Hot Deformation Behavior of As-cast AISI M2 High-speed Steel Containing Mischmetal

The hot deformation behavior of as-cast AISI M2 high-speed steel containing mischmetal （RE） has been investigated on a Gleeble-3500 simulator in the temperature range of 1000--1 150 ℃ and strain rate range of 0.01- 10 s 1 at true strain of 1.0. The mechanical behavior has been characterized using stress-strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the mechanism of flow insta- bility. The results show that the deformation activation energy decreases with increasing deformation temperature; the efficiency of power dissipation increases with decreasing strain rate and increasing temperature; flow instability is observed at low-to-medium temperature and higher strain rate region when the strain is smaller, but extends to lower strain rate and high temperature regions with the increment of strain, in which it is manifested as flow localization near the grain boundary. Hot deformation equations and processing maps are obtained. The optimal processing window is suggested and the deformation mechanism is dynamic recrystallization （DRX）.

Effects of mischmetal addition on phase transformation and as-cast microstructure characteristics of M2 high-speed steel

The influence of mischmetal （Ce-La） addition on phase transformation and as-cast microstructure characteristics of M2 high-speed steel （HSS） was investigated using Thermo-Calc software, differential scanning calorimetry, X-ray diffractometry and scanning electron microscopy with energy dispersive spectrometry. The results showed that the measured phase transition points of M2 HSS were broadly consistent with the theoretical results. After mischmetal addition, the liquidus peak temperature, the peak temperature of the eutectic precipitation of M6C and MC were all increased, especially for the M6C which was affected significantly and increased about 31 °C. The contents of Mo and V in the eutectic carbide decreased and that of Fe increased, while in the matrix, the Mo, V and Cr contents all increased slightly. Furthermore, the microstructure of as-cast dendrite and ledeburite were refined, the total eutectic carbide content decreased and distributed into a discontinuous network, the lamellar spacing of M2C was reduced and the lamellae became thinner.

Effect of Mischmetal on As-cast Microstructure and Mechanical Properties of M2 High Speed Steel

High speed steel has been widely used in various fields due to their excellent red hardness and good wear resistance. However, the influence of mischmetal （Ce-La） on the as-cast microstructures and mechanical properties of high speed steel has rarely been reported. Thus, the microstructure and mechanical properties of M2 high speed steel with addition of mischmetal （Ce-La） were investigated. The morphology and distribution of the eutectic carbides of the steel were observed by using optical microscopy and scanning electron microscopy, and the impact toughness and bending strength were tested. The results show that adding mischmetal has an obvious effect on the microstructure and mechanical properties of M2 high speed steel. The coarse eutectic structure is refined, the weak connection of the carbide networks is broken and the flake carbides become short and fine. More networks of eutectic carbides dissolve into the matrix. When a suitable adding content of mischmetal is selected, for example, 0.3 mass%, the impact strength and bending strength can increase by 27% and 10.76% compared with that without misehmetal, respectively.

Evolution of MnS and MgO·Al_(2)O_(3) inclusions in AISI M35 steel during electroslag remelting

Thermodynamics and kinetics of dissociation and precipitation of MnS inclusions,as well as the effect of reoxidation in liquid steel on MgO·Al_(2)O_(3)inclusions in AISI M35 steel during electroslag remelting(ESR)process were investigated.The inclusions found in the consumable electrode were MnS,MgO·Al_(2)O_(3)and MnS adhering to MgO·Al_(2)O_(3).MnS inclusions were nearly spherical and ellipse in morphology,and most of them were less than 2μm in size.MgO·Al_(2)O_(3)inclusions were polygonal and nearly spherical and most about 1-4μm in size.The inclusions in ESR ingot observed by scanning electron microscopy-energy-dispersive X-ray spectrometer were polygonal and nearly spherical MgO·Al_(2)O_(3).MnS inclusions in the consumable electrode were completely dissociated before the liquid film dripping into molten slag pool.The controlling step of MnS inclusions dissociation was the mass transfer of[Mn]in the liquid steel.During the solidification process,the thermodynamic driving force could not meet MnS inclusions precipitation before the solid fraction exceeds 0.996,and the kinetics condition is too poor for the growth of MnS inclusions in the steel when the solid fraction is larger than 0.996.MgO·Al_(2)O_(3)inclusions in ESR ingot originated from the remained MgO·Al_(2)O_(3)inclusions in consumable electrode and the fresh ones formed by the reaction between dissolved magnesium,oxygen and aluminum in liquid steel.