Research On Diagram Calculation And Microstructure Of High Boron High Speed Steel

High boron cast steel（0.4².0 wt.%B）is used at present as wear-resistant materials.Borides（such as Fe₂B）are widely used in wear-resistant materials as a hard phase with high hardness（HV1450¹800）and good heat stability.Through changing the content of boron and carbon to control the volume fraction and distribution of borides and carbides in matrix,reducing the use of expensive alloying elements such as tungsten,molybdenum,vanadium,niobium,cobalt,etc.high-boron high speed steel（HBHSS）has been developed that can be used as excellent wear-resistant material,meanwhile,the cost of production is decreased at the same time.However,the hardness of HBHSS alloy in high temperature decreased significantly and the wear resistance greatly reduced which made its promotion and application limited.Aluminum can improve the tempering stability,hardness and red hardness.In addition,the formation of Al₂O₃film on aluminum alloy can be combined with the Cr₂O₃ film,the combination further improve the wear resistance of the alloy in high temperature.This subject is started from the alloy design to calculate the phase diagram of HBHSS alloy and carry out the experiment to verify the correctness of calculated phase diagram.At first,the principle of phase diagram calculation and thermodynamic model was described,then the vertical sections of Fe-B pseudo-binary phase diagram with different boron contents have been calculated by use of Thermo-Calc software and the influence of boron content on the phase regions and the parameters of eutectic point have been analyzed.The eutectic point content of boron is about 1.89 wt.%and the eutectic temperature is about 1318℃under the equilibrium state.The phases are composed ofα、M₂B（M=Fe,Cr）、M₇C₃（M=Fe,Cr,Mo,V）、M₂₃C₆（M=Fe,Cr,Mo,V）andM₃C（M=Fe,Cr）.The effect of boron content on the change trend of phase content in HBHSS alloy system at 900℃have been analyzed and the equilibrium solidification processes of HBHSS alloy with different compositions have been simulated by Thermo-Calc.With the increase of boron content,the overall trend of carbides in the solidification structure is:MC+M₂₃C₆→MC+M₂₃C₆+M₇C₃→MC+M₇C₃→MC+M₃C.The vertical sections of Fe-B pseudo-binary phase diagram with different Al contents have been calculated by use of Thermo-Calc software and the influence of Al content on the phase regions.The parameters of eutectic point have been analyzed.With the increase of aluminum concentration,the boron concentration and temperature of the eutectic transition point have no significant change,but the region of austenite shrinks sharply.When the aluminum concentration comes to 1.5%,the austenite region shrinks to a small ring.The matrix of cast HBHSS isα-Fe,and hard phases are M₂（B,C）borides and M₇（B,C）₃-type boro-carbides.With the increasing of aluminum,the matrix microstructure will change from lath martensite+residual austenite（aluminum is 0.5%）into ferrite+pearlite（aluminum is 2.0%）.The calculated phase diagrams of alloy were compared with the experiment results.The chemical composition 0.4%C-6.0%Cr-4.0%Mo-1.0%Al-1.0%Si-0.5%Mn-1.0%V-x%B（x=1,2,3）of HBHSS are selected:The phase transition temperature were measured by DSC and the microstructure was detected by SEM,OM and XRD.The results indicate that calculated phase diagrams are agree well with the experimental results and furtherly prove the thermodynamics database of Thermo-Calc software is reliable.It can be used to help design the alloy composition and heat treatment process.The phase diagrams were calculated by Thermo-Calc software with elements concentration:0.4%C-6.0%Cr-4.0%Mo-1.0%Si-0.5%Mn-1.0%V-（1.0³.0）%B-x%Al（x=1.0,1.5,2.0）.The phase transition temperature were measured by DSC and the microstructure was detected by SEM,OM and XRD.The effect of aluminum on the phase diagram of high boron high speed steel and the influence of the as-cast structure were discussed.