The effect of partial replacement of Mo with W on simulated heat-affected zone (HAZ) toughness in 25Cr-base super duplex stainless steels has been investigated. The materials studied were Fe-25Cr-7Ni-3Mo-0.25N and two different W-bearing alloys, 2W2Mo and 3W1.5Mo. HAZs were simulated by a FUJI thermo-mechanical simulator. The cooling time from 1200 to 800℃, $Δt_{12/8}$, was varied between 5 and 270 sec. Charpy Ⅴ-notch impact toughness, hardness, ferrite content, XRD, average grain size and microstructures of the simulated samples were studied. Cooling rate and the addition of W were found to have strong effect on the microstructure of duplex stainless steels. All the simulated alloys exhibited a decrease in toughness at fast cooling due to high proportion of ferrite and intragranular precipitates. Impact toughness also decreased at slow cooling, $Δt_{12/8} = 270sec$, because of large matrix ferrite grain size despite higer austenite content. Peak impact toughness level was observed at cooling time of 33sec for all the alloys. It was also found that the substitution of W for Mo produces high impact toughness at all cooling conditions resulting from the increased austenite content, decreased intragranular precipitates and smaller ferrite matrix grain size. In conclusion, the optimum cooling time for welding of super duplex stainless steel according to this study is $Δt_{12/8} = 20~30sec$ and the addition of W has favorable effect on HAZ properties of super duplex stainless steel.
