Investigation On The Microstructure And Mechanical Properties Of Alloy Steels Used For Railway Frog

In this dissertation,the objects of research are high manganese austenite steels and carbide-free bainitic steels used for railway frog.The effect of microstructutre evolution and chemical composition partitioning on the conventional mechanical properties and fatigue properties has been explored.These subjects are investigated: cyclic deformation behavior of traditional high manganese steel(Mn12C1.2)and N+C synergistic enhancing high manganese austenite steel(Mn18Cr7C0.6N0.3);relationship between microstructure and properties in carbide-free bainitic steels(46Si MnCrAlMoNi and 46SiMnCrAlMoNi1);effect of tempering on the microstructure and properties of carbide-free bainitic steel(46SiMnCrAlMoNi1);microstructure evolution and atom partitioning behavior of carbide-free bainitic steels(34SiMnCrAlMoNi1 and 46SiMnCrAlMoNi1)during cyclic deformation;transformation behavior in carbide-free bainitic steel.Mn18Cr7C0.6N0.3 austenitic steel with N+C synergistic enhancing alloying which possesses better propertis than Mn12C1.2 steel is fabricated.Cyclic plastic and elastic strain controlled deformation researches show that Mn18Cr7C0.6N0.3 steel always shows cyclic softening caused by enhanced planar sliding due to the interaction between N+C and the substitutional atoms as well as the dislocation,which is totally different from cyclic hardening in Mn12C1.2 steel.Enhanced effective stress is obtained due to the solid solution strengthening effect caused by the short range order at low strain amplitude while this effect does not work at high strain amplitude.Significant planar slip characteristic can be observed resulting from low stacking fault energy and high short range order effects in Mn18Cr7C0.6N0.3 steel and finally the parallel or intersecting thin sheets with dislocation tangles separated by dislocation free sheets are obtained with the prolonged cycles under cyclic elastic or plastic strain controlled fatigue testing.The interaction between C atoms in the C-Mn cluster and dislocation is essential for its cyclic hardening in Mn12C1.2 steel.The developing/developed dislocation cells and stacking faults are the main cyclic deformation microstructures on the fractured sample surface.The research on the microstructure-properties relationship in 46 SiMnCrAlMoNi and IV 46SiMnCrAlMoNi1 bainitic steels shows that the sample after isothermal bainitic transformation at low temperature possesses the optimized balance of strength and toughness,which results from its fine microstructure and small amounts of retained austenite.When the isothermal bainitic transformation temperature is 350 ?,the highest fraction of retained austenite,the lowest carbon content in the retained austenite and the coarse film-like retained austenite are obtained,consequently leading to the lowest yield strength.However,the multi-scale distribution of the film-like retained austenite size in the sample after isothermal bainitic transformation at 350 ? can transform into martensite continuously during the straining,which is beneficial for obtaining a high ultimate strength and high elongation.The influence of tempering on the microstructure and properties in 46SiMnCrAlMoNi1 bainitic steel has been investigated.Results show that sample tempered at 340 ? occupies the optimal balance of strength and toughness on the basis of maintaining certain plasticity;the samples tempered at 320 ? and 360 ? with low and high yield ratio come second.The microstructure is not sensitive to the tempering process before 360?;when the tempering temperature increases to 450 ?,the bainitic ferrite plate coarsens significantly and the occurrence of a small quantity of carbide precipitation which accounts for its low toughness.Before 400 ?,the amount of retained austenite increases with the tempering temperature.The bainite transformation from the retained austenite dominated the tempering process as the diffusion coefficient of carbon is low.When tempered at 450 ?,the thin film-like retained austenite with higher carbon content prefers to decompose to the carbide precipitation due to the significantly increased diffusion coefficient of carbon which eventually leads to the decreased amount of retained austenite and lower carbon content within it.The cyclic deformation behaviors in 34SiMnCrAlMoNi1 and 46SiMnCrAlMoNi1 bainitic steels are explored.Results show that as decreasing isothermal bainitic transformation temperatures,cyclic hardening capacity decreases while cyclic softening degree increases.Fatigue damage capacity(W0)increases and fatigue damage dufusing capacity(?)decreases with the decrease in isothermal temperatures.Comprehensively considering the strength and plasticity,the fatigue properties increase within the number of cycles to failure,Nf<1000 while decrease at Nf>1000 with decreasing isothermal transformation temperatures.The substitutional atoms do not redistribute between the retained austenite and bainitic ferrite before and after cyclic deformation.The presence of carbon in bainitic ferrite and the bainitic transformation mechanism in medium carbon high silicon bainitic steel are explored.Most of the excess carbon in the baintic ferrite is located in the vacancy-carbon clusters while the minority exists in the ferrite lattice.Vacancy can act as the diffusion path and as the site for carbon.Based on the results of the atom probe and positron annihilation spectroscopy testing,a model presenting the effect of the interaction of vacancy and carbon on the bainite transformation is proposed.In the microstructures obtained by isothermal bainitic transformation at high temperature and low temperature,there exists non-partitioning behavior of substitutional atoms(Si/Al/Mn/Cr/Ni/Mo),which indicates the the carbide-free bainite transformation is dominated by shear mechanism.