Study On The Surface Strengthening Of U75V Rail By Laminar Plasma

As the China railway enters a new period of high-speed development,the research on reducing rail damage and improving service life becomes increasingly important,and becomes a hot topic in railway science and technology.In this thesis,study on the practical effect of the novel laminar plasma for the U75V rail surface strengthening was carried out from two perspectives of discrete surface quenching and surface cladding.The effect of discrete quenching on the U75V rail surface to improving the performance and service life of the rail was analyzed by studying the microstructure,hardness distribution and fatigue cracks of the rail.The effects of different preheating/post-heating temperatures on the microstructure and mechanical properties of the strengthened rail were studied,and a set of optimal surface cladding process parameters was obtained.At the same time,the difference between laser surface cladding and laminar plasma surface cladding is analyzed and compared in terms of the microstructure,hardness,tensile strength,bonding strength,wear resistance,corrosion resistance.The main results are as follows:The phase distribution and hardness distribution of the rails been discrete quenched were studied.It was found that the quenched pearlite transformed into cryptocrystalline martensite with high strength and certain toughness.The interface between quenching heat affected zone and pearlite matrix is interlaced.The hardness distribution has a high degree of consistency with the phase distribution,the strength of martensite reaches 920 HV,and the strength of pearlite matrix is around 350 HV.After the actual use of the discrete quenched rails,it was found that the service life was significantly improved.For the rails that have reached the service life,the study found that fatigue cracks basically germinated in the quenching zone,the cracks were deep and numerous,and the cracks on the substrate between the discrete quenching points were shallow and little.This shows that the laminar plasma discrete quenching technology can delay the appearance of cracks,alleviate the propagation of cracks in the substrate and improve the whole life of the rail.The bonding quality of the cladding layer and the substrate and the phase composition of the heat affected zone at different preheating and cooling methods were studied.It was found that using a constant temperature heating platform to preheat and post-heat the substrate meet the requirements of the overall performance of the rail.The copper heating platform is determined to be used to conduct the surface cladding on the rail surface with laminar plasma at different preheating/post-heating temperatures.The research shows that when the preheating/post-heating temperature is 340°C,the current of transferred arc is 60 A,the moving speed is 1.0mm/s,the main gas flow rate is 2.4 L/min,the shielding gas flow rate is 3 L/min,and the voltage of powder disk is 3 V,the powder gas flow rate is 0.144 m³/h,the scanning distance is 4 mm,and distance between generator and the substrate is 10 mm,the mechanical properties and corrosion resistance of the obtained cladding layer can achieve good results,in which the tensile strength reaches 1166 MPa,the self-corrosion voltage was-0.384 V,and the self-corrosion current density was 3.885(10^-6A/cm ²).The comprehensive performance of the samples processed at 340°C is the best.The microstructure and mechanical properties of the sample after laminar plasma and laser surface cladding were compared,and it was found that the cladding layer of the laminar plasma cladding sample had lower martensite content than that of the laser cladding layer.To some extent,this leads to corrosion resistance and wear resistance superior to those of laser cladding layers.In addition,in the bonding strength and tensile strength,laminar plasma cladding layer is superior to laser cladding layer,but the gap is not large.