The effects of Mo and W in duplex stainless steels on weldability and corrosion resistance of heat affected zone(HAZ) were investigated. Four different alloys, Fe-22Cr-5.5Ni-0.15N -3Mo(3Mo alloy), Fe-22Cr-5.5Ni-0.15N-3W, and two compositions with partial replacement of Mo with W, were prepared for this study. The weldability has been examined using multipass welds and simulated welding samples. The heat cycles of simulated welding were determined by thermo-mechanical simulation. Cooling times from 1200℃ to 800℃ were varied between 5 and 171 sec. Microstructure, phase balance, Charpy-V impact strength and pitting corrosion resistance were investigated.
In HAZ simulation test, It was found that the substitution of W with Mo resulted in increasing the Charpy impact energy and pitting potential of the simulated heat affected zone, particularly at high cooling rates. It was due to increasing the partially transformed austenite and reducing $Cr_2N$ precipitates in the ferrite matrix. In actual Gas Tungsten Arc Welding(GTAW) test, the increasing the partially transformed austenites and reducing the width of HAZ regions in the multipass welds were also observed.
To study the effect of cooling rates in a welding area, impact energies and pitting corrosion potential of the HAZ region of the each alloys were investigated. 3Mo duplex stainless steel showed the lower impact energy and pitting corrosion potential in comparison with that of the solution treated state. In addition, 3W duplex stainless steel showed the slightly lower impact energy and pitting corrosion potential compared with the parent plate.
W in HAZ delays γ to δ phase transformation during heating resulting in the partial dissolution of austenite phase, which increases austenite volume fraction. Because of these results, the partial dissolved austenite retards the coarsening of the ferrite phase by preventing the growth of δ-ferrite grain. During cooling period, the partial dissolved austenite retards the $Cr_2N$ phase precipitation due to the absorption of nitrogen in the austenite phase. Therefore, these factors play major roles in increasing the impact strength and corrosion resistance at the HAZ region.
