Characterization of phase transformation and strengthening mechanisms in a novel maraging steel produced using laser-based powder bed fusion

A novel maraging steel for additive manufacturing, Specialis (R), has been investigated to achieve a better understanding of the increase in hardness compared to conventional maraging steels. Phase transformations upon various heating rates have been evaluated by means of dilatometry and differential scanning caloriometry. The results were used to estimate the activation energy of precipitation formation. Microstructural characterization has been carried out by means of scanning electron microscopy and transmission electron microscopy, using electron backscatter diffraction, energy-dispersive X-ray spectroscopy, selected-area electron diffraction and electron energy loss spectroscopy. Two significant strengthening effects were identified in comparison to 18Ni300. Firstly, the elevated amount of Ti in Specialis (R) has increased the precipitation, raising the hardness through the Orowan mechanism. Secondly, a grain refinement has been observed as a result of the addition of V and Al, which has doubled the grain boundary strengthening mechanism known as the Hall-Petch effect. Finally, the calculated strength increase due to both mechanisms (159 MPa Hall-Petch + 328 MPa Orowan) was consistent with the difference observed in measured tensile strengths between Specialis (R) and 18Ni300 (395 MPa).