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Date 09-04-2005 13:33
For austenitic stainless steels, the Magna gage, Severn Gage, and the Ferrite Scope are all instruments used to measure ferrite content of SS filler metal deposits. All should be traceable to the latest revision of the AWS Standard ANSI/AWS A4.2. The Welding Research Council adopted the term Ferrite Number (FN) to be used in place of percentage ferrite to clearly indicate that the measuring instrument was calibrated to the WRC procedure. The Ferrite Number is to be used in the same way as was the previously used percentage-ferrite term.
In measuring ferrite in Duplex stainless steels, the FN can by calculated by using the Magna Gage with the counterweight (EFN). Since the original instrument only went to 28 FN, a counterweight is required to extend the measurement to a higher ferrite content.The relationship between percentage ferrite and FN can be determined as % ferrite = 0.55 EFN + 10.6.
Chuck