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.

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A heat treatment technique was applied on standard duplex stainless steel welds in order to investigate the influence of thermal treatments, e.g., multi-pass welding, on the microstructure. By using a stationary arc, a spatial steady-state temperature field ranging from liquidus to room temperature evolves within a single sample and results in a graded microstructure. The arc heat treatment was applied for 10 and 60 min respectively, and the experimental results were compared against thermodynamic calculations. Metallographic investigations revealed the formation of secondary phases within distinct zones. For the 10 min arc heat treated sample, the formation of sigma phase was observed in a temperature range of 730–1000 °C and chi phase was found above 700 °C. For the 60 min sample, sigma phase formed between 675 and 1025 °C, while chi phase formed above 600 °C. In both samples, transformation of chi phase to sigma phase as well as the formation of secondary austenite at 575–1100 °C was observed. Hardness measurements identified brittle regions, which correlated with the regions enriched in sigma phase and decomposition of ferrite. Compared to the initial microstructure, regions with secondary phases showed increased sensitization to local corrosion, when tested according to ASTM A262-Practice A.

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