Study On The Evolution Of Carbides In Directional Solidification Microstructure Of M2 High Speed Steel Under A Static Magnetic Field

M2 high speed steel is one of the important tool steels.However,due to its high content of carbon and alloy elements,it is easy to precipitate large-sized eutectic carbides in the solidified structure,which results in the decrease of its mechanical properties and service life.The key to promote the performance of high-speed steel is the improvement of the as-cast microstructure's properties,especially the morphology and size of eutectic carbides.In recent years,due to the character for the non-touch force,the applications of the magnetic field have attracted more and more attention.For example,magnetic damping and thermal electromagnetic effects are generated in the molten metal by the application of a static magnetic field.This shows that the magnetic field not only inhibits the flow but also generates a new flow.Besides,the migration process of the solute in the liquid metal was also effeced by the magnetic field.All of these effects will affect the morphology and dendrite size of the solidified macromicrostructure,which in turn change the metal.Mechanical properties.Therefore,this paper mainly studied the influence of magnetic field on the as-cast macromicrostructure of M2 high-speed steel.Consider that the growth direction of dendrites is multi-directional in the traditional solidification,and the transfer processes of solute and heat are relatively complex.In order to simplify the solidification process,the method of directional solidification was used to explore the influence of static magnetic field on the macro-microstructure of M2 high-speed steel.Firstly,the effect of different pulling speeds(20 ?m/s-10000?m/s)on the directional solidification macro-microstructure of M2 high speed steel was studied.The results showed that the increase of the pulling speeds promote the transition of the eutectic carbide from lamella to short rod.When the pulling speed was 500 ?m/s,the morphology of eutectic carbides were a mixture of lamella and short rod.When the pulling speed was less than 500 ?m/s,the eutectic carbides were the coarse lamella.However,when the pulling speed was much larger than 500 ?m/s,the morphology of the eutectic carbide was stabilized to a short rod shape.Both the lamellar eutectic carbide and the short rod-like eutectic carbide were M2 C carbide.Raising the pulling speeds promoted the precipitation of eutectic carbides,at the same time the percentage of the alloying elements in eutectic carbides declined.The spacing of(110)in martensite phases decreased from 2.0607 ? to 2.0553 ? as the pulling speed increasing from 20 ?m/s to 10000 ?m/s.This indicates that with the increase of cooling rate,the solid solubility of alloying elements in matrix structure may decrease.Secondly,the effect of the transverse static magnetic field(TSMF)(0 T-1 T)on the directional solidification macro-microstructure of M2 high speed steel was studied.The results showed that the morphology of eutectic carbide changed from lamella to fiber as the magnetic flux density(MFD)increasing from 0 T to 1 T.The transformation from lamella to fiber had been enhanced with the increase of MFD,and the distributed areas of fibrous eutectic carbides expanded as the increase of MFD.Solute elements moved and gathered at a certain region in the mushy zone under a TSMF.In the mushy zones,channel segregations were formed in the samples treated by 0.2 T and 0.4 T TSMF.When the MFD increased to 0.8 T or 1 T,the channel segregation disappeared.Morphology transition,solute elements movement and the formation of channel segregations were attributed to the thermoelectric magnetic force.This paper provided a new method to gain a high quality of M2 high speed steel with fibrous eutectic carbides distributed in the matrix assisted by a TEMF.Lastly,the effects of longitudinal static magnetic field(0 T-1.5 T)on the ·directional solidification microstructure of M2 high speed steel was studied.The results showed that with the increase of MFD,the columnar dendrites in directional solidification microstructure of M2 high speed steel were gradually destroyed,and the transformation from columnar crystals into equiaxed crystals(CET)occured.When the MFD was less than 0.1 T,the solidified microstructure was columnar,and when the MFD was large than 0.2 T,the solidified microstructure was equiaxed.In addition,we found that directional solidification under longitudinal magnetic field had little effect on the morphology of eutectic carbides.With the increase of MFD,the segregation of eutectic carbides disappears at the edge of the sample,and the total amount of carbides increases.In summary,the directionally solidified macro-microstructure of M2 high speed steel can be affected by the magnetic field and the pulling speed.This paper explored the influence of static magnetic field on the directionally solidified macromicrostructure of M2 high speed steel and the morphology,quantity and distribution of eutectic carbides.It is of great significance and value for improving the as-cast microstructure of high-speed steel and controling the morphology and distribution of eutectic carbides.