The fracture toughness of two experimental silicon-containing steels in the bainitic condition has been measured and related to the microstructural state of the steels. The optimum bainitic microstructure for high strength and high toughness combinations consists of bainitic ferrite and thin interwoven laths of retained austenite instead of cementite, this condition being achieved through the silicon addition to the steels. The thin films of retained austenite are thermally and mechanically stable and act to reduce the effective fracture grain size and also possibly help to blunt propagating microcracks; blockier volumes of retained austenite are unstable and hence not beneficial to toughness. The two experimental steels achieved strength and toughness values equal to, or better than, some commercial steels in the martensitic condition.
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