Precipitation and decomposition behaviors of carbides in AISI M2 high-speed steel with nitrogen and mischmetal

The behaviors of the precipitation and decomposition of carbides in AISI M2 high-speed steel modified by nitrogen and mischmetal were investigated using DSC, XRD, SEM and TEM. The as-cast microstructure of the experimental steel consists of dendrites of iron matrix, networks of eutectic carbides and secondary carbides. The average distance between networks is about 34 mu m. The carbides mainly include M2C, M(C,N) and M6C, and their relative contents are 58.5%, 30.3% and 11.2%, respectively. The average spacing between the M2C fibers is 1.5 mu m. The decomposition of M2C occurs from 897.2 to 1221.5 A degrees C (heating rate of 200 A degrees C/h). Some precipitated carbide particles occur in the M2C matrix after holding for 15 min at 1100 A degrees C. With increasing holding time, the carbide fibers neck down more and more obviously until they are broken down. The spectral peaks of M2C almost disappear after holding for 60 min. The spectral peaks of M6C gradually strengthen with the holding time, and the relative content of M6C increases to 79.8% after holding for 60 min. After holding for 180 min, the carbide fibers disappear, and the decomposition products consist of fine carbide particles (about 300 nm) and short rod-like carbides (about 3.5 mu m).