Microstructure, mechanical properties and strengthening mechanisms of friction stir welded Kanthal APMT™ steel

Kanthal APMT™ steel (Fe-22Cr-5Al-3Mo) is being investigated for possible use as an accident-tolerant fuel cladding material in advanced light water reactors. Generally, high-chromium ferritic steels do not have good weldability because of a variety of metallurgical issues. In the present study, friction stir welding (FSW), a solid-state joining process, was applied to a Kanthal APMT™ plate in a bead-on-plate configuration using a tool rotation rate of 600 RPM and a traverse speed of 25.4 mm/min. Microstructure and mechanical properties were evaluated to determine the weld quality and examine feasibility of applying FSW as a joining technique for this steel. Microstructural characteristics were examined by optical microscopy and electron microscopy. The stir zone (SZ) contained equiaxed grain structure with an average grain size of 13.7 μm. Interestingly, Vickers microhardness profile across the processed zone revealed no significant change in microhardness. Mini-tensile testing, however, showed a marked improvement in mechanical properties of the SZ in the longitudinal direction compared to the base metal. The results are discussed in the light of fundamental strengthening mechanisms.