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The investigation is just beginning

The investigation is just beginning

(OP)
The investigation is just beginning. I only have 2 pictures for now, and almost no details.

One brand new M36 "bolt" out of a litter of 26 snapped at installation.

The thread runout detail may not be the best. That will be part of the investigation.

The material nominally is hot rolled 1045, about Grade 8.8, but with yield strength more like 50% of UTS, not 80%.

The full diameter body bolts I've broken by tightening to maximum Moosage generally show some some necking as I recall.

RE: The investigation is just beginning

Tmoose,
were the bolts made in a far eastern country by any chance ?
B.E.

You are judged not by what you know, but by what you can do.

RE: The investigation is just beginning

Counterfeiting, while prevalent, is not exclusive to the far east.

Seems like a thorough investigation of materials, annealing, etc. is in order, particularly if another one does the same thing

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: The investigation is just beginning

Hydrogen embrittlement

RE: The investigation is just beginning

Looks awful intergranular, but the part looks plain, so maybe some sort of SCC or dirty steel. There is no way that you should be able to break an M36 without a torque multiplier unless you are Arnold S.

RE: The investigation is just beginning

Agree with screwman...intergranular corrosion and/or inclusions

RE: The investigation is just beginning

(OP)
The parts were made in early 2016 a shop in South Carolina, from hunks of 180 BHN 1045 HR steel bar provided by a US company with a custom material cert that states among other things, melted and made in the USA.

After machining about a year ago, and sitting in our warehouse the parts moved around by truck up to Canada where they were being installed Tuesday.

I learned that 6 bolts were installed fine, but the 7th the nutted stump snapped and flew away at high velocity awhile being tightened, allegedly to 1900 lb-ft.

I'm hoping to get lots more info over the next day or 2, and even the broken parts.

Wouldn't intergranular corrosion require a sophisticated nasty atmosphere to affect an unstressed HR carbon steel part?

RE: The investigation is just beginning

Isn't it a little cold in Canada this time of year?

Mike Halloran
Pembroke Pines, FL, USA

RE: The investigation is just beginning

This is why I prefer the use of pullers and then snugging the nut - less chance for fly-aways and much better tension control.

I'd look for hydrogen as well. We had some steel belleville washers that were embrittled and could be snapped like a potato chip with finger pressure.

I don't see much runout on the thread - maybe it's the angle the photo is taken, but if it is short, then that would have been a stress concentration that would push it over the edge.

Did they just stop advancing and cut a ring groove rather than spiraling out? Sometimes the body is relieved after the threads to make the stress transition smoother.

It also looks like a really sharp root, also a nasty stress concentrator. Maybe this was the first one and the cutter was duller for the rest.

I would not trust any of them without inspecting them.

RE: The investigation is just beginning

There's just nothing to be seen of the plastic properties (16% elongation at break) of the material as if per standard, this here is seemingly pure brittle fracture.
Additional questions
- Have these bolts been hardened or tempered?
- What was the lubricant when nuts were tightened?
- Is there a washer under the nut?
For metric threads, to smoothen the notch at the end of the thread by a defined thread runout there's DIN standard 76. However, the bolt would need to be checked for the smaller diameter in the runout's root. dg would be 30.7 mm, pls. refer att. pic.
Another direction for improvement might be the bolts material. The tightening torque being approx. adequate to a 8.8 quality M36 slightly oiled at assembly, then is the 1045 in the chosen state adequate to this? Perhaps you could contact a bolt manufacturer for advice?
Regards

Roland Heilmann
Lpz FRG

RE: The investigation is just beginning

Quote:

One brand new M36 "bolt" out of a litter of 26 snapped at installation.

Folks, read the original post this is a brand new bolt. It snapped on installation. Either improper heat treatment or hydrogen embrittlement. Corrosion????

RE: The investigation is just beginning

As Mike noted... any Charpy testing of the material done? The cross-section appears to have a bit of a flaw. If critical, then all should be replaced and/or tested.

Dik

RE: The investigation is just beginning

I see "cast" in the cert. I assume that means it is a casting. Is that typical for material this large and highly stressed?

RE: The investigation is just beginning

I'm not seeing that elongation at all on the failed part. The cert shows 18+% elongation and a 30% RA and it isn't there on the failed part. What sort of temperature was the part at when it broke? Could it have been below the brittle transition temperature? I find the yield point quite low for a 105ksi UTS, I would have expected to see something closer to 80ksi instead of the 55ksi that is listed on the cert.

RE: The investigation is just beginning

(OP)
The steel supplier said 1045-C1Q2 is their internal code for Hot rolled 1045 bar.

I believe the "continually cast" refers to the billet that was then patty-caked/hot rolled to an 8-1 reduction ratio.

I think Pretty much All us puny earthlings' steel forgings start their lives as castings.
https://www.youtube.com/watch?v=9AMbKpeJRoU

RE: The investigation is just beginning

(OP)
" I find the yield point quite low for a 105ksi UTS, I would have expected to see something closer to 80ksi instead of the 55ksi that is listed on the cert."

I'm Not very far into this, but I'm finding hot rolled materials often have similar Yield/UTS ratios close to 50%.

Here is good old A36.

RE: The investigation is just beginning

Send the failed piece out for met analysis to determine the root cause of failure.

RE: The investigation is just beginning

I had a number of high strength bolts snap while being tightenedg when the ambient temperature was around 10 degrees F. When the workers waited until the temperatures climbed up to around 30 degrees, no more broken bolts. The manufacturer sent a sales rep to witness a few bolts being installed in the early morning and just like the ticking of the clock, snap, snap, snap. Each one failed during installation. We waited until after coffee break when the temperature was not as cold and there were no more failures.

Best regards - Al

RE: The investigation is just beginning

I am not familiar with the fastener specs. Was this supposed to be Q&T, I see no HT notes at all.
Cut threads (I guess from the very sharp pictures) or rolled?
Were they even stress relieved after forming?
Not bad looking steel.
I sure didn't see 18% elong in the failure pictures.
Something doesn't add up.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: The investigation is just beginning

hi T.Moose

I did a quick calculation based on the 1900lbf-ft and the 55100 psi figure from the specification and it appears the tension in the bolts after tightening should be around 95% of the yield, now given that torqueing of bolts is subject to huge errors, I just wonder whether that particular bolt got overloaded because of some change in friction value or thread geometry etc.
I also note that the bolt failed during tightening and that is when the bolt see's the most stress because its subject to both tensile and shear stress until the wrench is removed.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

RE: The investigation is just beginning

(OP)
thanks all.

The attached image shows XRF test results for the broken bolt, and an old bolt.
It includes the material certs for the new bolts.

One thing that has me scratching my head is the different results for two locations on the broken new bolt.
Maybe the surface prep needed to be more severe.

The fact XRF does not measure carbon content makes me (an XRF ignoramus) wonder how useful it really can be.


More news at 11.

thanks

Dan T

RE: The investigation is just beginning

You are right, you need C to know much. That will take some OE work.
The other thing is that most people are not careful enough with surface prep, and they don't know how to turn on the 'high precision' mode.
Is your cert chem from the melt or a product check?
I will assume melt, so given the variation in a portable XRF and the allowances for check chem, it looks like a match to me.
I usually re-test the same spot 5 times and record each time. That gives a good idea of how precise the readings are.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: The investigation is just beginning

(OP)
" it looks like a match to me. "
Should I take that to mean the new, broken bolt //could be// 1045?

thanks

Dan T

RE: The investigation is just beginning

Nothing there clearly tells me that it isn't 1045.
I would be more concerned with mechanical properties and microstructure.
Though at some point a detailed analysis would be good to have.
Mix-ups have been known to happen.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: The investigation is just beginning

Let us know about your findings from your investigation particularly as alluded above about cold temperature condition and overtightening.

RE: The investigation is just beginning

The pictures in the OP show a failure right at the thread run-out. The thread also appears to be cut rather than rolled. And the transition from the cut thread root run-out to the bolt body looks like it creates a significant stress concentration. The corrective action would be to "waist" the bolt body profile to provide better stress distribution, and roll form the threads to provide better fatigue performance.

RE: The investigation is just beginning

where's the SEM test, charpy and tensile. what is the actual hardness of the bolt and what heat treat process was used?

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