Abstract
Ultrahigh-strength low alloy steels transformed below the martensite start temperature (MS) with a mixed microstructure consisting of lath martensite, lower bainite, and minor amounts of retained austenite are of interest in a variety of potential commercial applications as they exhibit good combinations of strength, ductility, and toughness. However, their use requires a knowledge of the resistance to crack propagation. Analysis of the microstructure and fractography show that it is possible to distinguish the fracture behavior of steels austempered below MS from those quenched and tempered. The sharp-notch plane-strain fracture toughness KIC has been determined by the notched tensile testing and modeled with different expressions from literature. Based on pioneer works of Irwin (1962) [20] and Naylor (1979) [22], KIC associated with austempering below MS has been modeled with the aspect ratio of the microstructural subunits and monotonic plastic zone size ahead of the crack tip. A comparison between existing expressions and the new one is conducted. It is found that modeling with the new expression delivers the best agreement with the experimental values.
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