Methods for the measurement of ferrite content in multipass duplex stainless steel welds

The phase balance is commonly determined as a part of the duplex stainless steel (DSS) welding procedure qualification. In this work, the aim was to find a reliable, but still fast method to measure the ferrite content of DSS welds. Four methods were compared: image analysis with light optical metallography, magnetic measurements with Feritscope and Magne-Gage, and X-ray diffractometry (XRD). Image analysis with a magnification of ×500 was concluded to be most accurate on condition that the image quality was sufficiently high. The best contrast for image analysis was achieved by covering all surfaces apart from the part of interest with adhesive tape and etching the sample in a modified Beraha II solution. The Feritscope systematically underestimated the average ferrite volume fraction compared with image analysis, and a correction factor of 1.1 is suggested. Magne-Gage in turn resulted in considerably higher ferrite numbers as compared with the Feritscope and would require a correction factor of 1.18. Besides the long duration of XRD measurements, the method proved unsuitable for ferrite measurement due to the coarse texture of the microstructure.