Study On Material Manufacturing And Properties Of Ductile Iron For Metro Wheels

With the rapid development of metro transit,the demand of wheel also increases quickly.It is significant to develop the wheels that meet the special requriements of ‘low speed and light load' for metro vehicles.Ductile iron,combining excellent mechanical properties,good wear resistance,fair machinability and low manufacturing cost,has been used extensively in many structural application.Therefore,it is of great importance to develop new ductile iron wheels that can meet the safety,comfort and economic requrements and have superior comprehensive properties.This paper aimed to develop new ductile irons for metro wheel and wstabilished a technical route combining thermal tests of small-sized specimens and numerical simulation of full-size wheels to find out the relationship between compositon,structure and properties.Furthermore,material design and manufacturing process of ductile iron for metro wheels were studied and wear behavior of ductile iron was also explored.Firstly,chemical composition of alloyed ductile iron was designed by FactSage thermodunamics software and orthogonal experiment.the effect of Si,Cu and Ni on matrix structure,mechanical properties and dry rolling-sliding wear performance were studied respectively.The results show that matrix structure with different ferrite and pearlite fractions can be otained through an adequate balance of alloying elements,whose competitive transformation is realted to the diffusion of carbon during eutectoid transformation.Besides,hardness,yield strength and ultimate tensile strength increase whili elongation decreases until the matrix becomes fully pearlite.The main wear mechanism is delamination under air cooling condition.Based on the results of property tests,ductile iron with 80% pearlite exhibits the optimun combination including utimate tensile strength of 676 MPa,elongation of 8.6% and average wear reate of 11 ?g/m,indicating its potential for application in metro wheels.Secondly,dual phase austempered ductile iron was produced through intercritical austempering and the effects of intercritical annealing temperature,austempering time and temperature on matrix structure were investigated.Furthermore,mehcanical properties and dry rolling-slidig wear performance were also conducted.The results show that short austempering time contributes to matensite transformation while long time leads to forming cementite.The optimun austempering time ranges 20 to 60 min.Besides,austempering temperature mainly affects the diffusion of carbon and transformation kinetics of bainite ferrite.As intercritical annealing temperature increases,ausferrite and high carbon retained austenite increase while proeutectoid ferrite decreases.In addition,dual phase austempered ductile iron shows an improvement in mechanical properties with respect to alloyed ductilr iron.Lower austempering temperature or higher intercritical annealing temperature results in the increase of ultimate tensile strength and yield strength and the decrease of elongation.Impact toughness shows a maximun value at the intercritical annealing temperature of 810 oC.The main wear mechanism under air cooling condition is delamination.According to the results of propertiy tests,dual phase austempered ductile iron,with 20% proeutectoid ferrite and 80% ausferrite,exhibits the optimun combination including utimate tensile strength of 746 MPa,elongation of 14%,impact toughness of 125 J and average wear reate of 11 ?g/m,suggesting the potential of this material for application in metro wheels..Based on the above research results,alloyed ductile iron,especially for dual phase austempered ductile iron,can be the potential materials for metro wheels.Then,manufacturing process was discussed and optimized combining thermal test of smallsized specimens and numerical simulation of full-size wheels.The results show that with the help of closed-raining casting system,samll riser and chilled rion,simultaneous solidification casting process can be realized,which reduces casting defects and improve the quality of wheel castings.According to the results of thermal test and numerical simulation,prediction models of graphite,as-cast matrix structure and intercritical austempering matrix structure were estabilished respectively and microstructure distributions of full-size ductile rion wheel were predicted.It can be found that graphites are uniformaly distributed in the matrix due to simutaneous solidification casting process.Besides,the fraction of pearlite increases along with the radial direction of the wheel.Intercritical austempering matrix structure has a certain depth of austemperability and can't obtain all dual phase austempered ductile iron.The center of hub and rim show a mixture of preeutectoid ferrite,pearlite and ausfeffite,which may lead to a decrease of mechanical properties.However,it is still higher than that of alloyed ductile iron and has little effect on the actual use of metro wheels.In summary,key issues of casting and heat treatment processes of new ductile iron were investigated in this paper and two kinds of potentia metro wheel materials,such as alloyed ductile iron and dual phase austempered ductile iron,were developed.Furthermore,manufacturing process of full-size ductile iron wheel was optimiized,which provides a complete theoretical basis and experimental basis for the development and application of new ductile ductile iron wheel.