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.
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）.
Ming-gui QUShu-hua SUNXing-hong BAIZhong-ping SHIYang GAOWan-tang FU
The microstructure and mechanical properties of medium carbon steel after cyclic heat treatment were in- vestigated. The effects of cyclic numbers and long time annealing on the microstructure and mechanical properties of the experimental steel were compared. A short duration （5 min） holding at 1023 K （above A1 temperature） and a short-duration （3 min） holding at 893 K are adopted in each cyclic heat treatment. The spheroidization is accelerated during cyclic heat treatment, and the spheroidizing ratio grows with cyclic numbers. After 12-cycle heat treatments, there are few incompletely spheroidized regions in the specimens, and cementite lamellae mostly change into cement- ite particles. The morphological character of cementite for 12 cycles is similar to that undergoing annealing for 10 h at 973 K. The strength of the experimental steel after 5-cycle heat treatment is the lowest in the following cyclic heat treatment, but it is still higher than that of specimens with subcritical annealing over a long period （10 h）. After 12- cycle heat treatment, the strength of the experimental steel is close to that of the normalized steel, and the plasticity is the best in all heat treated specimens.
Zhi-qing LÜHui-fang ZHANGQing MENGZhen-hua WANGWan-tang FU
