FRACTURE CHARACTERISTICS OF 475 degree C EMBRITTLED 30%CR FERRITIC STAINLESS STEELS.

Intergranular microcracks are initiated when yield stress increases with aging over the initial twinning stress detected on the stress-strain curve. The unstable fracture occurs with cleavage crack propagation when the criterion for propagation is satisfied. The critical yield stress for the ductile-brittle transition is slightly higher than the initial twinning stress, which varies linearly with the reciprocal square root of grain diameters and is insensitive to the aging temperature. For specimens with coarse grain sizes which have low initial twinning stress, the embrittling time is affected by the hardening behavior in the early stages of aging.