Research On Effect Of Solidification Pressure On The Cast Structure And Segregation Of M42 High Speed Steel

M42 ultrahard high speed steel has a high hardness while maintaining an excellent toughness.Its comprehensive performance is obviously superior to other high speed steels.It is widely used in cutting field,and has an irreplaceable position.However,there are many problems in the solidification structure of the M42 high speed steel prepared by the traditional smelting method,e.g.coarse cast dendrites,eutectic ledeburite and carbides,etc.which seriously affects the processing performance of M42 and restricts its application and popularization finally.As a special smelting technology of high-quality special steel,pressurized solidification technology has the functions of improving the interfacial heat-transfer conditions,refining dendrite structure,reducing or even eliminating porosity,porosity and segregation,and changing the morphology and distribution of intergranular precipitates.Therefore in this paper,solidification structure(dendrite,ledeburite,carbide,porosity and shrinkage)and segregation of M42 ingots under different pressures(0.1 MPa,1MPa and 2MPa)were studied systematically.And the change rule of cast structures and segregation were discussed in detail combined with the relation between interfacial heat transfer coefficient and solidification pressure.Some main conclusions are summarized as follow:(1).As solidification pressure increases,the primary and secondary dendrite arm spacing decreases,and the average thickness of eutectic ledeburite decreases accordingly.M2C eutectic ledeburite is the prevailing ledeburite structure.Some M6C eutectic ledeburite,existing mainly in the central region of ingot,decreases with increasing pressure.(2).Pressurized solidification can effectively reduce the thickness and inter-rods spacing of M2C carbide by increasing nucleation rate,and make compositions of M2C eutectic ledeburite distribute in a more homogeneous way by restricting diffusion of elements in liquid phase.(3).Segregation morphology of M42 ingot is ledeburite structure.And as solidification pressure increases,the volume fraction of whole segregation region and the large segregation point decreases,macro-segregation was suppressed further.(4).Elements C,Mo,W,V and Cr exist mainly in region between dendrites,while Fe and Co exist more in dendrites.Mo,V and W have an obvious dendritic segregation,and Mo is the most distinct one.Increasing pressure can aggravate the dendritic segregation by restricting the diffusion of elements.(5).Pressurized solidification can enhance the feeding capacity of liquid steel by increasing the feeding velocity and flow,and then reduce the volume of porosity and shrinkage largely.