What are you thoughts regarding these fasteners?

[jnelson33]

Hello all! This is my first post on the forum; I've used this forum countless times to help me in my line of work. Hoping I can get some general feedback on these custom fasteners.

We're an automotive, aftermarket performance parts company and we recently decided to purchase some hardware from an overseas vendor. They are essentially DIN 912 / ISO 4762 spec screws that we customized with Torx drivers instead of hex (allen) drivers. The exact spec is M4x0.7x30, 10.9 steel, white zinc plated. Awesomely detailed specs, I know, but that's what they provided.

One of our product assemblers brought a major issue to my attention upon entering the warehouse today. As you'll see from the pics, these screws are having major quality issues. The majority of them all shear right at the head to shank fillet, but the shape in which they do so can vary.

All of the shearing appears to be mostly sharp and crisp like a higher tensile material should be, but there are other breaks that jog up/down the axis more-so than some of the other screws that broke.

So, we're obviously trying to get a count of the bad screws, and rectify this somehow, but, why would this happen in general? I'd like to have a bit of ammo and feedback to converse better with the supplier. I'm thinking that the material's temperature was simply not in the right zone for the cold heading process when these were being made. I'm also wondering if the driver change, Torx plunge operation vs the hex plunge operation, has weakened the head by essentially thinning out the cross-section.

I'll wait to post more until I get some feedback, that way I'm not posting anything that won't facilitate good dialog.

 

[jnelson33]

I've come across the topic of hydrogen embrittlement, which can cause delayed cracking; seems like one valid idea. One would think that a fastener company would know about this however.

 

[mrfailure]

I would suspect hydrogen embrittlement also, but you should have them evaluated for failure analysis to identify mechanism and likely root cause(s).

 

[jnelson33]

Ok, thanks for the reply. I am trying to get a material card for the screws so that I can observe their hardness. ASTM F1941M states this is no doubt a concern for material compositions of 40+ HRC, so I'd like to find out if these screws fall into that hardness for certain.

Do you know any companies off-hand specializing in that in the state of Florida?

 

[mrfailure]

A Florida lab I found professional and have used in past is Martin Metallurgical in Stuart.

 

It does indeed appear most likely to be a case of hydrogen embrittlement. Plated fasteners are the classic case presented in textbooks as an example of HIC.
Since there will be financial ramifications I urge you to have the failure investigation performed by a 3rd party. It is important to determine whether this was a one-off process failure, or a wider lack of quality assurance, technical expertise and/or process control.
It should go without saying, but quarantine the entire lot and put any further deliveries on hold while you backtrack to investigate your previous installation of similar products from this supplier.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[swall]

I've seen heads come off like that due to hydrogen embrittlement and I have seen heads come off like that due to internal cracks from heading. Get the lab investigation done.

 

[DAVIDSTECKER]

That a look at the raw material chemical certificates.
If memory service me correctly carbon content should be 30 points or so, defiantly below 40 points, at least for hot dipped zinc galvanizing.
I make some parts that get HDG and use 4130 to prevent hydrogen embrittlement.

 

[3DDave]

I'd scratch the torx process from the list. It would be a consideration if the fracture was in the side or top of the head, but this is not there.

The cleanliness of the breaks suggests hydrogen embrittlement; it may be they are more sensitive to hydrogen due to unrelieved stress from the heading process which is why they don't fail elsewhere. It looks like there is no elongation.

It's a shame; those look like very nicely made parts.

 

[EdStainless]

Were these baked after plating? As is commonly required for fasteners.
Were these tested by the manufacturer? In tension with a wedge under the head?

If you take a couple dozen from one lot and bake half of them (400F overnight, or 600F for 1hr) then load test all of them. See if the bake helps. It may not, because they may have acid cleaned the steel prior to heading. The acid cleaning can result in hydrogen absorption, and then they could get cracks when they headed. Then the platting just covered it up.

I have had this happen with other alloys and it was related to poor heading process creating internal defects.
Serious lab work is in order.

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

 

[jnelson33]

I really appreciate all of this feedback, and I'm also pleased that we've converged on the same problem that's likely affecting the screws.

I've created a proper screw request sheet in a much more methodical manner than was previously done.

The screw is now fully spec'd with items like ISO/ASME fastener standards for the geometric sizing, ISO spec for alloy steel compositions, and ASTM specs for zinc plating and avoiding hydrogen embrittlement. Lastly, and more importantly, I've told them the ASTM testing methods I'd like the screws to be tested and certified to, with certificate provided for testing results.

My boss doesn't think it's worth testing the current lot since the supplier has already agreed to "hopefully" rectify this issue, and if they follow my spec sheet, it hopefully should rectify this issue.