The Measurement of Ferrite Number (FN) in Real Weldments — Final Report

Abstract

The amount of ferrite in stainless steel weld metals can have an influence on a wide range of properties including corrosion resistance, toughness, long term high temperature stability, resistance to hot cracking etc. It is therefore essential for some applications, that the ferrite level is controlled within specified limits and that methods are available for checking that specified levels have been achieved. A Round Robin involving 17 laboratories in 8 countries was organised within Commission II of IIW. The objectives were to compare measurements of ferrite number (FN) on real welds using magnetic instruments. Five different stainless steel welds covering a range of FN values from zero to about 45 were tested in different locations and with different surface finishes. In addition each laboratory carried out a chemical analysis and used the WRC-92 diagram to predict the FN values from the chemical composition of the weld metal. Measured and predicted results were compared. The results revealed variations in measured FN in different locations in the weld, particularly in regions of high dilution. The tests also showed that there was an effect of surface finish. A detailed analysis of all the results indicated that scatter of about ± 20% of the measured value should be expected when testing real welds. This is in contrast to scatter of about ± 10% when laboratory specimens are used. Predicted FN values using chemical composition and the WRC-92 diagram showed scatter of approximately ± 40% of the predicted value. The report recommends that when weld metal FN values are required they should be measured using calibrated magnetic instruments rather than be predicted from chemical compositions. Guidance on suitable testing locations and weld surface finishes are also given.