Microstructure And Properties Study Of WC/Bainitic Steel Fgm In Turnout Rail

With the developing of heavy load and high speed railway transportation,the squeezing and impact of the carrier on turnouts becomes more and more serious.The key parts of the turnouts are trend to fail because of wearing and fracture.The function gradient concept,therefore,is applied to prepare the high wear resistant and impact resistant functionally graded materials(FGM)suitable for the key parts of turnouts,which has great research significance.This project produced Bainitic steel matrix materials and WC/Bainitic steel composite materials with the method of ?preheat + laser deposition + on-line isothermal quenching?.The influence of laser deposition on the microstructure of the bainitic steel and the influence of WC content on the microstructure of the composites were analysed by optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS).The effects of laser deposition process on the mechanical properties of bainitic steel and the influence of WC content on the mechanical properties of composites were studied by hardness test,wear test and impact toughness test.According to the above experimental results,the reasonable composition of FGM was designed.Different of isothermal quenching treatment processes were applied on the FGM to study the effects of isothermal quenching treatment on FGM microstructure and properties,and finally optinized the heat treatment process.The conclusions of this paper can be drawn as following:(1)When laser single-layer deposition is applied to produce Bainitic steel,laser working time is short,the Bainite transformation mainly occurs in the subsequent isothermal quenching process,resulting in the formation of lower Bainite.The materials' average hardness(430HV0.2)is high,the wear resistance is 2.2 times of the substrate.When laser multi-layer deposition is applied to produce Bainitic steel,laser working time is long,Bainite transformation happens during laser deposition process,the main microstructure is the mixture of granular Bainite and lower Bainite.The average hardness of materials(336HV0.2)is higher than that of the substrate(285HV0.2).The wear resistance is 1.4 times of the substrate and the impact toughness is 4 times of the substrate.Bainite steel prepared by multi-layer deposition has high impact toughness and can be used as the core ductile layer of FGM.(2)WC content will change the phase transformation of composite materials.With the increase of WC content,the Bainite content decreases and block Martensite/Austenite mixed microstructure increases in the composite materials.In addition,a continuouse reticular distributed Ledeburite microstructure is generated,and the size of the Ledeburite is gradually growing with the increase of WC content.In terms of the properties,with more WC content,the hardness of the composites increases,the wear resistance first increases and then decreases,the impact toughness will deteriorates.To be specific,for 5vt%WC/Bainitic steel composites,the average hardness is 461HV0.2,the wear resistance is 2.8 times of the substrate and the impact toughness is 1.8 times of the substrate.It can be used as middle transition layer in FGM.As for 10vt%WC/Bainitic steel composites,the average hardness is 493HV0.2,the wear resistance is 4.2 times of the substrate and the impact toughness is equivalent to substrate.It can be used as a wear-resistant layer on the surface of FGM.Furthermore,when the content of WC is 15vt%,20vt%,the hardness and wear resistance of the composite substrate is greatly increased,however,the impact toughness is seriously deteriorated,which is not suitable for FGM.(3)The isothermal temperature has a significant impact on the microstructure and properties of FGM,yet the isothermal time has little effect on it.It has been determined that FGM has a ideal microstructure and properties with the isothermal temperature at 300? and isothermal time of 2 hours.The ductile layer,transition layer and wear-resisting layer microstructures are granular Bainite + lower Bainite +M/A structure,lower Bainite +M/A structure,M/A structure +lower Bainite + Ledeburite respectively.The overall material hardness presents a gradient change,the wear resistance is 4 times of the substrate and the impact toughness is 3 times of the substrate.