Page 414 - Failure Analysis Case Studies II

398

Fig. 4. (a) Optical micrograph and (b) SEM photograph showing microstructures featuring typical fibrous nature
comprising elongated ferrite and cementite lamella. The cracks along the drawing direction can be observed in (a).

test), and that under a slow strain rate test (such as in a standard tensile test) would show a
significant difference in toughness. The impact toughness results from both as-received and baked
specimens are shown in Table 1. It may be noted that although the specimen geometry was non-
standard for CVN toughness tests, the impact toughness values from identical specimens would
be useful for making comparisons. It was observed that both the lots showed an average value of
33 J under identical test conditions.
The nature of the stress-strain plots does not show any evidence of strain ageing. Figure 6, and
also Table 1, show the tensile properties of the baked as well as the as-received samples. A
noticeable increase may be observed in the % elongation and yield stress of the baked specimens,
suggesting hydrogen embrittlement to be operative in the as-received wire. A quantitative estimate
of the residual hydrogen in the wires could not be attempted because atomic hydrogen would