Evolution And Experimental Study Of Eutectoid Steel Rail Structure

In recent years,with the formation of high density large axle load and high speed simultaneously,hypereutectoid steel rail?BG400?has been widely concerned because of its high strength and good wear resistance.However,there are still some technical problems in BG400.During the heat treatment process,it is easy to form abnormal structures such as intergranular network carbide and martensite,which seriously shortens the service life of the rail.The pearlite laminates of heat-treated rails are widely spaced,which leads to the lack of hardness of rail tread.Therefore,this paper studies the above problems,and the results are as follows:The critical temperature of BG400 phase change was determined by quenching dilatometer L78,and its continuous cooling transition curve?CCT curve?was plotted and analyzed.The results show that the critical temperature A_c11 is 702?and A_cmm is 772.4?.The austenitizing temperature of the thermal simulation process should be selected as 900;The optimum cooling rate is in the range of 5?/s¹0?/s.The optimal isothermal temperature should be 600?,the fluctuation is no more than 30?;The isothermal time should be greater than 20s.The Gleeble-3500 thermal simulation test machine was used to carry out thermal simulation experiments under the conditions of cooling rate of 0.1?/s,0.5?/s,1?/s,3?/s,5?/s,8?/s,10?/s,isothermal temperature of 570?,600?,630?and isothermal time of 20s and 30s respectively.The microstructure parameters and hardness of different thermal simulation processes were analyzed and measured statistically.The results showed that when the cooling rate was 5?/s,8?/s,10?/s,the isothermal temperature was 600,and the isothermal time was 30s,the microstructure was pearlite,amorphous reticular carbide and other abnormal structures,the hardness reached more than 400HB,and the pearlite lamellar spacing was narrowed to 130nm¹10nm,reaching the sostenite level.With the refinement of pearlite lamellae spacing,the change rule of cementite thickness per unit length is the same as that of pearlite lamellae spacing,while the percentage of cementite content per unit area is related to the change of cementite thickness per unit length.The optimal thermal simulation process is:The cooling rate is 8?/s,the isothermal temperature is 600?,and the isothermal time is 30s.EBSD was used to characterize pearlite pellets at different isothermal temperatures,and the energy spectrum analysis of carbide particles and intercrystalline reticulated carbides was carried out and the mechanism was studied.The results show that the size relation of pearlite pellets at different isothermal temperatures is 600?<630?<570?.The nucleation growth of pearlite depends on interfacial diffusion and is affected by alloying element redistribution.Local alloying element aggregation will affect the transformation behavior of supercooled austenite to pearlite,which is the reason for formation of reticular carbide and special carbide.This study provides a scientific and reasonable theoretical basis for optimizing the BG400 rail production process and has important guiding significance.