Evolution mechanism of Mo-rich, B2-NiAl, and Cu-rich particles under aging and its influence on strength and elongation in a maraging hot work steel SDH88

Abstract

A novel maraging hot work steel, SDH88, is developed to achieve a balance between ultimate tensile strength (UTS), yield strength (YS), and elongation by combining nanometer-scale precipitates of B2-NiAl, Cu-rich, and M2C carbide. The peak-aging sample demonstrates a YS of ∼1281.8 MPa, a UTS of ∼1665.9 MPa, and an elongation (EL) of ∼17.5 % at room temperature. The microstructure and precipitation changes from peak aging (8h) to over-aging (48h) were examined using different techniques. Under peak aging condition, the precipitates consist of four nanoparticles: 9R-Cu, B2-NiAl, Cu-NiAl, and M2C carbides. However, at over-aging condition, the type of particles changes and consist of B2-NiAl, M6C, Laves phases and FCC-Cu. This study systematically investigates the precipitation evolution of Mo-rich, B2-NiAl, and Cu-rich particles from peak-aging to over-aging conditions. The high YS at peak aging condition is attributed to high precipitation strengthening of ∼553.3 MPa and dislocation strengthening of about 430.8 MPa. The number densities of NiAl (<3.8 nm) particles are more than that of Cu particles which indicates that the NiAl nanoparticles are the dominant strengthening phase. Finally, this work clearly demonstrates that the new alloy composition design allows the application of aging steels in die casting steels with great advantages and potential for obtaining balanced mechanical properties through the systematic study of the aging process and mechanism of precipitation evolution. Proposing a new approach for designing hot work die steel without quenching treatment during mold processing.