Effects of Holding Time on Microstructure and Mechanical Properties of QT500 Ductile Iron during Low Temperature Annealing

The ferrite-pearlite mixed matrix nodular cast iron(QT500) was annealed at 740 ℃ and held for 2 h, 4 h and 6 h, respectively.The effects of holding time on the microstructure and mechanical properties of the nodular cast iron samples were detected by metallography microscope and Vickers microhardness tester.The results show that with the increase of holding time, the decarbonization phenomenon occurs in ductile iron specimens, and the matrix structure gradually changes from ferrite-pearlite mixed matrix to ferritic matrix, and the ferrite grain coarsening phenomenon occurs.The maximum hardness of nodular cast iron is 197 HV after holding for 2 h.When the holding time is prolonged, the hardness begins to decrease gradually.When the holding time is 6 h, the hardness of nodular cast iron specimen is 138 HV, lower than that of original nodular cast iron specimen 148 HV.Therefore, ductile iron materials with desired properties can be obtained by changing the holding time in the low temperature annealing heat treatment.