Grendel Kaboom w/ Young's MFG Bolt

[Dcommoncents]

WOW that is a strange and catastrophic bolt failure. Just speaking intuitively, I would think some type of severe metallurgical problem would have to be in play for it to not only shatter all around the lugs, but also tear diagonally like that. Definitely staying tuned to learn more about this one.

[mdram]

heat treat failure?

[Bill3508]

WOW that is a strange and catastrophic bolt failure. Just speaking intuitively, I would think some type of severe metallurgical problem would have to be in play for it to not only shatter all around the lugs, but also tear diagonally like that. Definitely staying tuned to learn more about this one.

Yeah it's crazy how all of the lug breaks look identical. I assume they went first and the rest collapsed from there.

[Dcommoncents]

Yeah it's crazy how all of the lug breaks look identical. I assume they went first and the rest collapsed from there.

Yeah, my guess is that the lugs broke, resulting in a mess of lug bits in the barrel extension that kept the bolt from unlocking, and then the force of the cam pin trying to turn the bolt caused a second failure around the cam pin hole and the damage around the cam pin slot on your carrier.

[LRRPF52]

Looks like it was through-hardened so brittle, that it shattered like glass.

There needs to be a balance of surface hardness and substrate ductility on bolts, with a viable alloy.

If you heat treat any of the appropriate alloys to the extent that the part is brittle like that, it doesn't matter what alloy it was made from, how well you MPI it, or coat it with chrome.

It will shatter like glass.

A big thing to notice is whether the extractor is intact and in position or not when you see a kaboom. When a case fails as the cause of a catastrophic failure, the pressure relief is now managed by the path of least resistance after the high pressure gasket (your brass) has failed. The gas path normally blows the extractor out, blows the magazine out onto the ground, eggs out the lower receiver magazine well in many cases, eggs out the upper, and often cracks the bolt carrier.

None of that happened with this, and the extractor is sitting in the extractor pocket, still held in place by the pin, but all the bolt lugs have shattered off like crumbs. Brass looks really clean as well.

[grayfox]

Sorry that this all happened, but for certain it is a fascinating lesson in applied metallurgy.
Young's has been mentioned a couple of times in here re: failures, is there more of a history with them than with others, I wonder? Or just chalk it up to a few bad pieces...

[Bill3508]

Looks like it was through-hardened so brittle, that it shattered like glass.

There needs to be a balance of surface hardness and substrate ductility on bolts, with a viable alloy.

If you heat treat any of the appropriate alloys to the extent that the part is brittle like that, it doesn't matter what alloy it was made from, how well you MPI it, or coat it with chrome.

It will shatter like glass.

A big thing to notice is whether the extractor is intact and in position or not when you see a kaboom. When a case fails as the cause of a catastrophic failure, the pressure relief is now managed by the path of least resistance after the high pressure gasket (your brass) has failed. The gas path normally blows the extractor out, blows the magazine out onto the ground, eggs out the lower receiver magazine well in many cases, eggs out the upper, and often cracks the bolt carrier.

None of that happened with this, and the extractor is sitting in the extractor pocket, still held in place by the pin, but all the bolt lugs have shattered off like crumbs. Brass looks really clean as well.

Yeah this was not a normal kaboom. There was no gas from the failure introduced into the receiver. All the gas went forward which is lucky in hindsight

[Kswhitetails]

Very glad you weren’t hurt.

[204 AR]

So what has Young said about it?

[CVCOBRA1]

I see that there was a group buy of Young bolts on here last July. Don't know how many were sold but I would think if there was a problem in the machining it would show up on other bolts by now. Curious also what Young would say. A Google search doesn't really point to a serious problem.

[Bill3508]

So what has Young said about it?

Nothing yet. Called them and they received my email but had not looked at it yet.

[Bill3508]

I see that there was a group buy of Young bolts on here last July. Don't know how many were sold but I would think if there was a problem in the machining it would show up on other bolts by now. Curious also what Young would say. A Google search doesn't really point to a serious problem.

My bolt was purchased from them January 6, 2018.

[Clarence]

I agree with the posters that indicated brittle failure. Surface hardening processes make a hard surface and keep a ductile interior. But, the depth of the hard surface is dependent on time and temperature of treatment. At a point, the hardened portions are so deep that they leave an insufficient ductile portion, and this type of brittle failure occurs. I've seen similar failures in high speed gearing.

[CVCOBRA1]

I have to edit this as to the source. The group buy of the "Young" bolts was stated to come from this forum. Actually that info came from a Google search that pointed me to a different AR forum. After looking back on here in the Group Buy threads I can't see where any Young bolts were offered on this forum. As usual, you just can't believe everything you read on the internet.....Sorry about that

[sneaky one]

Get a current group buy bolt. It's made at the same company that Bill A used to make his bolts. Same as mtl. as the AA bolt.

[Bill3508]

I bought 3 of the monster bolts from the group buy here. Haven't tried them yet.

[Texas]

The shiny appearance of the sheared lug from the bolt would make me look at the Rc hardness of the bolt. I suspect that the bolt was improperly heat treated causing the steel to crystallize and crack with the first and second rounds leading to catastrophic failure on the third round. The case that failed appears to be due to the brass flowing and separating when it became too thin.

[ErikS]

I had a 5.56 bolt fail at the cam pin hole, I caught it before it let go. It looked nothing like that bolt. My guess without seeing test results is embrittlement from to long of a hardening process.

The 5.56 bolt I had fail was a BCM, so even top shelf companies can have failures.

[LRRPF52]

I had a 5.56 bolt fail at the cam pin hole, I caught it before it let go. It looked nothing like that bolt. My guess without seeing test results is embrittlement from to long of a hardening process.

The 5.56 bolt I had fail was a BCM, so even top shelf companies can have failures.

Especially when you HPT a bolt, you're asking for it to break earlier than it should.

Some of the best companies in the AR15 and AR10/SR25 world refuse to HPT bolts for that reason, like Diemaco, KAC, etc.

I cracked 5.56 bolts at the cam pin hole as well.

This problem is metallurgy though.

A little history of bolts (5.56)
The US Army and USAF did a joint study in the early 1960s after the first production models of AR15s had been fielded and started to accumulate a round count on them. They were seeing bolt failures enough to initiate fleet testing, and found that hard chrome plating plus inadequate 8620 alloy were contributing to broken bolts.

The conclusions of that study showed most rifles cracked at the cam pin hole, and the others sheared the lugs adjacent to the extractor. Their solution was to use a 9310 alloy even back then, and drop the hard chrome plating of bolts due to hydrogen embrittlement tendencies of the process. This is why the Colt 601 (Green rifle M16) and Colt 602 had chromed bolts, which were later replaced with phosphated bolts. Phosphating is basically micro-fracturing of the surface, so not the best coating or surface treatment either.

At the end of the day, the main focus of the AR15 was to be a lightweight rifle. The stress-bearing components were scaled down from the AR10 for a different cartridge with lower chamber pressure (.222 Remington), and then pressure was increased with the .222 Remington Special (renamed the 5.56x45 and type-classified as M193), followed by a propellant change without consultation or notification of the designers shortly after that. Bolt thrust became an issue for guns shot in volume with the early chromed bolts.

These issues were solved by the time the Colt 603/M16A1 was standardized, since better alloy and processed were used, as well as the taming of the cyclic rate with a heavier buffer. You would rarely see broken bolts in 20" RLGS M16s, M16A1s, or M16A2s. Once the carbines became the focus for production, broken bolts started occurring more frequently, primarily in SOCOM since they actually train with tons of live fire. The Canadians said they scratched their heads when they saw so many reports of broken bolts, since they weren't seeing it in their guns and clients, who shoot just as much (SAS, SBS). Their bolt metallurgy processes are different than the US TDP/Mil-Std, with the absence of HPT being the most notable difference, tighter controls on heat treating and dimensions being the others.

[Sean Boburk]

The brass of the failed round...looks like the imprint of the extractor or ejector hole on the back of it. Also looks like a flat primer. If not overpressure (hot round, etc), what would cause that?

Sean