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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 844; p. 143167
• Main Authors: Paul, Moses J., Muniandy, Yokasundery, Kruzic, Jamie J., Ramamurty, Upadrasta, Gludovatz, Bernd
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 02-06-2022; Elsevier BV
• Subjects: 18Ni-300 maraging steel; Aging; Aging (materials); Anisotropy; Bonding strength; Cellular structure; Crack propagation; Damage tolerance; Ductility; Fracture toughness; Heat treating; Laser powder bed fusion; Maraging steels; Mechanical properties; Melting; Micro- and meso-structure; Microstructure; Molybdenum; Phase transitions; Powder beds; Precipitates; Solution heat treatment; Strength and fracture resistance curve (R-curve) behavior; Tensile tests; Thermal cycling; Titanium; Mechanical properties; Laser powder bed fusion; Micro- and meso-structure; 18Ni-300 maraging steel; Strength and fracture resistance curve (R-curve) behavior
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2022.143167

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.
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•Strength, ductility and fracture toughness are analyzed in two orthogonal directions.•Aging results in nano-precipitates that enhance strength but reduce ductility and fracture resistance.•Solution treatment removes AM-characteristics resulting in reduced damage-tolerance.•Thermal cycling enables transformation-induced plasticity improving fracture toughness.•Anisotropy in failure characteristics is low relative to many LPBF-processed alloys.