depending on final use. In other words, a certain % of Ferrite could be useful for autogenous welding but if this % were little bit higher ,it could be deleterious for hot working ( forging , hot rolling .. ) , for highest polishabilty, in case of soaking for a long time at temperature in the range of sigma phase precipitation temperature.
In terms of NDT, higher level of Ferrite could cause some difficulties in terms of Eddy current testing MPI by encircling coils ( similr to Metal positive inspection).
Edstainless is right but his sentence doens't mean Ferrite is every time "dangerous". There are a lot of specifications imposes a limit of Ferrite but the main important thing remains in following question: is this limit an useful and mandatory requiremnt or is according to a tradition handed down from year to year in the stainless steels story ?
If you inform us what is the final use,we would give ,about % Ferrite, the right answer.
There only exist limits for dF in some specific cases.
And there are two different issues.
One is the calculated dF amount at equilibrium.
The second is the actual amount of dF in the material. Even alloys with high calculated dF can be annealed thoroughly and result in undetectable amounts of dF in the final product.
One other application that requires low residual ferrite is exposure to acids (such as orange juice).
Chemistry of autogenously welded tubulars and castings are manipulated by the manufactures to provide ferrite. I've seen ferrite in 316 castings routinely above 8%.
The calculated ferrite number that we see for 316L sheet from US mills is usually 5-10%.
The measurable amount in annealed sheet is so low that it is undetectable. The same goes for properly annealed welded tubing.
