Effects Of Micro-Alloying Elements On Mechanical And Fatigue Properties Of Hypereutectoid Rails Steel

As a new generation of pearlite rail,hypereutectoid steel rail has high strength,hardness and wear resistance due to high carbon content.However,the subsequent brittleness increase and the weakening of local performance are easy to cause instantaneous failure of the rail during use.In this thesis,1#,2#containing Ni,3#Ni-Ce,4#Cu-Ce supereutectoid rail steel as the research object,the effect of microalloying elements on the microstructure,mechanical properties and fatigue properties of experimental steel after heat treatment process optimization was studied.Combined with transmission electron microscope analysis,the mechanism of microalloying elements in supereutectoid rail steel was studied.The results of heat treatment optimization show that the mechanical properties of 1#hypereutectoid rail steel are the best at 560℃for 30s,the sheet spacing is 99nm,and the tensile strength is 1289MPa.The mechanical properties of 2#Ni-containing,3#Ni-Ce and 4#Cu-Ce super-eutectoid rail steel were the best when they were kept at 540℃for 60s.The tensile strength reached 1437MPa,1420MPa and 1358MPa,respectively.The elongations were 11.2%,13.8%and 12.2%,respectively.The reduction rates of cross-section were 44.8%,48.3%and40.2%,respectively.The chip spacings were 81nm,70nm and 69nm,respectively.Among them,the comprehensive mechanical properties of 3#Ni-Ce super-eutectoid rail steel were the best.The results show that Ni and Cu elements improve the tensile strength of heat treated rail steel,and the strengthening effect of Ni element is greater than that of Cu element.Ce element can significantly improve the plasticity of eutectoid rail steel,pearlite lamellar refinement effect is the most obvious.The results of fatigue crack propagation behavior show that the optimization of heat treatment process reduces the pearlite spacing and fatigue striation spacing of hypereutectoid rail steel,and the fatigue performance is significantly improved.Among them,the fatigue cycle number N of 2#steel containing Ni is the highest,reaching 104 million times,and the crack growth rate is the slowest,m=1.759.WhenΔK=10MPa·m^1/2andΔK=13.5MPa·m^1/2,the fatigue crack growth rate reaches 7.6m/Gc and 13.2m/Gc,respectively.This is due to the high content of lamellar cementite and the relatively thick layer of cementite after heat treatment of2#steel,which effectively inhibits the crack growth and has the minimum fatigue striation spacing.TEM analysis results show that heat treatment promotes the microalloying effect of rare earth element Ce,causes lattice distortion,destroys the growth mode of local cementite and ferrite,improves the microstructure morphology of some pearlite and improves the plasticity of3#Ni-Ce steel.After heat treatment,the lamellar structure of cementite in 2#Ni-containing steel is more and thicker,and the ability to resist fatigue crack propagation is enhanced,so that 2#Ni-containing steel has the best fatigue performance.In addition,（Fe,Cr）₇C₃in 2#Ni-containing steel and 3#Ni-Ce steel can jointly hinder the dislocation movement with the cementite layer,and the solid solution strengthening effect of Ni can significantly improve the strength of the steel.（Fe,Cr）₇C₃can also cause crack deflection and reduce the crack growth rate,which is helpful to improve the fatigue performance of steel.Ce refines Nb C precipitates in3#Ni-Ce and 4#Cu-Ce steels,promotes the uniform distribution of Nb C in ferrite matrix and enhances the precipitation strengthening effect.Heat treatment can promote the segregation of Ce in grain boundaries,give full play to the role of Ce refining pearlite spacing,purifying and strengthening grain boundaries,so that 3#steel has the optimal mechanical properties.