Research On Fracture Mechanism And Fatigue Performance Of Ferrite Ductile Iron

The impact on microstructure, tensile performance and fatigue performance at room temperature of different pearlite content of ferrite ductile iron was studied in this paper, in order to get the micro-mechanism of the crack initiation in ductile iron, pearlite-ferrite ductile iron and ferrite ductile iron were observed in undirectional tensile test, and the influence of pearlite content and cast defects on crack initiation was analyzed.The tensile properties of different content pearlite ductile iron were analyzed, and the impact on the tensile strength,0.2%yield strength, elongation at fracture of the content pearlite in ductile iron were studied.Microscope observations of pearlite-ferrite ductile iron were performed during uniaxial tensile test, and the studies have showed crack initiated on the interface of graphite elements and ferrite matrix in the stage of elastic-plastic deformation, and the formation of crack was mainly due to debonding between graphite elements and ferrite caused by the inconsistent deformation. Crack probably initiated on the interface of graphite elements and pearlite matrix, but this more rarely happens, finally the aggregation of micropore formed the larger crack, leading to final failure.For ferrite nodular iron, crack initiated on the interface of graphite elements and ferrite matrix with plastic deformation of ferrite matrix in the stage of elastic-plastic deformation, only corresponding to very high deformation, matrix plastic deformation became evident. Stress concentrated caused by defects, micropore were developed between graphite and ferrite that near the defects in the first place, so defects only accelerated the crack initiation and propagation.The fatigue performance of different content pearlite ductile iron was analyzed in this paper, and the tensile fatigue limits of stress ratio0.1of40%,10%,5%pearlite content in ductile iron were305MPa,276.67MPa and269.00MPa acquired by up-and-down method. The tensile fatigue behaviors of40%,10%,5%pearlite content in ductile iron with mean stress were193.78MPa,194.09MPa,191.49MPa.The fatigue fracture analyzed by scanning electron microscopy, the studies have suggested river pattern mainly originated from graphite, and the direction had obvious changes when came across the graphite, namely cleavage crack of nodular iron in the process of propagation would significantly change, thus the fatigue crack propagation velocity was reduced. For pearlite-ferrite ductile iron and ferrite ductile iron, the fatigue crack initiated mainly in shrinkage cavity and inclusion defects on the edge of specimen, and the microscopic mechanism of fracture was micropore aggregation fracture, the micropore was mainly shrinkage cavity and the stripping of inclusion and graphite or substrate interface, and the fracture of the pearlite-ferrite and ferrite ductile iron was toughness fracture.