Age Challenged Rivets

New builder here (just starting an RV8/RV8A)--have a question for the experienced builders. I have lots of rivets (AN470 and AN426) that I bought 15+ years ago for a previous project. Does age have any affect on the strength etc of these rivets? Not sure whether I could use them on a new build or if I should just toss them out.

there has been discussion from time to time about "age hardened" rivets and that these might be annealed to restore their malleability, (I believe Dave Anders, who built a beatiful polished RV-4 some years ago suggested annealing the rivets before use) but I can tell you from experience that I have used and continue to use rivets in excess of 15 years of age and notice absolutely no issues.
Rivets can work harden if the rivet gun has insufficient pressure (too may stroke required to set) but that is a different matter.
Bill Brooks
Ottawa Canada
RV-6A still in progress

RIVET AGE

you could search an article in EAA's Sport Aviation, a guy used a little dental oven to anneal his old rivets, said they then drove like butter, much less effort, no cracked heads from excessive working etc.
Try some 'new' ones and see if you find a difference.?

flyboy1963 said:

you could search an article in EAA's Sport Aviation, a guy used a little dental oven to anneal his old rivets, said they then drove like butter, much less effort, no cracked heads from excessive working etc.
Try some 'new' ones and see if you find a difference.?

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if someone said "they drove like butter" i'd be worried they lost some of the temper required for strength. They are delivered from Van's with a certain hardness built in, and you don't want to reduce that.

old rivets

Boy I sure hope they don't go bad with age. I cringe at the thought of drilling out 16,000 plus rivet about 20 years from now to put in fresh ones!

I have some that are 10+ years old and I cannot tell them from the new ones. Build on!

I believe they do get harder as they age. I don't think 15 years will be an issue. AD rivets are pretty soft to begin with. The D rivets of old were much more prone to hardening and cracking. "Ice Box" rivets hardened so quickly you had to store them in the freezer. I have never used any of those, pretty odd thought. Still, rivets are pretty cheap, you could just buy new ones. I wouldn't worry about it however.

2017 alloy (AD rivets)

age hardens after heat treat and water quench at room temperature to most of it's strength in about three days. Older rivets are a little harder. Keeping on dry ice just after quenching like ice box rivets (DD 2024) will make them much easier to shoot. The problem is maintaining the range of temperature needed to heat treat without damaging the (molecular structure??) material.

AWM

I bought some rivets at the Boeing surplus store in Seattle some years back. They were hard as a rock and very difficult to set. I tossed 'em.

Rivets are relatively cheap. Why not go with some fresh ones and not think about it. Or try bucking a few on some scrap peaces and see how it goes.

Danny7 said:

if someone said "they drove like butter" i'd be worried they lost some of the temper required for strength. They are delivered from Van's with a certain hardness built in, and you don't want to reduce that.

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I heat treated some after I read that article back in the 80's. I was working on a 4 at the time. After heat treating them, I tested some for shear strength in some test coupons. They sheared right at book value.

They did squeeze and drive easier, but universal heads were an issue because the rivet set would deform the head since it was soft.

As stated, I think a couple weeks after the heat treat, the age hardening is pretty much done.

Old or possibly defective rivets:

See post #10 in this thread with close-up photos of two batches of rivets, one old and one new. By old, I'm talking 5 years or so in since the kit was delivered. Frankly I suspect they were only marginally within spec to begin with. I threw the old ones out.

Photo of problem rivet:

cracked rivets

I inherited some rivets of unknown age and found at least half of them developed cracks in the shop head after squeezing. Didn't try annealing them as they are so cheap to buy.

Thanks to everyone for their input, suggestions, observations, etc. At this point, think I will use the old rivets mostly for practice with my new squeezer. Will keep the rest as spares just in case I get short a few and don't want to wait for another Van's order. Happy Flying--

Sky

This is something I've meant to do for years as an experiment and finally have gotten around to it. Being a pack rat I have enough rivets to build a fleet of airplanes and do not want to throw any away just to get fresh ones. So for a couple of months I had some saved searches on eBay to buy a small dental burnout oven cheap, and one finally came my way. Last night I gave it a try. Turned the oven on and set it to 850 degrees. Made a "cup" out of aluminum foil and put about 200 3-4 flush rivets in it. Let them bake for 15 minutes, unplugged the oven, opened the door and grabbed the foil cup with a pair of needle-nose pliers and tossed them in a big cup of water next to the oven. I did this as quickly as possible; it took less than a second to get them in the quench bath.

I smashed the rivets on my vise per the advice in the old Sport Aviation article and found none of them cracked, indicating the annealing operation was a success. The ones that did crack were the ones that I didn't anneal.

So I drilled and dimpled a dozen holes in some scrap and drove annealed and un-annealed rivets, to see the difference.

The annealed rivets were much easier to drive. I'd estimate they're 40% softer. I will be annealing rivets in batches going forward when riveting skins.

Solution heat treatment, NOT annealing. Good info here:

http://www.mlevel3.com/BCIT/heat%20treat.htm

rocketbob said:

I smashed the rivets on my vise per the advice in the old Sport Aviation article and found none of them cracked, indicating the annealing operation was a success. The ones that did crack were the ones that I didn't anneal.

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Have you ever done the same test on a batch of new rivets?

Materials properties

I'd be concerned what happens to the material properties (i.e. yield and ultimate stress). These are known to change dramatically depending on the temper condition.

I couldn't find data for 2117 in both the -O and -T4 conditions, but here is data for 2017 to give you an idea of how the strength can change.

2017-T4:
Yield strength - 40,000 psi
Ultimate strength - 62,000 psi

2017-O:
Yield strength - 10,000 psi
Ultimate strength - 26,000 psi

Unless your heat treatment is a very controlled process (including follow-on testing), you may find your rivets to retain only a fraction of their intended strength.

With all the focus on safety, I really, really, want to discourage this process.

I can't remember where I saw it but there was someone that did lab testing with some coupons after annealing the rivets using the same process and found them to be right on spec after they age-hardened. The process I followed was straight out of the March 1993 issue of Sport Aviation where Dave Anders described how to do it.

On the older RV's I recall it was permissible to use pop-rivets for the lower skins so I doubt the tensile strength of solid rivets will ever be an issue even if they were under the specified strength numbers.

rocketbob said:

I can't remember where I saw it but there was someone that did lab testing with some coupons after annealing the rivets using the same process and found them to be right on spec after they age-hardened. The process I followed was straight out of the March 1993 issue of Sport Aviation where Dave Anders described how to do it.

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That was me, and they tested right at book value for shear strength. I kept the rivets in the freezer until I squeezed them. Took the test coupons in the next day and tested to failure. Pretty much dead on the book values based on the cross sectional area of the rivet hole. Old rivets were also tested and they did test higher than book.

"These (2117) are known as Natural Aging alloys and actually achieve 90% full strength in 24 hrs. " from my previous link.

Bob, if you keep saying annealing, I'm going to have to ...

Aluminum -T3 and -T4 tempers (AN426 and AN470 are T4) migrate toward the -T8 -T6 temper over long periods of time i.e. they get hard.

fl-mike said:

That was me, and they tested right at book value for shear strength.
Bob, if you keep saying annealing, I'm going to have to ...

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Heck you said you tested the annealed I mean solution hardened rivets right in this thread...

rocketbob said:

I can't remember where I saw it but there was someone that did lab testing with some coupons after annealing the rivets using the same process and found them to be right on spec after they age-hardened. The process I followed was straight out of the March 1993 issue of Sport Aviation where Dave Anders described how to do it...

Click to expand...

I can use my low-tech tensile tester to do pull tests to validate the shear strength if you want. All I need is two pieces of aluminum, say 1/16" thick, joined with the test rivets. The pieces should each have a 3/8" hole to connect to the yokes of the tester.

To see my tester in action, search YouTube on "breakotron."

Thanks, Bob K.

A friend and I were doing some repairs to a Cessna 140 about eight years ago and did some solution heat treating with an oven to some #6 AD rivets to make them easier to drive (some were back-riveted in tight areas). We did pretty much what you guys described; heating up to around 900F (I forget the exact temp) and quenching them in water. The rivets were definitely easier to drive after that!

I was curious about the effects on strength, and built a "pull tester" out of two four-foot lengths of angle iron and a fish scale, along with several identical aluminum plates to rivet together and shear apart with the levers. It was pretty similar to what Bob described (great videos BTW). Here's what I found:

1. Right after quenching, rivets that had been heat-treated had around 20% (IIRC) less shear strength than rivets that had not been heat-treated. That was expected anyway because the whole reason for this was to make them softer (hopefully temporarily).

2. After several days, the heat-treated rivets regained practically all of their shear strength if not driven.

3. After the same number of days, heat-treated rivets that had been driven were about 10-20% weaker than untreated rivets that had also been driven.

So it seemed like, after several days, the treated rivets were about as strong as untreated rivets, but only if they had not been driven. If the heat-treated rivets were driven, it seemed as if they never regained their full strength (at least not after about a month of waiting). This happened reliably with several samples. It's as if the work-hardening from driving somehow interfered with the age-hardening process.

It would be interesting for someone else to try this and see if they get the same result. For me, I was convinced enough that I spent many hours drilling out the heat-treated rivets and replacing them with untreated ones.

Steve