The Study On Microstructure And Fatigue Fracture Behavior Of U76CrRE Heat Treated Rail

In order to meet the higher requirements of the current national development for railway transportation,a rail and beam factory has developed a new generation of pearlescent rare earth rail U76CrRE.Because of its good mechanical properties,it has attracted extensive attention.However,the fatigue properties of U76CrRE rail steel are rarely reported.In this paper,the on-line heat treatment process will be optimized through the CCT curves and TTT curves of U76CrRE rail steel to further improve the mechanical properties of U76CrRE rail steel.Combined with alloying methods,1#and 2#test steels were contrasted and studied to explore the influence of quenching workmanship and alloy element content on the phase change and fatigue performance of U76CrRE rail steel.The results are as follows:CCT curve and TTT curve of U76CrRE rail steel were measured and drawn by quenching dilatometer.The results show that the critical cooling rate of pearlite transformation of U76CrRE rail steel is 14℃/s,and 580℃～540℃is the best isothermal temperature range of isothermal heat treatment process;The critical point of phase transition A_c1is 736.3℃,A_ccmis 780.9℃,A_rcmis 694.3℃and A_r1is 660.0℃.By optimizing the quenching workmanship,the microstructure of 1#test steel is refined and the performance of 1#test steel are enhanced.In the wake of enlargement of cooling speed before phase change,the drop of constant temperature and the curtail of constant time,1#the pearlite lamellar spacing of experimental steel increased from 153.8nm reduced to 67.4 nm;560℃-30 s is the best heat treatment process for 1#experimental steel.Specifically,the tensile strength is 1370 MPa,the hardness is 390 HB,the elongation after fracture is 12.57%,and the reduction of area is 41.32%.By measuring 1#fatigue cycle times and fatigue crack length of experimental steel after different quenching heat treatment,a-N curve and fatigue crack growth rate curve are drawn.The results show that with the enlargement of cooling speed before phase change and the drop of constant temperature,the number of fatigue cycles of 1#experimental steel increases and the fatigue crack growth rate decreases.1-4#（560℃-30 s）the experimental steel has the largest number of fatigue cycles（914000 times）,and has the best strength and toughness.1-4#experimental steel the fatigue crack growth rates of experimental steelΔk=10 MPa·m^1/2andΔk=13.5 MPa·m^1/2are 8.3 m/Gc and 19.5 m/Gc respectively.In the wake of enlargement of cooling speed before phase change and the drop of constant temperature,1#test steel fatigue macro fracture roughness decreases,fatigue micro fracture striation spacing and cleavage surface size decrease gradually.1-4#experimental steel the spacing of bright lines in zone I of experimental steel is 2.42μm.The distance between the bright lines in zone II is 3.65μm.The size of cleavage surface in Zone III is36.2μm.The results of high Cr 2#experimental steel show that the apparent activation energies of phase transformation of 1#,2#experimental steel are 216.9kj/mol and 354.8kj/mol respectively.2-1#the fatigue crack growth rates corresponding toΔk=10 MPa·m^1/2andΔk=13.5 MPa·m^1/2are 7.7 m/Gc and 14.6 m/Gc respectively.The increase of Cr content can hinder the occurrence of pearlite transformation,enlarge the activation energy of pearlite change and drop the change speed of pearlite change.In the wake of enlargement of Cr content,the mechanical properties and fatigue cycle times of 2#test steel increase,while the fatigue crack growth rate decreases to a certain extent.