Morphology and Properties of M2C Eutectic Carbides in AISI M2 Steel

In as-cast structure of AISI M2 steel, the predominant type of eutectic carbides is M2C, the morphology of which has crucial influence on the distribution and dimension of carbides in final products. In the present work, the morphology and properties of M2C carbides formed at different cooling conditions have been investigated by means of optical microscope (OM), scanning electron microscope (SEMI, energy dispersive Xray spectroscopy (EDS), transmission electron microscope ITEM), X-ray diffraction (XRD) and differential thermal analysis (DTA). With increasing cooling rates, the morphology of M2C transforms from the lamellar type to the rod-like one, and the two carbides show different growing characteristics during solidification. Compared with the lamellar carbides, rod-like M2C is less stable and decomposes faster at high temperatures, accelerating the separation and spheroidization of carbides, even after hot deformation. It is concluded that the formation of rod-like M2C in cast ingots promotes homogeneous distribution and refinement of carbides in the final products, favoring the improvement of mechanical properties of high speed steels.