Why in case of Stainless steel, microstructure is essentially stabilized to Austenite? 2

[Abhijeet242]

Ni or Mn are two Austenite stabilizing elements. In SS popularly 18/8 steel, where Ni is used as an Austenite stabilizing element,why is it essential to stabilize Austenite microstructure? Is it to avoid Carbide formation with Chromium? If yes, then how making Austenite stable helps to avoid Carbide formation?

 

[EdStainless]

Those are two separate issues. You want austenite because it is ductile and has get damage tolerance.
Elements that are Aus stabilizers also include N, C, and Cu.
Balancing these alloys is tricky, each element has solubility limits and also can have impact on properties ranging from strength to welding.
Commercial 18/8 alloys (304 or 316) are not very stable. Slight amounts of cold work will trigger martensite formation (note the magnetism).
Carbide formation is detrimental because it takes Cr out of the alloy and that lowers corrosion resistance. This is the worst right around the carbides. This local depletion is why you get intergranular corrosion (IGA) in grain boundaries (the carbides will form in the grain boundaries first).
There are SS grades with other structures, Ferritic (FeCr with very low C), Martensitic (basically high Cr steels), and Duplex (mixed Aus/Fer). Each has its own properties and reasons for use.

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[Abhijeet242]

Thank you for the reply @EdStainless. It is certain as you pointed out Stainless steel is available in Ferrite, martensite as well as duplex. However, when it comes to corrosion resistance except duplex, they are not as corrosion resistant as Austenitic SS grade. So that brings me to the question as is it because Austenite phase would not react to any heat treatment and so that it will not impair its Corrosion resistance for a range of operating condition? Is this the reason why Austenite is a choice? Because if for martensitic SS, we add sufficiently large quantity of Cr i.e. more than 18% and then it should give combination of both Strength as well as Corrosion resistance equivalent to Austenitic SS. Then why don't we do that popularly?

 

[EdStainless]

It is Cr (and Mo and N in some grades) that give SS the resistance to aqueous corrosion.
The Ni is only there for phase control.
There are no matrensitic or PH grades with high corrosion resistance because of the hardening mechanisms involved. You can't make these with very high Cr.
There are ferritic, autenitic, and duplex grades that all have corrosion resistance ranging from so-so clear up to seawater is no problem.
The ferritic alloys are the most efficient use of alloying, but they are very limited in product form, thickness, properties, and applications.
The best balance of effcient alloy use and properties would be the duplex alloys. And you get higher strength as well.


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P.E. Metallurgy, consulting work welcomed

 

[Carloshhead]

Hi there, I'm Carlo, and I'm working as a safety technician in a shipbuilding company. Our company mainly uses pipes for transferring gases and other chemicals used for manufacturing the parts of the ship. We mainly use steel pipes. Every month we inspect the lines to find out the faults. The previous day we examined and found that there is a starting of corrosion. Last year we had done a corrosion resistance coating, but I think it was not much effective. Now we are planning to approach a professional team for a detailed as well as advanced corrosion coating.

 

Technically, it is perfectly correct to say that nickel stabilizes the austenite phase. However it is better to avoid that term (outside the secret society of metallurgists), so that you do not confuse it with the different (but related) issue of stabilization of carbon.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

IM, good point. In presentations I always try to remember to say 'Ni promotes or enhances the formation of hte austenite'. Stability and passive are both words that are used two different ways in SS lingo.

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P.E. Metallurgy, consulting work welcomed