Effect of direct heat treatment and densification-heat treatment on microstructure and mechanical properties of laser powder bed fusion 8Cr4Mo4V steel

In this study, 8Cr4Mo4V steel powder was used as the feedstock to fabricate samples by laser powder bed fusion (LPBF), which were then subjected to direct heat treatment and densification-heat treatment to tailor the as-deposited microstructure. The microstructural evolution, strengthening mechanisms, and fracture mechanisms after different post-treatments were systematically investigated. The results indicate that the as-deposited samples exhibit elemental segregation and internal defects, which severely deteriorate the tensile properties. Both direct heat treatment and densification-heat treatment effectively eliminate elemental segregation and achieve microstructural homogenization. Although the primary carbides coarsen to approximately 2 μm after densification-heat treatment, the size of these carbides remains considerably finer than that of carbides in conventional forgings, with Mo₂C and VC as the carbide types. Direct heat treatment increases the ultimate tensile strength (UTS) by approximately 16%, as the stress concentration associated with residual defects still dominates crack initiation and propagation, resulting in predominantly brittle fracture. Compared with the as-deposited samples, densification-heat treatment can effectively eliminate internal microstructural defects. The yield strength (YS) and UTS attain 2129.46 MPa and 2449.90 MPa, increasing by approximately 142% and 133%, respectively. The enhanced YS is primarily attributed to dislocation strengthening and precipitation strengthening, with a measured hardness of 748.79 HV1. The dominant failure mode is brittle fracture, which is consistent with that of conventional forgings.