Effect of heat treatment on microstructure and mechanical properties of 18Ni-350 maraging steel fabricated by Wire and Arc Additive Manufacturing

Abstract

An 18Ni-350 maraging steel single wall structure was fabricated using Wire Arc Additive Manufacturing (WAAM). Due to the rapid cooling and cyclic heating, elements such as Ni, Ti, and Mo were segregated within the single wall, resulting in a microstructure characterized by coarse primary dendrites and secondary dendrites. Various heat treatment were devised for the samples, encompassing Solution Treatment (ST), Solution + Aging Treatment (SAT), and Direct Aging Treatment (DAT). The findings revealed that elemental segregation persisted when the ST temperature was below 1050 °C, but disappeared when it exceeded 1150 °C, withthe microstructure transitioning from dendritic to equiaxed grains. ST resulted in the decrease of microhardness and tensile strength, but an enhancement in elongation. The effect of DAT was unsatisfactory for the improvement of mechanical properties while SAT could significantly improve them. Transmission Electron Microscope (TEM) analysis showed that a uniform distribution of Ni3Ti and Ni3Mo strengthening phases was formed in the martensitic matrix after SAT. The ultimate tensile strength of the sample reached 2163 ± 42MPa under the optimal heat treatment condition (SAT 1150 °C-1 h + 480 °C-6 h).