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.