Microstructure, mechanical and corrosion behavior of duplex stainless steel/carbon steel dissimilar metal welds

The microstructure characteristics, mechanical properties and corrosion behaviour of dissimilar metals welds between SAF2205 duplex stainless steel and AS/NZS 3678-250 carbon steel were evaluated. Multipass gas tungsten arc welding (GTAW) process was employed to weld these steels using three different filler metals: ER2209, ER316L and INCONEL 82. For all these different weldments, the composition change from that of the fusion zone to that of the base metal lead to sharp changes in microstructures and hardness, and subsequently affected the corrosion potential and corrosion rate. It is believed that the formation of galvanic cells between the different zones of weld, and localized galvanic cells between the different metallurgical phases within a zone, increased the corrosion rate in this zone. It was found that narrow hardened transition zones adjacent to the carbon steel fusion zones were formed. In tension tests, all weldments failed in the heated affected zone (HAZ) of the carbon steel adjacent to the fusion zone, with ductile fracture. The HAZ of the carbon steel also showed the highest corrosion rates and lower corrosion potential as compared to those in all other regions. The ferrite reformation and larger grain size in this zone, were the prime factors that increased the brittleness and reduced the corrosion resistance of this zone.