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

Abstract

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 μm. 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 μm. The decomposition of M₂C occurs from 897.2 to 1221.5 °C (heating rate of 200 °C/h). Some precipitated carbide particles occur in the M₂C matrix after holding for 15 min at 1100 °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 μm).

Keywords

high-speed steel / metastable carbides / decomposition / phase transformation refinement

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