Morphology, microstructure and decomposition behavior of M2C carbides in high speed steel

The morphology, microstructure and decomposition behavior of M2C carbides in high speed steels with different chemical compositions have been investigated by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction and X-ray diffraction. The results show that the morphology and substructure of M2C carbides are very sensitive to chemical compositions of high speed steels. M2C carbides present the plate-like shape in tungsten-molybdenum steel and present the polycrystal orientation in the eutectic cell. In contrast, they show the fibrous shape in molybdenum-base steel and exhibit the monocrystal orientation. Plate-like and fibrous M2C carbides are both metastable and decompose into M6C together with MC at high temperatures. MC nucleates inside the plate-like M2C while it is formed at the fibrous M2C/matrix interface during the decomposition process. Such differences are expected to arise from different compositions of plate-like and fibrous M2C carbides.