AMS QQ-P-416 history of bake times, carburized parts 4

[dave048]

I have a pinion gear made out of 9310. It is carburized and final tempered at 300 deg F (Rc62). Currently the post plate hydrogen embrittlement bake in AMS-QQ-P-416 for this type of part says 23 hours at 275F. I have evidence that this used to be 12 hours. Is that true? and further back in time was it another bake time?
Thanks,

 

[TVP]

The original QQ-P-416 revision F (1991) shows 23 hours in Table 1:

http://assist.daps.dla.mil/docimages/A/0000/0005/1006/000000134400_000000180284_TTMGJYCHYL.PDF

 

[israelkk]

I believe it is unwise to use Electro deposited cadmium plating per QQ-P-416 on parts heat treated to more than 160 ksi (the carburized layer strength is way higher and probably over 300 ksi). You are risking cracking the carburized layer even before you bake them. You should use vaccum deposited cadmium such as per SAE-AMS-C-8837 or diffused nickel cadmium per AMS 2416, etc.

 

[dave048]

Aerospace parts are routinely core hardened then carburized. The carburized surface is the only surface that reaches 62 Rc. Many are Cadmium plated too.

 

[dave048]

Thanks TVP.
I have one other question that could could cost someone thier job (fortunately not me)and a lot of money.

I know that the bake is to prevent embrittlement and to prevent a decrease in ductility caused by the plating process.
The question is; What would happen to a cadmium plated part that was only baked for 5 hours at 275 deg F?

These gears have been in the field for a few years with no problem or evidence of cracks on the ones that have been returned for regular maintanence.
I know they will be reinspected and probably scrapped. The question is whether the recall should take place during regular maintenance or immediately. I have to suggest one or the other.

 

[unclesyd]

Some of you more familar with the Mil Spec jargan take a look at note three (3) on page 15 of the reference by TVP.
I read that to say the if the operating temperature is less tha 275F then the min can be 5 hrs at 275F.

 

[CoryPad]

unclesyd,

That is for stress relief before plating. The embrittlement relief (after plating) is 23 hours.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TVP]

Dave,

Did your company agree to meet the requirements of AMS-QQ-P-416 based on its inclusion on an engineering drawing or specification? If the answer is yes, then your company is obligated to at least notify your customer and discuss the issue. Perhaps increased inspection will be sufficient instead of complete replacement.

 

[dave048]

Thanks, TVP.
This is a overhaul facility.
What I was wondering is what would happen to the gears. But it's probably too hard to say. Apparently they won't crack under normal usage but we are not sure whether they have experienced heavy loads. Then the question is What now? Do we suggest scrapping the gears or NDT inspect and re-use.
Is there a document that tells us what would happen to the gear at different bake times? Maybe the research that led to the 23 hours called out in the spec?

TO unclesyd and corypad,
I think that same misinterpretation is what got us into trouble in the first place.
Thanks to all of you.

 

[swall]

Dave048--keep in mind that hydrogen embrittlement is mainly a failure mode that occurs under sustained tensile stress. With a gear, this is not the sort of loading you would have, except possibly due to residual stresses somewhere at the case to core interface. Hydrogen embrittlement also tends to exhibit "infant mortality" behavior. You will get the failures early on, say within 200-400 hours. You may not have as big of a problem as you fear. But, as TVP said, you need to bring the issue to your customer's attention.

 

[dave048]

swall- thanks for the post

I am very interested in the "infant mortality". Is there a paper that explains that condition? This condition seems to explain some of what we have experienced.
Also is there some work that explains how the 23 hours bake was selected in QQ-P-416? Maybe something that will explain results when various bake times were tested.

Yes,the customer will have to be notifed but that is being taken care of, I am in an engineering advisory position with this company.
I have access to some vary talented gear designers and metalurgists. The reason I am asking for information is that I am a Manufacturing Engineer and would like to bring something to the table when I discuss this with them.
I doubt if they have the ENG-Tips resource of you talented people.
Thanks guys

 

[CoryPad]

dave048,

The subject of Hydrogen Embrittlement (or Hydrogen Assisted Cracking or Stress Corrosion Cracking) has been studied extensively, and published widely. I am not sure there is a single reference that will answer your questions, but there are many references from which you can learn. A search of this site will reveal many references. You can use Google Scholar to find scientific literature. ASTM, ISO, and SAE all have consensus standards covering this subject. Good luck.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[israelkk]

I have seen high strength parts shuttered or cracked between the plating and the baking. I still believe that a low hydrogen coating or a vaccum coating is the solution.

 

[dave048]

israelkk-
Thanks for the post but changing the type of plate process is not an option. This is an overhaul facility. The OEM sets the plate process type.
Electrodeposited cadmium plate combined with carburized surfaces is very common in aerospace gearing. The process is very mature. There is a switch away from cadmium plate lately but its due to evironmental concerns not because of cracking before the bake.
Is the bake you are talking about done within 4 hours of plating?
Maybe you could start another thread to discuss your experience. I would be interested in discussing it.

All-
This has been a very elightening and intersting discussion. I now have a lot of leads to investigate.