Investigation on microstructure and mechanical properties of electron-beam-welded joint of reduced activation ferritic/martensitic steel fabricated by selective laser melting

The microstructure and mechanical properties of the complex low-activation ferritic–martensitic components of the China Fusion Engineering Test Reactor (CFETR) were examined in this study. The quenching–tempering treatment of CLF-1 steel formed by selective laser melting (SLM) (referred to as the QTed material) was carried out to analyze the effects of heat treatment on the microstructural and mechanical properties. Meanwhile, the microstructural and mechanical characteristics of CLF-1 steel electron beam welding (EBW) joints after quenching–tempering (referred to as the EBWed material) were investigated. Compared with the untreated sample (referred to as the SLMed material, with an ultimate tensile strength (UTS) = 979.8 ± 12.7 MPa and elongation (EL) = 3.7% ± 0.4%), the quenching–tempering treatment improved the EL of CLF-1 steel effectively (QTed material, 22.3% ± 0.1%), while it decreased the UTS correspondingly (505.6 ± 11.8 MPa), which mainly was due to the change in the grain size and structure during the heat treatment. The special microstructural distribution characteristics of the CLF-1 fabricated via SLM determined that the fracture mechanism was transgranular ductile/intergranular brittle mixed fracture. After electron beam welding of the QTed material, the tensile properties of the joint (UTS = 710.6 ± 21.4 MPa, EL = 9.7% ± 0.1%) were between those of the SLMed and QTed materials. The fracture appeared in the heat-affected zone (HAZ), proving that the strength of the weld was higher than that of the base metal. The microstructure of the weld was almost completely composed of coarse lath martensite, and the accumulation of dislocation clusters, precipitated M23C6-type carbides, and precipitated MX-type carbides contributed to the improvement of the yield strength (YS). The HAZ could be divided into three regions based on the different heating temperatures, and every region had different grain structures composed of mainly martensite and ferrite. The contributions of four strengthening mechanisms to the YS of CLF-1 steel were analyzed quantitatively, which lays a foundation for further improving the performance of CLF-1 steel and improving its application feasibility on the first wall of a fusion reactor.