Stainless steel is not a single material but the name for a family of corrosion resistant steels. Like many scientific discoveries the origins of stainless steel lies in a serendipitous accident. In 1913 Sheffield, England, Harry Brearley was investigating the development of new steel alloys for use in gun barrels. He noticed that some of his samples didn’t rust and were difficult to etch. These alloys contained around 13% chromium.
The first application of these steels was in cutlery for which Sheffield subsequently became world famous. Simultaneous work in France led to the development of the first austenitic stainless steels.
Worldwide demand for stainless steel is increasing at a rate of about 5% per annum. Annual consumption is now well over 20 million tonnes and is rising in areas such as the construction industry and household appliances. New uses are being continuously found for the attractive appearance, corrosion resistance, low maintenance and strength of stainless steel. Stainless steel is more expensive than standard grades of steel but it has greater resistance to corrosion, needs low maintenance and has no need for painting or other protective coatings. These factors mean stainless steel can be more economically viable once service life and life-cycle costs are considered.
The advantageous properties of stainless steels can be seen when compared to standard plain carbon mild steel. Although stainless steels have a broad range of properties, in general, when compared with mild steel, stainless steels have:
~ Higher corrosion resistance
~ Higher cryogenic toughness
~ Higher work hardening rate
~ Higher hot strength
~ Higher ductility
~ Higher strength and hardness
~ A more attractive appearance
~ Lower maintenance
All stainless steels are iron-based alloys that contain a minimum of around 10.5% Chromium. The Chromium in the alloy forms a self-healing protective clear oxide layer. This oxide layer gives stainless steels their corrosion resistance. The self healing nature of the oxide layer means the corrosion resistance remains intact regardless of fabrication methods. Even if the material surface is cut or damaged, it will self heal and corrosion resistance will be maintained.
Conversely, normal carbon steels may be protected from corrosion by painting or other coatings like galvanising. Any modification of the surface exposes the underlying steel and corrosion can occur.
The corrosion of different grades of stainless steel will differ with various environments. Suitable grades will depend upon the service environment. Even trace amounts of some elements can markedly alter the corrosion resistance. Chlorides in particular can have an adverse effect on the corrosion resistance of stainless steel.
Grades high in Chromium, Molybdenum and Nickel are the most resistant to corrosion.
Cryogenic (Low Temperature) Resistance
Cryogenic resistance is measured by the ductility or toughness at sub zero temperatures. At cryogenic temperatures the tensile strengths of austenitic stainless steels are substantially higher than at ambient temperatures. They also maintain excellent toughness.
Ferritic, martensitic and precipitation hardening steels should not be used at sub-zero temperatures. The toughness of these grades drops significantly at low temperatures. In some cases this drop occurs close to room temperature.