Replacing Bolts

[charlie]

I was quite surprised to be informed by Trevor from Davies Aircraft Corp that I may replace some bolts on my microlight wings, but I should not electro-plate them as this weakens the steel and make it brittle. Was told to keep it black, ie grade 12.9. Pros apparently bake them after adonizing to re-instate to the original grade.

How many of us know this? I really did not and what about poor bikers that change bolts and brackets to beautify their bikes?
Comments and suggestions welcome.

[KFA]

We use cadmium on our 8.8 and 12.9 bolts and have never had a problem with any of them in 18 years. This explain it quite well. Chrome plating in structural bolts are a no no

"Plating causes problems with high-alloy steels due to hydrogen embrittlement, if appropriate measures are not taken after plating to "bake out" the hydrogen. This is especially true of chrome plating which tends to lock in the hydrogen. Plating does not adversely effect the mild steel used for 8.8 fasteners. The torque-tension relationship is greatly affected by plating due to its effect on the friction coefficient. Cadmium plating reduces the friction by 25% and zinc plating increases the friction up to 40%. This requires a corresponding 25% reduction or 40% increase in required torque for the same tension. Stainless steel fasteners have a friction coefficient about two times the corresponding plain steel fastener. This does not mean that stainless fasteners require double the specified torque since they usually cannot achieve the strength of a steel fastener."

I must just add to this that the cadmium embrittlement does happen but only at temps above 190 deg C, so in high heat applications these plated bolts also need to be baked out. Hydrogen embrittlement starts normally starts at around 205 degr C. The effects of hydrogen and cadmium ebrittlement is highly increased with the increase in bolt strength.

[Wagtail]

I decided to quickly respond because this is an extremely important questions which can have SIGNIFICANT safety implications.

Thanks KFA, nicely simplified and to the point, all I can add is PLEASE guys be very carefull. This is not a story, someone thinks he is doing good and then.......

We regularly have fasteners tested and EVERY SINGLE Zn plated or Cr plated 12.9 we have tested FAILED. Some were "guaranteed" to have been baked.....( de-brittled) It is not that the bolts are just weaker, the scary stuff is they become as brittle as glass. Elongation ( stretch ) can easily drop to 1-3 % where in a normal High tensile bolt the Elongation must be more than 10 % ( for a 12.9 )

Cadmium plated bolts are generally higher value items and the process of Cd plating is extremely regulated so it is only done by big industry players.

To add to the initial posting..If I have to choose I will rather choose a slightly rusted "black" 12.9 but NEVER EVER will I trust a shiny 12.9 bolt......

[ystervark7]

Just some comments ( quoted from this site:
http://www.synergysupplycm.com/pdf/Hydr ... 20like.pdf )

There are several classic ind i c a t o r s o f a r e a l h y d r o g e n embrittlement failure. If any of these factors are missing, then the failure must be attributable to something other than hydrogen embrittlement. These five characteristics are:
1. The failure must be a DELAYED failure. The delay is generally from one to 24 hours after installation. If the failure occurs during installation, it is definitely NOT caused by hydrogen embrittlement.
If the delay is a week or later after installation, the cause is probably stress corrosion and not hydrogen
embrittlement. The failure modes and metallurgical appearances are identical in stress corrosion failures
and hydrogen embrittlement failures. The primary indication of stress corrosion is that the delay is longer
than 24 to 48 hours after installation. More details on stress corrosion will be dealt with in a later article.
2 . The fasteners must be hardened to at least Rockwell C37. Unhardened fasteners never suffer from
hydrogen embrittlement. Fasteners that have a hardness of Rockwell C36 or less are extremely unlikely to
ever suffer from hydrogen embrittlement.
The greater the fastener’s hardness above Rockwell C36, the greater the chance that the part will experience a failure attributable to hydrogen embrittlement.
Inch socket head cap screws, L-9 bolts, metric property class 12.9 screws and bolts, and spring steel
washers and roll pins are the common fasteners which
are most susceptible to hydrogen embrittlement failures because of their high hardness.
3. The failed parts must be electro-plated. The creation of the hydrogen in the metal fastener is caused
by the cleaning processes where acids are used and not by the actual application of the plating. I have heard
of many cases where non-electroplated parts were suspected of hydrogen em-brittlement, but I am not aware
of any that were confirmed to have failed from hydrogen embrittlement.
4. The appearance must be that of an “intergranular” failure. Look closely at the surface of the broken areas
in the photograph in this article. The surface of the failure looks relatively smooth with a texture that looks
like the surface of emery cloth. If you look at it under magnification, you see that the surface has a crystalline appearance with many sharp faces or facets. Some describe the appearance as being similar to broken rock candy.
A bolt or screw failure caused by excessive torque or tensile stresses produces a failure surface that has
peaks and valleys in it which is referred to as “dimpling.” This is also technically referred to as a “ductile”
failure as opposed to a “brittle” failure which is characteristic of a hydrogen embrittlement failure. If you look
at the failure surface of a bolt or screw failure surface
that has peaks and valleys and/or a swirling appearance. The failure is highly unlikely to be attributable to
hydrogen embrittlement.
5. The failure location is either where the fastener’s head connects to the body or in the threads within two
thread pitches above where the bolt’s thread engages the mating thread

also see that hydrogen embrittlement are often false blamed.
http://www.synergysupplycm.com/pdf/Hydr ... 20Info.pdf

It is still better to avoid using plated high strength bolts but if they have been in your plane for a while it is very unlikely that they will fail due to hydrogen embrittlement.

[Martinvt]

Hi there,

Does Nickel plating have the same effects on the bolts?

I can do Nickel plating and was just wondering if it can also weaken the bolts??

Thanks

[wiskeyfoxtrot]

Would it not be better to use "aircraft spec" bolts on any heavy stress area ? These bolts are tried & tested for this type of application.

Just a thought ?

Later

[KFA]

Would be ideal to use aircraft spec bolts but most local manufacturers use metric bolts and AN bolts are imperial. Not sure about the nikel plating so do not want to even guess an answer.

[wiskeyfoxtrot]

Manfred at Aviate products ( 011 ) 9489883 ( used to manufacture the Raptor trikes ) used only aviation spec bolts on the Raptors, mabey he could help with suppliers ?