inconel 718 threaded bolt@ 1000 F + 0.2 atm O2, gas embrittlement ?

[davefitz]

Can anyone advise if there is a real issue with embrittlement of inconel 718 at 1000F (538 C) at 0.2 atm O2?

Our application is bolting for a flange , external to a boiler. ASME sect II provides allowable stresses for such bolting to over 1100F without any notes or advice.

We heard recently that GE 7FA gas turbine blades have failed when using INco 718 at 1100 F at 3.5 atm O2. Inco 718 is frequently used in aircraft bolting applications without issues.

"Nobody expects the Spanish Inquisition! "

 

[desertfox]

Hi davefitz

I am not aware of any problems and this product catalogue suggests the material is okay for the temperature:-

http://www.tachen.com/catalog/pdf/nickel_alloy/special_metals_product_handbook.pdf

I assume the bolts have been checked for the stress at temperature, ie I assume there is a designed preload so that nobody can over tighten the bolts on assembly.

desertfox

 

[EdStainless]

The way that the bolts are manufactured is critical.
If they had any sharp features or if the heat treatment wasn't right then all sports of things can go wrong.

On ones that have failed have you seen a failure report? The first thing that I would do is grab the intact portion and run a stress rupture on it.

This alloy is very reliable, the properties repeat with great precision.
Make sure that the bar stock is from a domestic source, has been vacuum remelted, and that stress rupture tests were run on actual fasteners.
I see very little risk.

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Plymouth Tube

 

[metengr]

http://www.dtic.mil/dtic/tr/fulltext/u2/a340457.pdf

 

[davefitz]

Thanks for the referenced report and other comments. The issue was raised initially due to graphic presentations of turbine failures by R. Munson, and a more recent paper by David Woodford "gas Phase Embrittlement and time dependent cracking of nickel based superalloys"( corrosion 2005 # 05418).

Regardless of what metallurgical solution we have applied to this issue ( P91-316 H dmw at GT24 HP OTC) there is always a "fly in the ointment". I will attempt to find a method to pro-rate the oxidation rate at 0.2 atm O2 /1000F down from the known failure history of the 7FA ( 22,000 hrs at 1100 F at 3.5 atm O2) and determine if it is neccesary to use a low temperature vacuum funace application of aluminum ( Oerlikon process,ion injection, 1-3 um thick) , but that of course may induce other issues.

Its too bad the nimonic bolts are not listed in sect II.

"Nobody expects the Spanish Inquisition! "