Not that it matters very much, but I have to disagree that the Grade 8
bolt will fail "suddenly"....although it certainly is less ductile
than the Grade 5, it is not terribly so. Onset of yield for the Grade
8 is at 130 kpsi, with tensile failure expected at 150 kpsi. By way
of comparison, the Grade 5 yields at 92 kpsi, with tensile failure at
120 kpsi. The Grade 8 will also deform prior to failure, not just
give a brittle snap. The primary difference between the two is that
Grade 8 yields and fails at higher stress, not that the Grade 8 is
significantly more brittle (although, as I mentioned, it is somewhat
more brittle than the Grade 5). The good news for us is that we'll
probably never even see yield loads for the bolts on our cars if we
use Grade 5, excepting of course impact loads which nobody likes to
think about anyway.
Both grades are heat treated and tempered at a minimum 800 deg F temp,
quenched in oil or water (Grade 5) or oil only (Grade 8). Chemical
composition varies slightly between the grades in sulphur and
phosphorus. Grain structure is specified more closely for Grade 8.
Fascinating tech trivia? Not really. The short answer has already
been given: you're probably ok using either grade. I personally use
Grade 8 where the loads look high to me, and whatever's laying around
for other locations. I do like the Grade 8's finish on the bolts I
buy locally, but that's a manufacturer's option and not a specified
requirement for the grade. The best advice is to use what was there
originally unless you have some compelling reason to dispute the
designers of the car. Aircraft grade fasteners are overkill for the
street and would only make sense on a raced vehicle, or to satisfy
some compulsion to gild the lily...... Gold-plated Lucas bullet
connectors, anyone?
MD
78 Midget
---Steve Byers <byers@cconnect.net> wrote:
>
> Ulix -
> Both Grade 5 and Grade 8 fasteners are made of carbon steel, with
Grade 5
> being specified as "carbon steel", and Grade 8 as "medium carbon
steel" (
> I don't have the specific composition). Grade 8's are heat treated
to a
> higher strength than Grade 5's, and the increase in strength is
accompanied
> by a decrease in ductility, which is generally true for heat-treatable
> materials.
>
> If you load-tested a Grade 5 bolt and a Grade 8 bolt of equal sizes,
you
> would see both of them perform the same up to the yield strength of
the
> Grade 5. Then the Grade 5 would fail plastically (i.e., permanent
stretch)
> and fracture at a higher load. The Grade 8 bolt would continue up
to a
> higher yield load than the Grade 5, of course (it's stronger), but
when it
> failed it would do so suddenly.
>
> I think for most of our automotive applications, Grade 5 is
completely
> adequate, and Grade 8's aren't going to fracture even though they
are less
> ductile (because they are never going to see their ultimate load
anyway).
> Grade 8's are more expensive, so take your choice.
>
>
> Steve Byers
> Havelock, NC USA
> '73 Midget GAN5UD126009G "OO NINE"
> "It is better to remain silent, and be thought a fool
> than to speak, and remove all doubt" -- Mark Twain
>
>
> ----------
> > From: Ulix Goettsch <ulix@u.washington.edu>
> > To: Steve Byers <byers@cconnect.net>
> > Cc: Lancer7676@aol.com; ckotting@iwaynet.net; carlson@navtech.com;
> spridgets@autox.team.net
> > Subject: Re: Sway bar mounting bolts
> > Date: Friday, September 25, 1998 2:20 PM
> >
> > Steve,
> >
> > My question here is if a grade 8 fastener is stronger
> > because it has been heat treated and is thus more brittle, or
could it be
> > that it is really made of a more expensive alloy that is stronger
but has
> > a comparable ductility to a grade 5 fastener.
> > I have no basis for this theory, but I wouldn't be surprised if
the story
> > of the brittle grade 8 bolt is an urban legend...
> >
> > Ulix
> >
> >
> >
> >
>
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