Fracture toughness is one of the most critical material properties determining the lifetime of tools and machine parts as well as their in service safety. A material class known to obtain potentially very high fracture toughness are nanostructured carbide free bainitic steels which have a microstructure comprised of very fine bainitic ferrite and film like retained austenite. High values of fracture toughness have been attributed to the presence of retained austenite which is a metastable phase undergoing stress/strain induced transformation to very hard martensite which exerts a crack blunting effect. However high retained austenite is known to be detrimental to the steels impact toughness and promote notch sensitivity. To improve the materials resistance to impact loading KAB steels have been developed which transform all the retained austenite into bainite during prolonged natural aging. These steels have shown considerable improvements in impact toughness however the behaviour of this microstructure in the presence of a fatigue crack is not fully understood and has been evaluated in the current work.