Effect of as cast microstructure on tensile properties of Austempered Ductile Iron

The percentage of pearlite in as cast ductile iron with MS = 0.25 cm, Ms = 0.50 cm, Ms = 0.75 cm, Ms = 1.00 cm and Ms = 1.25 cm is 73.7%, 22.6%, 7.4%, 6.6% and 6.0%, respectively. Therefore, the quantity of pearlite in as cast ductile iron can be indirectly characterized by casting modulus Ms. Several tensile test bars with wall thickness of Φ 7cm were cut from ductile iron castings with different as cast microstructure, and were Austempered at 920 ℃ / 2H + 280 ℃ / 1.5h.

Although the tensile strength RM, elongation A and area reduction Z of as cast ductile iron with modulus Ms = 0.25cm are lower than those of as cast ductile iron with MS = 0.50cm and Ms = 0.75cm after isothermal transformation, the yield strength Rp0.2, yield ratio and elastic modulus E are relatively higher. Among them, Rp0.2 can reach 1268.8 MPa, yield strength ratio can reach 0.96, e can reach 117.1 MPa. This shows that ADI obtained by isothermal transformation of MS = 0.25cm ductile iron has strong resistance to elastic deformation, is not easy to occur plastic deformation, and has good rigidity and reliability. However, it is found that the strength and toughness of ADI obtained by isothermal transformation of as cast ductile iron with MS = 0.50cm and Ms = 0.75cm is relatively better than that of as cast ductile iron with MS = 1.00cm and Ms = 1.25cm after isothermal transformation.

The main reason is that, on the one hand, the amount of pearlite in the as cast structure of MS = 0.25cm ductile iron is the most, the nodularization rate of graphite is the highest, and the number of graphite balls is the largest, which makes the interface area of α – Fe / Fe3C and graphite ball / matrix which can be nucleated by austenite is the largest, and the size of high temperature austenite grain is the most and the smallest, Ms = 0.50cm The as cast microstructure of nodular iron with MS = 0.75cm, Ms = 1.00cm and Ms = 1.25cm is the second, while the amount and size of high temperature austenite grains are less and larger after austenitizing. The finer the grain size of high temperature austenite is, the smaller the ferrite size is after austempering, and the higher the tensile strength of ADI material is.

On the other hand, the carbon content in high temperature austenite of nodular iron with different modulus is different due to the difference of as cast structure. After holding at 920 ℃ for 2 h, the carbon content in high temperature austenite of MS = 1.25cm ductile iron is the highest, and decreases gradually with the decrease of nodular iron modulus. The lower the carbon content in high temperature austenite, the lower the carbon content of untransformed austenite in ADI matrix after austempering. Under the action of applied tensile stress, the retained austenite with lower carbon content will partly undergo γ → m transformation, and the strength of M is higher than that of γ, thus enhancing the tensile strength of the material. In addition, the spheroidization rate and spheroidization number of graphite in as cast ductile iron with MS = 1.25cm are the lowest, and with the decrease of MS, the spheroidization rate and spheroidization number of graphite increase. When ADI material is subjected to tensile stress, the lower the spheroidization rate of graphite is, the easier the crack begins to crack at the grain boundary of graphite ball, resulting in the decrease of tensile strength. To sum up, the number of pearlite, the number of graphite spheres and the spheroidization rate are the key factors affecting the mechanical properties of ADI.

It should be noted that the tensile test bar is cut from the thin-walled ductile iron test bar, and the tensile property of the isothermally quenched ductile iron obtained is worse than that of the sample obtained from the conventional single cast or attached cast ductile iron test block, but it does not affect the variation of the tensile property with the process factors.

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