Effect of heat treatment on microstructure and mechanical properties of selective laser-melted PH13-8Mo stainless steel

PH13-8Mo stainless steel powder with high sphericity and well fluidity was prepared by the plasma rotating electrode process. The formed parts with ultra-high purity were manufactured by selective laser melting. The tensile and impact mechanical properties of the printed parts under three heat treatment regimes were compared to those without heat treatment. The microstructure, grain orientation and phase composition were characterized by electron-backscatter diffraction, X-ray diffraction and transmission electron microscopy. The characterization results reveal that the addition of heat treatment makes the grains enlarge, and after different heat treatments, the average grain size from 1.51 µm in the printed state increases to 2.78, 3.09 and 2.06 μm, respectively. The formed parts are mainly composed of martensite and retained austenite. Moreover, the NiAl and M23C6 precipitates form, which are the major strengthening phases of PH13-8Mo stainless steel. The optimal heat treatment process is 925 °C × 1 h water cooling (WC) + 0 °C × 2 h air cooling (AC) + 540 °C × 4 h AC. Using this heat treatment process, PH13-8Mo formed parts have the optimal comprehensive mechanical properties: the tensile strength, yield strength and impact energy KU2 are 1492 MPa, 1432 MPa and 63 J, respectively. The strengthening and toughening via heat treatment are mainly attributed to dislocation strengthening, the formation of NiAl and M23C6 precipitates and the change of volume fraction of the retained austenite.