New insights into the microstructure of M2 high-speed steel

Abstract

In this work, the microstructure of the heat-treated AISI M2 high-speed steel of different initial structural states, wrought and as-cast, was investigated in detail down to the atomic scale, using advanced electron microscopy techniques. Significantly different amount of the secondary carbide particles of a size up to 300 nm in the microstructure of both steels was established and described. In the tempered matrix, the formation of the ultrafine sub-grains with a size in the range of 3–15 nm and the substructure of the α-Fe consisting of nano-sized elongated plates about 200–300 nm in length and 7–30 nm in thickness were proven. Fine dispersions of the secondary hardening carbides along the matrix sub-grain boundaries and inside the carbide clusters were thoroughly investigated and identified as vanadium-rich MC carbide. Extremely fine precipitates at the boundaries of the nano-sized elongated plates in the substructure of the tempered α-Fe were revealed in both steels. The precipitates, regularly dispersed in the tempered matrix and containing carbon, were enriched primarily in iron, molybdenum, chromium, vanadium, and, to a lesser extent, in tungsten, as proven by the combined EDX and EELS analysis. At the atomic scale, the precipitates were found to be mostly disordered solids without a crystalline structure. However, some of the precipitates exhibited traces of crystalline structure, which likely corresponds to Fe21(W,Mo)2C6 phase.