Failed Fasteners

[kek78]

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

 

[swall]

What is the operating environment? I would suspect an SCC failure rather than HE.

 

[kek78]

swall,
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

 

[deanc]

http://assist.daps.dla.mil/docimages/0001/59/31/1291A.PD3

http://www.uspto.gov/web/offices/tac/fqa/index.html

Where are your bolts from....China,Tiawan perhaps?

http://www.boltscience.com/

http://www.tribology-abc.com/sub9.htm

 

[redpicker]

You mention a metallurgical evaluation. Did that include a chemical analysis and microsctucturial analysis?

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

 

[kek78]

redpicker,
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

 

[TVP]

Hydrogen can enter steel both during initial processing (melting, acid cleaning, electroplating) as well as during service. Does the failed fastener show any evidence of corrosion (SCC, pitting, etc.)? Perhaps hydrogen was produced as a result of corrosion of the fastener-- black oxide has essentially no corrosion resistance when the temperature, humidity, and environment differs from ambient.

 

[CoryPad]

The likely cause is Hydrogen Assisted Cracking (or Stress Corrosion Cracking). As stated by TVP, black oxide will offer no corrosion protection. The HAC/SCC can occur with no obvious/visible signs of red rust corrosion. This one batch of parts may be more susceptible due to initial hydrogen concentration from processing plus added hydrogen concentration from environment.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[kek78]

TVP,
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

 

[stanweld]

It sometimes amazing what a few points of Rockwell hardness can do on like processed steels. Hardness below a threshhold value = no failures; hardness above the threshhold = 100% failures under the same service criteria.

 

[deanc]

I have observed a number of problems lately with failed bolts.

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.

 

[IRstuff]

Counterfeit bolts have been problematic for decades

TTFN

FAQ731-376

 

[EdStainless]

Take a close look at the threads. Cut or rolled? If they are rolled are the smooth and clean?
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

 

[kek78]

Thanks everybody for their help. I think as CoryPad indicated I have some kind of Hydrogen Assisted Cracking going on. The source is unknown and without processing history I probably will never find out the source of it.

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

 

[strider6]

sorry, maybe the thread is closed, but i didn't find in the post what is the materials of the fastener?

regards
Strider

http://www.corrosionist.com

 

[kek78]

The chemistry of the failed fastener is the following:
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

 

[monkeydog]

What is the material supposed to be?

 

[kenvlach]

But, where's the analysis for hydrogen???

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://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=10340

“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.

 

[deanc]

What are the head markings on the bolt?

 

[MDAPSEY]

The processing before the Black Oxide can cause Hydrogen Embrittlement. Acid is used to clean/Etch the parts prior to processing.