You buy a Magnagage. The need for a small amount of ferrite in a stainless weld involves a great reduction in hot-cracking tendency. Some of the harmful impurities partition to the ferrite phase.
"When the eagles are silent, the parrots begin to jabber."
Winston Churchill
The amount of ferrite present in austenitic or duplex stainless steels is called "FN" or Ferrite Number. For austenitic SS, a small amount of ferrite will decrease the tendency for hot cracking during solidification. Company specifications should have a required FN range in their welding specs. Too low of a number may indicate that there are hot cracks. Too high of a number may decrease the corrosion resistance, or the ferrite can convert to sigma at higher temperatures. Ferrite is magnetic whereas austenite is not.
Duplex SS nominally contains 50% ferrite/austenite, although the acceptable range for ferrite is much broader than exactly 50%.
There are several ways to measure the FN. As mentioned by Metalguy, you can use a Magne-Gage. I have used a Severn Gage and a Feritscope.
For lab or qualification work you can take micros and determine the ferrite volume by counting.
The FN and the actual volume fraction of ferrite are not exactly the same, so pick one system and stick with it.
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Still trying to help you stop corrosion.
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Ferrite levels in materials are important for all of the reasons already mentioned. Measurement of ferrite using a Ferritscope or Severn Gauge is a good idea - especially for duplex materials.
Cast austenitic stainless steels typically have anywhere from 8-18% ferrite in them. This is due to the alloy balance (chemically), but is also advantageous when welding the materials and during the solidification. As a result, cast "stainless steels" (austenitic) will draw a magnet lightly.
Duplex stainless steels are even more important to measure the ferrite. The reason is that the ferrite can vary over a wide range (from as low as the low 40's to as high as the 60% range). Lower ferrite readings in duplex stainless steels can indicate a real potential problem. Improperly heat treated (insufficient cooling rate) duplex stainless steels can have sigma phase or other deleterious phases that can embrittle the material and cause catastrophic mechanical failures (not to mention the loss of the desired corrosion resistance).
