Stainless Steel is a generic name for a family of corrosion resistant alloys of materials containing more that 11 % Chromium. A more appropriate name for this group of materials would be “Corrosion Resistant Steel” that have various properties, according to the alloying elements and methods of production.
Corrosion resistance is obtained by the alloying elements in particular the Chromium addition through a self repairing system. The material forms a very thin layer of chromium oxide that is inert from attack in most environments. When the oxide layer is broken through what ever method the chromium within the material re-oxidates with the atmosphere, forming another resistant layer
Types of Stainless Steel
They are many grades available that generally are categorised into the 5 basic forms
Ferritic
Austenitic
Martensitic
Duplex
Precipitated Hardened
Ferrite
At ambient temperatures, iron crystallises into the body centre cubic structure non as thealpha ferrite – iron stage when referring stainless steel alloys. The material is magnetic that is destroyed when transforming to the beta stage at the Currie point of 769º C. The structure still remains ferritic up to 910 º C. The body centre cubic crystalline structure is characterised by moderate ductility which is important when referring to ferritic stainless steel. The basic composition of ferritic stainless steels is 12 – 18% Chromium with low carbon content.
Basic Properties
Moderate to good corrosion resistance, depending on level of chromium
Magnetic
May not be hardened by heat treatment
Difficult to weld
Not as good formability as austenitic grades
Typical Applications
Automotive Exhaust Systems
Floppy disk hubs
Civil Construction Equipment
Austenite
At temperature above 910 º C a very stable structure becomes face centre cubic. This is none as the gamma stage – iron or austenite when referring to stainless steel. This material is characterised by high ductility and is non magnetic. The basic composition of austenitic stainless steel is 18 % Chromium and 8 % Nickel.
