Ultrastrong and ductile synergy of additively manufactured H13 steel by tuning cellular structure and nano-carbides through tempering treatment

Microstructural evolution and mechanical properties of H13 steel fabricated by selective laser melting (SLM) with subsequent tempering treatment were systematically examined. It was found that the microstructure of the as-SLMed H13 samples consisted of cellular structures, lath martensite and high-volume fraction of retained austenite. After tempering at 600 degrees C for 1 h, the nanoscale Cr23C6 particles were detected at the boundaries of the partially dissolved cellular structures. The fine grains, the retained cellular structures, and the formation of Cr23C6 carbides significantly improved the mechanical properties of the H13 steel. A superior mechanical properties, including the yield strength (YS) of 1647 +/- 29 MPa, ultimate tensile strength (UTS) of 2013 +/- 35 MPa and elongation (El) of 4.1 +/- 0.3% have been achieved in the SLMed H13 steel after tempering at 600 degrees C for 1 h. With the increase of tempering temper-ature to 700 degrees C, the cellular structures were completely dissolved and the high number density of coarse Cr23C6 carbides were formed, which led to the decrease of UTS at 1083 +/- 21 MPa, while the elongation was significantly improved to 12.3 +/- 1.2% due to the recovery of dislocation density and the decomposition of martensite in the H13 steel.