Hardening steels by the generation of transient phase using additive manufacturing

In the present work, the tensile properties of 15–5 PH steel fabricated by selective laser melting (SLM) were examined with respect to the transient austenite phase. Compared with the 8%-transient-phase sample, the 18%-transient-phase one shows higher ultimate tensile strength and relatively low yield strength, as well as hardening behavior. We conducted in-situ neutron-diffraction study to examine the microstructure evolution for mechanistic understanding. After applying the external load, most non-equilibrium, retained austenite in the 8%-transient-phase sample transforms before the yield strength, whereas in the 18%-transient-phase sample only 50% of the austenite transforms. Accompanying with the phase transformation, a decrease in the dislocation density and the dislocation strain energy-assisted phase transformation of the ferrite phase are found in the 8%-transient-phase sample even before yielding, which, however, is not the case in the 18%-transient-phase sample. The study demonstrates the SLM enables tuning the amount of transient-phase ratio and coherency between phases to realize a mechanical property control for additive manufactured steel.