Effect of the Rotation Speed during Friction Stir Welding on the Microstructure and Corrosion Resistance of SAF 2707 Hyper Duplex Stainless Steel

Microstructure and corrosion resistance of a friction stir welded SAF 2707 hyper duplex stainless-steel are investigated. Friction stir welding is performed at a constant welding speed of 100 mm min(-1) and rotation speeds of 200, 300, and 500 rpm using a tungsten-rhenium-based tool. Microstructure evolution of the joint is analyzed by metallographic microscope, scanning electron microscopy, and electron backscatter diffraction. Corrosion resistance is determined by electrochemical potentiodynamic polarization and electrochemical impedance spectrum. All stir zones have finer grains of ferrite and austenite phases because of the plastic deformation and dynamic recrystallization. This characteristic leads to the relatively higher corrosion resistance of the stir zone at 500 rpm than that of the base metal. Nevertheless, sigma phase is observed at the bottom of the stir zone at 200 and 300 rpm, this characteristic would degrade the corrosion resistance despite the smaller grain size of the ferrite and austenite phases. The bottom of stir zone at 200 rpm shows the poorest corrosion resistance, it has the largest amount of sigma phases than the other samples.