< WEATHERING STEELS
STAINLESS STEELS (ALLOY STEELS, AUSTENITIC, 18/8, 3XX SERIES)
Shortly before the onset of the First World War a series of special steel alloys was developed in Sheffied, Britain. The principal alloying elements added into these steels were CHROMIUM and NICKEL. The resulting material variations are known today as STAINLESS STEELS. Stainless steel is used in a variety of art foundry situations - either as a alloy for casting sculpture, or more commonly, as a wrought material for constructing stud fixings and structural supports.
There are three basic types of stainless alloy in common use. The first two types are the MARTENSITIC and FERRITIC alloys. Both of these are designated by the number prefix ’4’ in the three digit ISO identifying code used for stainless products. These two steels are alloyed with chromium, usually to a level somewhere between 11% and 18%. Both grades are almost exclusively used for special applications including (most recognisably), cutlery manufacture. The martensitic and ferritic alloys are typically very hard, prone to embrittlement, and do not weld especially well (these are similar characteristics to those found in LOW ALLOY CARBON STEELS). The ‘4’ series stainless steels are usually distinguished by their ability to attract a magnet, it is this quality which most obviously separates these alloys from the third type, a chromium/nickel alloy mix known as AUSTENITIC stainless, which at best has only a low magnetic potential.
Unlike martensitic and ferritic steels, austenitic stainless steels are alloyed with significant quantities of nickel in addition to chromium. The nickel content provides the alloy with a degree of ductility and it is this material that allows austenitic stainless steels to be worked, formed and annealed using similar workshop methods to those used for MILD or LOW CARBON STEELS (including rolling, forging, pressing, deep drawing etc). The most common grades of austenitic contain approximately 18% chromium and 8% nickel additions, hence the alternative term for this alloy as 18/8 STAINLESS. Austenitic grades are identified by an ISO code commencing with the number ‘3’ (though some rare austenitics are designated as ‘9’ series).
Like mild carbon steels, stainless steel can be a problematic material to cast due to it’s shrinkage and hot tearing tendencies. Difficulty in obtaining a good quality reproduction in a stainless steel is further compounded by the presence of chromium and nickel elements, both of which can be subject to partial loss during melting procedures. For this reason founders who cast stainless usually melt the charge in ELECTRICAL INDUCTION type furnaces. It is important to bear in mind that this type of advanced equipment is not available in the most art and design focused foundries. In addition, the small capacity of many induction furnaces may significantly limit the maximum size of design work that can be cast in a single pour.
Most cast stainless steel sculptures are finished to a MIRROR POLISH or similarly high state of surface condition. Completing a work to a satisfactory standard of polish can be both an expensive and time consuming undertaking. The appearance of stainless steel as cast in the foundry or straight from mill rolling, is a dull white-grey. Unless pre-polished by machine, MILL SCALE on stainless requires a great deal of surface working to remove and then develop a full mirror effect. HOT FORMED material (rolled, cast, forged etc) is especially difficult to polish, so COLD REDUCED wrought stock is used where possible if a high finish is required.
Cast stainless material is always hot formed, so POLISHING cast works is not an especially easy undertaking. Despite the difficulties involved, highly polished stainless steel has the potential to create memorable sculptural images. ‘Rabbit’ (1986) by Jeff Koons (1955-), and ‘Turning the World Inside Out’ (1995), by Anish Kapoor (1954-), are both examples of highly polished artworks cast in stainless steel. Artists desiring a ‘silvered’ reflective surface on smaller works might also consider using a more readily available cast metal such as aluminium or copper based alloy. Both of these materials can be more easily cast and worked than stainless, then electroplated with a nickel or chromium plate.
Wrought stainless steel has an important role in the foundry as a material for constructing fixings, armatures and similar structural supports. It is also often used for special internal features set within sculptures, including pipework for water. Stainless is especially useful if copper or another conduit material is deemed insufficiently long-lasting. Stainless is excellent for making ancillary items because of its strength and inherent resistance to corrosion. The extra expense incurred by specifying stainless fixtures and fittings over mild steel equivalents can often be offset, at least in part, by the improved performance of the material in service. Savings can also be made in not having to apply a protective (PAINT or GALVANISED) coating to exposed stainless steel surfaces.
Like cast versions, wrought stainless steel is designated by various BS, ASTM and ISO specification codes. The most important grades for general art founding and fabrication purposes are the ISO 304 and 316 versions. 316, and 316 L (LOW CARBON) stainless steels are considered superior materials, this grade contains between 16-18% chromium, 10-14% nickel and 2-3% molybdenum. The higher alloy content in 316 stainless makes it suitable for use in most of the interior and exterior settings in which art and architectural designs are displayed. The uses for the less expensive 304 grade are rather more limited as this version is not suitable for use in exposed or external environments.
Stainless sculptures formed in ‘3’ series austenitics can be readily welded by any electric arc process with a matching filler rod or electrode. TIG and MMA techniques are the most popular and satisfactory for joining artworks, though wire is widely available for MIG welding. Gas welding can be done using a slightly carburising flame setting, together with a fluxed protected filler rod and joint area.
One notable drawback in welding stainless steel constructions, is that the high thermal resistance of the material renders it prone to HEAT DISTORTION. Countering distortion requires the use of fabrication techniques that take this effect into account – namely the careful balancing of input heat and the use of jigs or other workpiece restraints as necessary. A high quality TIG butt weld in stainless plate requires the joint to be BACK-PURGED (ie the underside of the joint is protected and then flooded with inert gas during welding). Back-purging prevents the UNDERBEAD from being contaminated and oxidised by atmospheric gases. As stainless steels are primarily selected for their ability to resist corrosive environments, they are not considered suitable for PATINATION.
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