Improving the Impact Toughness of FASTCOOL50 Die Steel by Increasing High‐Angle Grain Boundaries Density

Hot work die steel FASTCOOL50 was developed by Rovalma S.A., this steel indicates high thermal conductivity while low impact toughness. Herein, a novel strategy is proposed to improve impact toughness by refining the effective grain size and by reducing the forging temperature. Samples with different microstructural characteristics are prepared by varying the forging and heat treatment processes. The crack propagation path and mechanism are analyzed in detail by electron backscatter diffraction; the relationship between yield strength/cleavage fracture stress and martensitic block size ( d b ) is established; and the strong dependence of cleavage fracture stress on d b is revealed. Compared to traditional forging process with of d b 1.61 μm, the novel strategy can decrease the d b to 0.86 μm. Thus, this improve the impact toughness by 4 times. This phenomenon is attributed to the refinement of the d b and the increase in the proportion of high‐angle grain boundaries, increasing the energy consumption of crack propagation. Owing to its excellent strength, toughness, and thermal conductivity, FASTCOOL50 has been manufactured into casting pins and successfully applied in gigacasting molds, achieving a service life of over 30 000 cycles while also addressing the issue of shrinkage in cast parts.