Another effect of multilayer welding is the formation of secondary austenite due to reheating by deposition of subsequent beads. In the case of multilayer welding, this means that the nickel content in the subsequent weld beads increases and more austenite can be formed. To ensure sufficient austenite formation, however, the specially developed fillers are over-alloyed by 2–4% with nickel. įiller metals for DSS are mainly of matching composition or slightly over-alloyed compared with the base material to compensate element loss and segregation in the weld metal. A final post-weld heat treatment dissolves intermetallic phases, and the austenite content increases, but is for practical reasons rarely used in practice. The joint preparation affects the dilution from the base material. This is in turn a function of the composition of the steel grade, filler metal, and shielding gas.
The weld metal chemical composition, however, has the largest influence on the final microstructure. If the temperature is lower than room temperature, preheating up to 50–80 ☌ can be performed to guarantee dry joint surfaces. Although preheating is rarely applied for welding DSS, it might be necessary if the plates are very thick and/or heavily restrained to reduce cooling rates and stress levels. The microstructure of welds can be influenced by the heat input and cooling rate, mainly by adjusting the arc energy and limit the inter-pass temperature to 100–150 ☌. When welding, the solidification is normally fully ferritic and austenite forms upon cooling. While the base metal consists of approximately equal proportions of ferrite and austenite, the weld metal can show a significantly wider range. The corrosion resistance increases with the chromium, molybdenum, and nitrogen content, but high levels of chromium and molybdenum also increase the risk of formation of intermetallic phases at an elevated temperature. As compared with ferritic alloys, DSSs are more ductile, less sensitive to hydrogen embrittlement and more resistant to localized and general corrosion. The high chromium content and alloying with nitrogen contribute to high strength and the resistance to intergranular corrosion, and stress-corrosion cracking is better than for most austenitic grades. The properties are being determined by the chemical composition and the phase balance. Besides the long duration of XRD measurements, the method proved unsuitable for ferrite measurement due to the coarse texture of the microstructure.ĭuplex stainless steels (DSSs) combine the advantages of ferritic and austenitic stainless steels.
Magne-Gage in turn resulted in considerably higher ferrite numbers as compared with the Feritscope and would require a correction factor of 1.18. The Feritscope systematically underestimated the average ferrite volume fraction compared with image analysis, and a correction factor of 1.1 is suggested. 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. Image analysis with a magnification of ×500 was concluded to be most accurate on condition that the image quality was sufficiently high. Four methods were compared: image analysis with light optical metallography, magnetic measurements with Feritscope and Magne-Gage, and X-ray diffractometry (XRD). In this work, the aim was to find a reliable, but still fast method to measure the ferrite content of DSS welds. The phase balance is commonly determined as a part of the duplex stainless steel (DSS) welding procedure qualification.
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