| With the development of high-speed trains in China,the maximum speed of the trains increases from 200 to 350km·h-1.High-speed trains usually use disc brake.During emergency braking,the brake disc absorbs most friction heat energy resulting from the kinetic energy of the trains.For high-speed trains with a speed of 300km·h-1 and more,the heat load resulting from emergency braking becomes larger and the temperature even exceeds the A3 point of brake disc steel.After repeated braking cycles,thermal damages,like thermal fatigue cracks and hot spots,will occur on the friction surface of the brake disc,which harms the lifespan of brake discs and poses a threat to train safety.Therefore,a higher requirement is suggested for the steel for brake discs due to the harsh service environment.By considering the increasing demand for performance of brake disc steel,such as strength,toughness and thermal fatigue resistance,the contents of alloying elements will be adjusted on the basis of 28CrMoV steel for brake discs,and so a higher A3 point temperature and thermal conductivity is obtained.A systematic study will be carried out on the evolution of microstructure,the precipitation behavior of second phase particle,and the changes in mechanical properties and thermal fatigue resistance.Thermo-Calc thermodynamic software is used to calculate the compositions and precipitation behaviors of the equilibrium precipitation phases in brake disc steel.The effects of alloying elements on the the A3 point temperature and thermal conductivity are also studied.The results show that the contents of V and Mo in the steel should be increased,while the contents of Cr and C are opposite.The equilibrium precipitates phases are V(C,N),(Mo,V)C,M7C3,M23C6 and MnS in the base steel.Increasing the contents of V and Mo promotes the precipitation of V(C,N)and(Mo,V)C phases,and improves the A3 point temperature.Moreover,Increasing V mainly forms precipitates and so has little effect on thermal conductivity.However,an increase in the contents of Cr and C promotes the precipitation of M23C6 and M7C3 phases,and reduces A3 piont temperature.Moreover,the increased Cr mainly dissolves in the matrix and so reduces the thermal conductivity.The A3 point temperature of the base steel is 840?.The austenite grain growth behavior of Cr-Mo-V steel is studied based on thermodynamic calculation results,and the static continuous cooling transformation(CCT)behaviors of undercooled austenite are also studied by thermal dilatometer.The results show that increasing V content refines austenite grains at the temperature up to 1000?,while the addition of Nb has a good refinement effect in the temperature range of 900?1200?.The abnormal grain growth behaviors in the tested steels are affected by the dissolution and coarsening of V-riched M8C7(V(C,N))or NbC particles.During the cooling transformation,with increasing cooling rate,the ferrite content decreases,while the content of bainite increases first and then decreases,and final microstructure is completely transformed into martensite.The V addition mainly increases the precipitation amount of V,so the dissolved C content is reduced and the amount of M8C7 increases.Therefore,Ac1,Ac3 and Ms point temperatures increase,the CCT curves are shifted to left,and critical cooling rate of total martensite formation is increased form 10 to 15?· s-1.The effect of Nb is similar to that of V,but its effect is weaker due to its small amount(0.045%).The quenched and tempered microstructures of the tested steels are tempered martensite.When the quenching temperature is in the range of 880?900? and the content of V is 0.31%?0.49%,the tested steels have a good combination of strength and toughness.The precipitates are mainly M8C7,(Mo,V)C,M7C3,and M23C6 after being quenched and tempered.Increasing V content refines martensite and promotes the precipitation of(Mo,V)C,meanwhile,the precipitation of large size carbides,such as M23C6 and M7C3,is suppressed.Therefore,the strength of the tested steel increases and the impact energy changes little,However,when the quenching temperature is 920?940?,the V addition results in a significant increase in(Mo,V)C content;therefore,the impact energy drops rapidly.Tempering experiment results show that,the impact energy is less than 100J when the tempering temperature does not exceed 600?,but the yield strength is less than 1000MPa when being tempered at 700?.Therefore,the tempering temperature should be 650? to meet the performance requirements.Moreover,there is a negative linear relationship between the yield strength and the logarithm of time,and so tempering time should not exceed 2h.The optimum amount of Nb in the tested steel is 0.025%.The addition of 0.025%Nb refines martensite and increases the proportion of high-angle grain boundaries(HAGB),so the yield strength and impact energy increase simultaneously.In the early stages of tempering,the addition of 0.045%Nb suppresses the precipitation of(Mo,V)C and promotes the precipitation of large-size NbC,therefore,the strength and toughness are reduced simultaneously.During long-time tempering,the addition of 0.045%Nb promotes the tempering stability due to the suppression of precipitation and coarsening of M23C6 and M7C3 and the precipitation strengthening of M8C7 and NbC.Moreover,the impact energy of the steel with 0.045%Nb addition is not much different from that of the Nb-free steel,but it is still smaller than that of the steel with 0.025%Nb addition.The high temperature performance tests of the tested steels,like thermal fatigue property,oxidation resistance and high temperature mechanical properties,are conducted by thermal fatigue tester,muffle furnace and high-temperature tensile tester.Thermal fatigue test results show that,the additions of 0.025%Nb and 0.18%V(0.31%to 0.49%)improve the thermal fatigue properties of the tested steel.The additions of Nb and V suppress the precipitation and coarsening of large-size carbides and increase the contents of M8C7 and NbC,at the same time,the distribution of precipitates is more even,which improves the microstructure stability and the hardness of the tested steel.Therefore,the length of main crack in the tested steel is reduced and the thermal fatigue resistance is improved.The increase in Cr content inhibits the oxidation behavior of the tested steel.The oxide layer is divided into two layers.The outer layer is loose and mainly Fe2O3,while the inner layer is denser and has a higher Cr content,which is consist of FeCr2O4 and some FeO.The Cr addition increases Cr content in the inner layer,and so the inner layer becomes denser and its thickness decreases,which improves the oxidation behavior of the tested steel.High temperature mechanical propertity experiments show that,increasing the contents of V and Mo improves the high temperature strength of the tested steel when the experimental temperature does not exceed 400?,while the additions of Cr and Nb have little effect. |
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