Enhanced tensile properties of Fe–Ni–C steel resulting from stabilization of austenite by friction stir welding

• Friction stir weld (FSW) of Fe–Ni–C steel were studied.
• Austenite stabilizes due to the plastic deformation during the FSW.
• Stability of the austenite depends on the tool rotation speed.
• The austenite promotes both the strength and elongation due to the TRIP.

The austenite in the stir zone (SZ) of Fe–24wt%Ni–0.1%C alloy fabricated by friction stir welding (FSW) was successfully stabilized. The volume fraction of austenite increases with decreasing rotation speed of the FSW tool. Electron backscatter diffraction (EBSD) measurements clarified the Kurdjumov–Sachs orientation relationship between the retained austenite and the martensite phase; this orientation relationship indicated that a phase transformation occurs after stirring and that austenite is plastically deformed during this transformation. Additional EBSD measurements clarified that more dislocations remain and that the austenite grain is refined when the rotation speed is decreased. These lattice defects provide a barrier to the martensite transformation. The retained austenite exhibited the transformation-induced plasticity (TRIP) effect when the SZ was evaluated using tensile tests. As a result, SZs with a substantial amount of retained austenite exhibit both high strength and ductility.