Microstructural effects on the tensile and fracture behavior of selective laser melted H13 tool steel under varying conditions

The microstructural-mechanical correlative study has been conducted for characterization of selective laser melted H13 tool steel. Transformation behavior from austenite to martensite has been observed with partitioning of C in matrix with correlative atomic diffusivity during selective laser melting process. During solidification, columnar grain structures are formed due to epitaxial growth following the build direction of H13 tool steels. Columnar microstructures are mostly composed of martensite with small amount of retained austenite. Supercooling of H13 with high laser scan speed increased the nucleation sites, which reduced the diameter of columnar grain. During tensile test, deformation appeared in grain boundary while there was no significant martensitic phase transformation confirmed by X-ray diffraction (XRD) method and electron backscattered diffraction (EBSD) analysis. S2 (scan speed of 200 mm/s specimen had the better tensile property with tensile strength of 1700 MPa and elongation of 1.6% than the rest(< 1300 MPa, 0.3%). Fractographic observation confirmed that porosity, pore size, and pore types are key metallic parameters affecting to tensile properties. Various carbides were identified by transmission electron microscopy (TEM) and 3D atom probe tomography (APT) analysis to confirm these microstructural characteristics and fracture behaviors of selective laser melted H13 tool steels.