Effect of Heat Treatments on the Microstructure and Properties of 18Ni300 Maraging Steel Produced by Selective Laser Melting

The microstructure and tensile properties of 18Ni-300 maraging steel manufactured by selective laser melting (SLM) were investigated after different heat treatments and compared to the original samples. Heat treatment alters the microscopic morphology of the original sample, and the differences in the cross-sectional and longitudinal sectional morphology of the original sample become indistinguishable after heat treatment. Cellular and long strip structures can be observed in the original and aged samples. After solution aging, the cellular and long strip structures completely disappeared, being transformed into parallel and almost equal-length plate martensite. Additionally, inverted austenite and Ni₃(Ti, Al, Mo) precipitates were present. The microhardness increased from 310 HV to 710 HV, the nanohardness rose to 7.7 GPa, tensile strength reached 2068 MPa, and elongation to fracture improved to 4.5%. These optimal properties were achieved with solution treatment at 820 °C for 2 h and aging at 490 °C for 7 h.