Effect of Cooling Rates on the Characteristics of Carbides during Solidification of D2 Cold-Work Die Steel

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 degrees 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.