The term Ferrite Number, designated as  FN, has been widely adopted in metal fabrication & filler manufacturing industries as a relative measure for quantifying ferrite content with Austenitic and Duplex Austenitic-Ferritic Steel Weld Metal, utilizing a tool known as “feritscope ® (Ref: 1 )  which works based on magnetic induction principle. The magnetic pull –an attraction of ferromagnetic phase available in the austenitic & ferritic austenitic microstructure gives a strong response to the probe in contact with the surface of the sample. Ferrite is a ferromagnetic phase” which gives a response to the magnetic probe of the instrument. (Ref 2 ). The Ferrite Number (FN), instead of Ferrite content approach has been developed to decrease the variations in measured values of ferrite content on a given sample by various labs. To add in that, FN Measurement is a non-destructive method, reliable and accurate method.

The WRC, IIW and the ASME Code strongly recommend the specification of ferrite in stainless steel weld metals by FN, not by % Fe.

Curious readers are requested to read the very exhaustive answer provided by Dr D.J.Kotecki, AWS, Welding journal, Stainless Steel Q &A Columnist. Pl. See AWS WJ December 2000 issue. 

As per many works of literature, (Ref: – 3,4), the deviation between FN & ferrite volume per cent takes place after 8FN approximately. For example, weld metal with 16 FN contains approximately 13.8 vol %. ferrite content. (Ref. 4  )

Weld-metal ferrite content can also be predicted using constitution diagrams, such as Schaeffler-DeLong diagram but the latest is the WRC-1992 diagram (Ref: 3  )

For more information regarding the approximate conversion of FN to wt% Fe, Refer Post by Thomas Eun (Ref: 4)

Calibrated Fisher Feritscope Instrument Model Fischer 2531 (Source Ref:-5)

Ref:1

https://www.fischer-technology.com/fileadmin/content/1-filebase/3-products/2-pdf/2-portable/en/BROC_FMP30_FERITSCOPE_902-039_en.pdf

Ref:2

“Standard Procedures for Calibrating Magnetic Instruments to Measure the Delta Ferrite Content of Austenitic and Duplex Austenitic-Ferritic Steel Weld Metal,” AWS 4.2-91, AWS, 1991.

Ref: 3

Kotecki DJ and Siewert TA (1992); WRC-1992 constitution diagram for stainless steel weld metals: a modification of the WRC-1988 diagram, Welding Journal, 71(5), pp. 171-s to178-s.

Ref: 4

https://www.eng-tips.com/viewthread.cfm?qid=34136

Ref: 5

PK Nanavati and Sanjay N Soman, (2018); An Experimental Comparative Review of Ferrite Measurement Techniques used in Duplex Stainless Steel (UNS S32205) Welds, INDIAN WELDING JOURNAL Volume 51 No. 2, April 2018, DOI: 10.22486/iwj/2018/v51/i2/170309