Effects of post homogeneity heat treatment processes on microstructure evolution behavior and tensile mechanical properties of laser additive manufactured ultrahigh-strength AerMet100 steel

Ultrahigh-strength AerMet100 steel plate was fabricated by laser additive manufacturing process (LAM) followed by subsequent heat treatments. Microstructures and tensile mechanical properties of the steel were examined using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and universal mechanical testing machine. The results indicate that post-LAM heat treatments greatly change alloy carbide characteristics, decrease austenite content and prompt the formation of fine equiaxed prior-austenite grains for LAM AerMet100 steel. Compared to fine short rod-like Nb-rich MC carbides and needle-like M₃C carbides in as-deposited specimens, alloy carbides in tempered specimens have the large changes, which include fine spherical Nb-rich MC carbides, fine rod-like M₂C carbides, large-size spherical Mo-rich M₆C carbides and Cr-rich M₂₃C₆ carbides. And the fine rod-like M₂C carbides are dispersive distribution and coherency with the martix martensite, resulting in strong coherent strain strengthening. After proper post homogeneity heat treatment processes, tensile mechanical properties of LAM AerMet100 steel have the comprehensive improvement, which are comparable to those of the forged ones. In comparison with low ductility of as-deposited specimen, the higher ductility of tempered specimen is mainly related to its extremely strong strain-strengthening capability achieved by dispersive precipitation of M₂C carbides. With the increase of homogenization, the further improved ductility of tempered specimen mainly ascribe to the decreased amount and size of large-size alloy carbides (especially Mo-rich M₆C carbide).