Influence of normalizing and tempering treatment on microstructures and mechanical properties of reduced activation ferritic-martensitic steel fabricated by laser directed energy deposition

Reduced activation ferritic-martensitic (RAFM) steel with high strength was prepared by utilizing laser directed energy deposition (L-DED) technology, followed by normalizing and tempering (N&T) to enhance ductility and toughness with underlying mechanisms unraveled through detailed microstructural analysis. Owing to ultrafast heating and cooling features in laser processing, the L-DEDed specimen exhibits a complicated heterostructure, including ultrafine martensite with high-density of dislocations, a small amount of retained δ-ferrite and austenite, and a few second-phase particles at grain boundaries. After the N&T heat treatment, the L-DED-N&Ted specimen exhibits a more uniform martensitic microstructure with a typical hierarchical feature, along with the presence of dense Cr-rich M23C6-type and Ta-rich MX-type precipitates distributed at the prior austenite grain boundaries and martensitic lath boundaries. The L-DEDed specimen exhibits ultra-high tensile strength (∼1.6 GPa) yet with relatively low elongation (∼6 %) and limited impact toughness (∼20 J/cm2). For the L-DED-N&Ted specimen, its tensile strength decreases but still has a considerably high value of ∼800 MPa, along with largely enhanced elongation (almost doubled) and impact toughness (2.5–3.5 times increased). The high strength of L-DEDed specimen is mainly attributed to strengthening due to dislocations, solid-solution and hetero-deformation, with its lower impact toughness due largely to the existence of δ-ferrite and austenite. The N&T heat treatment produces a more homogeneous microstructure for the RAFM steel along with the elimination of δ-ferrite and austenite. Such microstructural characteristics facilitate suppressing crack initiation and rapid propagation, remarkably enhancing the ductility and impact toughness of the RAFM steel.