Calculation Of Equilibrium Phase For High Carbon Alloy Steel With Multiple Types Of Ultra-fine Carbides And Examination In Heat Treatment Process

On the basis of the composition of the high carbon alloy steels with multiple types of ultra-carbides,the perpendicular section diagram,the characteristic diagram of phase and the composition of carbides and matrix for Fe-Cr-W-Mo-V-Si-Mo-Ni-C alloy system were calculated by Theme-Calc software.The reasonable project of alloy design for high carbon alloy steels with multiple types of ultra-fine carbides is carried out properly owing to the calculational results.It is important for composition design, structure design,technics design and performance forecast of the alloy steels.In this paper,the microstructure and hardness of a medium-alloy high carbon steel DM8A were studed detailedly.There are multiple types of carbides(M₃C,M₂₃C₆,M₇C₃, M₆C,MC)in the annealed steel.Above Al temperature,γ+M₂₃C₆+M₆C+MC phase region exists and M₆C carbide gradually substitutes M₂₃C₆ carbide.Below Al temperature,α+M₆C+M₂₃C₆+M₇C₃+M₃C+MC phase region exists and M₇C₃ carbide substitules M₂₃C₆ carbide in the range of 600℃to 700℃.The tempering hardness at temperature 200℃～350℃of steel DM8A quenched is 62～54HRC.In the tempering process,the amount of the carbide M₃C precipition is the most and the amount of carbide M₆C and M₇C₃ precipition is also increased,so tempering resistace of the steel DM8A is higher.The carbides transformation in heat treatment process for medium_alloy steel can bedescfibed approximatirely by calculating based on phase equilibrium thermodynamic.The microstructure and hardness of high alloy and high carbon steel DM6S were studed,the result show that there are multiple types of carbides in the annealed steel adding alloy elements which were selected quantitatively depend on phase equilibrium calculation.Due to different crystal structure of the carbides and different thermodynamics and dynamics characteristics of the carbides nucleation and growth, there are different ultra-fine carbides in the high carbon and high alloy steel.When the content of the W and Mo elements in the steel is rational,the amount of M₆C carbide in steel DM6S exactes much,and the M₂₃C₆ and M₇C₃ carbides are restricted,which makes the annealing carbides refine.The V content in the steel should be controlled in order to avoid appearing eutectic MC(VC)carbides.The tempering carbide phase containt M₂₃C₆,MC and MC stable carbides and M₇C₃ and M₃C metastable carbides. The tempering hardness of steel DM6S at temperature 520～550℃is 60～63 HRC. The carbides transformation in heat treatment process for high-alloy steel is agreement with calculational results in contrast to medium-alloy steel.