Synopsis: A study has been made of the true fracture ductility in static tension at room temperature of a 0.42% C-Ni-Cr-Mo steel having mixed structure of martensite and bainite. It was found that the lower bainite of low volume fraction associated with lower-tempered martensite, had a very detrimental effect on the ductility. This could be due to the fact that crack initiation occured by brittle-fracture of bainite parallel to tensile axis as a result of higher plastic restraining the bainite

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... y martensite during plastic deformation. When the upper bainite associated with the tempered martensite, a detrimental effect was found on the ductility regardless of volume fraction of bainite and tempering conditions. The detrimental effect results from the fact that, in low volume fraction of the bainite, crack initiation occurs by brittle-fracture of the bainite, and that with increase in the bainite, it occurs by brittle-fracture of martensite, it being attributable to higher stress concentration in the vicinity of two-phase interface as a result of unequal strain of two-phase at an earlier stage of plastic deformation.