Effect of martensite morphology and volume fraction on the low-temperature impact toughness of dual-phase steels

Abstract The influence of martensite morphology and volume fraction on low-temperature impact toughness of dual-phase (DP) steels was investigated by impact tests, nanoindentation tests, atom probe tomography (APT) and microstructural examination. The APT results of medium-C martensite show that many small-sized C clusters are formed at the twin grain boundaries inside the lath, which increase nanohardness and decrease impact toughness. The impact test results indicate that the low-temperature impact toughness of lath martensite is significantly superior to ferrite. Refining the lath martensite substructure can improve the low-temperature impact toughness of DP steels. However, martensite twins obviously reduce the low-temperature impact toughness of DP steels. It is further found that the impact toughness of DP steel is affected by the toughness of ferrite and martensite, and conforms to rule of mixtures. Finally, the low-temperature impact fracture mechanism of DP steels was observed, and it is found that the large-size ferrite grains in DP steels preferentially occur cleavage fracture. As the impact temperature decreases, the fracture mode of DP steel changes from dimple plus quasi-cleavage fracture to brittle cleavage fracture.