Cad-plate fatigue life 7

[macoovacany]

As standard practise, we specify the cad-plate of all steel parts we design to QQ-P-416F. However, it has come to our attention that this may dramatically reduce the fatigue life.
On page 287 of "Mechanical Engineering Design" by Shigley and Mischke it states "Metallic coatings, such as... cadmium plating, reduce the endurance limit by as much as 50 percent."
As a note, AN bolts are CAD plated.
Any thoughts, comments or, hopefully, an explanation?

 

[tbuelna]

macoovacany:

Cadmium plating provides excellent corrosion protection, that's why it was widely used. Cadmium can also be toxic, so that's why it is not being used for new designs. The typical material now used for medium strength fasteners in the aircraft world is A286 cres (usually with silver plating on the nut to prevent galling).

I've never seen where the fatigue endurance limit was adversely affected by a properely applied plating. However, plating ferrous materials with tensile strengths above about 150ksi requires a post plating embrittlement relief. This consists of baking the part in an oven, for a specific time and temperature immediately after plating, to drive off any hydrogen in the surface of the part.

Here's a link to a typical embrittlement relief spec:

http://www.raytheonaircraft.com/scm...rements for Chemical Processes Revision -.pdf

Hope that answers your question.

Terry

 

[yates]

I am not aware of any literature covering a possible reduction in fatigue life of steel bolts due to cad. plating. As a manufacturer, I have never observed such a reduction in specification imposed fatigue-life tests. However, one should be careful - these tests are manufacturing acceptance tests which are really accelerated tests - the loads used are much higher than in service and are applied for less cycles.
Providing post-plate embrittlement relief and pre-plate stress relief has been carried out properly, I can see no reason for the assertion stated in your 'Mechanical Engineering Design' Is this a recent publication ? Furthermore, how can cad. plate be deleterious to fatigue resistance when the very fact that it is there to prevent corrosion means it must afford some protection against corrosion-induced fatigue initiation.
The best thing is to get away from cad. plate on steel bolts altogether and use A286 as tbuelna suggests, or precipitation-hardening steels (15-5PH, 17-4PH, 17-7PH)if even higher strength is required.

 

[tbuelna]

Yates:
The precipitation hardening CRES alloys 15-5 or 17-4 are never used for fasteners, due to stress corrosion problems under sustained tension loads. A286 (or sometimes even 718 Inconel) is the material of choice. I have ocassionally seen 13-8Mo used, since it seems to exhibit fairly low notch sensitivity.

As a side note, most aircraft structural fasteners (Huck bolts, Hilocks, etc) are now titanium alloys. The cost break point on a typical commercial aircraft, like a 737, is about $300 per pound of aircraft weight. Titanium materials will cost about $50 per pound, so their use becomes quite cost effective.

regards,
Terry

 

[graemew]

tbuelna,
Just a little note about silver plated nuts.
They were by Textron for the assembly of the BAe146 wing skins to the spars and the result was a huge problem of multiple site corrosion damage inside the wings on the spar flanges. It has resulted in the scrapping of badly affected aircraft and mayor repairs on others and just about finished off the BAe 146 as a design.
Use silver plated nuts in high temperature applications (like on engines ), but NEVER use them on aluminium, especially in fuel tanks (just look at where silver sits in the galvanic series ).
Cad., Cat-Ti, or IVD aluminium is definitly the way to go for fasteners, although Airbus has recently been having notable success with a kind of ferrous coating on clip nuts (I don't recall all the detail but it was covered in an article in their company magazine a couple of years ago ).
If you seek more info about the BAe146 problems talk to National Jet in Adelaide, Australia.
Regards,

 

[avio]

Cadmium plating to some high strength ferrous alloys can indeed cause some embrittlement; a relief in an oven can be a remedy to avoid this, like already mentionned in other replies. Another possibility is to have the part first Nickel plated; then Cadmium plated. The Ni thickness must be such that the Cadmium diffuses into the Nickel and does not diffuse into the base material and cannot cause this embrittlement. If you have access to P&W docs, check SPOP 25 in the SPM.

 

[Frederick]

Cad plating as well as any electroplating and some electroless plating will induce hydrogen into the metal which causes the embrittlement. So nickle plating can be detrimental also. Proper and prompt baking after plating will "drive" the hydrogen out. Hydrogen embrittlement is so insidious that it can literally rip components apart all by itself. It is necessary to avoid this at all costs on any machine where failure could cause injury or loss of life. For instance, it can be induced by certain common cleaning chemicals.

If Cad plating is followed by proper post plating treatment, its use will have minimal affect on the fatigue life. Look out for other platings, such as chrome, though, as chrome plating is full of small cracks that can propogate into the metal. They will reduce fatigue life just by being there

 

[jetmaker]

Another note to add...

Do not use Cad plating in parts that are subject to extreme temperatures as this can result in cad diffusion into the base material and result in cad-embrittlement.

 

[avio]

Correct, that why the spop 25 needs the Ni layer, to have the Cd diffused into th Ni , avoiding the diffusion in the base material

 

[aviat]

The following article warns about the use of cad. plated bolts in titanium structure.

http://www.eaa1000.av.org/technicl/corrosion/ticad.htm

 

[pipsqueek]

if the question is related to electo plating, not just cadmium, then in some circumstances fatigue life can be affected, eg. chromium plating. In this case the chrome is subject to fatigue cracking long before the parent material, but the crack in the plating can propogate into the parent across the plating surface.

 

[DELTA98]

Hi-Kote 1 is a phenolic-based aluminum coating with excellent corrosion and high temperature resistance (to 450° F). It is a solvent based material that is applied by spraying and is then oven cured to fully develop it's properties. It has excellent resistance to fuel, hydraulic fluid and solvents, and contains softeners to reduce the brittleness of the coating and to impart it's excellent adhesive properties. The corrosion protection is powered via chromated compounds that have been long accepted as effective corrosion inhibitors.

Hi all
When I last worked @ Boeing we were using the Hi KOTE FASTENERS ON THE C-17. Though Lightening strike were still being tested for commerical use aircrafts. Here is a link you may find intresting.

http://www.hi-shear.com/fastener_hk_hk1.htm

Hi-Kote 1 is used as a protective coating on Titanium, Nickel based (Inco 718) and Cres alloy (A-286, 300 series stainless) products, most of which are externally threaded fasteners. Hi-Kote 1 facilitates the installation of fasteners as it's durable surface prevents galling during push in. It has also been successfully used on aluminum rivets to provide primary sealing around the fastener and eliminate the need for wet sealant.

Hi-Kote 1 is widely accepted as the leader in protective aluminum coatings for aircraft fasteners, and conforms to the requirements of Hi-Shear Specification 294.

 

[Kenneth]

Regarding titanium and cadmium, note that "high" temperatures are not required for failure:

http://www.dstan.mod.uk/data/00/970/01040208.pdf

I have also seen fastener service failures of cadmium plated fasteners that initiated at localized plating "arc burns".

 

[Kenneth]

The link in the post immediately above no longer works (for some reason). Here's one that does:

http://www.dstan.mod.uk/data/00/970/01040308.pdf

 

[WKTaylor]

Guys per AMS-QQ-P-416.... I think this spec paragraph says-it-all...

3.2.1 Stress relief treatment: Unless otherwise specified (see 6.2), parts which have been machined,
ground, cold formed, or cold straightened after heat treatment, shall receive a stress relief
treatment in accordance with Table I or Table IA, as applicable, prior to shot peening, cleaning, and
plating for the relief of damaging residual tensile stresses. For fastener hardware, cold working of
the head-to-shank fillet radius and thread rolling after heat treat produce compressive stresses and
a stress relief treatment is not required on parts thus affected.


Regards, Wil Taylor