I'm going to say that it's highly unlikely that the steel alloy is at fault. Specialty steel mills produce thousands of tons of vacuum-arc-furnace processed alloy steels for use in winding large coil springs, and controlling alloy composition within tight limits is well understood.
Spring failure modes are also well understood, and if we could look closely at several samples of broken springs, we could very likely determine the root cause of the failures.
More likely contributors are:
1. A spring design that results in higher than typical stress levels, but does not result in fractures unless some other factors are present that further increase the stress levels;
2. A stress riser (small surface defect like a nick or notch) that was created during the spring winding process, or during the installation of the spring on the strut;
3. Related to
#2 would be a defect in the powdercoat paint that, in a highly corrosive environment (like winter-salted roads) results in a small, localized, but deep corrosion pit that functions as a stress riser.
4. Some defct in the heat-treating process. Depending on how the springs are quenched, the first part of the spring that contacts the quenching medium (typically oil) will see the greatest thermal shock, with slightly less shock as the rest of the spring gets submerged. If all springs were oriented identically as they were quenched, this could help explain the consistent location of the crack.
It looks like almost all the failures are occurring at first turn at the bottom of the spring. A local zone of excessive hardness caused by improper heat-treatment could explain the consistent location, Random pinhole defects in the powdercoat paint would be unlikely to occur at the same location, but a consistently-located nick in the paint could be caused at multiple points during the painting, baking, and spring installation processes.
All the photos of broken TTUE springs I've seen on the Forum show severe rusting at the point of the fracture. I have not seen any photos of broken springs from Southern or Western areas that are generally dry, and where no exposure to road salt occurs. If the root cause is a defect in the heat treatment, then springs should be breaking regardless of environmental exposure.
We need more miles on more TTUEs to be able to see a clear pattern as to what conditions contribute to fracture, and a detailed failure analysis conducted by a metallurgist at a failure analysis lab.