Effects of tool rotation speed on the mechanical properties and microstructure of friction stir welded ODS steel

The friction stir welding (FSW) method was used to form joints in an oxide dispersion strengthened (ODS) steel with tool rotation speeds ranging from 250 rpm to 400 rpm to investigate the effect of rotation speed of the tool on the mechanical properties and microstructure of the joints. The hardness of the stirred zone (SZ) of the ODS steel joints was reduced after welding at all rotation speeds because of recrystallization induced by the heat generated during the FSW process. The recrystallization was accompanied by a change in the grain morphology from elongated grains to isotropic grains. The sizes of recrystallized grains increased with increasing rotation speed of the tool. Oxide particles, however, were slightly coarsened by the FSW process, and there was no systematic relationship between the particle size and the rotation speed. The hardness of the SZ of ODS steel joints followed the Hall-Petch relationship. Tensile tests of the FSW ODS steel joints revealed that the change in the ultimate tensile strength is similar to the change in the hardness, suggesting that the joint strength was reduced by grain growth but not by degradation of the fracture strength.