Summary

What is stainless steel ?

Stainless steel is the generic name for a number of different steels used primarily for their resistance to corrosion. The one key element they all share is a certain minimum percentage (by mass) of chromium: 10.5%.

Although other elements, particularly nickel and molybdenum, are added to improve corrosion resistance, chromium is always the deciding factor. The vast majority of steel produced in the world is carbon and alloy steel, with the more expensive stainless steels representing a small, but valuable niche market.

Carbon steels without any protection will form a coating of rust which will in a sense protect the rest of the steel. So constantly removing the rust exposes a new fresh layer of steel to be attacked. This is called general corrosion .

The stainless steel advantage

The success of stainless steel is based on the fact that it has one unique advantage. The chromium in the stainless steel has a great affinity for oxygen, and will form on the surface of the steel at a molecular level a film of chromium oxide. Stainless steel particularity is described as a passive, tenacious and self renewing layer.

Stainless steel is more expensive that using ordinary steel, not just because of the higher cost of stainless steel, but also because it is more difficult to machine.

The development of stainless steel The inventor of stainless steel, Harry Brearley, was born in Sheffield, England in 1871. By years of private study and night school he became an expert in the analysis of steel and its production.

One of Brearley's research was to look for a steel with better resistance to erosion, not corrosion. As a line of investigation he decided to experiment with steels containing chromium, as these were known to have a higher melting point than ordinary steels.

Using first the crucible process, and then more successfully an electric furnace, a number of different melts of 6 to 15% chromium with varying carbon contents were made. The first true stainless steel was melted on the 13th August 1913. It contained 0.24% carbon and 12.8% chromium.

Within a year of Brearley's discovery, Krupp in Germany were experimenting by adding nickel to the melt. Brearley's steel could only be supplied in the hardened and tempered condition; the Krupp steel was more resistant to acids, was softer and more ductile and therefore easier to work. From these two inventions, just before the First World War, were to develop the "400" series of martensitic and "300" series of austenitic stainless steels.

The First World War largely put a halt to the development of stainless steel, but in the early 1920s a whole variety of chromium and nickel combinations were tried including 20/6, 17/7 and 15/11. Dr W.H. Hatfield, who is credited with the invention in 1924 of 18/8 stainless steel (18% chromium, 8% nickel) which, with various additions, still dominates the melting of stainless steel today. Dr Hatfield also invented 18/8 stainless with titanium added, now known as 321.

Product characteristics

Stainless steel is selected for use compared to other materials for a number of different reasons, not just its resistance to corrosion. These include: Its aesthetic qualities: it can be polished to a satin or mirror finish. "Dry Corrosion" occurs to steel at higher temperatures. Non-contamination of the liquids stainless comes into contact with. Weight savings: as thinner sections .

Applications

Very large amounts of stainless steel are used in food production and storage. The most commonly used grades are 304 and 316. Whereas 304 is used for normal temperatures and acid concentrations, 316 is used for harsher environments. Very often in food production stainless is used not because the food itself is corrosive but the use of stainless allows for faster and more efficient cleaning.

The pumping and containment of oils, gases and acids has created a large market for stainless tanks, pipes, pumps and valves. Special grades of stainless have been developed to have greater corrosion resistance : These are used in de-salination plants, sewage plants, offshore oil rigs, harbour supports, the storage of dilute nitric acid and ships propellers. Architecture is a growing market. Many modern buildings use stainless for cladding.. The low maintenance cost and anti-vandal characteristics of stainless provides a growing market in public transport, ticket machines and street furniture.

The nuclear power industry uses large quantities of stainless, often specified with a low cobalt content, for both power generation and radiation containment.