposed to, a rich rust oxide patina naturally develops without unduly compromising the longevity of the artwork. The weathered patina process relies on an ongoing cycle of WETTING followed by DRYING; the steel will nornally deteriorate if continually wet or if exposed to high saline and other corrosive atmospheres. To encourage an even development of the ‘patina’, sculptures made in weathering steels can be lightly shot-blasted prior to final installation. The surface of the metal may also be treated with a mild SPIRIT OF SALTS (a weak solution of hydrochloric acid), to accelerate the oxide’s development, though excessive treatment can potentially induce acid pitting.
Weathering steels are only usually available as COLD REDUCED or HOT ROLLED sheets and plates. Cor-ten can be supplied in thicknesses of up to 1 inch (25mm), other versions vary from about 3 - 20mm. Unfortunately this means that, the extensive ranges of cross sectional shaped stock (angles, hollow sections, beams etc) associated with standard grades of steel, are not available in weathering steels.
Welding procedures for weathering steels are broadly the same as for other carbon steels, though some of the techniques used to weld the high strength (LAS) steels with LOW HYDROGEN rods may apply. Standard steel filler rods, wire and electrodes can be used for welding weathering steels (with dilution helping to retain the steel's properties), though matched fillers specific to weathering steels are preferred for welding thicker sections – this ensures metallurgical compatability. For TIG and MIG processes an ER80S-G type filler wire provides a suitable match, and for MMA welding, a nickel/copper bearing mild steel low hydrogen (BASIC) electrode will normally be recommended.
With the exception of weathering steels, nearly all carbon steel sculptures intended for long term exposure out of doors are subjected to some form of protective surface treatment. Suitable treatments include painting, stove enamelling and hot dip galvanising to name a few. These various coatings prevent the premature corrosion of the design and inhibit rust seepage. Due to the difficulties of combining carbon steels with non-ferrous metals (because of cross contamination and ELECTROGALVANIC CORROSION), suitable insulation measures and fittings should be used if (for example), a bronze cast is to mounted on a steel base.
STAINLESS STEELS >
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