Effect of heat treatment on the strength and fracture resistance of a laser powder bed fusion-processed 18Ni-300 maraging steel

Engineering materials processed using additive manufacturing (AM) techniques such as laser powder bed fusion (LPBF) often exhibit unique microstructures and defects that must be controlled to obtain peak performance in mechanical properties. Towards this end, we here investigate how three heat-treatments (direct aging, solution treatment + aging, and thermal cycling + aging) impact microstructure evolution and corresponding mechanical performance of LPBF-processed 18Ni-300 maraging steel. Specifically, we focus on how strength and fracture toughness measured in two orthogonal directions correlate with phase transformation kinetics as well as micro- and meso-structural features and compare findings with results from as-built and conventionally processed material. Aging results in the formation of Ni–Ti and Mo-rich nano-precipitates that enhance strength but reduce both ductility and fracture resistance when compared to as-built material. With the solution treatment, however, AM-characteristics such as cellular sub-structures and melt-pool boundaries dissolve almost entirely and as a result the damage-tolerance capacity of the material degrades and becomes comparable to cast material. Thermal cycling, on the other hand, results in the formation of reverted austenite thereby enabling the transformation-induced plasticity (TRIP) effect. This leads to extensive gains in ductility that are accompanied by only moderate reductions in strength while fracture toughness is improved significantly. While anisotropy in the obtained data from both the tensile tests and the crack resistance curve (R-curve) measurements is noticeable, it is low relative to many LPBF-processed alloys due to the strong bonding characteristics between the individual layers.