Today's austenitic-ferritic (duplex) steels have considerably better weldability than earlier grades. They can be welded more or less as common austenitic steels.
Besides being susceptible to intergranular corrosion, the old steels were also susceptible to ferrite grain growth in the heat affected zone (HAZ) and poor ferrite to austenite transformation, resulting in reduced ductility.
Today's steels, which have higher nickel content and are alloyed with nitrogen, exhibit austenite transformation in the HAZ that is sufficient in most cases. However, extremely rapid cooling after welding, for example in a tack or in a strike mark, can lead to an unfavourably high ferrite content.
When welding duplex 2205 in a conventional way (0.5-2.5 kJ/mm) and using filler metals at the same time, a satisfactory ferrite-austenite balance can be obtained. For the super duplex stainless steel SAF 2507 a somewhat different heat input is recommended (0.2-1.5 kJ/mm). The reason for lowering the minimum value is that this steel has much higher nitrogen content than 2205. The nitrogen favours a fast reformation of austenite, which is important when welding with a low heat input. The maximum level is lowered in order to minimize the risk of secondary phases.
The steels are welded with austenitic-ferritic filler metals. Welding without filler metal is not recommended without subsequent quench annealing. Nitrogen affects not only the microstructure, but also the weld pool penetration. Increased nitrogen content reduces the penetration into the parent metal. To avoid porosity in TIG welding it is recommended to produce thin beads. To achieve the highest possible pitting corrosion resistance at the root side in ordinary 2205 weld metals, the purging gas should be 90%N + 10% H2 or mixtures of Ar + N2/H2. The use of H2 in the shielding gas is not recommended when welding super duplex steels.