Effect of a direct aging heat treatment on the microstructure and the tensile properties of a 18Ni-300 maraging steel produced by Laser Powder Bed Fusion

The microstructure and the tensile properties of a 18Ni-300 maraging steel manufactured by Laser Powder Bed Fusion (L-PBF) were investigated after a Direct Aging Treatment (DAT) and compared to those obtained after a conventional Solution and Aging Treatment (SAT). DAT results in a 5% lower strength and a 70-80% higher ductility than SAT. The crystallite size of martensite laths and the characteristics of the submicrometric compounds precipitated during aging are very similar between the two variants. The different properties are attributed to the higher content of austenite (both residual and reverted) in DAT material, resulting from the Ni, Ti and Mo segregation during L-PBF. Such an austenite increases tensile ductility through a Transformation Induced Plasticity (TRIP) effect that was demonstrated by investigating the microstructure with Transmission Electron Microscopy (TEM) close to the fracture surface where deformation is localized.