| In the present work, mechanical properties (strength and toughness, etc) of medium carbon silicon steel according to the prior austenite grain size and the change of structures tempered at various temperatures were studied, using primarily electron microscopy. As-quenched martensitic structure consits mainly of packets of laths, and auto-tempered precipitate (ε-carbide) could be also observed within some of laths. ε-carbide was precipitated within laths when tempered at below 300℃, while cementite begins to form first along lath boundaries tempered at 400℃ and develop in spheroidized shape at higher tempering temperature. Packet size was related to yield stress and toughness. And it was apparent for silicon to retard the softening of tempered carbon steel. Tempered martensite embrittlement occurred at around 400℃ tempering and resulted in interlath cleavage of fracture mode. Such a considerable decrease in toughness is thought to be attributed to the formation of cementite in interlath boundaries, since retained austenite in lath boundaries is thermally unstable with respect to tempering temperature. |
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