Characteristics of carbides, such as their shape, size, type, and structure, significantly affect the plasticity and toughness of D2 cold‐work die steel. The effect of variations in the cooling rate on characteristics of carbides during solidification of D2 cold‐work die steel is investigated. Characteristics of carbides solidified at various cooling rates are quantified by scanning electron microscopy, energy‐dispersive spectroscopy, and X‐ray diffraction. The quantity of M7C3 carbides increases, but their size and the layer spacing decrease as the cooling rate increases in the range of 0.3–4 °C s−1. The types of carbide are not affected by the cooling rate. The main type is M7C3, with small amounts of M23C6 and MC. The carbides mostly comprise hexagonal hollow bars. There are a few lamellar and curved bar structures. The formation mechanism of hexagonal hollow bar carbides is clarified. The growth of hexagonal hollow bar M7C3 carbides is primarily affected by the chromium content and temperature gradient. Carbides form planar shapes together with nucleation masses and subsequently grow within a lateral envelope of helical dislocations. The interior cavity of M7C3 bar carbides shrinks and becomes filled with austenite as the cooling rate increases, which results in thin hexagonal hollow bar carbides.
Investigating the fracture mechanisms of carbides is of paramount importance for optimizing hot working processes in die steels. This work establishes a comprehensive criterion equation to predict the behavior of carbide fracture in D2 cold‐work die steel during hot working process. High‐temperature laser confocal microscope is employed to prepare samples with varying solidification rates. Subsequently, these samples are subjected to a range of heating temperatures spanning from 1000 °C and 1100 °C. The samples are analyzed utilizing scanning electron microscopy energy spectrum analysis and X‐ray diffraction. The activation energy of M7C3 carbides is calculated through the utilization of kinetic equations and the Arrhenius law. Additionally, a criterion model for the fracture behavior of M7C3 carbides in D2 cold‐work die steel is established. The results reveal that thin hollow rod‐shaped carbide is easy to fracture. Moreover, the activation energy for phase transition of M7C3 carbides in D2 cold‐work die steels is determined to be 226.974 KJ mol−1. Furthermore, the fracture behavior of carbides in D2 cold‐work steel is found to be influenced by various factors such as heating temperature, deformation rate, and peak stress. Finally, the criterion model demonstrates substantial agreement with the experimental observations gathered, affirming its reliability and accuracy.
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