Effects of rotation speed on microstructure and hardness of friction stir welded ODS ferritic steel

One of the critical issues in the development and use of nanostructured oxide dispersion strengthened (ODS) steels is the welding process. Friction stir welding (FSW) has been considered to be a promising way to weld ODS steels. So far, there are only few works focused on FSW of ODS steels. In FSW method, the effect of rotation speed of tool is important, because the rotation speed remarkably influences the microstructure of the welded ODS steels. The present paper investigated the microstructural and mechanical properties of an ODS ferritic steel subjected to FSW under varying rotation speeds (250, 300, 350, 400 rpm). It was found by hardness measurement and microstructural examination that the hardness of the SZ (stir zone) obviously dropped as the rotation speed increased, which was attributed to the grain growth and coarsening. The EBSD results revealed that the size and orientation of the grains within SZ changed drastically, according to the difference in the rotation speeds. More detailed analysis to correlate the hardness change with microstructure changes by FSW is now under way to understand the factor controlling hardness of ODS steel.