I don't think that zinc plating involves any high temperatures for
electroplating. A prep acid bath may have been part of the process, as a
cleaning step prior to plating.

If surface embrittlement had been a factor, I think cracking, probably
leading to a full break/snap would have taken place.
Most quality hex keys/allen wrenches are already fairly hard, though I would
think of the twisting failure as a feature for tighening fasteners, to let
the user know the screw was as tight as it should be.

I agree with JR, that screws this size shouldn't be torqued to the tighness
of cracking when they let loose.
I prefer to use medium-hold blue threadlocker for small fasteners.. easy
enough to remove for disassembly when required.

In high quality equipment (aerospace), the countersink angles of the screw
head and the base material could create a taper lock, but that's probably
not the case in this situation. Medium hold threadlocker would likely
eliminate a taper lock if applied to the countersink area.

All that may have happened was that the key had a soft spot, or that it was
one of the Unbrako-type keys.

I'd feel fortunate that the key twisted insead of snapping off, which can
result in a very sharp projection which fingers may have come in conact
with.
Additionally fortunate that the screw didn't require drilling to remove it.

--
WB
..........


"Joseph Gwinn" wrote in message
...
In article ,

The hex socket screw seems OK, and has been done and undone many times so
far.
If I wear the 10-32 screws out, I'll just replace them.

The Craftsman hex key is standing up nicely too.

The weak link is the old hex key. I wonder if the plating had something
to do
with it, as they may have had to heat the keys to allow hydrogen from
plating to
diffuse away, to avoid hydrogen embrittlement. The Craftsman hex keys are
black
oxide finished, as are the hex socket screws.

http://en.wikipedia.org/wiki/Hydrogen_embrittlement

Joe Gwinn