Stainless steel is composed of an alloy containing Iron with a minimum of 10.5% Chromium. Chromium is the key as it produces a thin layer of oxide on the surface known as the "passive layer". This layer prevents any corrosion of the surface. If you increase the amount of Chromium, you will increase the resistance to corrosion.
What To Choose?
In order to decide which type of stainless steel to use is based on a combination of the following factors:
Corrosive environment? – Atmospheric, water, concentration of particular chemicals, chloride content, presence of acid.
Temperature of operation? – High temperatures accelerate corrosion rates require a higher grade. Low temperatures will require a tough austenitic steel.
Strength required? – Higher strength can be obtained from the austenitic, duplex, martensitic and PH steels. Other processes such as welding and forming often influence which of these is most suitable. For example, high strength austenitic steels produced by work hardening would not be suitable where welding was necessary as the process would soften the steel.
Welding? - Austenitic steels are generally more weldable than the other types. Ferritic steels are weldable in thin sections. Duplex steels require more care than austenitic steels but are now regarded as fully weldable. Martensitic and PH grades are less weldable.
Forming? – Austenitic steels are the most formable of all the types being able to undergo a high degree of deep drawing or stretch forming. Generally, ferritic steels are not as formable but can still be capable of producing quite intricate shapes. Duplex, martensitic and PH grades are not particularly formable.
Product Form? – Not all grades are available in all product forms and sizes. For example forms can come in sheet, bar, tube. In general, the austenitic steels are available in all product forms over a wide range of dimensions. Ferritics are more likely to be in sheet form than bar. For martensitic steels, the reverse is true.
Customer Performance Expectations? – Must be considered and this factor is often missed in the selection process. Particularly, what are the aesthetic requirements as compared to the structural requirements? Design life is sometimes specified but is very difficult to guarantee.
Surface finish is at least as important in many applications, particularly where there is a strong aesthetic component.
Availability?. There may be a perfectly correct technical choice of material which cannot be implemented because it is not available in the time required.
Cost? Sometimes the correct technical option is not finally chosen on cost grounds alone. However, it is important to assess cost on the correct basis. Many stainless steel applications are shown to be advantageous on a life cycle cost basis rather than initial cost.
The final choice will almost certainly be in the hands of a specialist but their task can be helped by gathering as much information about the above factors. If you miss something, sometimes it can be the difference between a successful and unsuccessful application.
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