Microstructure and low-temperature impact fracture behavior of QT400-18AL containing Ni

Abstract

The effects of Ni content on the microstructures and low-temperature impact fracture behaviors of QT400-18AL ferritic ductile cast iron were systematically investigated by XRD, SEM, EBSD, and low-temperature impact test technique. The results demonstrate that the average diameter of graphite spheres and HAGBs increase with the increase of Ni content, whereas the nodule count, the crystal lattice constant and average grain size of the ferrite phase decrease. Additionally, the UTS, YS, and hardness of the QT400-18AL at room temperature increase with the increasing Ni content. The Ni-0.75 cast iron has the best room temperature mechanical properties with UTS of 391 MPa, YS of 253 MPa, and hardness of 140 HB. The average impact energy of the QT400-18AL decreases with the decrease in test temperatures and increase with the increase in Ni content at the same test temperatures. The fracture morphologies show that the ductile fracture zones increase with the increase in Ni content at the same impact temperature. Moreover, they decrease with the decreasing impact temperatures at the same Ni content, and the fracture types change from ductile fracture to ductile-brittle mixed fracture. The Ni-0.75 cast iron exerted satisfactory impact performance at −40 °C owing to the optimized combination of GBCD and dislocation density (KAM value), and its low-temperature impact energy was 12.64 J.