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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 specification. 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. Ferrite scope is used to measure the Ferrite content.
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