Abstract
The effect of the nitrogen content in the shielding gas and its effect on temperature distributions at the welded zone of thin sheets of duplex stainless steel have been evaluated. The duplex stainless steels have many features due to unique microstructural combination of austenite and ferrite grains. The phase balance can be easily shifted depending on the welding parameters. Two sheets were welded using pure argon and pure argon plus 2% of nitrogen as shielding gas. The thermal profile had shown that N2-supplemented shielding gas lead to higher peaks of temperature using similar welding parameters. Microstructural examination showed that the austenite phase in the weld increased with the presence of nitrogen in the shielding gas. The added nitrogen promoted primary austenite formation and slightly increases the microhardness at the solidified zone. Microhardness mapping and metallographic imaging presented information about microstructures, confirming the formation of secondary phases during thermal cycle in the temperature range 850 °C and 950 °C. Control of ferrite amounts in the welds is essential mainly to improve mechanical properties and corrosion resistance of welding zones.
Highlights
Duplex stainless steels (DSS) have basically a microstructure consisting of austenite and ferrite
2% nitrogen is recommended in pure argon mixture for shielding gas and optimizing the phase balance, impact toughness and corrosion resistance of DSS13
In this region of the sheet (HAZ), the holding time were maintained between 500 °C and 600 °C for 6 s, approximately, for both TK1 and TK2 thermocouples that leads to precipitations of secondary phases as α’, σ, ε and π . 15,22,25 The TK3 thermocouple presented a peak of temperature around 760 °C at 3 mm away from the center of welding line
Summary
Duplex stainless steels (DSS) have basically a microstructure consisting of austenite and ferrite. The new technique using addition of nitrogen and lowering of carbon content made it possible to produce much cleaner steels improving high temperature stability as shown by duplex structure[1,2,3,4,5]. DSS presents satisfactory weldability, it is possible to occur the formation of excessive ferrite, nitrides and intermetallic phases in the solidifying zone (SZ) and in the heat affected zone (HAZ)[1,2,10]. These formations of deleterious phases need to be avoided and a balanced ferrite / austenite microstructure must be maintained[11]
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