Microstructure Of M2 High Speed Steel Research Articles

New insights into the microstructure of M2 high-speed steel

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.

Increasing Solidification Rate of M2 High-Speed Steel Ingot by Fusible Metal Mold

As one of the most popular used high-speed steels, M2 possesses excellent hardness and toughness as cutting tools. Networks of eutectic carbides distributing in the inter-dendritic region are academically considered to be the typical microstructure of M2, which can be refined by increasing cooling rate. In this paper, a novel casting method named fusible metal mold (FMM) is employed to refine the microstructure of M2 high-speed steel. Results show that FMM casting method can improve cooling rate by 100% without any contamination of the melts’ composition.

Feasibility of partial remelting process in globular microstructure preparation for semi-solid processing of M2 high-speed steel

Feasibility of partial remelting process in producing the feedstock for semi-solid processing of M2 high-speed steel (HSS) has been investigated. The effect of reheating parameters, that is, heating rate, holding temperature and holding time on final microstructure of M2 HSS, has been studied. For this purpose, a series of experiments based on Taguchi method was employed to determine the required reheating cycles. The experiments were carried out in a resistance furnace and in an argon-controlled atmosphere to avoid any oxidation and decarburization. During any reheating cycle, the temperature of surface and core of the samples were recorded and compared with the furnace temperature. Microstructural evolution of the samples was then studied and the formation of a globular microstructure in holding time has been evaluated. The particle shape factor and globule size in each stage of experiments have been derived and its relation to other parameters has been studied. Finally, appropriate reheating cycle and parameters has been determined and two-dimensional plots of parameters versus shape factor are introduced as a process window for reheating cycle derived.

Rapid solidification of M 2 high-speed steel by laser melting

The effects of laser surface melting and rapid solidification on the microstructure of M2 high-speed steel (HSS) have been investigated. A solid state pulse Nd-YAG laser of wavelength 1.06 μm, maximum power of 100 W, beam diameter ∼1 mm, and pulse duration of 0.8 and 2.5 ms. Optical, scanning electron microscopy, and X-ray diffraction techniques were used to evaluate the microstructure and identify the phases. Results show that laser surface melting has led to a complete dissolution of the carbides and re-solidification of cellular/dendritic structure of a very fine scale surrounded by a continuous interdendritic (or intercellular) network of carbides eutectic. The phases appeared were mainly δ ferrite and M 6C and small amount of austenite γ. Laser surface melting leads to a refinement of the microstructure and altering the morphology and the distribution of the phases. The microhardness of this material was not increased after laser melting.