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

Highlights: • 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 A_c1 in the stir zone enabled impact toughness matching with the base metal. Bead-on-plate friction stir welding was conducted on 6 mm thick plate of Reduced Activation Ferritic–Martensitic Steel employing polycrystalline cubic boron nitride tool with rotational speeds of 200, 300, 500 and 700 rpm and traverse speed of 30 mm/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 700 rpm 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 300 rpm led to martensite formation and simultaneous recovery, recrystallization and grain growth in SZs with wide distribution in grain size whereas SZ of 200 rpm 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 200 rpm weld exhibited higher impact toughness in the absence of martensite in SZ.