Microstructure evolution of 24CrNiMoY alloy steel parts by high power selective laser melting

24CrNiMoY alloy steel used for brake disc was prepared by high power selective laser melting (HP-SLM) in order to increase the productivity. In this work, the microstructure and mechanical properties of as-fabricated part were characterized. Effects of the location of as-fabricated 24CrNiMoY part (top, middle and bottom) and scanning speed on microstructure evolution were studied. The results of this work indicated that the as-fabricated part microstructure mainly consisted of granular bainite and lath bainite. Demonstrating that bainite was obtained by a large temperature gradient and sufficient holding time by experiment. With increasing scanning speed, the size $d_B$ of the bainitic ferrite lath beam spacing and the size ${\varnothing }_B$ of the mixed structure of polygonal ferrite and quasi-polygonal ferrite were gradually refined and larger, respectively. The HP-SLM-produced parts were dominated by high-angle (>15°) grain boundaries (HAGBs) and the contents of HAGBs decreased at first while increased (62.3%, 63.5%, 50.1%) when scanning speed varied from 5 to 9mm/s. Supersaturated solid solution, dislocation, and subgrain cell structure, which results in a high microhardness and fine strength.