Impact Toughness and Fatigue Crack Propagation in Carbide-Free Bainite: The Adverse Role of Retained Austenite and Martensite-Austenite Islands

The deformation behavior of carbide-bearing bainite (CBB) and carbide-free bainite (CFB) under various loading conditions is investigated. Retained austenite (RA) in CFB enhances strength and ductility only under uniaxial tension through gradual strain-induced martensitic transformation, yet deteriorates impact toughness and fatigue crack resistance. CBB, however, shows superior impact toughness and stronger resistance to crack propagation with effective fatigue crack deflection at prior austenite grain boundaries (PAGB). Martensite-austenite (MA) islands and unstable austenitic constituents induce intergranular fracture in CFB by forming a brittle network along the PAGBs when combined with localized deformation. Furthermore, weakened PAGBs fail to provide an effective barrier against transgranular fracture. In this case, rapid PAG debonding through MA islands leads to cleavage fracture in Charpy tests, whereas ductile fracture occurs in crack propagation tests. These results highlight the need to carefully design CFB microstructures, particularly RA, to achieve optimal mechanical performance for specific applications.