The effect of heat treatments on mechanical properties of M789 steel fabricated by laser powder bed fusion

Highlights: • Nano-scale, plate-like and spherical precipitates forms during aging of M789 steel. • The ideal temperature to form Ni₃Ti precipitates (η-phase) is at 500 °C. • Retained austenite can easily be formed when aged above 500 °C. • The Ni₃Ti precipitates provide a net increase of 779 MPa in tensile strength. -- Abstract: The microstructural evolution during the aging heat treatment of a newly developed steel (designed to be employed in the laser powder bed fusion), known as M789, was evaluated. The electron-backscatter diffraction (EBSD) analysis suggests that the as-printed sample contains a fully martensitic phase with random crystallographic texture. The samples were then solutionized at 1000 °C for 1 h, followed by aging at a temperature range of 400–600 °C and holding times from 40 to 120 min. The maximum hardness and tensile strength were achieved at aging temperatures of 450 °C and 500 °C for 2 h, with the latter temperature generated the highest amount of Ni₃Ti precipitates (η-phase) with no traces of reverted austenite. Then, the heat-treated sample underwent transmission electron microscopy (TEM) and atom probe tomography (APT) analyses, which revealed the nano-scale, plate-like and spherical precipitates with number densities of 1.11 × 10²⁴ m⁻³ and 0.93 × 10²⁴ m⁻³, respectively. These precipitates contributed to the increase in tensile strength, from 1019 MPa (as-printed) to 1798 MPa (heat-treated), a net increase of 779 MPa. As a consequence of enhanced strength, the elongation decreased from 16% to 9%. Generally, the results of all experiments are consistent with simulations; the errors were identified to be less than 5%.