Effect of Heat Treatment Condition on Microstructural and Mechanical Anisotropies of Selective Laser Melted Maraging 18Ni-300 Steel

Abstract

18Ni-300 maraging steel produced by the selective laser melting (SLM) process has a unique microstructure that is different from that of the same alloy processed by conventional methods. In this paper, maraging steels were fabricated by the selective laser melting process and their microstructures and mechanical properties were investigated in terms of post heat treatment conditions. Moreover, the effect of different heat treatments on the mechanical anisotropy was studied in detail. The micro Vickers hardness in the as-built state was around 340 Hv and could be increased to approximately 600 Hv by aging heat treatments. It was found that the solution heat treatment was not necessary to obtain a fully hardened state. From tensile tests of the maraging steels heat treated with different conditions, it was found that the highest strength was achieved by aging and solution treatment (ST) temperatures lower than the commonly used temperatures. In the direction parallel to the laser scanning, the highest ultimate tensile strength was obtained when 450 °C aging was done without solution heat treatment. In the other two directions tested, i.e., directions normal to the building and 45 degrees to the laser scanning direction, the highest tensile strength was obtained when aging was done at 450 °C after 750 °C solution treatment.

Wear Behavior of Conventionally and Directly Aged Maraging 18Ni-300 Steel Produced by Laser Powder Bed Fusion

“…The maraging steel produced by the LPBF presented a hardness of ~340–350 Hv in as-built state and 570–590 Hv (i.e., ~1.7 times higher than in the as-built) after HT850 heat treatment. These results can be easily explained by the precipitation strengthening mechanism, i.e., a uniform distribution of fine Ni-rich intermetallic precipitates during the aging of a ductile, low-carbon martensite structure [ 12 , 17 , 22 , 23 , 24 ]. The hardness variations among the three directions are small and within statistical deviations.…”

“…In addition, the beads generated during the LPBF process are observed in the wear plane of each specimen (white line). Further details regarding the microstructure, and tensile behavior of the LPBF-processed maraging steels used in this study can be also found in [ 22 ].…”

“…Figure 4 b shows that the size of the flake-like debris is much larger in the case of BD-HT850 than in the other two directions. These results can be understood because the BD-HT850 sample has the lowest yield and tensile strength among the three types of HT850 samples [ 22 ]. Therefore, the local plastic deformation of the BD-HT850 sample would be more severe than the other two samples, resulting in a higher wear rate and larger flake-like debris because of the easier delamination fracture.…”

“…The specimens were wrapped in a protective heat-treatment foil to prevent oxidation of the sample surface. Note that additional details of the material, including the heat-treatment effect on the microstructure and tensile behavior, were reported in the previous study [ 22 ].…”

“…The specimens were wrapped in a protective heat-treatment foil to prevent oxidation of the sample surface. Note that additional details of the material, including the heat-treatment effect on the microstructure and tensile behavior, were reported in the previous study [22]. To examine the anisotropy of the wear behavior of the samples, three types of samples were built along with three directions with the LPBF building axis.…”