Abstract
Direct manufacturing techniques, such as directed energy deposition (DED), are able to produce complex components efficiently. In this study, microstructure evolution and impact toughness of DED 300M ultra-high strength steel are investigated. The results show that the microstructure of the as-deposited DED 300M ultra-high strength steel is mainly composed of martensite and some blocky bainite. The micro-segregation of elements is observed within the interdendritic area. After heat treatment, the microstructure becomes uniform and consists of martensite and lower bainite. The impact toughness of the as-deposited DED 300M ultra-high strength steel is 9 J/cm2, while it is significantly increased to 25 J/cm2 after heat treatment. Furthermore, it is observed that the fracture mode of the as-deposited sample is quasi-cleavage fracture. During the process of propagation, the main cracks would go across the martensite packet and deflect in the another one, and secondary cracks also deflected in the high-angle grain boundaries. By contrast, the fracture mode of heat-treated DED 300 M steel is ductile fracture.
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