Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting

Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers. Using atom-probe tomography (APT), we investigated how this nonconventional thermal history influences the phase-transformation behavior of maraging steels (Fe-18Ni-9Co-3.4Mo-1.2Ti) produced by SLM. We found that despite the "intrinsic heat treatment" and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe, Ni, Co)(3)(Ti, Mo), (Fe, Ni, Co)(3)(Mo, Ti), and (Fe, Ni, Co)(7)Mo-6 (mu phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations.