Failed Fasteners
Failed Fasteners
(OP)
I have several failed fasteners. After metallurgical evaluation the failure mode is classical grain boundary embrittlement (rock candy fracture surface, cracks along prior austenite grain boundaries in micro). Hydrogen embrittlement, right? However, these fasteners were not plated (cad, zinc, etc.), they only have a black oxidized coating. I would not think this type of coating process would be a source for hydrogen pick-up. Could this be temper embrittlement? The hardness on these fasteners are Rc 44 and they were in service around 4-6 months. The material appears to be clean from the micro (no massive inclusions). Any suggestions? Thanks





RE: Failed Fasteners
RE: Failed Fasteners
These fasteners are in a piece of equipment (they just hold it together) indoors at 65 F. Also, another piece of equipment (the same design) is right next to it and has been in service for 4 years without any failures. No corrosion by-products are seen in any of the fasteners or micro-sections. - kek78
RE: Failed Fasteners
h
Where are your bolts from....China,Tiawan perhaps?
http://www.boltscience.com/
http://www.tribology-abc.com/sub9.htm
RE: Failed Fasteners
One of my personal peeves is using Temper Embrittlement as a failure mechanism. TE is not a failure mechanism. TE can explain a low toughness condition, and low toughness can result in failures as you have described, but for TE to be the responsible mechanism, you would have to seriously entertain the idea that if the material had a low toughness condition for other reasons (chemistry, microstructure, processing--i.e. cast vs. powder vs. wrought) then failure would not have occured.
My point is that if the failure resulted from low toughness but is "blamed" on TE, actions taken to avoid TE will not prevent additional failures if the "new" material does not have adequate toughness. If toughness is important in the application, then it needs to be considered when the material decision is made.
The reason I feel the chemistry of the fasteners is important is that, with only a few exceptions, a low alloy steel at 44 HRC is going to have low toughness, regardless of whether TE is involved or not. If this is the situation, the solution would be to use a lower hardness (if the application will permit) or change the material to one that will have adequate toughness at that hardness.
rp
RE: Failed Fasteners
I have not got the results for chemistry yet, but it is getting done. The microstructure looked typical, nothing out of the ordinary except for the large crack across the cross sectional area of the fastener with the prior austenite grains falling apart and micro cracks following prior austenite grain boundaries. SEM showed typical "rock candy" appearance, again with cracks following prior austenite grains.
I have also performed evaluation on a fastener from the other piece of equipment next to the failed one (as mentioned above). This fastener has been in service for four years. Microstructure looks good and is the same at the failed fasteners. Hardness was a little lower at Rc 40.
It will be interesting to see what the chemistry results are. I have done both fasteners (good & bad). - kek78
RE: Failed Fasteners
RE: Failed Fasteners
Regards,
Cory
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RE: Failed Fasteners
The fasteners are clean of any signs of corrosion, SCC, pitting. Plus, if it was a corrosion problem, wouldn't the equipment (4 years in service) next to the failed equipment (6 months in service) have been susceptible to SCC also. This equipment is inside at a constant-dry temperature. I do not know the processing history of these fasteners since I did not manufacture the equipment and the equipment manufacture is not being helpful. Legal stuff could happen here. - kek78
RE: Failed Fasteners
RE: Failed Fasteners
The bolts were from a supplier in Taiwan. Markings are good, the requested MTRs looked good. However it turns out this supplier buys from about 200 different small factories and has the bolts marked with their stamp. I cannot share more due to legal issues.
So if your testing does not show the results you desire, you may still never get to the bottom of the problem.
I would suggest you only buy "registered" bolts listed in the Fastner Act. Cheaper is not always least costly.
RE: Failed Fasteners
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: Failed Fasteners
I had some Ni alloy bolts that had some nasty laps in the thread roots that caused no end of trouble.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm
RE: Failed Fasteners
Purhaps these fasteners were plated then the plating removed to add the black oxide at customers request. I have seen strange things like that happen before.
Anyway thanks again. - kek78
RE: Failed Fasteners
regards
Strider
http://www.corrosionist.com
RE: Failed Fasteners
C=0.36
Mn=0.73
P=0.015
S=0.006
Si=0.20
Cr=0.99
Ni=0.02
Mo=0.19
Cu=0.01
Fe=Rem
The chemistry of the good fastener is the following:
C=0.32
Mn=0.64
P=0.011
S=0.003
Si=0.16
Cr=0.99
Ni=0.09
Mo=0.20
Cu=0.14
Fe=Rem
kek78
RE: Failed Fasteners
RE: Failed Fasteners
Were the parts ordered to be processed per either MIL-DTL-13924D or AMS 2485J?
MIL-DTL-13924D COATING, OXIDE, BLACK, FOR FERROUS METALS.
http:
“3.2.1 Stress relief. Unless otherwise specified for a particular end item specification or drawing, after forming and hardening, and prior to cleaning and coating, objectionable residual stress in ferrous alloy parts having a hardness greater than 40 HRC shall be relieved by suitable heat treatment.”
“3.3 Application of black coatings. The coating shall conform to the class specified. The specified black coating shall be applied under controlled time and temperature conditions. All equipment together with solutions or baths shall be properly maintained and kept free of dirt or possible contaminants. The selected process shall not reduce the hardness of the parts being processed or expose the parts to temperatures in the temper brittle range of the material, nor shall it cause embrittlement of the steel.”
“3.10 Hydrogen embrittlement relief treatment. Steel parts that are surface or through hardened at 40 HRC and above shall be given a hydrogen embrittlement relief treatment after application of the oxide coating. Coated springs or other parts subject to flexure shall not be flexed prior to the embrittlement relief treatment. If an embrittlement relief treatment is required, it shall follow the chromic acid rinse. The embrittlement relief treatment precedes the supplementary preservative treatment.”
A typical black oxide process sequence for (non-stainless) steels is:
alkaline degrease, rinse,
hydrochloric acid pickle until all rust is removed (so if some rusty items processed in the same hoist load with your basket of bolts, lots of hydrogen), rinse,
hot black oxide, rinse,
dilute chromic acid rinse (per MIL spec but optional in practice),
water-displacing oil.
RE: Failed Fasteners
RE: Failed Fasteners
RE: Failed Fasteners
RE: Failed Fasteners
1) The loads applied to the rivets is to great (doubtfull due to your posts confirmation of the failure mechanism)
2) The rivets have failed to live up to it designated criteria.
As your left with problem (2), your solution is to change suppliers of rivets as is obvious that any failure in this way is down to a process problem rather than a loading/service problem. You could spend a lot of time/money trying to come to the actual mode of failure and the reasons behind it, or you could just envelope it within a "processing problem" and move on with better fasteners, (whilst getting money back for previous supplier)