Improving impact toughness of heavy section reduced activation ferritic martensitic CLF-1 steel joints with electron beam welding

Abstract In this paper, electron beam butt welding of the reduced-activation ferrite/martenstic CLF-1 steel with a thickness of 32 mm was performed. Furthermore, the impact toughness, microstructure, phase compositions and impact fractures of the joints with different preheating, post welding heat treatment (PWHT) and heat inputs were investigated systematically and the influencing rules and mechanisms were analyzed. The experimental results shown that, with the optimization of the processes parameters, well-formed joints without any defects such as pores, incomplete fusion, and cracks was obtained by electron beam welding, which also possessed favorable tensile and flexural performances. The impact absorbed energy of the joints was not remarkably improved with preheating or post welding heat treatment. As the heat input dropped from 2805 J mm−1 to 1440 J mm−1, the impact absorbed energy of the weld increased from 3.3–12.5 J to 275 J and δ-ferrite content reduced from 2.79% to 0.75% after PWHT at 740 °C for 4 hours. At a heat input of 1836 J mm−1, the impact absorbed energy of the weld was comparable to the value of BM, and the specimens were not fractured after impact test. Preheating and PWHT only changed the size of martensite laths and carbides without eliminating the residual δ-ferrites in the weld. In addition, the impact fractures exhibited brittle fracture characteristics. As the heat input reduced, the content of δ-ferrites in the weld decreased significantly, the martensite laths in the weld zone after thermal treatment were refined, and Ta-rich MX carbides in the laths were precipitated. Therefore, the impact toughness of the joints increased significantly and the impact fractures exhibited brittle fracture characteristics.