《Table 1:Shape factor change in different parts》

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《Microstructure and properties of ductile iron bars for plunger pump prepared by horizontal continuous casting process》

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Figure 9 shows the longitudinal section lateral surface microstructure of fracture of tensile and torsion samples.It can be seen that the crack propagation paths of both fracture openings are two types.One is along the ferrite boundary surrounding the graphite nodules，and the graphite nodules are well preserved，that is，the tensile or torsional fracture of the surface materials of the HCCDIBs，as shown in Figs.9（a1）and（b1）.The other mode is the transverse crack extending through the graphite nodules，as that at the 1/2 R and in the center of HCCDIBs shown in Figs.9（a2），（a3） ，（b2） and（b3）.It is obvious that the graphite nodules at the fracture are cut or pulled out，leaving a semi-closed ferrite ring of bull's eye graphite.These two types of cracks originate from the holes formed by the graphite nodules are debonding from the matrix，both of them can lead to material fracture[14].Under the action of a shear force，tiny pores in the necking zone develop into cracks.This type of expansion occurring at the graphite-ferrite interface usually requires less energy and the crack propagation in the necking zone is relatively easy[17].When the crack propagates across the cut graphite spheres，it easily forms a large crack because the graphite spheres are close to each other.In addition，it can be seen from Fig.9 that some graphite spheres near the edge of the fracture appear oval in the tensile direction or torsional shear direction due to the great hydrostatic pressure[18].A slight decrease in the average shape factor F can be seen as in Table 1.This is mainly because there are many crystal defects in graphite nodules in ductile iron，mainly the small-angle tilted grain boundary，and the grain boundary is movable and is rarely restricted in movement[17].Therefore，when the load exceeds the elastic limit，graphite nodules will undergo obvious deformation with the increase of the load.In addition，due to the graphite nodules being surrounded by ferrite，the ferrite space surrounding the graphite nodules is spherical，and the BCC structureα-Fe can easily undergo a plastic deformation along the direction of deformation under tensile stress or torsional shear stress，so theα-Fe wrapped graphite nodules can also undergo a collaborative deformation without cracking.