Whenever chromium comes into contact with oxygen, it forms a thin invisible layer of chromium oxide (also referred to as dichromium trioxide) (Cr2O3); the oxide skin. This thin layer protects the underlying metal against further oxidation.
The quantity of carbon determines the hardness of the steel. A high carbon steel will be difficult to process.
Those types of stainless steel that have a nickel content of 6-26% (the AISI 300 series) are classed as austenitic and non-magnetic when supplied. They offer excellent deformation properties (bending, deep drawing, expansion) and are also shock-resistant over the entire temperature range, both at very low and very high temperatures. By adding nickel to the steel in the above-mentioned quantities, the austenitic phase remains thermodynamically stable. Consequently, after heat treating austenitic stainless steel, the steel does not undergo any phase change during the cooling process. It is therefore impossible to harden austenistic stainless steel by heat treatment. The other elements, such as, for ex. chromium and molybdenum, increase the steel’s corrosion resistance and workability.
AISI 430, AISI 410 and AISI 409 (the 400 series) are examples of ferritic, as well as duplex types of stainless types. In contrast to the austenitic types of stainless steel, ferritic and duplex types of stainless steel are indeed magnetic, but their deformation rate is generally not so good.
Certain types of stainless steels are highly susceptible to chlorine. Mains water, swimming pool water, sodium hypochlorite (NaOCl), hydrogen chloride (HCl) and iron (III)chloride have a highly corrosive effect upon stainless steel. Pitting corrosion is a type of corrosion whereby the surface of the steel becomes pitted. If stainless steel AISI 304, for example, comes into contact with chlorinated water, either from drinking water or swimming pool water, the chlorine will attack the protective layer of chromium oxide in places. At first, this will cause a slight dip in the surface, in which more chlorine ions will collect, and the pitting will then preferably continue in this area, causing the pitting to penetrate deeper. Finally, the material will generally be of smooth appearance, but with some pitting in the surface. Pitting corrosion is characterised by holes appearing right next to welded seams.
The elasticity modulus of stainless steel is not the same as that of normal steel. The E modulus of SS is E = 195,000 MPa and that of ‘normal’ construction steel is: E = 210,000 MPa.
The tensile strength of stainless steel is a standard used to classify the mechanical properties of this type of steel. In practice, the yield point (sometimes also referred to as the 0.2% minimum proof point or Rp 02) is much more important. If the material reaches its tensile strength, it will already be plastically very deformed.