Over Ageing of Inco 718 6

[COKA]

We have a welded INCO 718 part that was over aged by 12 hours. It was supposed to be in the furnace for 5 hrs at 1400F and for 3 hrs at 1200F, but instead of 3 hrs, it was left for 15 hrs. The hardness was found to be about 28HRC after ageing. Any comments on this? Thanks...

 

[Metalguy]

What hardness is required? What is the service temp.?

 

[unclesyd]

Where did you get that heat treating procedure?

 

[COKA]

Metalguy and unclesid,
This is an APU part, and the procedure is from the OEM manual. I don't have more information then that, the APU Engineer is on Vacation! The APU is an engine, but I'm not sure where exactly this part goes within the engine. I assume that since the part is nickel based, it is used in high temperature areas. As for the exact service temperature, my guess would be as good as yours.... 1600°F .... 2000°F...?? Very hot...

 

[unclesyd]

The reason for the question about heat treatment is that it is new to me. We used a lot 718 parts. mostly special fasteners, that operated in the 1200°F-1350°F and none used this heat treatment.

I’m at a loss on this heat treatment call out, though it sounds like some precipitation procedure that’s not in my information. I don't think it hurt the material, but as I don't understand the heat treatment I can't say this without some reservation. Whether you accomplished the desired results of the specified heat treatment procedure is another question.

I would wait for the engineer in charge to make a decision or you could call the producer of the material and ask them.

Our heat treatment for all 718 parts was:
1 Hr @ 1775°F air cool; 8 Hrs @ 1325°F cool @ 100°F/Hr to 145°F and hold 8 Hrs and air cool in still air.

What is the end use of Auxiliary Power Unit?

 

[COKA]

Auxilary Power unit is the small engine at the back of the aircraft used to give power when the engines are not running. It is because of the APU that we have light, AC, TV and so on inside the plane. Also, the APU is used to start the first engine. The APU is started from the ground.

 

[Metalguy]

COKA,
I don't think you really have a problem here. There are actually 2 diff. "standard" precip. HT's for this alloy. The easy one is 8 hr.@ 1450 deg.F. The double HT as per Unclesyd is 8 hr.@ 1325, then slow cool to 1150 f for 8 more (he has a type in his post).

Your "oddball" HT seems to have been spec'd by someone wanting some combo of properties, and I doubt the overaging at that low temp. will have much effect.

BUT, if you accept/install these parts, make sure the responsible engineer gets informed BEFORE the APU is started!

 

[Metalguy]

The usual Rc for "correctly" aged 718 is ~40 (IIRC), so your part is quite a bit softer/weaker. Do you have a Rc requirement?

I see I made a typo in my previous post on Unclesyd's typo! Guess our fingers are overworked!

 

[unclesyd]

Do you have to have any certification on the APU?

What little experience I have had with military and the FAA it's going to be hard to get these parts by an inspection.

Metalguy,
I don't have to 1450°F precip. treatment in any of my information, all manner of solution treatments though. If it would be applicable in our case it would save a lot of down time if we ever needed 718 parts in a hurry. Do you have a reference as all my contacts at INCO, in fact everywhere, have gone on to better but not necessarily greener(multicolored) pastures.
Certainly would appreciate it.
Thanks

 

[Metalguy]

Unclesyd,

Here's one source I found.

http://www.allvac.com/allvac/pages/PDF/

tech/NI-135%20718-OP.pdf

 

[Metalguy]

Also, IIRC, the ASTM lists both HT for 718. The 1425 is better for SCC resistance, while the 1325-1150 is stronger.

 

[Metalguy]

Damn, make that 1450, not 1425!

 

[unclesyd]

COKA,
good luck in getting your parts ok'd.

Metalguy,
It looks with a little a more checking COKA might be ok.

This appear to be a modification of the heat treatment once recommended for improved cryogenic properties.

They still recommend a treatment similar to the one I posted for tensile strength and toughness at elevated temperature.

Our main use is for holding a shroud on an 17,000 Hp @1200°F and 12,000 RPM recovery turbine. We have to stick to the original heat treatment. There are 36 7/8" x 51/2" silver plated set in studs with an interference fit. They don’t come out if the nut end is damaged. We also use longer ones (24&quot to mate the wheel with a curvic coupling.

Again thanks for the info.

 

[kenvlach]

Metalguy’s link has a typo (blank space); should be

http://www.allvac.com/allvac/pages/PDF/tech/NI-135 718-OP.pdf

This data sheet gives the single step aging:
“HEAT TREATMENT
The heat treatment consists of solution treating followed by a single aging treatment. Solution treat at 1,850 - 1,900° F (1,010 - 1,038° C) for 1 hour. Rapid cool. Age at 1,425 - 1,475° F (774 - 802° C) for 6 -8 hours hours. Air cool.”
[The two stage aging procedure is also given, on page 2.]

“HARDNESS
The hardness in the solution treated condition is about 20-25 HRc which increases upon aging to about 40 HRc max.”

The 4-page technical data sheet at

http://www.alleghenytechnologies.com/ludlum/documents/al718.pdf

gives the two-step aging procedure and a few more details:

“The optimum temperature for annealing or solution treating Altemp 718 alloy is determined by the relative importance of short or long time elevated temperature mechanical properties….best long time stress rupture or creep properties are obtained by solution treating at 1900 to 1950°F (1038-1066°C) and aging. If excess phases are present in the microstructure, they are more readily dissolved by the higher temperature solution treatment.
The best aging treatment following solution treatment is to hold the alloy at 1325 to 1350°F (718-732°C) for 8 hours, followed by furnace cooling to 1150 to 1200°F (621-649°C), holding for 8 hours and then air cooling.”
This is also the procedure given in ASM Metals Reference Book, 3rd Edn.

Also,
“Altemp 718 alloy is subject to Laves phase (Fe₂Cb) formation during solidification. This phase reduces the strength and toughness of weldments. This phase is dissolved by a 1900-1950°F (1038-1066°C) solution heat treatment.”

COKA’S results suggest that the welding resulted in the formation of undesirable precipitates which grew during the over-aging. The HRC 28 value is way, way below the max. achievable, HRC 40 (roughly, a UTS of 130 vs. 180 ksi). My recommendation is to solutionize and re-age after approval of the APU Engineer. Possibly, an Engineering Change Order has to be run past the FAA, so mind your Ps and Qs!!!

 

[kenvlach]

Metalguy & unclesyd,
I should have mentioned your recommendations re the APU Engineer and the FAA. So, some stars.
Ken

 

[unclesyd]

COKA Let us know what the outcome of the parts are.
It might help someone later.


It is interesting as to the call-outs then and now on the heat treatment of 718. I dug out part of an original print (1980's) of one of the studs we used with the following heat treating procedure. All 718 parts for the machine were treated the same.

1 Hr @ 1775°F air cool; 8 Hrs @ 1325°F cool @ 100°F/Hr to 1150°F and hold 8 Hrs and air cool in still air.

The accompanying notes mention 3 times on holding temps to
± 5°F. There were only two heat treating companies in Florida approved to heat treat the studs. Nowhere is there a call-out on the hardness of the parts.

Anecdotal:
This thing, steam turbine, air compressor, and recovery turbine, was designed for AIRCO by ex Pratt & Whitney engineers. As a result we have model 1, # 1, and the only 1
It has a total of 25,000 Hp on a single shaft operating at 12,000 RPM @ 1200°F on one end and 600°F on the other end with every metal in the book in between.

 

[kenvlach]

unclesyd, many thanks for sharing your wealth of experiences and insights.

With regard to the change in solutionizing heat treat procedure. It seems that the higher solutionizing temperature is to better dissolve primary solidification phases, and the more rapid cooling minimizes formation of the Laves phase. The broadening of the ± 5°F heat treat regimes is a natural consequence of better knowledge of the phase diagram – knowing the temperatures of formation and dissolution of desired and undesired phases:

“Laves phase formation is a significant problem for superalloy 718. It forms during ingot solidification or welding. It is Fe₂(Nb,Mo) and is stabilized by Si. One of the functions of C addition (>0.12%) in superalloy 718 is to promote NbC formation during final solidification, thereby preventing the formation of Fe₂Nb eutectic phase. Alternatively, the problem of Laves phase formation can be reduced by lowering the Nb content in the weld or filler metal.” – Physical Metallurgy Handbook, pp. 10.79-10.80 (2003). References for this paragraph date from 1991-1999.

Reviews of the Laves phase in alloy 718 are given by J. N. Dupont et al., Met. and Mater. Trans., vol. 29A, pp. 2785-96 & 2797-2806 (1998).

Inconel/Allvac 718 is a relatively new alloy which has been intensively studied within the past 10 years. For an idea of the amount of research on the aspect above, see

http://ceo.tms.org:8765/site/query....&pw=100%&ws=0&qm=0&st=1&nh=100&lk=1&rf=0&rq=0

Finally, note that alloy 718 was derived from 625, and that newer alloys 706, PWA 1471 and Rene 220 have since been derived from 718. “As a natural derivative of Alloy 718, Alloy 706 has gained stature and usage in land-based combustion turbines for generating electricity.” – from a synopsis for FIFTH INTERNATIONAL SYMPOSIUM ON SUPERALLOYS 718, 625, 706, AND DERIVATIVES, June 17-20, 2001, Pittsburgh, Pennsylvania.

 

[Metalguy]

And if THAT post doesn't deserve a star, nothing does!

Merry Christmas!

 

[COKA]

Metalguy, kenvlach, unclesid,

Thank you all for the inputs, it is greatly appreciated! The APU Engineer is still on vacation. We have another part, the same as the part that was overaged, which was welded at the same locations and has to go through the same heat treatment process. I asked for hardness measurements after heat treatment for comparison. I will let you know of the outcome.

COKA

 

[COKA]

OK
Hardness was taken on the part#2, which was welded at same locations and was heat treated the same way, and it was found to be about 28.8HRC. There was no major changes in hardness. I do not see major problems occuring from this. The responsible Engineer will be advised. Thank you again for your help!