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    Stainless Steel
   
What is Stainless Steel?

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 the alpha 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

 
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.

   


Basic Properties

  • Excellent resistance to corrosion
  • Easily welded
  • Good formability and ductility
  • Non magnetic
  • May be hardened by cold working


Typical Applications

  • Food Processing Equipment
  • Pharmaceutical Processing Equipment
  • Surgical Implant
  • Semiconductor Plants
  • Automotive
  • Architectural
  • Chemical Plant & Equipment
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