Austenitic Stainless Steel

Based on their microstructure, stainless steels are divided into the following families:

• austenitic
• ferritic
• martensitic
• duplex
• precipitation hardenable

The division based on microstructure is useful because the members within one family tend to have similar physical and mechanical properties.  However, the properties for one family can be very different from the properties for another family.  For example, austenitic stainless steels are non-magnetic, while ferritic and duplex stainless steels are magnetic. 

Select properties of austenitic and ferritic stainless steels

Properties	Austenitic	Ferritic
Toughness	Very high	Moderate
Ductility	Very high	Moderate
Weldability	Good	Limited
Thermal expansion	High	Moderate
Stress corrosion cracking resistance	Low	Very high
Magnetic properties	Non-magnetic	Ferro magnetic

Because of their good mechanical properties and the ease of fabrication, austenitic stainless steels are much more widely used than ferritic stainless steels.  About 75% of all stainless steel used worldwide is austenitic and about 25% is ferritic.  The other families, martensitic, duplex and precipitation hardenable stainless steels each represent less than 1% of the total market.

Over all Stainless Steel have the following properties:

• Excellent corrosion resistance - Stainles steel are significantly more resistant to oxidation by water and biocides than carbon steel and cast or ductile iron.
• Corrosion protection – an internal and external coating is not required for SS, nor is cathodic protection. This reduces system cost and makes stainless steels more compatible with environment.
• Hygienic materials - due to their very high passive film stability, SS are basically inert in waters. Leaching of alloying elements (Cr and Ni) is within safe limits. As a result, they provide a better quality of drinking water.
• Coefficient of friction – lower friction compared to cement lined carbon steel pipes or to corroded carbon steel results in less loss of water pressure along the pipeline. This provides an opportunity to diminish the internal diameter of the pipe and save capital cost. This is also a reason for a lower adhesion of deposits as well as lower bacteria colonization. They are also tolerant to high flow rates.
• Good strength and ductility – SS, particularly the duplex materials, have higher mechanical properties when compared to carbon steel and cast iron pipes. This allows a reduction in the wall thickness of the pipe (with out any required corrosion allowance) and results in investment cost savings. The better ductility may also be important in terms of security, e.g. in the case of an earthquake.
• Ease of fabrication – the SS are adaptable materials: ease of forming, machining and welding, combined with their good ductility, permit easy installation and modifications of pipelines. They are also available in a wide variety of product forms.
• Ease of maintenance - with high corrosion resistance and easy welding, water leaks can be drastically reduced, probably from 25% down to less than 5% which leads to further cost savings. This argument is more and more important due to the increase in the cost of water and water treatment.
• Excellent durability - as SS are resistant to crevice corrosion, cavitation and wear in pure and polluted waters as well as in atmosphere (even polluted), they are cost effective for long term use and do not cause environmental pollution. 
• Recyclable - SS are 100% recyclable and a part of their initial cost may be recoverable when they are recycled.

Source:IMOA