In order to obtain a perspective of the development of stainless steels, it is appropriate to look back to the beginning of the century; stainless steels are actually no older than that. Around 1910 work on materials problems was in progress in several places around the world and would lead to the discovery and development of the stainless steels.
In Sheffield, England, H. Brearly was trying to develop a new material for barrels for heavy guns that would be more resistant to abrasive wear. Chromium was among the alloying elements investigated and he noted that materials with high chromium contents would not take an etch. This discovery lead to the patent for a steel with 9- 16% chromium and less than 0.70% carbon; the first stainless steel had been born.
The first application for these stainless steels was stainless cutlery, in which the previously used carbon steel was replaced by the new stainless.
At roughly the same time B. Strauss was working in Essen, Germany, to find a suitable material for protective tubing for thermocouples and pyrometers. Among the iron-base alloys investigated were iron-chromium-nickel alloys with high chromium contents. It was found that specimens of alloys with more than 20% Cr did not rust even after having been left lying in the laboratory for quite some time. This discovery lead to the development of a steel with 0.25% carbon, 20% chromium and 7% nickel; this was the first austenitic stainless steel.
Parallel with the work in England and Germany, F.M. Becket was working in Niagara Falls, USA, to find a cheap and scaling-resistant material for troughs for pusher type furnaces that were run at temperatures up to 1200°C. He found that at least 20% chromium was necessary to achieve resistance to oxidation or scaling. This was the starting point of the development of heat-resistant steels.
However, it was not until after the end of World War II that the development in process metallurgy lead to the growth and widespread use of the modern stainless steels.

Stainless steels can thus be divided into six groups: martensitic, martensitic-austenitic, ferritic, ferritic-austenitic, austenitic and precipitation hardening steels. The names of the first five refer to the dominant components of the microstructure in the different steels. The name of the last group refers to the fact that these steels are hardened by a special mechanism involving the formation of precipitates within the microstructure. Table 2 gives a summary of the compositions within these different categories.

Chromium (Cr) This is the most important alloying element in stainless steels. It is this element that gives the stainless steels their basic corrosion resistance. The corrosion resistance increases with increasing chromium content. It also increases the resistance to oxidation at high temperatures. Chromium promotes a ferritic structure.

Nickel (Ni) The main reason for the nickel addition is to promote an austenitic structure. Nickel generally increases ductility and toughness. It also reduces the corrosion rate and is thus advantageous in acid environments. In precipitation hardening steels nickel is also used to form the intermetallic compounds that are used to increase the strength.

Molybdenum (Mo) Molybdenum substantially increases the resistance to both general and localised corrosion. It increases the mechanical strength somewhat and strongly promotes a ferritic structure. Molybdenum also promotes the formation secondary phases in ferritic, ferritic-austenitic and austenitic steels. In martensitic steels it will increase the hardness at higher tempering temperatures due to its effect on the carbide precipitation.

Copper (Cu) Copper enhances the corrosion resistance in certain acids and promotes an austenitic structure. In precipitation hardening steels copper is used to form the intermetallic compounds that are used to increase the strength.

Manganese (Mn) Manganese is generally used in stainless steels in order to improve hot ductility. Its effect on the ferrite/austenite balance varies with temperature: at low temperature manganese is a austenite stabiliser but at high temperatures it will stabilise ferrite. Manganese increases the solubility of nitrogen and is used to obtain high nitrogen contents in austenitic steels.

Silicon (Si) Silicon increases the resistance to oxidation, both at high temperatures and in strongly oxidising solutions at lower temperatures. It promotes a ferritic structure.

Carbon (C) Carbon is a strong austenite former and strongly promotes an austenitic structure. It also substantially increases the mechanical strength. Carbon reduces the resistance to intergranular corrosion. In ferritic stainless steels carbon will strongly reduce both toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels carbon increases hardness and strength. In the martensitic steels an increase in hardness and strength is generally accompanied by a decrease in toughness and in this way carbon reduces the toughness of these steels.

Nitrogen (N) Nitrogen is a very strong austenite former and strongly promotes an austenitic structure. It also substantially increases the mechanical strength. Nitrogen increases the resistance to localised corrosion, especially in combination with molybdenum. In ferritic stainless steels nitrogen will strongly reduce toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels nitrogen increases both hardness and strength but reduces the toughness.

Titanium (Ti) Titanium is a strong ferrite former and a strong carbide former, thus lowering the effective carbon content and promoting a ferritic structure in two ways. In austenitic steels it is added to increase the resistance to intergranular corrosion but it also increases the mechanical properties at high temperatures. In ferritic stainless steels titanium is added to improve toughness and corrosion resistance by lowering the amount of interstitials in solid solution. In martensitic steels titanium lowers the martensite hardness and increases the tempering resistance. In precipitation hardening steels titanium is used to form the intermetallic compounds that are used to increase the strength.

Niobium (Nb) Niobium is both a strong ferrite and carbide former. As titanium it promotes a ferritic structure. In austenitic steels it is added to improve the resistance to intergranular corrosion but it also enhances mechanical properties at high temperatures. In martensitic steels niobium lowers the hardness and increases the tempering resistance. In U.S. it is also referred to as Columbium (Cb).