Figures 1 and 2 show cylindrical parts that were apparently intact prior to hardening heat treatment (austenitize, quench and temper).
Both have fairly severe geometrical notches. The part of Figure 1 has a small tab protruding in a radial direction (middle arrow). This tab would cool much faster than any of the surrounding material. The client did not want to spend any money, but they should have because I was younger and might not have noticed then what I noticed now.
The part was water quenched because they also did not want to spend money on unnecessary alloys. Why did THIS ONE (and the others in this batch that cracked) have a problem? I don’t know.
Maybe the alloy was a little richer (a little more copper residual?). Maybe the quench water was a little cooler. Maybe there was a seam or other discontinuity. I think that is likely because there are multiple steps, including one right above the tab, that seemed likely crack promoters, but there were no cracks.
And look at Figure 2. Again, we have the thread forms that make a natural notch, running circumferentially the entire length of the part. I have had this part for years but went to dig it out because apparently quench cracks are a hot topic at Industrial Heating. Again, as I was taking the photos for this blog, I decided to use a black Sharpie to highlight the depth of the crack on the cut surface. But when I went to wipe off the extra ink, I found a subsurface discontinuity (arrow). Maybe that explains the radial crack.
Check back later for part 2 of this discussion.