Microstructure, texture and mechanical properties of a nickel-free high nitrogen duplex stainless steel processed through friction stir spot welding

Abstract

A nickel-free high nitrogen duplex stainless steel holding high Mn-content was friction stir spot welded under the various rotational speeds. The microstructure/texture evolutions of the joints and the correlated mechanical properties were investigated in detail. The ferrite and austenite constituent massive phases were refined down to the mean grain size of 0.9 μm and 1.1 μm, respectively. The microstructure evolutions revealed the activation of continuous dynamic recrystallization as the main restoration mechanism. This was further verified through the formation of Β- and Α-fiber shear texture in the specimens processed under the various processing condition. Besides the activation of different restoration micro-mechanisms in both ferrite and austenite, dynamic phase transformation of ferrite to austenite was also contributed in grain refinement. The transformation was facilitated by providing more diffusional paths through substructure development and grain refinement. The local and bulk mechanical properties of the joints were also assessed. The outstanding tensile-shear peak load of 12.8 KN obtained for the sample welded at 400 rpm was discussed relying on the developed ultrafine-grained microstructure and the maximum peak extension of 1.7 mm achieved at 600 rpm was attributed to the role of ferrite phase in strain accommodation.

Keywords

• Duplex stainless steel
• Friction stir spot welding
• Ultrafine-grained microstructure
• Recrystallization
• Thermomechanical processing
• Texture