Are they asking for a calculated ferrite or measured with a ferrite scope or tester? We would calculate our ferrite for each heat and typically would be around 15 for CF8M. With a bit of adjusting to the Ni/Cr ratio and adding some nitrogen we could drop the ferrite down in the 3-10 range. I don't recall ever having an issue achieving standard tensile and yield requirements with the lower ferrite content.

As far as the corrosion properties, that is out of my area.

Bob

The problem starts with ASTM, which permits a crazy high maximum ferrite content. If I recall correctly from a failure investigation I did years ago, it is 35FN, but don't quote me on it.
I haven't made or spec'ed this material, but if I were I would certainly aim for a lower ferrite range closer to the one you quoted (although foundries might balk at aiming for something as low as 3FN). Ferrite enhances castability but it also adds strength. In some environments (e.g. urea) this comes at the cost of corrosion resistance.
I believe the Steel Founders Society (SFSA) has a publication on cast stainless steels.

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

I saw this requirement over 30 years ago from at least two petrochemical companies. It was not that difficult then to obtain at a modest cost increase.

Yes, ferrite hurts corrosion resistance. The ferrite that forms in SS will be very lean compared the to the austenite and even mild acids (orange juice, CO2 saturated water, soft drinks, and so on) will eat the ferrite out quickly.
I used to use a lot of cast SS parts, but most were solution annealed and they had to be less than 0.5 FN (measured). The as-cast parts were speced as 2-8 FN. The guys that we dealt with didn't flinch at this.
The biggest help of FN in castings is to help resist solidification cracking.

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P.E. Metallurgy