I don't know of any industry standards in this area, but our customers often specify 40-60% (Not FN) If you are interested in better impact properties, you may want the ferrite to be as low as 30%. I think though, if the material meets the mechanical properties, and the corrosion requirements, should it really matter that much what the phase balance is?
I agree with GRoberts - we would normally get asked for 40-60% or 35-55% as the ferrite range in duplex stainless. We find that 45-50% gives good impact properties, provided that chemistry and heat treatment are OK.
I also agree that ferrite percent probably doesn't matter if everything else is OK, but if that's what the customer wants, then that's what he's going to get. I guess that these requirements have grown out of a long period of alloy development and experience in service. There may also be an element of borrowing copies of other people's specifications and re-hashng them as your own until it become industry standard (we have seen this)
