On Wed, 20 May 1998, Gano, Ken wrote:
> Would powder coating the heads of bolts (400 degrees F., 15 minutes) have
> any adverse effect on the strength of grade 5 or grade 8 bolts? I wouldn't
> think so, but am just curious.
Well, according to my little reference book, grade 5 bolts are heat
treated Med. Carbon steel, and grade 8 are Alloy, Med. Carbon. Also from
the same book, 420 degrees F. is the heat at which a light temper forms.
So I think it may be possible to lose the temper on a high-grade bolt by
heating it.
My vote goes for testing it! (unless it's too expensive)- take 2 grade 5
bolts, heat one, and bend/break/mangle them both. If it's easy to
overtorque the heated one, then you can be assured that the heat buggers
them up.
If you are powdercoating at home it may also be useful to set an accurate
thermometer inside so that you can monitor the actual heat next to the
part in question.
ISTR that making the metal acheive a temper involves heating it up to
cherry red, then quenching in water (or letting it air-cool). Then
polish, cut, do-whatever-needs-to-be-done, then reheat the (polished)
metal until a color appears, ie. blue. Then quench again in water and
hey-presto, the metal is tempered.
To remove the temper, the metal is again heated to cherry-red and cooled
in air. To make the metal brittle, heat to cherry-red and quench in water
(hardened too much)
If the bolts are case-hardened then they are chucked into a high-carbon
compound after being heated to cherry red. The cooling process robs the
high-carbon compound of some of its carbon, which gets into the steel,
thus hardening it. So therefore the only hardened part is on the surface.
(NB I could be really really wrong here, so someone correct me if so!!)
Case hardening is perhaps the cheapest, and least effective forms of
hardening steel.
-Malcolm
Babylon Anonymous
I work for a retired metallurgical engineer...
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