E Rivets Accidentally Heat Treated

[ozstressman]

We have had a batch of E rivets (7050-T73) that have been supplied in bags labelled DD. The E rivets have then been mixed up with the DD rivets. The result is that a bunch of E rivets have been heat treated and installed as per the DD procedure. From everything I have read, E rivets are not supposed to be heat treated (e.g. T.O. 1-1A-9, Para 3-37 (change 2)).

After installation the E rivets are supposed to be 7050-T731 (Ref. MMPDS-01, Table 8.1.2(b)). However, our rivets have been solution heat treated, cold worked (during installation) and then naturally aged, which would indicate that they are now 7050-T3 (or T31?).

I cannot find any material properties for 7050-T3 etc, so I may have no choice but to instruct them to rip out all the affected rivets. Does anyone know of any information that might help me out?

Also, as an aside, the E rivets had a chem-film finish, so during heat treatment the finish was burnt and discoloured. I guess the bad news is that the tradesmen installed them despite their different appearance both before and after heat treatment. The good news is that at least we can easily tell which ones are the heat treated E rivets by their burnt appearance.

 

[rb1957]

whilst i'm sure will taylor will be able to give you a complete answer, here's my 2c worth ...

i'm assuming we're not talking about many rivets affected, maybe 100 or so ? what if you did your own shear test with some. i'd set up a symmetric double shear specimen to make the test simple. you've got MMPDS to tell you the E strength ... should be good to go. personally i think 6 samples should give you a meaningfull answer.

 

[rerig]

ozstressman

DD Rivets (2024-T4)
E Rivets (7050-T73)

What occured is, you have solution heat treated 7050 as if it were 2024. 920F then chilled to -10F.
The 7050 does not have a room temp ageing process to a "T" temper. The 7050 rivets were in an AQ condition (maybe "O" condition) The cold working did not bring the 7050 rivets to a -T3 condition. They did not have the precipitation heating process to bring to a "T" condition.
The "E" rivets should be very soft to drill out.

However, if the sample testing confirms a harder condition maybe it can be justified leaving them in.
I would not. (I'm open to verification)

Rerig

 

[WKTaylor]

ozstressman...

Sounds sadly familiar...

The 2024 [DD**] and 2017 [D**] rivets will respond to re-solution heat treatment and quench by spontaneously age hardening to -T42... assuming proper heat treating methods are used. When driven in the "AQ" [As Quenched, unstable temper] condition [usually within a few minutes of the quench... or longer if frozen] the combination of strain hardening [driving] and spontaneous age-hardening [precipitation hardening] should actually increase both the FTu & FTy of the rivet relative to "just" age hardening [by a few +KSI].

On the other hand, 7050-T73 [“E” for MS or “KE” for NAS] rivets when re-solution heat treated and quenched, will enter the stable "W" temper. Without further age hardening heat treatments, the rivet will essentially remain in a low strength slightly strain hardened state [after driving]... and are worthless for engineering purposes, since there are NO allowables for this "state/condition".

YOU SHOULD MAKE EVERY EFFORT TO ID WHERE THESE HEAT TREATED "E" RIVETS HAVE BEEN INSTALLED AND REMOVE THEM FOR SAFETY OF FLIGHT [AIRWORTHIENESS].

What is troubling is that these (3) rivet alloys mentioned above all have distinctive head markings exactly for the purpose of identification: DD [double raised dash]; D [single raised dot]; and E [raised ring].

** WARNING: ONLY RIVETS WITH AN ANODIZE FINISH SHOULD BE HEAT TREATED... ALL OTHER PROTECTIVE FINISHED WILL BE DESTROYED BY THE HIGH HEAT PROCESSING LEAVING THE RIVET VULNERABLE TO CORROSION! REASON: THE RESIDUE ON THE RIVET PROVIDES no ISOLATION [RIVET IS BARE] AND FINISHES [PRIMER OR SEALANT, ETC] WILL NOT ADHERE TO WHAT IS "LEFT".

Note: I have been in XX-ALC facilities and have seen "AD" and "D" rivets mixed together in stock bins on-the-floor [several areas]. When I mentioned this critical issue to the shop supervisors... they shrugged and said "guess that's why some are easier to drive". When I mentioned this to other engineers they gave me this "I've tried to do something about this... but the problem spontaneously re-occurs". SAD. And dangerous.

I am afraid that this is also a training and "professionalism" issue [=Production Acceptance Certification: PAC]. Some ALC mechanics just don't pay attention the same way licensed mechanics should: licensed mechanics understand that they are civilly and criminally liable if the work is substandard or improper parts are installed (civil aviation). It is obvious that the mechanics who selected the rivets for heat treatment, the heat treater who did the SHT/Q and the mechanics installing the rivets were oblivious to what the coding means RE the various alloys.... and the importance of following tech data. On operational Air Bases [with GI mechanics] this incident might actually be called a “HAP” [high accident potential] mishap.

Yes, I was an ALC aero engineer before being lured to a big company.



Regards, Wil Taylor

 

[ozstressman]

Thank you all for your input. I made sure that the problem was fully looked into, and luckily the installation of heat treated E rivets turned out to be an isolated occurence. There was only a few of the bad rivets sprinkled in one row of fasteners, and they were replaced.

Apparently, it is not uncommon for the E rivets to get mixed up in the DD rivets, but luckily they are easy to distinguish. All the trademen put their hands on their hearts and said that they always throw the burnt E rivets away. After inspections of the areas where they were working we didn't find any more heat treated E rivets so it looks like thats the end of that little problem. There still remains the root problem of parts control, and after this episode I hope that things are improved in that department.

It will be obvious from my post that my knowledge of metallurgy is quite basic, so I was interested to find out that the 7050 has no natural age hardening process. This means that any heat treated E rivets would remain dangerously soft and also without corrosion protection, so I'm glad the mistake was picked up.

Once again, thanks for your help

Regards

Oz