why 18 Cr in 18/8 steel

The question was why increasing to 18 Cr in austenitic staibnless steels when 11 is enough to make it stainless.
The answer is that when casting the steel it is better to have ferrite
forming from the liquid and that he auqtenite is formed as a solid state transformation at a lower temperature. If only 11 Cr there is no ferrite at high temperature.
The ferrite/austenite transformation is affected by many alloying elements and the balance between these phase is a key to controlling the properties of steel.
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