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• Excellent weld joints of high borated steels were produced by electron beam welding. • Weld joints with higher boron content tend to exhibit better mechanical properties. • Post weld heat treatment is a crucial step to fabricate high-performance weld joints. • A proper heat input is essential to obtain weld joints with better ductility. High borated stainless steels (HBSSs) are widely used in nuclear field because of excellent neutron-shielding ability. However, coarse network-like borides are prone to be formed in weld joints during welding process, deteriorating the ductility seriously. In this work, electron beam (EB) welding was adopted to join ∼3.5 mm thick HBSSs containing 0.30–2.12 wt. % boron. Benefiting from high energy density, fine γ-Fe matrix and network-like borides were obtained in fusion zone (FZ) and partially molten zone (PMZ) of weld joints. The effects of boron content, heat input and post weld heat treatment (PWHT) on the microstructure, mechanical properties and fracture behaviour of weld joints were investigated. As boron content increased, narrower PMZ was observed in weld joints owing to smaller solidification temperature range. The volume fraction and size of borides in PMZ/FZ raised, enhancing the strength and decreasing the ductility. Meanwhile, the fracture location of weld joints was closer to the interface of PMZ and base metal (BM) during deformation. Higher heat input brought a wider PMZ and larger penetration depth, but hardly changed the size and distribution of borides. Interestingly, it caused a steep slope of interfacial line between PMZ and BM, leading to the fracture along the interface with low ductility. After PWHT, the borides in PMZ/FZ were spheroidized, and the stress localization during deformation was weakened. Consequently, excellent weld joints exhibiting similar mechanical properties with base steels were produced. This systematic work provided a reference for welding other materials containing coarse and brittle secondary phases. [ABSTRACT FROM AUTHOR]