Stainless steel & type of stainless steel

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Stainless steel is a name given to a group of steel alloys with many differences in properties and behaviour having one property in common - resistance to corrosion.

When an Alloy of Steel contains more than approximately 10.5% Chromium it can be classed as a stainless steel. This is due to the fact that Chromium has a high affinity for oxygen and forms a tenacious, stable oxide film, which is resistant to further chemical or physical change. This film, known as the passive film, forms practically instantaneously in ordinary atmospheres and has the remarkable property of being self-healing and rebuilding when it has been removed.

The large group of stainless steels can be divided into two major groups, namely Austenitic and Ferritic. The Ferritic group can be split again into two groups, Martensitic and Ferritic. If you would like to know more about the different types of steels within these groups, please follow below.

Austenitic

This group of steel alloys contains chromium normally in the range 17-25% and nickel in a range 8-20%, with various additional elements to achieve the desired properties. In the fully annealed condition, the steel alloys exhibit a useful range of physical and mechanical properties. The mechanical properties can be can be increased with cold working. Welding of this group must be carried out with the correct methods but the low carbon content results in fewer problems than the Ferritic or Martensitic grades. Normally these steels are non-magnetic but when a significant amount of cold working is involved, as in centreless grinding, the magnetic permeability may be increased. If this group is included with the Ferritic and Martensitic groups it can be seen that the stainless steel alloys offer a great deal of versatility for applications within modern industry. The numbers listed below represent grades within British Standard 970(bar) and British Standard 1449 (sheet and plate). The figures in brackets after each number are the Euronorms currently being introduced to supersede British Standards.

Ferritic

This group contains a minimum of 17% chrome and carbon in the range of 0.08% - 2.00%. The increase in chromium imparts increased resistance to corrosion at elevated temperatures, but the lack of mechanical properties due to the fact that it cannot be heat treated, limits its applications. Like martensitics they are magnetic and the welding of this group should be carried out with the necessary precautions.

Martensitic

This group contains a minimum of 12% chrome and usually a maximum of 14% with carbon in the range of 0.08% - 2.00%. Due to the high carbon content of the steel, it responds well to heat treatment to give various mechanical strengths, such as hardness. The carbon, however, is detrimental when welding and care should be taken during this operation. In the heat-treated condition this group of steels show a useful combination of corrosion resistance and mechanical properties that qualify them for a wide range of applications. The numbers listed below represent grades within British Standard 970. The figures in brackets after each number are the Euronorms currently being introduced to supersede British Standards.