Any metallurgists on here - what's happened here?
Discussion
I’m no metallurgist but have spent the last 28 years inspecting stuff.
Cracks pretty much always have a start point, a sharp corner or a hole. A stress raiser for them to go from.
I wonder if this is cast and these are casting defects that have become more visible over time.
Sometimes an imperfect casting has surface cracking where the material has cooled too rapidly.
These may have been acceptable for the intended use, or it wasn’t inspected in the first place as they made them by the thousands.
Cracks pretty much always have a start point, a sharp corner or a hole. A stress raiser for them to go from.
I wonder if this is cast and these are casting defects that have become more visible over time.
Sometimes an imperfect casting has surface cracking where the material has cooled too rapidly.
These may have been acceptable for the intended use, or it wasn’t inspected in the first place as they made them by the thousands.
Interesting article about it here.
https://web.archive.org/web/20110716065232/http://...
Seems soaking it in white vinegar can stop the rot for a while.
https://web.archive.org/web/20110716065232/http://...
Seems soaking it in white vinegar can stop the rot for a while.
k Mazak machine tools had to do with metal rotting 
It is possible the wheel could be a zinc alloy as these were easily cast in complex shapes by die casting, pressure die casting etc. Zinc alloys can suffer Mazak Rot due to impurities. Other non zinc alloys also suffer similar problems due to impurities eventually causing problems.
Some alloys improve with age as they “age harden” due to microstructural changes over time.
Even stainless steel can have similar problems. “ Weld decay” can result in ss welds unless a correct filler metal alloy is used. Titanium, (in small quantities) is one such solution. It preferentially scavenges the low level of unwanted carbon away from the chromium. Chrome carbide forms a galvanic corrosion cell at the microstructure level if allowed to form. Hence the titanium, which has greater affinity for carbon than chrome so grabs it first.
These days plastics have replaced die cast metal for many components such as gears. Cheaper but can limit the life if in higher load environments.
Some alloys improve with age as they “age harden” due to microstructural changes over time.
Even stainless steel can have similar problems. “ Weld decay” can result in ss welds unless a correct filler metal alloy is used. Titanium, (in small quantities) is one such solution. It preferentially scavenges the low level of unwanted carbon away from the chromium. Chrome carbide forms a galvanic corrosion cell at the microstructure level if allowed to form. Hence the titanium, which has greater affinity for carbon than chrome so grabs it first.
These days plastics have replaced die cast metal for many components such as gears. Cheaper but can limit the life if in higher load environments.
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