You can measure austenite/ferrite spacing.
In these alloys the first concern is A:F ratio, and the second concern is testing for intermetallics.
After that little else matters.
As EdS said it is important to mesure the ferrite content in duplex stainless steels and if I am wright, to have a ferrite number above 8, is not acceptable.
“Duplex stainless steels (DSS) are characterized ideally by equal amounts of ferrite and austenite, which provides increased mechanical resistance (680 to 880 MPa ultimate tensile strength) due to the fine grain size, typical of these steels (Sedriks 1996; Souza et al. 2005). They present higher corrosion resistance when compared to ferritic stainless steels, promoted by high content of chromium, nitrogen and molybdenum, and, due to the presence of austenite, the DSS present good ductility and toughness (250 J impact toughness) (Young et al. 2007).
This behavior (corrosion resistance, strength) is promoted by the presence of approximately 50% volume fraction of ferrite (Kashiwar et al. 2012), and therefore the control of the ferrite content in DSS is necessary to ensure the desired properties.”
EdS is right, grain size in Duplex SS is not a concern, nor is strength. It's all about maintaining its exceptional corrosion resistance through all processing steps.
Don't venture into the realm of duplex stainless steel without expert guidance, especially if you intend welding.
"Everyone is entitled to their own opinions, but they are not entitled to their own facts."
The reason that A:F is so important is that the A and F have different solubility for various elements.
These alloys are designed so that at near 50/50 the aqueous pitting resistance of the two phases will be nearly identical.
But as you get further away one phase or the other becomes more dilute (for example nearly all of the N ends up in the austenite, so the concentration at 50% A vs 75% A will be significantly different).
This leads to one phase having lower pitting resistance and thereby lowering the corrosion resistance of the alloy.
This is why annealing temp is so critical, changing temp changes the A:F ratio.
In a similar way the formation of intermetallics will damage both the mechanical properties (lower toughness) and pitting resistance. Some of these compounds are easily corroded themselves, and in other cases it is a matter of local depletion akin to sensitization. In order to avoid these you have have a good cooling rate from anneal and watch your max service temp.
I should point out that corrosion resistance of a duplex in any environment other than aqueous chloride bearing needs to be tested carefully, this is the service that they were deigned for.
The lean duplex alloys are very easy to process and fairly forgiving.
2205 is not much harder, provided that you have a well balanced chemistry.
The superduplex grades are the graduate level work.
