Duplex and superduplex stainless steels belong to a group of high performance stainless steels regarding to corrosion and mechanical properties. These achievements are related to a suitable chemical composition and a balanced microstructure. During welding thermal cycles the microstructure changes and, consequently, corrosion and mechanical properties might be impaired due to a precipitation of intermetallic phases, chromium nitrides, carbonitrides and others. This precipitation is an issue to be addressed for multipass welding.

The objective of this work is to study chromium nitride and sigma phase precipitation in a simulated HAZ of multipass welding (three passes) of UNS S31803 and UNS S32550 duplex stainless steels with different heat inputs. The HAZ simulated region is inside the intermetallic and chromium nitride precipitation range (950 to 750C).

Thermal cycles were theoretically calculated using a Rosenthal solution of a thin plate welding heat transfer condition for different heat inputs (0.4 to 1.0 kJ/mm). The model was experimentally corrected to provide better relationship between real and simulated thermal cycles. Welding parameters for determining the thermal cycles were based upon a GTA welding parameter of a duplex stainless steel plate with 6.0mm thick. A dilatometer was used to simulate the welding thermal cycles. Samples geometry for dilatometric studies were 2mm in diameter per 13mm long. Precipitated phases were characterised by two techniques: precipitates extraction followed by X-ray diffraction in a Debye-Scherrer chamber and electron diffraction by TEM. Microstructures of simulated samples were analysed by optical and scanning electron microscopy.

Experimental correction to the Rosenthal solution of heat transfer generated a thermal cycling model close to a real one. Therefore cycled samples in a dilatometer using the cycles produced microstructures representative of real HAZ welds. Microstructural characterisation of simulated samples showed a balanced microstructure similar to base metal of both duplex grades. Discontinuous films of a precipitated phase at ferrite/ferrite grain boundaries and ferrite/austenite interfaces were observed only in a UNS S32550 duplex grade for all heat inputs simulated. This suggests that sigma phase and chromium nitride precipitation took place during sample thermocycling. X-ray diffraction of extracted precipitates and electron diffraction by TEM confirmed the presence of chromium nitrides and sigma phases in the samples. On the other hand, microstructural analysis of UNS S31803 simulated samples did not present precipitation of intermetallic phases in tested temperature range of HAZ. These results were confirmed also by X-ray and electron diffractions at TEM. Based on these results, UNS S31803 is more resistant than UNS S32550 to precipitation in multipass welding.