Study Of Fatigue Performance And Fracture Toughness Of Cu-alloyed ADI Treated By Two-step Austempering Process

Austempered ductile iron?ADI?is attractive material due its excellent comprehensive mechanical properties.It has broad application prospect on transmission components such as engine crankshafts and gears.Good fatigue performance and fracture toughness are important assurance of ADI.Enhancing strength,toughness and plasticity of ADI simultaneously is an effectively method to improve fatigue performance and fracture toughness.Therefore,in this paper,aiming to obtain ADI with high strength,plasticity and toughness,a novel two-step austempering process was adopt on Cu-alloyed ductile iron.The two-step austempering Cu-alloyed ductile iron?two-step Cu-alloyed ADI?was prepared.Optical microscope,scanning electron microscope,transmission electron microscope,X-Ray diffractometer,tensile test,plain strain fracture toughness test,high cycle fatigue test are carried out to investigate the effect of Cu on microstructure,fatigue performance and fracture toughness of two-step Cu-alloyed ADI.Then,on this basis,the effects of graphite morphology,quantity and matrix microstructure on the fatigue properties and fracture toughness of two-step Cu-alloyed ADI were studied.This research lays the theoretical and practical foundation for the development of low-cost,high-performance ADI.The main contents are follows:Microstructure of as-cast state Cu-alloyed ductile iron is consisted of spheroidal graphite and matrix?ferrite and pearlite?.As increasing of Cu content,the size of spheroidal graphite decreases,while the content of pearlite increases significantly.When Cu content is over 0.8%,the content of pearlite keeps constant.After two-step austempering,the matrix transforms into acicular ferrite and retained austenite.The austenite content and size increases with increase of Cu content,carbon content of austenite decreases on the contrary.Ultimate tensile strength,yield strength and impact energy initially increases and then decreases with increasing of Cu content,while elongation keeps constant.Fatigue limit increases at first as Cu content increasing from 0.2%to 1.4%,and then reduces slightly with further increasing of Cu content.Fracture toughness gradually increases with Cu content increasing.When Cu content reaches to1.4%,two-step Cu-alloyed ADI achieves best comprehensive mechanical properties.Thereinto,ultimate tensile strength is 1370MPa,yield strength is1270MPa,elongation is 6.5%,impact energy reaches to 123J,fatigue limit is505MPa,fracture toughness is 68.855MPa·m^1/2.The fatigue crack initiates at the boundary between the matrix and spheroidal graphitesurface.With appropriate Cu content,austenite content of two-step Cu-alloyed ADI is increased.Meanwhile,upper bainite and martensite transformation is suppressed.Therefore,the plasticity and toughness is enhanced significantly,which effectively improves fatigue performance and fracture toughnessOn the basis of the optimal Cu content 1.4%,effect of spheroidal graphite morphology and quantity on the fatigue performance and fracture toughness of two-step Cu-alloyed ADI were studied through regulating the nodulizer and inoculant addition.With increase of nodulizer from 0.7%to 1.5%,nodulizing grade is increased gradually from 6 grade to 1 grade.The morphology of graphite is respectively vermicular,cluster and spheroidal.With improvement of graphite shape,mechanical properties improves significantly.Best comprehensive mechiancal properties is achieved when the graphite morphology is spheroidal.Thereinto,ultimate tensile strength is 1267MPa,yield strength is 1150MPa,elongation reaches to 11%,impact energy is 102.5J,fatigue limit is 499MPa,fracture toughness reaches to 73.06MPa·m^1/2.With increase of inoculant from 0.6%to 1.6%,the amount of spheroidal graphite increases from 40 unit/mm² to121unit/mm².With increase of spheroidal graphite quantity,mechanical properties have little change.Therefore,the graphite morphology is the major factor to influence the fatigue performance and fracture toughness of two-step Cu-alloyed ADI,while effect of amount of spheroidal graphite is less.The matrix microstructure is separated by vermicular graphite,the stress concentration is strongly around vermicular graphite.Hence,fatigue crack is easily propagated and is easily divided into more fatigue crack initiations.The regular spheroidal graphite effectively decreases the stress concentration around graphite,the fatigue crack initiation is delayed.Moreover,the fatigue crack needs to propagate along the boundary of graphite and matrix at first when the graphite morphology is spheroidal.This also increases the resistance of fatigue crack initiation.The spheroidal graphite can increase the area of plasticity region at the tip of crack,therefore,the resistance of fatigue crack initiation and propagation is improved.The microstructure of matrix is regulated and controlled through adjusting the parameter of two-step austempering process.Effect of microstructure of matrix on fatigue performance and fracture toughness of two-step Cu-alloyed ADI is studied.The results indicates that the fatigue performance and fracture toughness is improved with increase of austenite content and its carbon content.The fatigue performance and fracture toughness are also improved with refinement of ausferrite.When the austenite is decomposed or carbide is precipitated,the fatigue performance and fracture toughness is dramatically weakened.A model is built between austenite content,carbon content of Fracture toughness is closely related to austenite content,carbon content of austenite and yield strength.Moreover,the fracture toughness of two-step Cu-alloyed ADI is related to austenite stability.With austenite stability decreasing,the fracture behavior need to simultaneously overcome the plasticity deformation work and phase transformation energy,which increases the fracture toughness of two-step Cu-alloyed ADI.With further decreasing of austenite stability,strain induced martensite transformation is taken place when the austenite deformation is inadequate.Therefore,the plasticity deformation work which is needed to overcome decreases.Moreover,the formation of martensite cause the internal stress increasing,which accelerates the crack propagate.Thus,fracture toughness is weakened when the austenite is excessively unstable.