Effect of Fatigue Damage on the Dynamic Fracture Toughness of En-8-Grade Steel

摘要

Machine components normally experience fatigue cycling during operation. Failure of these components is mostly due to fatigue. So, it is important to know the fatigue damage behavior and fatigue life of the material before selecting these steels for making different machine components. The En-8-grade (equivalent to SAE/AISI 1040) steel is generally used as a machine component in the annealed or hardened-and-tempered condition. The fatigue life (fatigue/endurance limit) is also dependent upon the tensile properties of any material. By suitable heat treatment, one can manipulate the tensile properties of any steel. The present work reports the effect of fatigue damage in En-8-grade heat-treated steel (annealed and hardened and tempered), under different cyclic loading conditions at room temperature (25 deg C), on the impact and dynamic fracture-toughness properties. The results indicate higher fracture toughness and impact toughness in hardened-and-tempered steel than in annealed steel. Cyclic hardening and softening occurs in both the hardened-and-tempered as well as the annealed steel. With the increase of peak stress and number of fatigue cycles, the K_(ID) and CVN values decrease in hardened-and-tempered steels. The results are discussed in terms of dislocations, slip bands, and their density, microstructure, and fracture morphology.