I didn't see anyone else answer this directly so I will say what I know or
at least what I think I know.
For modern bolts you would buy from a hardware store, this is what you see.
Grade 2 or lower- no marks on the head- avoid these if you can unless you
don't care if it breaks.
Grade 5- three radial dashes- one at every other point on a hex head.
Grade 5 bolts have a minimum tensile strength of 120 kpsi (1000 lb/in2)
Grade 8- six radial dashes- one at every point on a hex head. 150 kpsi
minimum strength.
For other types of fasteners there is no standard marking system that I am
aware of so, unless the quality of the bolt is clearly stated, assume the
worst.
Although some may argue, I don't consider a grade 5 bolt to be hardened.
You can cut one with a bolt cutter or a hacksaw. Grade 8, on the other
hand, will destroy a bolt cutter (or nut splitter) and the hacksaw works
poorly. On the other hand, grade 8 bolts don't rust as bad so the is less
chance of needing brute force.
There are several types of stainless steel hardware available. Most of the
stuff you see at a hardware store is pretty junky. 303 or 304 stainless is
a good grade and at 85 kpsi approaches grade 5 for strength. 316 stainless
is about as good as you can get for corrosion resistance but it probably
isn't worth the price above 303 or 304. The harder a stainless steel gets
(i.e. 400 series), the less "stainless" it becomes. Stainless fasteners
are often called "18-8". This is another name for 300 series stainless and
refers to the percentage of chromium and nickel in the alloy.
If you want to know more there is a decent web page:
http://www.rockfordinternational.com/html/profile.html
It is not real in depth but that is where I got the info above. If you
really want to know about what materials to use, pick up a copy of the
Machinery's handbook. I guarantee that you will learn something and be
entertained for hours.
For those of you who must have simply the best, Unbrako bolts are rated at
180 kpsi. Also there are many exotic material such as boron alloyed bolts,
Elgiloy, MP35N, and others for specific needs. MP35N, for instance, rivals
Grade 8 for strength while having excellent corrosion resistance. However,
it is nearly impossible to machine and, if it is exposed to certain
temperatures for too long, it can also go brittle quickly. Neat stuff
though. I expect that an MP35N bolt would be quite spendy.
Regards,
Bill Eastman
61 MGA with some titanium hardware because I had it lying around.
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