Letter 23078

Hydrogen de-embrittlement cycle for fasteners  

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We buy fasteners of 10.9 & 12.9 class from our vendors. These fasteners are zinc plated. We are facing a problem of delayed failure of these bolts. Our vendors are following hydrogen de-embrittlement cycle as per IS1367 Part 11. Inspite of this we are facing bolt failures problem. We would like to know whether any other standard or alternate process is available to overcome this problem of hydrogen de-embrittlement. We also would like to know whether there is any kind of testing/inspection available to test fasteners for hydrogen embrittlement relief treatment.

Regards,

V.GANESH
- Bangalore, Karnataka, India

Typical approach of platers that do not know the difference or do not care is to pickle the parts in acid for far too long because it makes their life easier. If 1 minute in the acid will not work, then they need to be precleaned better. This is supposed to be an activation step, not a pickel. Next is the common practice of waiting until you have a full load for the oven before baking. They need to get into the oven (with the heat on) as soon as possible. The bake cycle will remove a bit of the hydrogen, but can not really get to deep attack from setting around all day.You will have far less problems if they will do the above. When a current bake cycle does not work, extend the bake time. Also thermocouple the middle of the load in the oven. It can take hours for the middle to reach full temp. There is equipment for testing hydrogen embrittlement in a bolt, but it is very expensive and requires tender loving care in its operation. Notch bar testing will help a lot, if it is handled exactly the same as the bolts.

James Watts
- Navarre, Florida

+++ You might want to look into having your fasteners Mechanically plated. This process is free from hydrogen embrittlement. Another advantage of mechanical plating is that you can vary the thickness of the coating, any where from 5 micron to 50 microns. Simon Cook - Melbourne, Victoria, Australia

The original question states that there is a problem with a delayed failure of these bolts. Is it one or two bolts in a 'batch' or is it the whole batch that fails after some time? If the first, How can one test for HE if it's only one or two, that are going to fail? Also, What are the distinctive features of a HE fracture?

Bob Howell
- Toronto, Ontario, Canada

We need a simple test for checking the components are hydrogen deebrittlement has been carried out or not.
please reply.

R V Vijay Ganesan
manufacturing of auto components - Coimbatore, TamilNadu, India

Sustained load test as ASTM F606 [link is to spec at TechStreet] is a good test to prove the proneness of hydrogen embrittlement failure at fastener level. Sample size to be determined based on the batch size undergoing the surface treatment. Also if the UTS is more than 1350 MPa it is better to go with vacuum deposition techniques rather that plating.

Thomas Kurian
- Trivandrum

Similar problem is being traced out at our end.

We are doing Zinc (Green) passivation on the Class 12.9 bolts.

2 of such cases are found during inhouse assembly.

My question is how we can trace that the bolts are properly de embrittled or not?

What is the easy method for ensuring this.

Deepak Mumbare
- Pune, India

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