A critical assessment of the microstructure and mechanical properties of friction stir welded reduced activation ferritic–martensitic steel

Bead-on-plate friction stir welding was conducted on 6mm thick plate of Reduced Activation Ferritic–Martensitic Steel employing polycrystalline cubic boron nitride tool with rotational speeds of 200, 300, 500 and 700rpm and traverse speed of 30mm/min. The interface temperature between shoulder bottom and top surface of the plate was monitored by non-contact in-line thermography which served to identify the peak temperature attained in the stir zone (SZ). This temperature for 200, 300 and 500, and 700rpm was respectively below Ac1, between Ac1 and Ac3, and above Ac3. In the base metal (BM), the prior austenite grain and martensite lath boundaries were decorated with chromium and tungsten rich M23C6 precipitates while intra-lath regions revealed Ta and V rich MX type carbides. Rotational speeds greater than 300rpm led to martensite formation and simultaneous recovery, recrystallization and grain growth in SZs with wide distribution in grain size whereas SZ of 200rpm and BM possessed similar distribution. The grain boundary M23C6 dissolved and very fine needles of Fe3C precipitated in all SZs. The hardness of all SZs was unacceptably higher compared to the BM. The 200rpm weld exhibited higher impact toughness in the absence of martensite in SZ. [Display omitted] •Effects of rotational speed during friction stir welding of a ferritic–martensitic steel were investigated.•Distribution of grain size in stir zones varied as a function of rotational speed.•Grain boundary M23C6 precipitates were dissolved while Fe3C formed in stir zones at all rotational speeds.•High rotational speeds promoted martensite occurrence in the stir zones with a drastic reduction in impact toughness.•Peak temperatures below Ac1 in the stir zone enabled impact toughness matching with the base metal.