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Ferrite testing

Ferrite testing

Ferrite testing

Can some explain to me how do we perform a Ferrite test? and what is purpose having this test


RE: Ferrite testing

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

RE: Ferrite testing

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.

RE: Ferrite testing

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.

= = = = = = = = = = = = = = = = = = = =
Still trying to help you stop corrosion.
formerly Trent Tube, now Plymouth Tube
or edstainless@earthlink.net

RE: Ferrite testing

If you have any qty of testing to do, a ferritescope such as Fischer's MP-30 is well worth the cost.  The readings can be taken very quickly.   

RE: Ferrite testing

Before you embark on a test programme, it may be worth your while perusing ISO TR 22824.

Steve Jones
Materials & Corrosion Engineer

RE: Ferrite testing

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).

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