What is stainless steel?
Stainless steel is essentially a low carbon steel to which chromium has been added. It is this addition of chromium, in amounts greater than 10.5% by weight, that gives the steel its unique ‘stainless’, corrosion resisting properties.
The chromium content of the steel allows the formation of a tough, adherent, invisible, corrosion resisting chromium oxide film on the steel surface. If damaged mechanically or chemically this film is self-healing, provided that oxygen, even in very small amounts, is present. The corrosion resistance as well as other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen.
Benefits of stainless steels
The bright, easily maintained surface of stainless steel provides an attractive and contemporary appearance, ideal for a myriad of architectural applications.
Lower alloy grades resist corrosion in normal atmospheric and potable water environments, while the more highly alloyed grades can resist corrosion in many acids and alkaline solutions, and some chloride bearing environment, properties which are widely utilised in process plants.
Stainless Steels have high tensile strengths and excellent fatigue properties. Austenitic grades work harden with cold working, and duplex steels allow for reduced thicknesses over traditional grades. Substantial cost savings therefore result as well as increased competitiveness with alternative materials.
Toughness and impact resistance
The austenitic microstructure of the 300 series provides high toughness, from elevated temperatures to far below freezing, making these steels suited to all applications, including cryogenic ones.
Special high chromium and nickel alloyed grades resist scaling and retain strength at high temperatures, up to over 1100°C.
Ease of fabrication
Modern steel manipulation techniques mean that stainless steels can be cut, welded, formed, and fabricated as readily as traditional steels and other materials.
The easy cleanability of stainless steel surfaces makes it the first choice for strict hygiene conditions, such as in hospitals, kitchens, abattoirs and other food processing plants.
Low maintenance costs
Stainless steel normally requires only a periodic wash with soap and water to maintain its original finish.
Long term value
When the total life cycle costs are considered, stainless steel is often the least expensive material option.
Grades of stainless steel
Various grades of stainless steel are identified by their content of alloying elements. These affect their metallurgical microstructures (by which the types are designated) as well as their mechanical properties and corrosion resistance, which can be exploited by the engineer and architect.
Ferritic (AISI 400 series)
Typically containing 10.5% min chromium and low carbon (<0.12%). Microstructure of martensite and ferrite. Martensitic (AISI 400 series)
Typically containing 12-18% chromium and higher carbon (>0.12%). Microstructure of martensite and ferrite.
Austenitic (AISI 300 series)
Typically containing 17-26% chromium, 7-25% nickel and up to 7% molybdenum. Microstructure of austenite, so these grades are non-magnetic.
Typically containing 18-25% chromium, 4-7% nickel and up to 4% molybdenum. Microstructure of austenite and ferrite.
Typical grades used in architecture
The austenitic steels account for in excess of 70% of stainless steel production, and represent by far the greatest percentage of stainless steels used in the Architectural, Building and Construction markets. The remainder is primarily ferritic grade 430.
The grades of particular interest to the architect are listed below:
17%Cr7%Ni: A lower alloys grade which results in high strength and comparatively high ductility after cold working.
18%Cr9%Ni: The general-purpose grade with excellent workability, weldability and polishability.
18%Cr10%Ni2%Mo: Superior corrosion resistance to the otherwise similar properties of type Grade. The “marine grade” stainless steel.
Grade 304 and Grade 316 are used for internal and external applications.
17%Cr: Interior architectural applications. Generally, slightly lower corrosion resistance than Grade 304 with adequate ductility for most forming operations. Not as suitable for welding operations.
Forms and finishes
Stainless steel comes in a wide variety of product forms and standard surface finishes.
A complete range of products encompassing sheet, coil, strip, plate, bar, tube, pipe, etc. is very important to meet the diversity of architectural applications.
Surface finishes which impart stainless steel with unique aesthetic appeal vary according to product form. The most common finishes however range from 2B, a silvery grey general purpose cold rolled finish, through BA (Bright Annealed), Hairline and No.4 (both with visible parallel polishing marks) and No.8, the most reflective finish, buff polished for high quality architectural applications.
Other non standard finishes are available on application including embossed and patterned finishes.
To many of these, colours such as gold, bronze, blue, red, green and black may be added by chemical process.