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Hydrogen embrittlement and brake hardware


An ongoing discussion beginning back in 2007 ...


Q. I am Dhanasekaran working for manufacturer of hydraulic brake systems. We are using bolts and nuts for our assemblies. I want to know about Hydrogen embrittlement problem in fasteners (particularly if hydrogen embrittlement problem is there in bolts or nuts whereby it will break, for example, after applying torque preferably in hours).
2. Raw material specification which will have more susceptibility to Hydrogen embrittlement.
3. Whether components having hardness less than 35 Hrc should have Hydrogen de-embrittlement process done after plating.
Thank you.

Dhanasekaran [last name deleted for privacy by Editor]
user of plated components - Chennai, India


A. There is huge amounts of information about de-embrittlement on this website, including the whys and wherefores.
As a start try these letters: 23078, 32819, 1373, 44599, 1216.

If, after reading the information, you are still stuck then let us know with the specifics of your problem and I'm sure there will be plenty of people prepared to help.

Brian Terry
Aerospace - Yeovil, Somerset, UK

Hydrogen embrittlement relief for parts having areas of different hardness?

July 26, 2017

Q. We received a gear shaft for hard chromium plating per AMS2406 [affil. link to spec at Techstreet] on some area on the shaft. The part is made of steel. The hardness of the shaft area to be plated is 30-32 HRC. The hardness of the gear teeth (carburized) is more than 60 HRC.
What should be the proper HE relief baking time and temperature?
Should it be suitable for 30-32 HRC steel, or it should be suitable for carburized steel?
Is it necessary to perform HE tests to approve the process as "non-embrittlement", even if the plating will be performed in a soft steel area of the part?

Alex Sirota
- Lod, Israel

August 9, 2017

A. Hi Alex!

In order to answer this question, I need to know two things:

- Is the carburized gear in contact with the bath? Do you mask it before plating? How?
- Do you require to bake parts below 36HRC?

Hope you can solve this issue!


Daniel Montañés
TEL - N FERRARIS - Cañuelas, Buenos Aires, Argentina

August 9, 2017

Q. Hello, Daniel
Thank you for your response.
The carburized area of the part is masked with wax and do not contact with the solutions.
AMS2406 [affil. link to spec at Techstreet] spec requests to bake steel parts with hardness HRC 40 and higher and carburized parts with all levels of hardness.
The first question is: if I plate the soft area of the carburized part, what is the technological/metallurgical explanation for hydrogen relief bake?
The second and more important question is: if I plate the soft area of the carburized part, have I to approve the plating process for hydrogen embrittlement? When I plate the same steel without hardened area, I do not have to perform HE tests. But if the same steel shaft has hardened area on its edge, and it was masked during the plating, I have to perform HE tests to comply with the spec?
Thank you again for your advice.

Alex Sirota [returning]
- Lod, Israel

August 16, 2017

A. Hi Alex!

If you mask the hard part of the shaft, then hydrogen is not in contact with the hard part, and there is no hydrogen absorption in the hard part of the shaft.

Hydrogen has a high affinity for some interstices in the hard steel structure. It is believed that atomic hydrogen migrates to these interstices and combines with another atomic hydrogen and forms hydrogen gas in the structure, thus forming some inner pressure in the structure. In tension, these additional forces induce a tenacity drop and a high fragility.

When you bake the parts within a few minutes/hours since you plated them, hydrogen is in the atomic form, and it is 'more favourable' for it to go outside and combine in molecular form. The bake temperature must be enough to make the coating permeable to hydrogen, so it can escape. If it is not, blistering could appear because hydrogen pops out and can't get outside because of the coating.

There are many books on hydrogen embrittlement and not every one has the same explanation, they are all theories! If I made a mistake in some of this, or something is not understood, ask me!

If you need, you can make the plating without baking (mask the hard part as you are), and test the gear for hydrogen. You can use a mechanical method (stress the gear up to the force it will be making in use, maintain this 100 hours and see if it cracks) or a manometric method (put the part in glycerin or silicone oil, heat it to bake temperature and see if there are bubbles). Neither method guarantees that there will not be embrittlement, but if it fails it shows you are doing something wrong.

Hope it works! Regards,

Daniel Montañés
TEL - N FERRARIS - Cañuelas, Buenos Aires, Argentina

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