Additive manufacturing Cr-Mo-Si-V steel: Systematic parameter assessments, precipitation behavior of in-situ VC-M23C6 and strengthening mechanisms

To obtain the H13 steel with defect-structure-performance compatibility fabricated by laser powder bed fusion (LPBF), a systematic optimisation framework was employed to get optimal process window in this paper. Subsequently, the microstructural evolutions, nanoprecipitation behaviors and strengthening mechanisms of H13 steels built at recommended parameters were in-depth analyzed. The evolutions of submicron sized cellular and columnar dendritic crystal were explained as well as the phase transformation process including lath martensite with a twin substructure and the carbon-rich residual austenite (RA) films were revealed. Two nano carbides, MC (rich in V and Mo) as well as M₂₃C₆ (rich in Cr and Mn), with diameters of 10–40 nm, were precipitated due to the intrinsic heat treatment (IHT) in LPBF process. Cr₂₃C₆ particles preferentially nucleated at the grain and subgrain boundaries due to the presence of crystalline defects such as dislocations and stacking faults caused by lattice distortion. It then grew by alloying elemental depletion while remaining semi-coherency with the α-Fe matrix. VC particles nucleated in situ within M₂₃C₆ as V atoms accumulated and replaced M atoms in the M₂₃C₆ lattice. With the growth of VC nuclei, the strain energy caused by local lattice misfit increased. This was offset by the development of self-accommodating twins featuring a long-period stacking order substructure within the VC particles. Eventually, spheroidal VC with twin structure were embedded in or adjacent to M₂₃C₆ carbides, and the orientation relationships for VC/M₂₃C₆ and VC/α-Fe were revealed. The as-built H13 steel exhibited excellent hardness and strength compared to wrought H13 steel, which was mainly attributed to dislocation strengthening and grain boundary strengthening, with precipitation strengthening playing a secondary role due to the low amount of nanoprecipitates. The low elongation (El) at fracture was closely related to the instability of RA films as well as the residual stress.