Wire-arc additive manufacturing and post-heat treatment optimization on microstructure and mechanical properties of Grade 91 steel

Wire-arc additive manufacturing (WAAM) of Grade 91 steel (P91) is demonstrated for the first time. This alloy has been widely used in pressure vessel components of thermal and nuclear power plants. The post-WAAM heat treatment process is critical to the anisotropy elimination and property improvement, which is designed and optimized with the help of the CALPHAD (CALculation of PHAse Diagrams)-based thermodynamic modeling in this work. The microstructure and mechanical properties of the as-built and post-heat-treated WAAM P91 steel are studied. The results show that the as-built P91 steel fabricated by WAAM has a much higher strength than traditional wrought material, with an ultra-fine and high-density dislocation embedded martensite lath and quantities of δ-ferrite. The heterogeneous structure resulting from the WAAM process is removed after homogenization at 1200°C for 2 h. The microstructure is optimized with the very fine martensite lath and rational prior austenite grain size (PAGS). The segregated MX observed in the matrix of the as-built material is dissolved back to the matrix during homogenization, which is beneficial to the dispersive precipitation of M23C6 and MX in the grain boundaries and matrix during the following aging. By optimizing homogenization (1200°C for 2 h) and aging (760°C for 2 h) heat treatments, the WAAM P91 steel demonstrates excellent mechanical properties with the ultimate tensile strength of 774 MPa, yield strength of 686 MPa and ductility of 19.4%, which far surpasses the mechanical properties of standard wrought P91 steel used in the industry. The strengthening mechanism is elucidated to be the synergistic effect of MX and M23C6 precipitation.