Effect of Nb/V Alloying on the Microstructure and Mechanical Properties of SiMo Ductile Iron

Three kinds of SiMo ductile iron samples with Nb/V alloying were prepared by the sand casting method. The microstructure and mechanical properties were evaluated via optical microscopy (OM), scanning electron microscopy (SEM) and an electronic universal testing machine. The metallographic analysis showed that Nb/V addition had a negligible effect on the graphite morphology and nodularity of the SiMo ductile iron, but the pearlite content increased and the size of the ferrite decreased. There are many Nb/V-rich carbides inside the ferrite grains and at the interface of the pearlite in ductile iron containing Nb/V. The tensile testing results showed that the SiMo ductile iron sample with 0.17% V addition had better mechanical properties at room and high temperatures. For SiMo ductile iron, the tensile strength (Rm), yield strength (Rp0.2), elongation after fracture (A) and Brinell hardness at room temperature are 631 MPa, 442 MPa, 8.9% and 222 HBW, respectively. The Rm, Rp0.2 and A at 650 degrees C were 202 MPa, 131 MPa and 21.8%, respectively. The better high-temperature performance of the sample containing 0.17% V is mainly attributed to the ability of V element to promote the precipitation and refinement of carbides, which can effectively improve the high-temperature strength of the matrix.