Hardening steels by the generation of transient phase using additive manufacturing

Abstract

In this work, 15-5 PH steel samples were fabricated using selective laser melting (SLM). The tensile properties were compared for the two different orthogonally-built systems for comparing anisotropy. The horizontally-built samples showed greater ultimate tensile strength, hardening behavior, and less yield strength as compared with the vertically-built sample. We applied in-situ neutron-diffraction experiments subjected to continuous tension at room temperature to examine both specimens. The neutrons penetrate the bulk samples and show the phase evolution of non-equilibrium retain-Austenite. More non-equilibrium phase is found in the horizontally-built sample than that of the vertically-built. While most non-equilibrium phase transforms at the yielding point of the vertically-built sample, only 50% of the non-equilibrium retained Austenite transform at the yielding point of the horizontally-built sample. Accompanying with the phase transformation, the Ferrite phase shows different microstructure evolution in two systems. In the vertically-built sample, there is a decrease in the dislocation density of the Ferrite phase before yielding, but it is not the case in the horizontally-built system. The dislocation strain energy assisted transformation is found in the vertically-built sample of the Ferrite phase even before yielding, but it is not the case in the horizontally-built system. The results suggest the selective laser melting can tune the amount of phase ratio and coherency between the phases for different tension behaviors of the metals, which open new dimension to customize local anisotropy for additive manufactured steel.