Investigations Of High Strength And Toughness Bainitic Steel Used For Frog

Frog is one of the key components of railway line,which is subjected to huge alternating impact load and compressive stress during service,therefore material with high strength,high toughness and good wear resistance is required to guarantee the safety and efficiency for railway.Air-cooled bainite steel has high strength?more than 1200MPa?,good toughness?more than 40J at room temperature?,good weldability and other benefits,so it is widely considered as a new generation of structural materials for rail and frog.However,the development of high load railway makes greater demands on the strength,wear and fatigue resistance of frog,and the development of jointless track also requires well weldability.There are two methods of improving the performance of bainite rail,alloying method and heat treatment method.Alloying method is a way to increase the strength,hardness and toughness by adding Mn,Cr,Mo,Ni and other alloying elements.But if the addition of alloying elements is too much,segregation will tend to occur,which is bad for homogeneity and stability of mechanical properties and microstructure.In the practical production,heat treatment method mainly realizes the microstructure control of bainite steel by isothermal treatment and controlled cooling.In order to obtain good bainite structure,it is necessary to reduce the alloying element content in bainite steel to improve the homogeneity and stability of bainite microstructure,and adjust the performance and microstructure of baintic steel by combining with heat treatment method to reach better match of strength and toughness.In this paper,nine kinds of bainite steels have designed.Raw material were made by melting in the vacuum induction melting furnace and forging,and the strength,hardness and impact toughness of different bainite steels were evaluated.Light microscope,scanning electron microscope,transmission electron microscope and X-ray diffractometer have been used to study the microstructures of the materials.The influences of C,Cr,V,Ni,Al etc.on the properties and microstructure of the materials have discussed.Results of the trials indicated that the microstructures of the testing bainite steels were chiefly consist of carbide free baintie and M/A island,and the filminess retained austenite?RA?mainly dispersed between baintie ferrite laths.A few of them existed in bainite ferrite lath at 20°⁴0°degree angles.With the carbon content added,M/A island and retained austenite contents increased,the strength of bainitic steel enhanced and ductility and toughness first rose and then dropped.With the chromium content added,the microstructure was refined and the strength of bainitic steel increased,A and Z first rose and then dropped.Both V and Ni could enhance the strength,hardness,toughness of bainitic steel.Al and Si had similar functions,it could inhibit the precipitation of carbide to form carbide free bainite.But the addition of Al led to coarsening and the strength of the steel was lower than 1250MPa,far below strengths of other bainitic steels with no addition of Al,which is more than 1400MPa).By optimizing the alloying elements,the optimum composition which owned good combination of strength,toughness and plasticity was obatined,and the mechanical properties were as follows:R_m=1467MPa,R_p0.2=850MPa,A=17.0%,Z=45.5%,KU₂=52J,HRC=44.4.The test bainitic steels were heat treated,the process was:heating to 900?and holding for 1.5h then cooling with furnace to room temperature+heating to 900?and holding for1.0h then air cooling to room temperature+heating to 300?and holding for 3⁴h then air cooling to room temperature.The microstructures have been refined by heat treatment at different degrees:the thickness of retanined austenite between banite ferrite laths decreased and the lath became long and thin.The results of XRD test showed that the retained austenite contents of the tested bainite steels after heat treatment were decreased,the content of the steel with 0.287?wt.%?carbon decreased by 35.0%and the content of the steel with0.227?wt.%?carbon decreased by 63.8%.The test results of mechanical properties showed that heat treatment had different effects on the mechanical properties of test bainite steels.On the basis of laboratory trial production,the industrial trial of bainite steel for frog was carried out.The feature points of phase transition of the bainite steel were obtained by measuring the continuous cooling transformation curve,they were:Ac₁=730??Ac₃=873??Ms=320?.From the CCT diagram,in the cooling rate range of 0.05?/s¹.0?/s,the bainite steel could mainly obtain bainite and when the cooling rate was greater than 1.5?/s,the microstructure was low carbon martensite primarily.The content of low carbon martensite increased gradually with the increase of cooling rate.Heat treatment optimization trials were developed,and the results showed that annealing could refine the grain of bainite steel,which was helpful to improve the ductility of steel;if the normalizing temperature was too high,it would make the grain coarse,which would harm the ductility of bainite steel.Therefore,the optimized heat treatment process was obtained.The bainite steel in this paper has been used in the manufacture of alloy steel combined frog with wing rail inlaid,and the application denoted that the surface quality of the wing rail and the center of turnout were good,with less abrasion and bare of spalling defects,in six months to one year service process.