Study On Phase Equilibrium And Related Properties Of Fe-Co-Cr-Mo System

Carbon-free precipitation hardening high-speed steel Fe-Co-Mo is an important tool steel.After heat treatment,this steel has extremely high hardness and good tempering resistance.It is used in the cutting of titanium alloys and nickel-based super alloys.It has unique advantages in application.However,as a typical iron-based alloy,it has the disadvantage of poor corrosion resistance.To improve the poor corrosion resistance of Fe-Co-Mo alloy,adding Cr element is a feasible solution.In order to understand the influence of phase relationship on the properties of Fe-Co-Cr-Mo quaternary alloy,the phase relationship of Fe-Co-Cr-Mo quaternary system and its related ternary system was studied.In this study,the equilibrium alloy method was used to determine the correlation phase diagram,and the scanning electron microscope,energy spectrometer and X-ray diffractometer were used to analyze the samples,and the correlation three in the Fe-Co-Cr-Mo quaternary system were determined.The isothermal section of the element system at 1150℃,and on this basis,the isothermal section of the Fe-Co-Cr-Mo quaternary system with a Fe content of 65 at.%and a Cr content of 9 at.%was measured.In order to study the effect of different Cr element content on the corrosion resistance and hardness of Fe-Co-Cr-Mo alloy,Fe-Co-Cr-Mo alloys with different Cr element contents were prepared,and the different Cr element contents were tested by Rockwell hardness tester.The hardness of the Fe-Co-Cr-Mo alloy was tested,and the corrosion resistance of the Fe-Co-Cr-Mo alloy was tested using an electrochemical workstation,and the following conclusions were obtained.（1）There are three three-phase regions in the isothermal section of the Fe-Co-Mo ternary system at 1150℃,namelyμ+（Mo）+σ（Co₂Mo₃）,μ+α+γandμ+θ（Co₉Mo₂）+γ.The 1150℃ isothermal section of the Co-Cr-Mo ternary system has 6 three-phase regions,which areμ+（Mo）+σ（Co₇Cr₈）,μ+（Mo）+σ（Co₂Mo₃）,γ+σ（Co₇Cr₈）+R,μ+（Mo）+R,μ+γ+R andμ+θ（Co₉Mo₂）+γ.The isothermal section of the Fe-Cr-Mo ternary system at 1150℃ has four three-phase regions,which areσ+α+（Mo）,μ+（Mo）+R,μ+α+σ,andσ+α+R.（2）The cobalt-rich angle of the isothermal section of Fe-Co-Cr-Mo quaternary system with Fe content of 65at.%has one four-phase area and three three-phase areas,which areμ+α+γ,μ+α+R,R+α+γthree three-phase areas,and four-phase areaμ+γ+α+R.The isothermal section of the Fe-Co-Cr-Mo quaternary system with a Cr content of 9at.%has one four-phase area and five three-phase areas.The four-phase area isμ+γ+α+R,and the three-phase area includesμ+α+γ,μ+α+R,R+α+γ,and R+μ+γ.（3）Adding Cr element to Fe-Co-Mo alloy can improve the corrosion resistance of the alloy.Through electrochemical test,it can be known that in 3.5wt.%Nacl solution,the self-corrosion current density of 12.5wt.%Cr alloy is the smallest.Its corrosion resistance is the best.According to the test results of the Rockwell hardness tester,after the addition of Cr element exceeds 10.5wt.%,the hardness of the alloy begins to gradually decrease.Therefore,the maximum addition of Cr element in the alloy should not exceed 9at.%.