Microstructural evolution and mechanical properties of a newly designed steel fabricated by laser powder bed fusion

Abstract

The M789 steel was designed by voestalpine group based on Grade 250 maraging steel to maintain good printability and increase the corrosion resistance. In this work, the M789 steel was manufactured by laser powder bed fusion process. The microstructural evolution of additively manufactured M789 steel was studied using optical and electron microscopy techniques. In the as-printed condition, the epitaxial grain growth with cellular dendritic structure was detected. The primary dendrite arm spacing was measured to be 0.42 ± 0.12 μm, suggesting the cooling rate is in order of 106 °C/s during solidification. This high cooling rate eliminates the occurrence of precipitation. The electron backscatter diffraction maps indicate the formation of elongated columnar grains with significant low-angle grain boundaries. After solution annealing and aging treatment, large needle-like martensitic structures were observed with fewer LAGBs than the as-printed condition. Spherical ETA-Ni3(Ti,Al) precipitates (η-phase) were formed during the heat treatment, leading to a significant increase in hardness, tensile strength, and yield strength compared to the as-printed condition. The tensile strength and yield strength along the building direction are 1798 ± 4 and 1714 ± 13 MPa, respectively. However, the elongation and toughness in the heat-treated condition are lower than that of the as-printed condition. Thermodynamic and mechanical properties simulations were also carried out, and the results are consistent with the experimental data.