Enhanced mechanical performance in additively manufactured AF9628 steel: Role of cellular segregation in phase transformation

Abstract

In this study, AF9628 high-strength low-alloy steel was fabricated using laser melting deposition. The initial non-equilibrium microstructure exhibited cellular segregation of elements such as Cr and Mo, with cellular structure dimensions of 20–30 μm. Samples were heated to 950 °C and held there for 10 min before being cooled at four different cooling rates-that is, 0.5, 1, 10, and 50 °C/s-to research the impact of cooling conditions on the microstructure. The results demonstrated that under specific cooling conditions (1 °C/s), cellular segregation could effectively limit the growth of bainitic/martensitic substructures. This becomes an effective component interface that can hinder the phase transformation, in addition to the grain boundaries. When the cooling rate was too fast or too slow, martensite and bainite grew throughout the cellular structure, the phase-transformation restriction by cellular segregation was limited. At a cooling rate of 1 °C/s, its mechanical properties (tensile strength of 1865 MPa and elongation of 10.3 %), exceeding those of samples prepared by other heat treatment routes. The results of grain reconstruction confirmed the restrictive influence of cellular segregation on bainitic/martensitic transformation. This approach to microstructure control opens up new routes for the optimization of the microstructure and properties in additive manufacturing of low alloy steel.