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A study has been made of stress-corrosion fracture surfaces produced in a Type 310S and a Type 304 steel tested in boiling aqueous magnesium chloride (154°C). During cracking, small load pulses were periodically superimposed onto an otherwise constant load, and, in the case of transgranular cracking, these produced markings on the fracture surfaces that delineated the instantaneous position of the crack front. A one-to-one correspondence was observed between these crack-front markings and the number of load pulses for time intervals between pulses exceeding 15 s. For smaller intervals, there were fewer markings than pulses. Moreover, the distance between the markings did not decrease to zero as the intervals decreased below 15 s but attained an approximately constant value. These observations are considered to indicate that transgranular cracking is a discontinuous process and that crack advance involves brittle mechanical fracture. No crack-front markings were detected on intergranular stress-corrosion surfaces, presumably because the crack tip is normally blunt in this case. The authors suggest that intergranular stress-corrosion cracking in this system occurs by continuous anodic dissolution at the crack tip, probably by the film-rupture model.
austenitic stainless steels, stress-corrosion cracking, fractography, materials, materials science
research assistant, Lehigh University, Bethlehem, Pa.
a metallurgist, National Bureau of Standards, Washington, D.C.,