Stainless Grade Equivalents

Bob

McMaster-Carr. mcmaster.com

Bob AZ

A search for "grade 8 stainless" on their site only shows hex nuts. In the suggested search items it shows 18-8, but does not say if that is the same strength as Grade 8 bolts.

It's not. Not even close.

I think that a Grade 8 in stainless is one very special bolt -- either a

I did find some reference in a UK welding group that seemed to indicate that A4-80 was close, but the only bolts I've found in that so far are metric.

Aha. Here's a precipitation-hardening bolt (17-4 PH) that claims to be as strong as Grade 8:

There's also a standard for them -- ASTM F593 -- that relates the strength of stainless fasteners to those of carbon and alloy steels.

You can get 17-4 PH stainless fasteners from McMaster that have approximately the same tensile strength as grade 8 screws. Only $7.06 _each_ for a 3/8-16x1 hex head cap screw. Even MIL spec 18-8 and 300 series stainless fasteners have approximately the same tensile strength as grade 2 fasteners. There's probably nothing wrong with the "Hillman crap."

IIRC A4 is 316 and those fasteners won't be anywhere near a grade 8. I've seen someone recently confuse metric 8.8 designation with grade 8, a big difference. An 8.8 fastener is about the same as a grade 5.

LOL. All Hillman stuff unless labeled otherwise is grade 2 or worse. (mostly a lot worse) As somebody who has been shooting screws my entire life for various applications I never saw so many broken screws until Hillman started dominating the market with their "we will fill it for you" marketing. Its easier for resellers, but point blank package screws were a lot better quality 20 years ago.

Ok.... now I have my two grades. LOL. 316 is about Grade 5, and 17-4 is about Grade 8. I am guessing the 316 would be full hardness to make grade 5 since it's a work hardening spring steel stainless.

Thanks guys.

A typical 316 SS screw has a tensile strength comparable to a grade 2 carbon steel fastener, around 70,000 psi.

So are you saying that 18-8 which is a the most common stainless hardware is even lower strength than grade 2?

Now according to this chart 18-8 is about the same as grade 5.

Sigh. Back to square one.

This person claims some, but not all 316 bolts are close, but slightly less tensile strength than grade 5.

I'm saying I don't think you can count on common 18-8, 304, or 316 fasteners to be any stronger than grade 2. Individual lots or samples may be either side of grade 2's, but absent some sort of spec beyond a simple material designation, there are no guarantees.

"18-8" is a generic name for basic austenitic stainless steel -- something like 303 - 304, but not necessarily. It is not a technical specification, only an indication that it contains 18% chromium and 8% nickel. The rest of the alloy is up for grabs.

When you see 18-8 used to describe kitchen utensils or flatware, you know only that it's basic stainless. If you see it used to describe fasteners or other technical items, you know either that (1) the seller doesn't know which way is up; (2) the seller doesn't know what the hell the material is; or (3) the seller knows what it is, but it's so bad that he doesn't want to tell you.

You can't heat-treat 316, except to anneal it.

Hi-strength stainless bolts are made from heat-treatable grades, either

Thanks. Sometimes a "Grade 8" is the bolt for the job, and sometimes a "Grade 5" is better. I was hoping to find an equivalent for both, but that seems to be not practical. Grade 2 is rarely the best choice except as a shear bolt on farm equipment. Looks like I have the choice of going with quality (ARP) or unknown when it comes to stainless but no other way to grade.

Stainless presents one issue that may or may not be important in a given application. The issue is elongation; when weaker bolts are better in an application, it's usually because their greater elongation -- their ability to stretch, plastically, or to bend -- is important because of the nature of the load.

Austenitic stainless -- the 300 series -- is not very strong in terms of ultimate tensile strength, but a combination of good basic elongation, combined with pretty extreme work-hardening ability, produces a very big spread between yield strength and ultimate tensile strength. This can be very useful in those applications where a bolt may be overloaded and should yield, rather than break. A slightly bigger bolt of a weaker material may substantially outperform a smaller but higher-strength bolt.

The hardenable grades of martensitic (400 Series) and precipitation-hardening stainless *can* have very good elongation, but you have to check the specs on specific grades if it's an important issue for you. The elongation of regular alloy steel bolts is well-known and widely published, if you want to compare them.

Otherwise, it's mostly a matter of how much money you want to spend for the sake of corrosion resistance.

agreed.

compare any old stock box of fasteners to what you get a home depot these days.

Hell, even even for bulk wood bolts, I have to hand sort the stuff a high percentage is just deformed.