Aging Behaviour and Mechanical Performance of 18-Ni 300 Steel Processed by Selective Laser Melting

Abstract

An 18-Ni 300 grade maraging steel was processed by selective laser melting and an investigation was carried out on microstructural and mechanical behaviour as a function of aging condition. Owing to the rapid cooling rate, the as-built alloy featured a full potential for precipitate strengthening, without the need of a solution treatment prior to aging. The amount of reversed austenite found in the microstructure increased after aging and revealed to depend on aging temperature and time. Similarly to the corresponding wrought counterpart, also in the selective laser-melted 18-Ni 300 alloy, aging promoted a dramatic increase in strength with respect to the as-built condition and a drop in tensile ductility. No systematic changes were found in tensile properties as a function of measured amount of austenite. It is proposed that the submicrometric structure and the phase distribution inherited by the rapid solidification condition brought by selective laser melting are such that changes in tensile strength and ductility are mainly governed by the effects brought by the strengthening precipitates, whereas the concurrent reversion of the γ-Fe phase in different amounts seems to play a minor role.

Crystallographic Features of Microstructure in Maraging Steel Fabricated by Selective Laser Melting

“…The retained γ phases often appeared along the grain boundaries in the α-Fe matrix. This result strongly agrees with the results of previous studies [9,10,21]. Notably, the fine γ grains were often localized at the melt pool boundaries.…”

“…The melt pools were approximately 50-100 µ m high ( Figure 3a) and approximately 50 µ m wide (Figure 3b). They contained elongated cellar structures with a mean spacing of approximately 500 nm (Figure 3c,d), as reported in the literature [10,11,[19][20][21]. The elongation direction of the cellar structure appears to be independent of the presence of the melt pool boundaries.…”

“…The strength level of SLM-fabricated maraging steels is approximately 1 GPa [9][10][11]. The subsequent aging at elevated temperatures (~460 • C) enhances the precipitation of fine intermetallic phases within the martensite structure, which further strengthens the materials [9][10][11]. However, most previous studies have investigated the effect of laser scanning conditions and aging treatments on the mechanical properties (strength and fatigue) of SLM-fabricated maraging steels [11][12][13][14].…”

“…During the cooling process, the locally irradiated areas are rapidly quenched from the austenitic region to reach temperatures that are lower than the martensite start temperature, resulting in the formation of a martensite structure that contributes strengthening materials. The strength level of SLM-fabricated maraging steels is approximately 1 GPa [9][10][11]. The subsequent aging at elevated temperatures (~460 • C) enhances the precipitation of fine intermetallic phases within the martensite structure, which further strengthens the materials [9][10][11].…”

Comparison of Maraging Steel Micro- and Nanostructure Produced Conventionally and by Laser Additive Manufacturing

“…The almost complete lack of interstitial alloying elements leads to a good weldability of this class of alloys [7]. This, in turn, makes them amenable to metal additive manufacturing (AM) processes, in particular Laser Metal Deposition (LMD) and Selective Laser Melting (SLM) [8,9,10,11,12,13,14,15,16]. Since these processes involve a small melt pool generated by a laser beam for the consolidation of powder feedstock to a dense material, they share similarities with micro-welding processes.…”