Global demand for stainless steel is soaring, with a total production in excess of 18million metric tonnes in 2004 representing an increase of 7.3percent over the almost 17 million etrictonnes produced in 2003.

Moreover, growth in Europe is remaining strong (4.6percent year-on-year), indicating that the figures are not purely a result of the rapidly expanding Asian market. These figures from the International Stainless Steel Forum (ISSF) are even more impressive when seen in the context of previous years' growth. If cold rolled products are taken as typical of all demand, the curve is heading upwards at a healthy rate and there is every reason to expect that it will continue to do so.

One of the reasons for this strong demand is the wide variety of applications for which the family of metals known as stainless steel is suitable. The architectural and process industries account for much of the demand, but stainless steel is also used in medical equipment, prosthetics, marine equipment , household goods, automotive, aerospace, nuclear and countless general engineering applications.

With an increasing interest in whole-life costs and the need for products to be recycled at the end of their service life, stainless steel is becoming the material of choice with greater regularity. Where other grades of steel are used, the costs associated with corrosion over the lifetime of a component can be vast, partly due to the need for periodic maintenance, but also because of the added expense incurred when a component fails unexpectedly, or because corrosion renders parts difficult or impossible to dismantle when required.

Nevertheless, stainless steel is not totally immune to corrosion; the thin protective layer of chromium-rich oxide can only repair itself if there is sufficient oxygen available, and some particularly aggressive environments or process media require specialist grades of stainless steel if they are to survive. Additions of alloying elements such as nickel, molybdenum and nitrogen can also enhance other properties such as formability, strength and cryogenic toughness.

The history of stainless steel only dates back less than 100 years, but developments have been virtually non-stop. Recently, highly alloyed superaustenitic and superduplex grades have become available, offering excellent corrosion resistance and better weldability.

Nevertheless, the evolutionary story of stainless steel is not just about alloying. In many cases stainless steel is selected partly for its aesthetic properties, and pattern-rolled stainless steel has been available for many years. Traditionally the choice of finishes has been restricted to Linen, Diamonds, Squares and Leather Grain, but ThyssenKrupp Nirosta has recently launched six new alternatives that will appeal to architectural and engineering users, as well as product designers.

The new finishes are called Microchecker, Microlinen, Austenite, Triangles, Waterfall and Haze. Four of them -- Microchecker, Microlinen, Haze and Austenite -- have been developed for small surfaces, whereas Triangles and Waterfall are suitable for larger applications.

Microchecker is based on the teardrop finish familiar from hot-rolled stainless steel strip with its raised elliptical teardrops. While the original coarse and functional hot-rolled version is used mainly for anti-slip floors, Microchecker is a delicate pattern on a shiny surface with teardrops measuring less than five millimetres long. It is said that this high-class finish triggers industrial associations and is suitable for conveying a high-quality technical impression.

Microlinen, as the name suggests, is a caled-down version of the traditional Linen pattern with closely alternating dots and dashes. Microlinen is around 50percent smaller, making for a finer, denser, higher-quality appearance. One particular advantage of this finish is that it loses nothing of its quality and attractive looks when the sheet undergoes heavy forming.

The new Haze finish, which is already being used in the manufacture of stainless steel sinks, is also capable of withstanding heavy forming operations. It has a dense surface texture composed of alternating vertical and horizontal raised bars a few tenths of a millimetre in size. Compared with the common finishes used for stainless steel sinks, Haze is claimed to offer advantages in terms of scratch resistance and cleaning.

Austenite is a very striking pattern based on the microstructure of an austenitic stainless steel. The grain boundaries are represented by slightly raised bars and the different shades of the individual grains by closely spaced grooves hatched into the surface. The Austenite pattern plays with the light to create a dazzling finish and is suitable, for example, for producing special design effects on trim strips or switch trim plates.

Triangles is described as a calm, stylish pattern consisting of juxtaposed equilateral triangles around two millimetres in height. The sides of the triangles are represented by bars while the faces are recessed. Triangles is suitable for large applications, such as in elevators, escalator panelling or construction elements.

Waterfall has also been designed for large applications and creates an impression of many small water droplets running down a smooth surface. The texture is deeply embossed and therefore insensitive to fingerprints. Because of its non-directional reflection properties, it offers the same appearance from every viewpoint.

ThyssenKrupp Nirosta supplies these design finishes as standard on strip and sheet made of Nirosta4016, 4301 and 4401, or other grades can be supplied on request.
It has been said that a material is only as good as the methods available for joining it.

Not surprisingly, welding is commonly used with stainless steel, even though the process of locally melting and recasting the metal inevitably results in metallurgical changes that are not necessarily favourable. Manual metal arc welding was the dominant process for stainless steel well into the 1980s, and it is still widely used today.

Submerged arc welding has been highly successfully used for semi-automated welding, and cored wires are now popular too. One of the main developers of welding processes and consumables for stainless steel has been ESAB, which has closely followed the development of new grades of stainless steel.

Laser and electron beam welding certainly have niche applications, though it is unlikely that they will challenge the dominance of conventional fusion welding processes for larger sections. While friction stir welding is proving successful for aluminium, its use on stainless steel is less so, due to problems with tool life and welding speed. On the other hand, laser-hybrid welding is showing good results, providing most of the advantages of laser welding (such as good penetration) with the good bridging ability of MIG (metal inert gas) welding. High-quality welds can be produced relatively quickly, with the added benefit of being able to use a consumable wire if extra material is required or the weld metal composition needs to be modified.

On a smaller scale, selective laser sintering is maturing within the field of rapid product development as a process for creating mould tools, prototype components and even production-quality components where small quantities are required. This additive process can build parts with excellent mechanical properties, fine detail and very good surface quality. Machines such as the Eosint M270 from EOS (Electro Optical Systems) have a build volume of 250x250x215mm and a layer thickness of just 20--100microns, depending on the material being used.

Another process suitable for building stainless steel parts was reported in the April 2004 issue of European Design Engineer. Cold gas dynamic manufacturing (CGDM) uses a high-pressure (10-35bar) gas stream that is heated up to 600°C and accelerated through a convergent-divergent (de Laval) nozzle to supersonic velocities as high as 2000m/s. Metallic particles less than 50microns in diameter are entrained into the gas stream and accelerated due to the drag force exerted on them in the high-velocity gas flow. When the particles impact on a substrate, their kinetic energy causes them to deform plastically -- at temperatures believed to be below their melting point -- so that they adhere to the surface.

Whatever happens to the global supply of stainless steel, it seems that demand will continue to increase for the foreseeable future. Furthermore, it is likely that there will continue to be new grades capable of higher performance, and process enhancements will enable production costs to be reduced. In turn, this will lead to more consumption.

The future for stainless certainly looks bright, shiny and ready to resist attack from alternative materials.