Technically, the cracks discussed in part 1 are quench cracks because they showed up after quenching. If we had looked at the crack surface with a scanning electron microscope, presumably it would have looked something like Figure 3. But if there had been an oxide seam that promoted the crack, it might not have had such nice, sharp features. And the cross section including the crack profile would have looked more like Figure 4 than Figure 5. Figure 5 is actually probably corrosion damage because a quench crack would not usually have multiple continuous layers of loose grains. But I could not find a better cross-section image of an intergranular crack profile.
Figures 6 and 7 show features on another cylindrical part: a row of closely spaced flat-bottom holes in a protruding flange. I believe this is a true, classical quench crack. The thin ligaments between the flat-bottom holes probably would cool much more quickly than any other area on this part. Once the ligament cracked, the crack kept propagating into the thin, flat bottom of the holes.
Or maybe the thin flat-bottomed holes were cooled rapidly enough to crack on their own. I haven’t figured out what the direction of the crack would be if the hole bottoms cracked first. But note that there are cracks coming from both directions toward the center of the hole. This leads me to believe the thin ligaments between the holes cracked first. The way to figure it out would be to open the cracks and look at the crack surface features. However, this was another low-budget (no-budget) job donated to my “museum of failure!”
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