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Austenitic Stainless Steels

Introduction

Stainless steel is the name given to a family of corrosion and heat resistant steels containing a minimum of 10.5% chromium. Just as there is a range of structural and engineering carbon steels meeting different requirements of strength, weldability and toughness, so there is a wide range of stainless steels with progressively higher levels of corrosion resistance and strength. This results from the controlled addition of alloying elements, each offering specific attributes in respect of strength and ability to resist different environments. The available grades of stainless steel can be classified into five basic families: ferritic, martensitic, austenitic, duplex and precipitation hardenable.

Austenitic stainless steels

Austenitic stainless steels have a “face-centred-cubic” (fcc) crystal structure, which is the same as that of pure iron above the A3 temperature of 910oC. Pure iron maintains this structure until it reaches the A4 temperature of 1390oC, where it reverts to the BCC structure.

In this type of stainless steel there are basically two main alloying elements, i.e. Chromium, and Nickel. The classic austenitic stainless steel is the “18:8” alloy which contains 18% Chromium and 8% Nickel. In these alloys they mainly rely on Nickel additions to stabilise the Austenitic/FCC structure, but other elements such as Manganese, Copper and Nitrogen also exhibit this property of stabilising the Austenitic structure. These steels are not heat treatable, so can not be hardened in this manner, however depending upon their exact composition they can show very high levels of work hardening.

Because of their face centred cubic structure they are highly ductile and can be readily formed into a variety of shapes.

Their composition can be altered to maximise formability, whether this is for deep drawing or stretch forming.

Their face centred cubic structure also means that they are non-magnetic, but certain compositions within this group can become weakly magnetic, especially after cold working. However, it is also possible to manipulate their composition so that they do not become even slightly magnetic after such working.

Austenitic stainless steels generally exhibit good weldability and very good toughness even down to cryogenic temperatures, i.e. they do not display a ductile to brittle transition with decreasing temperature.

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