Exploring the effect of the as-built microstructure on the martensite to austenite reversion sequence in a selective laser melted ¹⁸Ni maraging steel

Full text: During selective laser melting, the non-equilibrium rapid solidification conditions promote local elemental segregation, leading to a characteristic microstructure composed of cellular walls. These walls can display either low carbon BCC martensite or FCC retained austenite crystal structures, thus affecting the subsequent isochronal or isothermal martensite to austenite phase transformation mechanism. In the present study, the effect of the non-homogeneous as-built microstructure on the martensite-to-austenite reversion phenomena was studied for a ¹⁸Ni maraging steel fabricated by selective laser melting. In-situ synchrotron X-ray diffraction was used to retrieve the austenite volume fraction and lattice parameter evolution during the physical simulation of continuous heating to the fully γ field; and during isothermal tempering cycles throughout the inter-critical α+γ field. The effect of the as-built microstructure on the diffusive- and displacive-driven austenite reversion mechanisms was studied by applying slow (0.167°C.s-1), intermediate (5°C.s^-1) and ultra-fast (500°C.s-1) heating rates. For slow diffusive-controlled austenite reversion in the as-built state, the α+γ inter-critical field was expanded when compared to a solution treated sample. This was associated to the synergic effects of compositional segregations (anticipating reversion) and pre-existing retained austenite (delaying solubilization). These two factors, however, were irrelevant when the massive-driven reversion mechanism was dominant upon heating rates at and above 5°C.s-1. Within the α+γ inter-critical field, the as-built state did not fundamentally alter the overall reversion kinetics, the capacity to reach thermodynamic equilibrium nor affected the total volume fraction of reverted austenite formed right before cooling. (author)