Stainless Steel Alloys

The Stainless Steel Family
A short description of the various grades of stainless steel and how they fit into
distinct metallurgical families. It has been written primarily from a European
perspective and may not fully reflect the practice in other regions.
Stainless steel is the term used to describe an extremely versatile family of
engineering materials, which are selected primarily for their corrosion and heat
resistant properties.
All stainless steels contain principally iron and a minimum of 10.5% chromium. At this
level, chromium reacts with oxygen and moisture in the environment to form a
protective, adherent and coherent, oxide film that envelops the entire surface of the
material. This oxide film (known as the passive or boundary layer) is very thin (2-3
namometres). [1nanometre = 10-9 m].
The passive layer on stainless steels exhibits a truly remarkable property: when
damaged (e.g. abraded), it self-repairs as chromium in the steel reacts rapidly with
oxygen and moisture in the environment to reform the oxide layer.
Increasing the chromium content beyond the minimum of 10.5% confers still greater
corrosion resistance. Corrosion resistance may be further improved, and a wide
range of properties provided, by the addition of 8% or more nickel. The addition of
molybdenum further increases corrosion resistance (in particular, resistance to pitting
corrosion), while nitrogen increases mechanical strength and enhances resistance to
pitting.
Categories of Stainless Steels
The stainless steel family tree has several branches, which may be differentiated in a
variety of ways e.g. in terms of their areas of application, by the alloying elements
used in their production, or, perhaps the most accurate way, by the metallurgical
phases present in their microscopic structures:
 Ferritic
 Martensitic (including precipitation hardening steels)
 Austenitic
 Duplex steels, consisting of mixture of ferrite and austenite