Morphology,microstructure and decomposition behavior of M2C carbides in high speed steel

The morphology,microstructure and decomposition behavior of M2C carbides in high speed steels with different chemical compositions have been investigated by scanning electron microscopy,transmission electron microscopy,electron backscatter diffraction and X-ray diffraction.The results show that the morphology and substructure of M2C carbides are very sensitive to chemical compositions of high speed steels.M2C carbides present the plate-like shape in tungsten-molybdenum steel and present the polycrystal orientation in the eutectic cell.In contrast,they show the fibrous shape in molybdenum-base steel and exhibit the monocrystal orientation.Plate-like and fibrous M2C carbides are both metastable and decompose into M6 C together with MC at high temperatures.MC nucleates inside the plate-like M2C while it is formed at the fibrous M2C/matrix interface during the decomposition process.Such differences are expected to arise from different compositions of plate-like and fibrous M2C carbides.

A New Approach for Refining Carbide Dimensions in M42 Super Hard High-speed Steel

Obtaining small carbides is crucial but difficult for high-speed steels.A new approach for refining carbide dimensions in M42 super hard high-speed steel by increasing cooling rate and spheroidizing treatment was proposed.The morphologies and properties of eutectic carbides formed at different cooling rates were investigated by means of scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,X-ray diffraction（XRD）,transmission electron microscopy（TEM）,electron back-scattered diffraction（EBSD）and differential scanning calorimeter（DSC）.The results show that eutectic carbides change from a lamellar shape into a curved-rod shape as cooling rate increases.Despite different morphologies,the two carbides are both of M2C type with a hexagonal close-packed structure and display a single crystal orientation in one eutectic colony.The morphology of M2C mainly depends on the growing process of eutectic carbides,which is strongly influenced by cooling rate.Compared with lamellar carbides,M2C carbides with curved-rod shapes are less stable,and decompose into M6 C and MC at lower temperatures.They are more inclined to spheroidize during heating,which ultimately and distinguishably refines the carbide dimensions.As small carbides are much easier to dissolve into matrices during austenization,the process described herein improves the supersaturation of alloying elements in martensite,which leads to an increment of hardness in M42 steel.

Phase Transformation of a Cold Work Tool Steel during Tempering

The hardness and microstructure evolution of a 8% Cr cold work tool steel during tempering for 40 h were investigated. Transmission electron microscope examinations showed that M_3C carbides precipitated from supersaturated martensite after tempering at 350 ℃. When the tempering temperature was higher than 520 ℃,the M_（23）C_6 carbides precipitated to substitute for M_3C carbides. After ageing at the temperature of 520 ℃ for 40 h,it was observed that very fine and dense secondary Mo_2C precipitates were precipitated. Thus,it can be concluded that the early stage of Mo_2C-carbide precipitation is like to be Gunier-Preston（ G-P） zone formed by [Mo-C] segregation group which is responsible for the secondary hardening peak at 520 ℃. Overageing at 700 ℃ resulted in recovery of martensitic microstructure and precipitation of M_（23）C_6 carbides.When ageing at 700 ℃ for more than 20 h,recrystallization occurred,which resulted in a change of the matrix morphology from martensitic plates to equiaxed ferrite. It was noticed that the size of recrystallized grain / subgrain was very fine,which was attributed to the pinning effect of M_（23）C_6 precipitates.