Effect Of Pre-fatigue On Mechanical Properties And Microstructure Of QT400 Ductile Iron

Nodular cast iron is one of the metal materials widely used in production and life.Research on its fatigue property is helpful to timely maintenance and replacement of the material in application,so as to ensure the safety of production and life.In this paper,the mechanical properties and microstructure of QT400 ductile iron under pre-fatigue condition are studied in detail.The mechanical properties,fracture mechanism and internal dislocation structure of QT400 under the pre-fatigue condition of fatigue life fraction D(=N_i/Nf)were analyzed by means of scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The experimental results show that the yield strengthσ_s of the sample with fatigue life fraction D of 5%,20%and 50%is 328.5 MPa,229.5 MPa and 380.8 MPa,respectively,which is 24.4%higher than that of the sample without pre-fatigue(D is 0%)(σ_s is 264.0MPa).Then decreased by 13.1%,and finally increased by 44.2%;The fatigue life fraction D is 5%,20%and 50%respectively.The tensile strengthσ_b of the pre-fatigue sample is353.6MPa,269.1MPa and 436.1MPa,respectively.Compared with the non-pre-fatigue sample(σ_b is 390.7MPa),the tensile strengthσ_b of the pre-fatigue sample decreases by 9.5%and 31.1%first.Then it increases by 11.6%.The fatigue life fraction D is 5%,20%and 50%,respectively.Theδof the pre-fatigue sample after breaking is 11%,10%and 15%,respectively.Theδof the unpre-fatigue sample after breaking is 13.4%,which firstly decreases by 17.9%and 25.4%,and then increases by 11.9%compared with the unpre-fatigue sample.According to SEM observation of tensile test fracture,the characteristics of quasi-cleavage fracture and micropore aggregation fracture exist in tensile test fracture of specimens with different fatigue life fraction D after pre-fatigue test and without pre-fatigue test,and the characteristics of quasi-cleavage show an increasing trend after pre-fatigue.After the pre-fatigue test,the dislocation aggregation caused damage to the matrix due to the movement towards the junction of the matrix and the graphite spheres or inclusions of the second phase.The average diameter of the dimple at the tensile fracture of the pre-fatigue specimen increased compared with that of the specimen without the pre-fatigue test.TEM analysis showed that the structural changes of QT400 dislocation and casting defects caused the changes of yield strength,tensile strength and elongation after fracture.Compared with non-pre-fatigue specimens with fatigue life fraction D of 5%and 50%,the yield strength increases due to the increase of dislocation density.Pre-fatigue specimens with 5%dislocation gathered in the veins of the parallel structure and channel structure,dislocation at grain boundary plug product reciprocating motion capture each other appear dislocation dipole.When D is 50%,dislocation density increases,and dislocation entanglement occurs at the junction of grain boundaries,second phase graphite pellets or inclusions and matrix,leading to an increase in yield strength.D is the presence of inclusions and shrinkage porosity and other casting defects in the matrix structure of 20%pre-fatigue samples,which are prone to form stress concentration in the process of pre-fatigue experiments,causing huge damage to the matrix,resulting in the decrease of yield strength,tensile strength and elongation after fracture of QT400.