Microstructure and corrosion behaviour of alloy 690–SUS 304L butt joints formed by electron beam welding

Abstract

This study investigates the microstructure and corrosion behaviour of alloy 690–SUS 304L stainless steel dissimilar weldments formed by electron beam welding (EBW). Slow strain rate tensile tests (SSRTs) are performed in air and in a 0·01M Na₂S₂O₃ + 1 wt-%NaCl corrosive environment. The microstructure, mechanical properties and corrosion behaviour of the weldments are analysed. Experimental results of this study reveal that the precipitation of chromium carbide is suppressed owing to the effect of rapid solidification of EBW in fusion zone (FZ). In addition, there is no chromium depletion found at the grain boundary at all. The interdendritic region of the FZ contains fine precipitates, which are a mixture of Ti rich phases with a rectangular shape and Cr rich phases with an oval shape. Fewer interdendritic phases are formed in the root than in the cap owing to the higher cooling rate in the root region of the weldment. Fracture tests indicate that all the specimens ruptured in the FZ by SSRTs preformed either in air and or in a corrosive environment. The tensile strength and total tensile elongation of the specimen deformed in the 0·01M Na₂S₂O₃ + 1 wt-%NaCl corrosive environment are lower than those of the specimen deformed in air. Fractographic investigations revealed that the fracture mode, in good accordance with mechanical behaviour and performance, transits from a ductile dimple rupture in the specimens tested in air to a mixed rupture of dimples and corrosion formation on fractured surface which obviously assisted the cracking in the specimens tested in the corrosive environment.