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Full penetration friction stir welding was conducted on 12 mm thick reduced activation ferritic-martensitic steel at tool rotational speeds of 500 and 900 rev min −1 . Comparator welds at 500 rev min −1 were also produced in 6 mm thick reduced activation ferritic-martensitic steel plate to evaluate section thickness effects. Increase in section thickness led to an increase in heat input, which strongly influenced the microstructure evolution in stir zone (SZ), thermomechanical affected zone and the overall hardness in the SZ of this steel. In the as-welded condition, the base metal microstructure was significantly altered and resulted in carbide-free grain boundaries. The desirable microstructure and mechanical properties were achieved by subjecting the as-welded joints to appropriate post-weld heat treatments.

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