Defining acceptable Delta ferrite amounts in 316L SS 1

[rbrez]

Recently I had a corroded section of 316L SS analysed by a lab who did a chemical analysis and microexamaination on the metal.
They found the material's chemical composition within ASTM standards but the nickel was at minumum 10%.
The microexamination found the material to have excessive delta ferrite.
I asked if there was a standard to refer to that would state what might be an acceptable (average) amount of delta ferrite in 316L. They could not give me a standard. They said most companies have already defined what they will accept as standard either as a "count" over an area or as "percentage" based, on their experience.

1) can anyone assist me in finding a standard (ASTM, NACE?)
2) if not, what would you normally expect and how would you define it - count or percentage?

Thanks

 

[mcguire]

Some users of 316 have standards on a maximum amount. The practical way to measure is by a magnetic device, translated to volume percentage. Bear in mind, however, that the experts do not agree on the adverse effect of ferrite. There is no question that ferrite will have lower nitrogen and have a lower pitting resistance number than its neighboring austenite, but if the ferrite is high because the molybdenum or chromium is elevated over the minimum, the entire alloy benefits.
That said 10% is unusually high for cold-rolled material, less so for hot-rolled and not surprizing for a weld or a casting. Which do you have?

 

[rbrez]

Thank you for the reply. The material was 3/16" thick for a tank. I have no information if the material was hot or cold rolled.
The moly was 2.1 (spec is 2 - 3) and chromium was 16.43 (spec is 16 to 18).
The lab did not give me a count or percentage. They just said "large amounts".
That is why I am looking for some "standard" ASTM or NACE standard to refer to.
The product is sodium chlorate, so I really want to minimize any delta ferrite.

 

[mcguire]

3/16 plate means hot-rolled 98% of the time. A better post hot rolling anneal would have dissolved most, if not all, of the ferrite. Given that chemistry I would anticipate sub-par performance from the ferrite phase. Look for a critical pitting temperature of around 5C versus 10-15C if it were optimally annealed.